SICStus Prolog User`s Manual

SICStus Prolog User`s Manual

SICStus Prolog User’s Manual

by the Intelligent Systems Laboratory

Swedish Institute of Computer Science

PO Box 1263

SE-164 29 Kista, Sweden

Release 3.8

October 1999

Swedish Institute of Computer Science

[email protected]

http://www.sics.se/sicstus/

Swedish Institute of Computer Science

PO Box 1263

SE-164 29 Kista, Sweden

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by SICS.

Introduction

Introduction

Prolog is a simple but powerful programming language developed at the University of Marseilles [Roussel 75], as a practical tool for programming in logic [Kowalski 74]. From a user’s point of view the major attraction of the language is ease of programming. Clear, readable, concise programs can be written quickly with few errors.

For an introduction to programming in Prolog, readers are recommended to consult [Sterling

& Shapiro 86]. However, for the benefit of those who do not have access to a copy of this book, and for those who have some prior knowledge of logic programming, a summary of the language is included. For a more general introduction to the field of Logic Programming see [Kowalski 79]. See

Chapter 3 [Prolog Intro], page 39 .

This manual describes a Prolog system developed at the Swedish Institute of Computer

Science. Parts of the system were developed by the project “Industrialization of SICStus

Prolog” in collaboration with Ericsson Telecom AB, NobelTech Systems AB, Infologics AB and Televerket. The system consists of a WAM emulator written in C, a library and runtime system written in C and Prolog and an interpreter and a compiler written in Prolog. The

Prolog engine is a Warren Abstract Machine (WAM) emulator [Warren 83]. Two modes of compilation are available: in-core i.e. incremental, and file-to-file. When compiled, a predicate will run about 8 times faster and use memory more economically. Implementation details can be found in [Carlsson 90] and in several technical reports available from SICS.

SICStus Prolog follows the mainstream Prolog tradition in terms of syntax and built-in predicates, and is largely compatible with DECsystem-10 Prolog and Quintus Prolog.

1

2 SICStus Prolog

Acknowledgments

Acknowledgments

The following people have contributed to the development of SICStus Prolog:

Christian Holzbaur, Key Hyckenberg, Per Mildner, Hans Nilsson, Mats

The Industrialization of SICStus Prolog (1988-1991) was funded by

Ericsson Telecom AB, NobelTech Systems AB, Infologics AB and

Televerket under the National Swedish Information Technology

Program IT4.

The development of release 3 (1991-1995) was funded in part by

Ellemtel Utvecklings AB

This manual is based on DECsystem-10 Prolog User’s Manual by

D.L. Bowen, L. Byrd, F.C.N. Pereira,

L.M. Pereira, D.H.D. Warren

See

Chapter 31 [CLPQR], page 313 , for acknowledgments relevant to the clp(Q,R) constraint

solver.

See

Chapter 32 [CLPFD], page 337

, for acknowledgments relevant to the clp(FD) constraint solver.

UNIX is a trademark of Bell Laboratories. MSDOS and Windows is a trademark of Microsoft Corp. OS/2 is a trademark of IBM Corp.

3

4 SICStus Prolog

Notational Conventions

Notational Conventions

Keyboard Characters

When referring to keyboard characters, printing characters are written thus: a, while control characters are written like this: ^A. Thus ^C is the character you get by holding down the h

CTL i key while you type c. Finally, the special control characters carriage-return, line-feed and space are often abbreviated to h

RET i

, h

LFD i and h

SPC i respectively.

Mode Spec

When introducing a built-in predicate, we shall present its usage with a mode spec which has the form name(arg, ..., arg) where each arg denotes how that argument should be instantiated in goals, and has one of the following forms:

:ArgName This argument should be instantiated to a term denoting a goal or a clause or a predicate name, or which otherwise needs special handling of module prefixes.

The argument is subject to module name expansion (see

Section 4.5 [Meta Exp], page 57 ).

+ArgName

This argument should be instantiated to a non-variable term.

-ArgName This argument should be uninstantiated.

?ArgName

This argument may or may not be instantiated.

Mode specs are not only used in the manual, but are part of the syntax of the language as well. When used in the source code, however, the ArgName part must be omitted. That is, arg must be either :, +, -, or ?.

Development and Runtime Systems

The full Prolog system with top-level, compiler, debugger etc. is known as the development system.

It is possible to link user-written C code with a subset of SICStus Prolog to create runtime systems.

When introducing a built-in predicate, any limitations on its use in runtime systems will be mentioned.

Function Prototypes

Whenever this manual documents a C function as part of SICStus Prolog’s foreign language interface, the function prototype will be displayed in ANSI C syntax.

5

6 SICStus Prolog

ISO Compliance

SICStus Prolog provides two execution modes: the iso mode, which is fully compliant with the International Standard ISO/IEC 13211-1 (PROLOG: Part 1—General Core), and the sicstus mode, which supports code written in earlier versions of SICStus Prolog. The execution mode can be changed using the Prolog flag language; see

Section 7.6 [State Info], page 137 . Note, however, that SICStus Prolog does not offer a strictly conforming mode

which rejects uses of implementation specific features.

To aid programmers who wish to write standard compliant programs, built-in predicates that are part of the ISO Prolog Standard are annotated with [ISO] in this manual. If such a predicate behaves differently in sicstus mode, an appropriate clarification is given. For the few predicates that have a completely different meaning in the two modes, two separate descriptions are given. The one for the iso mode is annotated with [ISO only], while the sicstus mode version is annotated with [SICStus only].

Chapter 1: Glossary

1 Glossary

abolish advice-point

A special case of breakpoint, the advice breakpoint. It is distinguished from spypoints in that it is intended for non-interactive debugging, such as checking of program invariants, collecting information, profiling, etc.

alphanumeric

An alphanumeric character is any of the lowercase characters from a to z, the uppercase characters from A to Z, the numerals from 0 to 9, or underscore (_).

ancestors An ancestor of a goal is any goal which the system is trying to solve when it calls that goal. The most distant ancestor is the goal which was typed at the top-level prompt.

anonymous

An anonymous variable is one which has no unique name, and whose value is therefore inaccessible. An anonymous variable is denoted by an underscore (_).

argument See predicate, structure, and arity.

arity atom

To abolish a predicate is to retract all the predicate’s clauses and to remove all information about it from the Prolog system, to make it as if that predicate had never existed.

The arity of a structure is its number of arguments. For example, the structure customer(jones,85) has an arity of 2.

A character sequence used to uniquely denote some entity in the problem domain. A number is not an atom. Examples of legal atoms are: hello * := ’#$%’ ’New York’ ’don\’t’

See

Section 3.1.1.3 [Atoms], page 40

. Atoms are recognized by the built-in predicate atom/1.

atomic term

Synonym for constant.

backtrace A collection of information on the control flow of the program, gathered by the debugger. Also the display of this information produced by the debugger. The backtrace includes data on goals that were called but not exited and also on goals that exited nondeterministically.

backtracking

The process of reviewing the goals that have been satisfied and attempting to resatisfy these goals by finding alternative solutions.

binding The process of assigning a value to a variable; used in unification.

blocked goal

A goal which is suspended because it is not instantiated enough.

body The body of a clause consists of the part of a Prolog clause following the ‘:-’ symbol.

7

8 SICStus Prolog breakpoint buffer

A description of certain invocations in the program where the user wants the debugger to stop, or to perform some other actions. A breakpoint is specific if it applies to the calls of a specific predicate, possibly under some conditions, otherwise it is generic. Depending on the intended usage, breakpoints can be classified as debugger breakpoints, also known as spypoints, or advice breakpoints, also called advice-points; see

Section 6.5 [Breakpoints], page 75 .

A temporary workspace in Emacs that contains a file being edited.

built-in predicate

A predicate that comes with the system and which does not have to be explicitly loaded before it is used.

callable term

A callable term is either a compound term or an atom. Callable terms are recognized by the built-in predicate callable/1.

character code

An integer which is the numeric representation of a character. SICStus Prolog supports character codes in the range 0..2147483647 (i.e. 2^31-1). However, to be able to input or output character codes larger than 255, one needs to use the appropriate wide character external encoding.

character code set

A subset of the set {0, ..., 2^31-1} that can be handled by the external encoding.

SICStus Prolog assumes that the character code set is an extension of the ASCII code set, i.e. it includes codes 0..127, and these codes are interpreted as ASCII characters character-conversion mapping

SICStus Prolog maintains a character-conversion mapping which is used while reading terms and programs. Initially, the mapping prescribes no character conversions. It can be modified by the built-in predicate char_conversion(In,

Out), following which In will be converted to Out. Character coversion can be switched off by the char_conversion Prolog flag.

character-type mapping

A function mapping each element of the character code set to one of the character categories (layout, letter, symbol-char, etc.), required for parsing tokens.

clause A fact or a rule. A rule comprises a head and a body. A fact consists of a head only, and is equivalent to a rule with the body true.

conjunction

A series of goals connected by the connective “and” (that is, a series of goals whose principal operator is ‘,’).

compactcode

Virtual code representation of compiled code. A reasonable compromise between performance and space requirement. A valid value for the compiling

Prolog flag.

Chapter 1: Glossary 9 compile To load a program (or a portion thereof) into Prolog through the compiler.

Compiled code runs more quickly than interpreted code, but you cannot debug compiled code in as much detail as interpreted code.

compound term

A compound term is a functor together with zero or more arguments. For example, in the term father(X), father/1 is the functor, and X is the first and only argument. The argument to a compound term can be another compound term, as in father(father(X)). Compound terms are recognized by the builtin predicate compound/1.

console-based executable

An executable which inherits the standard streams from the process that invoked it, e.g. a UNIX shell or a DOS-prompt.

constant An integer (for example: 1, 20, -10), a floating-point number (for example: 12.35), or an atom. Constants are recognized by the built-in predicate atomic/1.

consult creep

To load a program (or a portion thereof) into Prolog through the interpreter.

Interpreted code runs more slowly than compiled code, but you can debug interpreted code in more detail than compiled code.

What the debugger does in trace mode, also known as single-stepping. It goes to the next port of a procedure box and prints the goal, then prompts you for input. See

Section 6.2 [Basic Debug], page 71

.

cursor cut

The point on the screen at which typed characters appear. This is usually highlighted by a line or rectangle the size of one space, which may or may not blink.

Written as !. A built-in predicate that succeeds when encountered; if backtracking should later return to the cut, the goal that matched the head of the clause containing the cut fails immediately.

database The Prolog database comprises all of the clauses which have been loaded or asserted into the Prolog system or which have been asserted, except those clauses which have been retracted or abolished.

database reference

A compound term denoting a unique reference to a dynamic clause.

debug A mode of program execution in which the debugger stops to print the current goal only at procedures which have spypoints set on them (see leap).

debugcode

Interpreted representation of compiled code. A valid value for the compiling

Prolog flag.

declaration

A declaration looks like a directive, but is not executed but conveys information about procedures about to be loaded.

deinit function

A function in a foreign resource which is called prior to unloading the resource.

10 SICStus Prolog determinate

A procedure is determinate if it can supply only one answer.

development system

A stand-alone executable with the full programming environment, including top-level, compiler, debugger etc. The default sicstus executable is a development system; new development systems containing pre-linked foreign resources can also be created.

directive A directive is a goal preceded by the prefix operator ‘:-’, whose intuitive meaning is “execute this as a query, but do not print out any variable bindings.” disjunction

A series of goals connected by the connective “or” (that is, a series of goals whose principal operator is ‘;’).

dynamic predicate

A predicate that can be modified while a program is running. A predicate must explicitly be declared to be dynamic or it must be added to the database via one of the assertion predicates.

encoded string

A sequence of bytes representing a sequence of possibly wide character codes, using the UTF-8 encoding.

escape sequence

A sequence of characters beginning with \ inside certain syntactic tokens (see

[Escape Sequences], page 578 ).

export A module exports a procedure so that other modules can import it.

external encoding (of wide characters)

A way of encoding sequences of wide characters as sequences of (8-bit) bytes, used in stream input and output.

fact A clause with no conditions—that is, with an empty body. A fact is a statement that a relationship exists between its arguments. Some examples, with possible interpretations, are: king(louis, france).

% Louis was king of France.

have_beaks(birds).

% Birds have beaks.

employee(nancy, data_processing, 55000).

% Nancy is an employee in the

% data processing department.

fastcode Native code representation of compiled code. The fastest, but also the most space consuming representation. Only available for Sparc platforms. A valid value for the compiling Prolog flag.

filename An atom or a compound term denoting the name of a file. The rules for mapping such terms to absolute filenames are described in

Section 7.1 [Input Output], page 98 .

floundered query

A query where all unsolved goals are blocked.

Chapter 1: Glossary 11 foreign predicate

A predicate that is defined in a language other than Prolog, and explicitly bound to Prolog predicates by the Foreign Language Interface.

foreign resource

A named set of foreign predicates.

functor The name and arity of a compound term. For example, the compound term foo(a,b) is said to have “the functor foo of arity two”, which is generally written foo/2.

garbage collection

The freeing up of space for computation by making the space occupied by terms which are no longer available for use by the Prolog system.

generalized predicate spec

A generalized predicate spec is a term of one of the following forms. It is always interpreted wrt. a given module context:

Name all predicates called Name no matter what arity, where Name is an atom for a specific name or a variable for all names, or

Name/Arity the predicate of that name and arity, or

Name/(Low-High)

Name/[Low-High] the predicates of that name with arity in the range Low-High, or glue code Interface code between the Prolog engine and foreign predicates. Automatically generated by the foreign language interface as part of building a linked foreign resource.

goal

[Spec,...,Spec] the set of all predicates covered by the Specs.

A simple goal is a predicate call. When called, it will either succeed or fail.

A compound goal is a formula consisting of simple goals connected by connectives such as “and” (‘,’) or “or” (‘;’).

A goal typed at the top level is called a query.

ground

Name/[Arity,...,Arity] the predicates of that name with one of the given arities, or

Module:Spec specifying a particular module Module instead of the default module, where Module is an atom for a specific module or a variable for all modules, or head

A term is ground when it is free of (unbound) variables. Ground terms are recognized by the built-in predicate ground/1.

The head of a clause is the single goal which will be satisfied if the conditions in the body (if any) are true; the part of a rule before the ‘:-’ symbol. The head of a list is the first element of the list.

12 SICStus Prolog hook predicate

A hook predicate is a procedure that somehow alters or customizes the behavior of a hookable predicate.

hookable predicate

A hookable predicate is a built-in predicate whose behavior is somehow altered or customized by a hook predicate.

import Exported procedures in a module can be imported by other modules. Once a procedure has been imported by a module, it can be called, or exported, as if it were defined in that module.

There are two kinds of importation: procedure-importation, in which only specified procedures are imported from a module; and module-importation, in which all the predicates made exported by a module are imported.

indexing The process of filtering a set of potentially matching clauses of a procedure given a goal. For interpreted and compiled code, indexing is done on the principal functor of the first argument. Indexing is coarse w.r.t. big integers and floats.

init function

A function in a foreign resource which is called upon loading the resource.

initialization

An initialization is a goal that is executed when the file in which the initialization is declared is loaded, or upon reinitialization. A initialization is declared as a directive :- initialization Goal.

instantiation

A variable is instantiated if it is bound to a non-variable term; that is, to an atomic term or a compound term.

internal encoding (of wide characters)

A way of encoding wide character sequences internally within the Prolog system.

SICStus Prolog uses a technique known as the UTF-8 encoding for this purpose.

interpret Load a program or set of clauses into Prolog through the interpreter (also known as consulting). Interpreted code runs more slowly than compiled code, but more extensive facilities are available for debugging interpreted code.

invocation box

Same as procedure box.

leap What the debugger does in debug mode. The debugger shows only the ports of procedures that have spypoints on them. It then normally prompts you for input, at which time you may leap again to the next spypoint (see trace).

leashing Determines how frequently the debugger will stop and prompt you for input when you are tracing. A port at which the debugger stops is called a “leashed port”.

linked foreign resource

A foreign resource that is ready to be installed in an atomic operation, normally represented as a shared object or DLL.

Chapter 1: Glossary 13 list A list is written as a set of zero or more terms between square brackets. If there are no terms in a list, it is said to be empty, and is written as []. In this first set of examples, all members of each list are explicitly stated:

[aa, bb,cc] [X, Y] [Name] [[x, y], z]

In the second set of examples, only the first several members of each list are explicitly stated, while the rest of the list is represented by a variable on the right-hand side of the “rest of” operator, |:

[X | Y] [a, b, c | Y] [[x, y] | Rest]

| is also known as the “list constructor.” The first element of the list to the left of | is called the head of the list. The rest of the list, including the variable following | (which represents a list of any length), is called the tail of the list.

load To load a Prolog clause or set of clauses, in source or binary form, from a file or set of files.

meta-call The process of interpreting a callable term as a goal. This is done e.g. by the built-in predicate call/1.

meta-predicate

A meta-predicate is one which calls one or more of its arguments; more generally, any predicate which needs to assume some module in order to operate is called a meta-predicate. Some arguments of a meta-predicate are subject to module name expansion.

mode spec module

A term name(arg, ..., arg) where each arg denotes how that argument should be instantiated in goals. See

[Mode Spec], page 5 .

A module is a set of procedures in a module-file. Some procedures in a module are exported. The default module is user.

module name expansion

The process by which certain arguments of meta-predicates get prefixed by the source module. See

Section 4.5 [Meta Exp], page 57

.

module-file

A module-file is a file that is headed with a module declaration of the form"

:- module(ModuleName, ExportedPredList).

which must appear as the first term in the file.

multifile predicate

A predicate whose definition is to be spread over more than one file. Such a predicate must be preceded by an explicit multifile declaration in all files containing clauses for it.

mutable term

A special form of compound term which is subject to destructive assignment.

See

Section 7.8 [Modify Term], page 149 . Mutable terms are recognized by the

built-in predicate is_mutable/1.

name clash

A name clash occurs when a module attempts to define or import a procedure that it has already defined or imported.

14 SICStus Prolog occurs-check

A test to ensure that binding a variable does not bind it to a term where that variable occurs.

one-char atom

An atom which consists of a single character.

operator A notational convenience that allows you to express any compound term in a different format. For example, if likes in

| ?- likes(sue, cider).

is declared an infix operator, the query above could be written:

| ?- sue likes cider.

An operator does not have to be associated with a predicate. However, certain built-in predicates are declared as operators. For example,

| ?- =..(X, Y).

can be written as

| ?- X =.. Y.

because =.. has been declared an infix operator.

Those predicates which correspond to built-in operators are written using infix notation in the list of built-in predicates at the beginning of the part that contains the reference pages.

Some built-in operators do not correspond to built-in predicates; for example, arithmetic operators. See

[Standard Operators], page 581

for a list of built-in operators.

pair A compound term K-V . Pairs are used by the built-in predicate keysort/2 and by many library modules.

parent port

The parent of the current goal is a goal which, in its attempt to obtain a successful solution to itself, is calling the current goal.

One of the five key points of interest in the execution of a Prolog procedure.

See

Section 6.1 [Procedure Box], page 69

for a definition.

pre-linked foreign resource

A linked foreign resource that is linked into a stand-alone executable as part of building the executable.

precedence

A number associated with each Prolog operator, which is used to disambiguate the structure of the term represented by an expression containing a number of operators. Operators of lower precedence are applied before those of higher precedence; the operator with the highest precedence is considered the principal functor of the expression. To disambiguate operators of the same precedence, the associativity type is also necessary. See

Section 3.6 [Operators], page 49 .

predicate A functor that specifies some relationship existing in the problem domain. For example, < /2 is a built-in predicate specifying the relationship of one number being less than another. In contrast, the functor + /2 is not (normally used as) a predicate.

Chapter 1: Glossary 15

A predicate is either built-in or is implemented by a procedure.

predicate spec

A compound term name/arity or module:name/arity denoting a predicate.

procedure A set of clauses in which the head of each clause has the same predicate. For instance, a group of clauses of the following form: connects(san_francisco, oakland, bart_train).

connects(san_francisco, fremont, bart_train).

connects(concord, daly_city, bart_train).

is identified as belonging to the procedure connects/3.

procedure box

A way of visualizing the execution of a Prolog procedure, A procedure box is entered and exited via ports.

profiledcode

Virtual code representation of compiled code, instrumented for profiling. A valid value for the compiling Prolog flag.

profiling The process of gathering execution statistics of parts of the program, essentially counting the times selected program points have been reached.

program A set of procedures designed to perform a given task.

PO file A PO (Prolog object) file contains a binary representation of a set of modules, predicates, clauses and directives. They are portable between different platforms, except between 32-bit and 64-bit platforms. They are created by save_files/2, save_modules/2, and save_predicates/2.

QL file query

A QL (quick load) file contains an intermediate representation of a compiled source code file. They are portable between different platforms, but less efficient than PO files, and are therefore obsolescent. They are created by fcompile/1.

A query is a question put by the user to the Prolog system. A query is written as a goal followed by a full-stop in response to the Prolog system prompt. For example,

| ?- father(edward, ralph).

refers to the predicate father/2. If a query has no variables in it, the system will respond either ‘yes’ or ‘no’. If a query contains variables, the system will try to find values of those variables for which the query is true. For example,

| ?- father(edward, X).

X = ralph

After the system has found one answer, the user can direct the system to look for additional answers to the query by typing ‘;’.

recursion The process in which a running procedure calls itself, presumably with different arguments and for the purpose of solving some subset of the original problem.

region rule

The text between the cursor and a previously set mark in an Emacs buffer.

A clause with one or more conditions. For a rule to be true, all of its conditions must also be true. For example,

16 SICStus Prolog has_stiff_neck(ralph) :hacker(ralph).

This rule states that if the individual ralph is a hacker, then he must also have a stiff neck. The constant ralph is replaced in has_stiff_neck(X) :hacker(X).

by the variable X. X unifies with anything, so this rule can be used to prove that any hacker has a stiff neck.

runtime kernel

A shared object or DLL containing the SICStus virtual machine and other runtime support for stand-alone executables.

runtime system

A stand-alone executable with a restricted set of built-in predicates and no toplevel. Stand-alone applications containing debugged Prolog code and destined for end-users are typically packaged as runtime systems.

saved-state

A snapshot of the state of Prolog saved in a file by save_program/[1,2].

semantics The relation between the set of Prolog symbols and their combinations (as

Prolog terms and clauses), and their meanings. Compare syntax.

sentence A clause or directive.

side-effect A predicate which produces a side-effect is one which has any effect on the

“outside world” (the user’s terminal, a file, etc.), or which changes the Prolog database.

simple term

A simple term is a constant or a variable. Simple terms are recognized by the built-in predicate simple/1.

source code

The human-readable, as opposed to the machine-executable, representation of a program.

source module

The module which is the context of a file being loaded. For module-files, the source module is named in the file’s module declaration. For other files, the source module is inherited from the context.

SP term ref

A “handle” object providing an interface from C to Prolog terms.

spypoint A special case of breakpoint, the debugger breakpoint, intended for interactive debugging. Its simplest form, the plain spypoint instructs the debugger to stop at all ports of all invocations of a specified predicate. Conditional spypoints apply to a single predicate, but are more selective: the user can supply applicability tests and prescribe the actions to be carried out by the debugger. A generic spypoint is like a conditional spypoint, but not restricted to a single predicate. See

Section 6.5 [Breakpoints], page 75 .

Chapter 1: Glossary 17 stand-alone executable

A binary program which can be invoked from the operating system, containing the SICStus runtime kernel. A stand-alone executable is a development system

(e.g. the default sicstus executable), or a runtime system. Both kinds are created by the spld utility. A stand-alone executable does not itself contain any Prolog code; all Prolog code must be loaded upon startup.

static predicate

A predicate that can be modified only by being reloaded or by being abolished.

See dynamic predicate.

stream An input/output channel. See

Section 7.1 [Input Output], page 98

.

stream alias

A name assigned to a stream at the time of opening, which can be referred to in I/O predicates. Must be an atom. There are also three predefined aliases for the standard streams: user_input, user_output and user_error.

string A special syntactic notation which is, by default, equivalent to a list of character codes e.g.

"SICStus"

By setting the Prolog flag double_quotes, the meaning of strings can be changed. With an appropriate setting, a string can be made equivalent to a list of one-char atoms, or to an atom. Strings are not a separate data type.

subterm selector

A list of argument positions selecting a subterm within a term (i.e. the subterm can be reached from the term by successively selecting the argument positions listed in the selector). Example: within the term q, (r, s; t) the subterm s is selected by the selector [2, 1, 2].

syntax The part of Prolog grammar dealing with the way in which symbols are put together to form legal Prolog terms. Compare semantics.

system encoding (of wide characters)

A way of encoding wide character strings, used or required by the operating system environment.

term A basic data object in Prolog. A term can be a constant, a variable, or a compound term.

trace A mode of program execution in which the debugger creeps to the next port and prints the goal.

type-in module

The module which is the context of queries.

unblocked goal

A goal which is not blocked.

unbound A variable is unbound if it has not yet been instantiated.

unification The process of matching a goal with the head of a clause during the evaluation of a query, or of matching arbitrary terms with one another during program execution.

18 SICStus Prolog

The rules governing the unification of terms are:

• Two constants unify with one another if they are identical.

• A variable unifies with a constant or a compound term. As a result of the unification, the variable is instantiated to the constant or compound term.

• A variable unifies with another variable. As a result of the unification, they become the same variable.

• A compound term unifies with another compound term if they have the same functor and if all of the arguments can be unified.

unit clause

See fact.

UTF-8 encoding

See internal encoding variable A logical variable is a name that stands for objects that may or may not be determined at a specific point in a Prolog program. When the object for which the variable stands is determined in the Prolog program, the variable becomes instantiated. A logical variable may be unified with a constant, a compound term, or another variable. Variables become uninstantiated when the procedure they occur in backtracks past the point at which they were instantiated.

Variables may be written as any sequence of alphanumeric characters starting with either a capital letter or _; e.g.

X Y Z Name Position _c _305 One_stop

See

Section 3.1.1.4 [Variables], page 40 .

volatile Predicate property. The clauses of a volatile predicate are not saved in savedstates.

windowed executable

An executable which pops up its own window when run, and which directs the standard streams to that window.

zip Same as leap mode, except no debugging information is collected while zipping.

Chapter 2: How to Run Prolog 19

2 How to Run Prolog

SICStus Prolog offers the user an interactive programming environment with tools for incrementally building programs, debugging programs by following their executions, and modifying parts of programs without having to start again from scratch.

The text of a Prolog program is normally created in a file or a number of files using one of the standard text editors. The Prolog interpreter can then be instructed to read in programs from these files; this is called consulting the file. Alternatively, the Prolog compiler can be used for compiling the file.

2.1 Getting Started

Under UNIX, SICStus Prolog is normally started from one of the shells. On other platforms, it is normally started by clicking on an icon. However, it is often convenient to run SICStus

Prolog under GNU Emacs instead. A GNU Emacs interface for SICStus Prolog is described later (see

Section 2.11 [Emacs Interface], page 29

). From a shell, SICStus Prolog is started by typing:

% sicstus [options] [-a argument...] where flags have the following meaning:

-f

-i

Fast start.

Don’t read any initialization file (‘~/.sicstusrc’ or

‘~/.sicstus.ini’) on startup. If the flag is omitted and this file exists, SICStus

Prolog will consult it on startup after running any initializations and printing the version banners.

Forced interactive. Prompt for user input, even if the standard input stream does not appear to be a terminal.

Use malloc()/free() in the memory manager’s bottom layer.

-m

-l prolog-file

Ensure that the file prolog-file is loaded on startup. This is done before any initialization file is loaded.

-r saved-state

Restore the saved state saved-state on startup. This is done before any prologfile or initialization file is loaded.

-a argument...

where the arguments can be retrieved from Prolog by prolog_flag(argv,

Args), which will unify Args with argument... represented as a list of atoms.

-B[abspath]

Creates a saved state for a development system. This option is not needed for normal use. If abspath is given, it specifies the absolute pathname for the saved state. NOTE: There must not be a space before the path, or it will be interpreted as a separate option.

20 SICStus Prolog

-R[abspath]

Equivalent to the -B option, except that it builds a saved state for a runtime system instead.

Under UNIX, a saved state file can be executed directly by typing:

% file argument...

This is equivalent to:

% sicstus -r file [-a argument...]

NOTE: As of release 3.7, saved-states do not store the complete path of the binary sp.exe.

Instead, they call the main executable sicstus, which is assumed to be found in the shell’s path. If there are several versions of SICStus installed, it is up to the user to make sure that the correct start-script is found.

Notice that the flags are not available when executing saved states—all the command-line arguments are treated as Prolog arguments.

The development system checks that a valid SICStus license exists and responds with a message of identification and the prompt ‘| ?- ’ as soon as it is ready to accept input, thus:

SICStus 3.8 (SunOS-5.5.1-sparc): Thu Aug 19 16:25:28 MET DST 1999

Licensed to SICS

| ?-

At this point the top-level is expecting input of a query. You cannot type in clauses or directives immediately (see

Section 2.3 [Inserting Clauses], page 22

). While typing in a query, the prompt (on following lines) becomes ‘ ’. That is, the ‘| ?- ’ appears only for the first line of the query, and subsequent lines are indented.

2.1.1 Environment Variables

The following environment variables can be set before starting SICStus Prolog. Some of these override the default sizes of certain areas. The sizes are given in bytes, but may be followed by K or M meaning kilobytes or megabytes respectively.

SP_CSETLEN

Selects the sub-code-set lengths when the EUC character set is used. For the details, see

Section 10.4 [WCX Environment Variables], page 223

.

SP_CTYPE

Selects the appropriate character set standard: The supported values are euc

(for EUC), utf8 (for Unicode) and iso_8859_1 (for ISO 8859/1). The latter is the default. For the details, see

Section 10.4 [WCX Environment Variables], page 223

.

SP_PATH

This environment variable can be used to specify the location of the Runtime

Library (corresponding to the third argument to SP_initialize()). See sec-

Chapter 2: How to Run Prolog 21

TMPDIR tion “Setting SP PATH under UNIX” in SICStus Prolog Release Notes, for more information.

If set, indicates the pathname where temporary files should be created. Defaults to ‘/usr/tmp’.

GLOBALSTKSIZE

Governs the initial size of the global stack.

LOCALSTKSIZE

Governs the initial size of the local stack.

CHOICESTKSIZE

Governs the initial size of the choicepoint stack.

TRAILSTKSIZE

Governs the initial size of the trail stack.

PROLOGINITSIZE

Governs the size of Prolog’s initial memory allocation.

PROLOGMAXSIZE

Defines a limit on the amount of data space which Prolog will use.

PROLOGINCSIZE

Governs the amount of space Prolog asks the operating system for in any given memory expansion.

PROLOGKEEPSIZE

Governs the size of space Prolog retains after performing some computation.

By default, Prolog gets memory from the operating system as the user program executes and returns all free memory back to the operating system when the user program does not need any more.

If the programmer knows that her program, once it has grown to a certain size, is likely to need as much memory for future computations, then she can advise Prolog not to return all the free memory back to the operating system by setting this variable. Only memory that is allocated above and beyond PROLOGKEEPSIZE is returned to the OS; the rest will be kept.

Send bug reports to [email protected]

. Bugs tend actually to be fixed if they can be isolated, so it is in your interest to report them in such a way that they can be easily reproduced.

The mailing list [email protected]

is a moderated mailing list for communication among users and implementors. To [un]subscribe, write to [email protected]

.

2.2 Reading in Programs

A program is made up of a sequence of clauses and directives. The clauses of a predicate do not have to be immediately consecutive, but remember that their relative order may be important (see

Section 3.3 [Procedural], page 45

).

22 SICStus Prolog

To input a program from a file file, just type the filename inside list brackets (followed by

. and h

RET i

), thus:

| ?- [file].

This instructs the interpreter to read in (consult) the program. Note that it may be necessary to enclose the filename file in single quotes to make it a legal Prolog atom; e.g.

| ?- [’myfile.pl’].

| ?- [’/usr/prolog/somefile’].

The specified file is then read in. Clauses in the file are stored so that they can later be interpreted, while any directives are obeyed as they are encountered. When the end of the file is found, the system displays on the standard error stream the time spent. This indicates the completion of the query.

Predicates that expect the name of a Prolog source file as an argument use absolute_file_name/2 (see

Section 7.1.5 [Stream Pred], page 118

) to look up the file.

This predicate will first search for a file with the suffix ‘.pl’ added to the name given as an argument. If this fails it will look for a file with no extra suffix added. There is also support for libraries.

In general, this query can be any list of filenames, such as:

| ?- [myprog,extras,tests].

In this case all three files would be consulted.

The clauses for all the predicates in the consulted files will replace any existing clauses for those predicates, i.e. any such previously existing clauses in the database will be deleted.

Note that consult/1 in SICStus Prolog behaves like reconsult/1 in DEC-10 Prolog.

2.3 Inserting Clauses at the Terminal

Clauses may also be typed in directly at the terminal, although this is only recommended if the clauses will not be needed permanently, and are few in number. To enter clauses at the terminal, you must give the special query:

| ?- [user].

| and the new prompt ‘| ’ shows that the system is now in a state where it expects input of clauses or directives. To return to top level, type ^D. The system responds thus:

{user consulted, 20 msec 200 bytes}

Chapter 2: How to Run Prolog 23

2.4 Queries and Directives

Queries and directives are ways of directing the system to execute some goal or goals.

In the following, suppose that list membership has been defined by loading the following clauses from a file: member(X, [X|_]).

member(X, [_|L]) :- member(X, L).

(Notice the use of anonymous variables written ‘_’.)

2.4.1 Queries

The full syntax of a query is ‘?-’ followed by a sequence of goals. The top level expects queries. This is signaled by the initial prompt ‘| ?- ’. Thus a query at top level looks like:

| ?- member(b, [a,b,c]).

Remember that Prolog terms must terminate with a full stop (., possibly followed by layout text), and that therefore Prolog will not execute anything until you have typed the full stop

(and then h

RET i

) at the end of the query.

If the goal(s) specified in a query can be satisfied, and if there are no variables as in this example, then the system answers yes and execution of the query terminates.

If variables are included in the query, then the final value of each variable is displayed

(except for variables whose names begin with _). Thus the query

| ?- member(X, [a,b,c]).

would be answered by

X = a

At this point the system is waiting for input of either just a h

RET i or else a ; followed by h

RET i

. Simply typing h

RET i terminates the query; the system responds with ‘yes’. However, typing ; causes the system to backtrack (see

Section 3.3 [Procedural], page 45 ) looking for

alternative solutions. If no further solutions can be found it outputs ‘no’.

The outcome of some queries is shown below, where a number preceded by _ is a systemgenerated name for a variable.

| ?- member(X, [tom,dick,harry]).

X = tom ;

X = dick ;

24 SICStus Prolog

X = harry ; no

| ?- member(X, [a,b,f(Y,c)]), member(X, [f(b,Z),d]).

X = f(b,c),

Y = b,

Z = c yes

| ?- member(X, [f(_),g]).

X = f(_A) yes

| ?-

Directives are like queries except that:

1. Variable bindings are not displayed if and when the directive succeeds.

2. You are not given the chance to backtrack through other solutions.

2.4.2 Directives

Directives start with the symbol ‘:-’. Any required output must be programmed explicitly; e.g. the directive:

:- member(3, [1,2,3]), write(ok).

asks the system to check whether 3 belongs to the list [1,2,3]. Execution of a directive terminates when all the goals in the directive have been successfully executed. Other alternative solutions are not sought. If no solution can be found, the system prints:

{Warning: Goal - goal failed} as a warning.

The principal use for directives (as opposed to queries) is to allow files to contain directives which call various predicates, but for which you do not want to have the answers printed out. In such cases you only want to call the predicates for their effect, i.e. you don’t want terminal interaction in the middle of consulting the file. A useful example would be the use of a directive in a file which consults a whole list of other files, e.g.

:- [ bits, bobs, main, tests, data, junk ].

If a directive like this were contained in the file ‘myprog’ then typing the following at toplevel would be a quick way of reading in your entire program:

| ?- [myprog].

Chapter 2: How to Run Prolog 25

When simply interacting with the top-level, this distinction between queries and directives is not normally very important. At top-level you should just type queries normally. In a file, queries are in fact treated as directives, i.e. if you wish to execute some goals then the directive in the file must be preceded by ‘:-’ or ‘?-’, otherwise it would be treated as a clause.

2.5 Syntax Errors

Syntax errors are detected during reading. Each clause, directive or in general any term read in by the built-in predicate read/1 that fails to comply with syntax requirements is displayed on the standard error stream as soon as it is read, along with its position in the input stream and a mark indicating the point in the string of symbols where the parser has failed to continue analysis, e.g.:

| member(X, X$L).

{SYNTAX ERROR: in line 5 (within 5-6)}

** , or ) expected in arguments ** member ( X , X

** here **

$ L ) .

if $ has not been declared as an infix operator.

Note that any comments in the faulty line are not displayed with the error message. If you are in doubt about which clause was wrong you can use the listing/1 predicate to list all the clauses which were successfully read in, e.g.

| ?- listing(member/2).

NOTE: The built in predicates read/[1,2] normaly raise an exception on syntax errors (see

Section 7.5 [Exception], page 134 ). The behavior is controlled by the flag syntax_errors

(see prolog_flag/3).

2.6 Undefined Predicates

There is a difference between predicates that have no definition and predicates that have no clauses. The latter case is meaningful e.g. for dynamic predicates (see

Section 5.2

[Declarations], page 62

) that clauses are being added to or removed from. There are good reasons for treating calls to undefined predicates as errors, as such calls easily arise from typing errors.

The system can optionally catch calls to predicates that have no definition.

First the user defined predicate user:unknown_predicate_handler/3 (see

Section 7.5 [Exception], page 134

) is called. If undefined or if the call fails the action is governed by the state of the unknown/2 flag which can be:

26 SICStus Prolog trace error which causes calls to undefined predicates to be reported and the debugger to be entered at the earliest opportunity.

which causes calls to such predicates to raise an exception (the default state).

See

Section 7.5 [Exception], page 134 .

warning which causes calls to such predicates to display a warning message and then fail.

fail which causes calls to such predicates to fail.

Calls to predicates that have no clauses are not caught.

The built-in predicate unknown(?OldState, ?NewState) unifies OldState with the current state and sets the state to NewState. The built-in predicate debugging/0 prints the value of this state along with its other information. This state is also controlled by the flag unknown

(see prolog_flag/3).

2.7 Program Execution And Interruption

Execution of a program is started by giving the system a query which contains a call to one of the program’s predicates.

Only when execution of one query is complete does the system become ready for another query. However, one may interrupt the normal execution of a query by typing ^C. This

^C interruption has the effect of suspending the execution, and the following message is displayed:

Prolog interruption (h or ? for help) ?

At this point, the development system accepts one-letter commands corresponding to certain actions. To execute an action simply type the corresponding character (lower or upper case) followed by h

RET i

. The available commands in development systems are: e h

?

a c b d z t aborts the current computation.

continues the execution.

exits from SICStus Prolog, closing all files.

lists available commands.

invokes a recursive top-level.

switch on the debugger. See

Chapter 6 [Debug Intro], page 69

.

If the standard input stream is not connected to the terminal, e.g. by redirecting standard input to a file or a pipe, the above ^C interrupt options are not available. Instead, typing

^C causes SICStus Prolog to exit, and no terminal prompts are printed.

Chapter 2: How to Run Prolog 27

2.8 Exiting From The Top-Level

To exit from the top-level and return to the shell, either type ^D at the top-level, or call the built-in predicate halt/0, or use the e (exit) command following a ^C interruption.

2.9 Nested Executions—Break and Abort

The Prolog system provides a way to suspend the execution of your program and to enter a new incarnation of the top level where you can issue queries to solve goals etc. This is achieved by issuing the query (see

Section 2.7 [Execution], page 26 ):

| ?- break.

This invokes a recursive top-level, indicated by the message:

{ Break level 1 }

You can now type queries just as if you were at top-level.

If another call of break/0 is encountered, it moves up to level 2, and so on. To close the break and resume the execution which was suspended, type ^D. The debugger state and current input and output streams will be restored, and execution will be resumed at the predicate call where it had been suspended after printing the message:

{ End break }

Alternatively, the suspended execution can be aborted by calling the built-in predicate abort/0.

A suspended execution can be aborted by issuing the query:

| ?- abort.

within a break. In this case no ^D is needed to close the break; all break levels are discarded and the system returns right back to top-level. I/O streams remain open, but the debugger is switched off. abort/0 may also be called from within a program.

2.10 Saving and Restoring Program States

Once a program has been read, the system will have available all the information necessary for its execution. This information is called a program state.

The state of a program may be saved on disk for future execution. The state consists of all predicates and modules except built-in predicates and clauses of volatile predicates, the current operator declarations, the values of all writable Prolog flags except debugging, source_info, and the user_* stream aliases (see

Section 7.6 [State Info], page 137

), any blackboard data (see

Section 7.11 [Blackboard Primitives], page 152 ), internal database

data (see

Section 7.10 [Database], page 152 ), and profiling data (see

Section 7.15 [Profiling], page 158 ), but no information for source-linked debugging.

28 SICStus Prolog

To save a program into a file File, type the following query. On UNIX platforms, the file becomes executable:

| ?- save_program(File).

You can also specify a goal to be run when a saved program is restored. This is done by:

| ?- save_program(File, start).

where start/0 is the predicate to be called.

Once a program has been saved into a file File, the following query will restore the system to the saved state:

| ?- restore(File).

If a saved state has been moved or copied to another machine, the path names of foreign resources and other files needed upon restore are typically different at restore time from their save time values. To solve this problem, certain atoms will be relocated during restore as follows:

• Atoms that had ‘$SP_PATH/library’ (the name of the directory containing the Prolog

Library) as prefix at save time will have that prefix replaced by the corresponding restore time value.

• Atoms that had the name of the directory containing File as prefix at save time will have that prefix replaced by the corresponding restore time value.

The purpose of this procedure is to be able to build and deploy an application consisting of a saved state and other files as a directory tree with the saved state at the root: as long as the other files maintain their relative position in the deployed copy, they can still be found upon restore.

NOTE: Foreign resources, see

Section 8.2 [Calling C], page 166 , are unloaded by save_

program/[1,2].

The names and paths of the resources, typically ‘$SP_PATH/library’ relative, are however included in the saved state. After the save, and after restoring a saved state, this information is used to reload the foreign resources again. The state of the foreign resource in terms of global C variables and allocated memory is thus not preserved.

Foreign resources may define init and deinit functions to take special action upon loading and unloading, see

Section 8.2.6 [Init and Deinit Functions], page 173 .

As of SICStus Prolog 3.8, partial saved states corresponding to a set of source files, modules, and predicates can be created by the built-in predicates save_files/2, save_modules/2, and save_predicates/2 respectively. These predicates create files in a binary format, by default with the prefix ‘.po’ (for Prolog object file), which can be loaded by load_ files/[1,2]. For example, to compile a program split into several source files into a single object file, type:

| ?- compile(Files), save_files(Files, Object).

Chapter 2: How to Run Prolog 29

For each filename given, the first goal will try to locate a source file with the default suffix

‘.pl’ and compile it into memory. The second goal will save the program just compiled into an object file whose default suffix is ‘.po’. Thus the object file will contain a partial memory image.

2.11 Emacs Interface

This section explains how to use the GNU Emacs interface for SICStus Prolog, and how to customize your GNU Emacs environment for it.

Emacs is a powerful programmable editor especially suitable for program development. It is available for free for many platforms, including various UNIX dialects, Windows and

MacOS. For information on obtaining Emacs, see www.emacs.org

. For information specific to GNU Emacs or XEmacs, see www.gnu.org

and www.xemacs.org

respectively.

The advantages of using SICStus in the Emacs environment are source-linked debugging, auto indentation, syntax highlighting, help on predefined predicates (requires the SICStus info files to be installed), loading code from inside Emacs, auto-fill mode, and more.

The Emacs interface is not part of SICStus Prolog proper, but is included in the distribution version of the mode written by Masanobu Umeda. Contributions has also been made by

Johan Andersson, Peter Olin, Mats Carlsson, Johan Bevemyr, Stefan Andersson, and Per have been borrowed (with permission) from Oz.el by Ralf Scheidhauer and Michael Mehl, the Emacs major mode for the Oz programming language. More ideas and code have been taken from the SICStus debugger mode by Per Mildner.

2.11.1 Installation

See section “The Emacs Interface” in SICStus Prolog Release Notes , for more information about installing the Emacs interface.

There are some differences between GNU Emacs and XEmacs. This will be indicated with

Emacs-Lisp comments in the examples.

2.11.1.1 Customizing Emacs

Version 20 of GNU Emacs and XEmacs introduced a new method for editing and storing user settings. This feature is available from the menu bar as ‘Customize’ and particular

Emacs variables can be customized with M-x customize-variable. Using ‘Customize’ is the preferred way to modify the settings for emacs and the appropriate customize commands will be indicated below, sometimes together with the old method of directly setting Emacs variables.

30 SICStus Prolog

2.11.1.2 Enabling Emacs Support for SICStus

Assuming the Emacs interface for SICStus Prolog has been installed in the default location, inserting the following lines in your ‘~/.emacs’ will make Emacs use this mode automatically when editing files with a ‘.pl’ extension:

(setq load-path

(cons (expand-file-name "/usr/local/lib/sicstus-3.8/emacs") load-path))

(autoload ’run-prolog "prolog" "Start a Prolog sub-process." t)

(autoload ’prolog-mode "prolog" "Major mode for editing Prolog programs." t)

(setq prolog-use-sicstus-sd t)

(setq auto-mode-alist (cons ’("\\.pl$" . prolog-mode) auto-mode-alist)) where the path in the first line is the file system path to ‘prolog.el’ (the generic Prolog mode) and ‘sicstus-support.el’ (SICStus specific code). For example, ‘~/emacs’ means that the file is in the user’s home directory, in directory emacs. Windows paths can be written like ‘C:/Program Files/SICStus/emacs’.

The last line above makes sure that files ending with ‘.pl’ are assumed to be Prolog files and not Perl, which is the default Emacs setting. If this is undesirable, remove that line.

It is then necessary for the user to manually switch to prolog mode by typing M-x prologmode after opening a prolog file, for an alternative approach, see

Section 2.11.4 [Mode Line], page 33 .

If the shell command sicstus is not available in the default path, then it is necessary to set the value of the environment variable EPROLOG to a shell command to invoke SICStus

Prolog. This is an example for C Shell: setenv EPROLOG /usr/local/bin/sicstus

2.11.1.3 Enabling Emacs Support for SICStus Documentation

It is possible to look up the documentation for any built in or library predicate from within

Emacs (using C-c ? or the menu). For this to work Emacs must be told about the location of the ‘info’-files that make up the documentation.

The default location for the ‘info’-files are ‘<prefix>/lib/sicstus-3.8/doc/info/’ on

UNIX platforms and ‘C:/Program Files/SICStus/doc/info/’ on Windows.

Add the following to your ‘~/.emacs’ file, assuming INFO is the path to the info files, e.g.,

‘C:/Program Files/SICStus/doc/info/’

(setq Info-default-directory-path

(append Info-default-directory-path ’("INFO"))) for GNU Emacs, or

(setq Info-directory-path

(append Info-directory-path ’("INFO")))

Chapter 2: How to Run Prolog 31 for XEmacs. You can also use M-x customize-group h

RET i info h

RET i if your Emacs is new enough. You may have to quit and restart Emacs for these changes to take effect.

2.11.2 Basic Configuration

If the following lines are not present in ‘~/.emacs’, we suggest they are added, so that the font-lock mode (syntax coloring support) is enabled for all major modes in Emacs that support it.

(global-font-lock-mode t) ; GNU Emacs

(setq font-lock-auto-fontify t) ; XEmacs

(setq font-lock-maximum-decoration t)

These settings and more are also available through M-x customize-group h

RET i font-lock.

If one wants to add font-locking only to the prolog mode, the two lines above could be replaced by:

(add-hook ’prolog-mode-hook ’turn-on-font-lock)

Similarly, to turn it off only for prolog mode use:

(add-hook ’prolog-mode-hook ’turn-off-font-lock)

2.11.3 Usage

A prolog process can be started by choosing Run Prolog from the Prolog menu, by typing

C-c h

RET i

, or by typing M-x run-prolog. It is however not strictly necessary to start a prolog process manually since it is automatically done when consulting or compiling, if needed. The process can be restarted (i.e. the old one is killed and a new one is created) by typing C-u C-c h

RET i

.

Programs are run and debugged in the normal way, with terminal I/O via the *prolog* buffer. The most common debugging predicates are available from the menu or via keybindings.

A particularly useful feature under the Emacs interface is source-linked debugging. This is enabled or disabled using the Prolog/Source level debugging menu entry. It can also be enabled by setting the Emacs variable prolog-use-sicstus-sd to t in ‘~/.emacs’. Both these methods set the Prolog flag source_info to emacs. Its value should be emacs while loading the code to be debugged and while debugging. If so, the debugger will display the source code location of the current goal when it prompts for a debugger command, by overlaying the beginning of the current line of code with an arrow. If source_info was off when the code was loaded, or if it was asserted or loaded from user, the current goal will still be shown but out of context.

Note that if the code has been modified since it was last loaded, Prolog’s line number information may be invalid. If this happens, just reload the relevant buffer.

32 SICStus Prolog

Consultation and compilation is either done via the menu or with the following key-bindings:

C-c C-f

Consult file.

C-c C-b

Consult buffer.

C-c C-r

Consult region.

C-c C-p

Consult predicate.

C-c C-c f

Compile file.

C-c C-c b

Compile buffer.

C-c C-c r

Compile region.

C-c C-c p

Compile predicate.

The boundaries used when consulting and compiling predicates are the first and last clauses of the predicate the cursor is currently in.

Other useful key-bindings are:

M-n

M-p

M-a

M-e

M-C-c

M-C-a

M-C-e

M-C-h

M-{

M-}

M-h

M-C-n

M-C-p

M-;

Go to the next clause.

Go to the previous clause.

Go to beginning of clause.

Go to end of clause.

Mark clause.

Go to beginning of predicate.

Go to end of predicate.

Mark predicate.

Go to the previous paragraph (i.e. empty line).

Go to the next paragraph (i.e. empty line).

Mark paragraph.

Go to matching right parenthesis.

Go to matching left parenthesis.

Creates a comment at comment-column. This comment will always stay at this position when the line is indented, regardless of changes in the text earlier on the line, provided that prolog-align-comments-flag is set to t.

C-c C-t

C-u C-c C-t

Enable and disable tracing, respectively.

Chapter 2: How to Run Prolog 33

C-c C-d

C-u C-c C-d

Enable and disable debugging, respectively.

C-c C-z

C-u C-c C-z

Enable and disable zipping, respectively.

C-x SPC

C-u C-x SPC

Set and remove a line breakpoint. This uses the advanced debugger features introduced in SICStus 3.8,

Section 6.5 [Breakpoints], page 75 .

C-c C-s

Insert the PredSpec of the current predicate into the code.

C-c C-n

Insert the name of the current predicate into the code. This can be useful when writing recursive predicates or predicates with several clauses. See also the prolog-electric-dot-flag variable below.

C-c C-v a

Convert all variables in a region to anonymous variables. This can also be done using the Prolog/Transform/All variables to ’_’ menu entry. See also the prolog-electric-underscore-flag Emacs variable.

C-c ?

Help on predicate. This requires the SICStus info files to be installed. If the

SICStus info files are installed in a nonstandard way, you may have to change the Emacs variable prolog-info-predicate-index.

2.11.4 Mode Line

If working with an application split into several modules, it is often useful to let files begin with a “mode line”:

%%% -*- Mode: Prolog; Module: ModuleName; -*-

The Emacs interface will look for the mode line and notify the SICStus Prolog module system that code fragments being incrementally reconsulted or recompiled should be imported into the module ModuleName. If the mode line is missing, the code fragment will be imported into the type-in module. An additional benefit of the mode line is that it tells

Emacs that the file contains Prolog code, regardless of the setting of the Emacs variable auto-mode-alist. A mode line can be inserted by choosing Insert/Module modeline in the Prolog menu.

2.11.5 Configuration

The behavior of the Emacs interface can be controlled by a set of user-configurable settings.

Some of these can be changed on the fly, while some require Emacs to be restarted. To set a variable on the fly, type M-x set-variable h

RET i

VariableName h

RET i

Value h

RET i . Note that variable names can be completed by typing a few characters and then pressing h

TAB i

.

To set a variable so that the setting is used every time Emacs is started, add lines of the following format to ‘~/.emacs’:

34 SICStus Prolog

(setq VariableName Value)

Note that the Emacs interface is presently not using the ‘Customize’ functionality to edit the settings.

The available settings are: prolog-system

The Prolog system to use. Defaults to ’sicstus, which will be assumed for the rest of this chapter. See the on-line documentation for the meaning of other settings. For other settings of prolog-system the variables below named sicstus-something will not be used, in some cases corresponding functionality is available through variables named prolog-something.

sicstus-version

The version of SICStus that is used. Defaults to ’(3 . 8). Note that the spaces are significant!

prolog-use-sicstus-sd

Set to t (the default) to enable the source-linked debugging extensions by default. The debugging can be enabled via the Prolog menu even if this variable is nil. Note that the source-linked debugging only works if sicstus-version is set correctly.

pltrace-port-arrow-assoc

[Obsolescent]

Only relevant for source-linked debugging, this controls how the various ports of invocation boxes (see

Section 6.1 [Procedure Box], page 69 ) map to arrows

that point into the current line of code in source code buffers. Initialized as:

’(("call" . ">>>") ("exit" . "+++") ("ndexit" . "?++")

("redo" . "<<<") ("fail" . "---") ("exception" . "==>")) where ndexit is the non-determinate variant of the Exit port. Do not rely on this variable. It will change in future releases.

prolog-indent-width

How many positions to indent the body of a clause. Defaults to tab-width, normally 8.

prolog-paren-indent

The number of positions to indent code inside grouping parentheses. Defaults to 4, which gives the following indentation.

p :-

( q1

; q2, q3

).

Note that the spaces between the parentheses and the code are automatically inserted when h

TAB i is pressed at those positions.

Chapter 2: How to Run Prolog 35 prolog-align-comments-flag

Set to nil to prevent single %-comments to be automatically aligned. Defaults to t.

Note that comments with one % are indented to comment-column, comments with two % to the code level, and that comments with three % are never changed when indenting.

prolog-indent-mline-comments-flag

Set to nil to prevent indentation of text inside /* ... */ comments. Defaults t.

prolog-object-end-to-0-flag

Set to nil to indent the closing } of an object definition to prolog-indentwidth. Defaults to t.

sicstus-keywords

This is a list with keywords that are highlighted in a special color when used as directives (i.e. as :- keyword). Defaults to

’((sicstus

("block" "discontiguous" "dynamic" "initialization"

"meta_predicate" "mode" "module" "multifile" "public" "volatile")))) prolog-electric-newline-flag

Set to nil to prevent Emacs from automatically indenting the next line when pressing h

RET i . Defaults to t.

prolog-hungry-delete-key-flag

Set to t to enable deletion of all white space before the cursor when pressing the delete key (unless inside a comment, string, or quoted atom). Defaults to nil.

prolog-electric-dot-flag

Set to t to enable the electric dot function. If enabled, pressing . at the end of a non-empty line inserts a dot and a newline. When pressed at the beginning of a line, a new head of the last predicate is inserted. When pressed at the end of a line with only whitespace, a recursive call to the current predicate is inserted.

The function respects the arity of the predicate and inserts parentheses and the correct number of commas for separation of the arguments. Defaults to nil.

prolog-electric-underscore-flag

Set to t to enable the electric underscore function. When enabled, pressing underscore (_) when the cursor is on a variable, replaces the variable with the anynomous variable. Defaults to nil.

prolog-old-sicstus-keys-flag

Set to t to enable the key-bindings of the old Emacs interface. These bindings are not used by default since they violate GNU Emacs recommendations.

Defaults to nil.

36 SICStus Prolog prolog-use-prolog-tokenizer-flag

Set to nil to use built-in functions of Emacs for parsing the source code when indenting. This is faster than the default but does not handle some of the syntax peculiarities of Prolog. Defaults to t.

prolog-parse-mode

What position the parsing is done from when indenting code. Two possible settings: ’beg-of-line and ’beg-of-clause. The first is faster but may result in erroneous indentation in /* ... */ comments. The default is ’beg-of-line.

prolog-imenu-flag

Set to t to enable a new Predicate menu which contains all predicates of the current file. Choosing an entry in the menu moves the cursor to the start of that predicate. Defaults to nil.

prolog-info-predicate-index

The info node for the SICStus predicate index. This is important if the online help function is to be used (by pressing C-c ?, or choosing the Prolog/Help on predicate menu entry). The default setting is "(sicstus)Predicate Index".

prolog-underscore-wordchar-flag

Set to nil to not make underscore (_) a word-constituent character. Defaults to t.

2.11.6 Tips

Some general tips and tricks for using the SICStus mode and Emacs in general are given here. Some of the methods may not work in all versions of Emacs.

2.11.6.1 Font-locking

When editing large files, it might happen that font-locking is not done because the file is too large. Typing M-x lazy-lock-mode results in only the visible parts of the buffer being highlighted, which is much faster, see its Emacs on-line documentation for details.

If the font-locking seems to be incorrect, choose Fontify Buffer from the Prolog menu.

2.11.6.2 Auto-fill mode

Auto-fill mode is enabled by typing M-x auto-fill-mode. This enables automatic line breaking with some features. For example, the following multiline comment was created by typing M-; followed by the text. The second line was indented and a % was added automatically.

dynamics([]).

% A list of pit furnace

% dynamic instances

Chapter 2: How to Run Prolog 37

2.11.6.3 Speed

There are several things to do if the speed of the Emacs environment is a problem:

• First of all, make sure that ‘prolog.el’ and ‘sicstus-support.el’ are compiled, i.e.

that there is a ‘prolog.elc’ and a ‘sicstus-support.elc’ file at the same location as the original files. To do the compilation, start Emacs and type M-x byte-compilefile h

RET i path h

RET i

, where path is the path to the ‘*.el’ file. Do not be alarmed if there are a few warning messages as this is normal. If all went well, there should now be a compiled file which is used the next time Emacs is started.

• The next thing to try is changing the setting of prolog-use-prolog-tokenizer-flag to nil. This means that Emacs uses built-in functions for some of the source code parsing, thus speeding up indentation. The problem is that it does not handle all peculiarities of the Prolog syntax, so this is a trade-off between correctness and speed.

• The setting of the prolog-parse-mode variable also affects the speed, ’beg-of-line being faster than ’beg-of-clause.

• Font locking may be slow. You can turn it off using customization, available through

M-x customize-group h

RET i font-lock h

RET i

. An alternative is to enable one of the lazy font locking modes. You can also turn it off completely, see

Section 2.11.2 [Basic

Configuration], page 31 .

2.11.6.4 Changing Colors

The prolog mode uses the default Emacs colors for font-locking as far as possible. The only custom settings are in the prolog process buffer. The default settings of the colors may not agree with your preferences, so here is how to change them.

If your emacs support it, use ‘Customize’, M-x customize-group h

RET i font-lock h

RET i will show the ‘Customize’ settings for font locking and also contains pointers to the ‘Customize’ group for the font lock (type)faces. The rest of this section outlines the more involved methods needed in older versions of Emacs.

First of all, list all available faces (a face is a combined setting of foreground and background colors, font, boldness, etc.) by typing M-x list-faces-display.

There are several functions that change the appearance of a face, the ones you will most likely need are: set-face-foreground set-face-background set-face-underline-p make-face-bold make-face-bold-italic make-face-italic make-face-unbold make-face-unitalic

38 SICStus Prolog

These can be tested interactively by typing M-x function-name. You will then be asked for the name of the face to change and a value. If the buffers are not updated according to the new settings, then refontify the buffer using the Fontify Buffer menu entry in the Prolog menu.

Colors are specified by a name or by RGB values. Available color names can be listed with

M-x list-colors-display.

To store the settings of the faces, a few lines must be added to ‘~/.emacs’. For example:

;; Customize font-lock faces

(add-hook ’font-lock-mode-hook

’(lambda ()

(set-face-foreground font-lock-variable-name-face "#00a000")

(make-face-bold font-lock-keyword-face)

(set-face-foreground font-lock-reference-face "Blue")

))

Chapter 3: The Prolog Language 39

3 The Prolog Language

This chapter provides a brief introduction to the syntax and semantics of a certain subset of logic (definite clauses, also known as Horn clauses), and indicates how this subset forms the basis of Prolog.

3.1 Syntax, Terminology and Informal Semantics

3.1.1 Terms

The data objects of the language are called terms. A term is either a constant, a variable or a compound term.

3.1.1.1 Integers

The constants include integers such as

0 1 999 -512

Besides the usual decimal, or base 10, notation, integers may also be written in other base notations. In sicstus mode, any base from 2 to 36 can be specified, while in iso mode bases 2 (binary), 8 (octal), and 16 (hex) can be used. Letters A through Z (upper or lower case) are used for bases greater than 10. E.g.

15 2’1111 8’17 16’f % sicstus mode

15 0b1111 0o17 0xf % iso mode all represent the integer fifteen. Except for the first, decimal, notation, the forms in the first line are only acceptable in sicstus mode, while those in the second line are only valid in iso mode.

There is also a special notation for character constants. E.g.

0’A 0’\x41 0’\101 are all equivalent to 65 (the character code for A). ‘0’’ followed by any character except \

(backslash) is thus read as an integer. If ‘0’’ is followed by \, the \ denotes the start of an escape sequence with special meaning (see

[Escape Sequences], page 578 ).

3.1.1.2 Floats

Constants also include floats such as

1.0

-3.141

4.5E7

-0.12e+8 12.0e-9

Note that there must be a decimal point in floats written with an exponent, and that there must be at least one digit before and after the decimal point.

40 SICStus Prolog

3.1.1.3 Atoms

Constants also include atoms such as a void = := ’Algol-68’ []

Atoms are definite elementary objects, and correspond to proper nouns in natural language.

For reference purposes, here is a list of the possible forms which an atom may take:

1. Any sequence of alphanumeric characters (including _), starting with a lower case letter.

2. Any sequence from the following set of characters:

+-*/\^<>=~:[email protected]#$&

This set can in fact be larger; see

[Token String], page 574

for a precise definition.

3. Any sequence of characters delimited by single quotes. If the single quote character is included in the sequence it must be escaped, e.g. ’can\’t’. Backslashes in the sequence denote escape sequences (see

[Escape Sequences], page 578

).

4. Any of: ! ; [] {}

Note that the bracket pairs are special: [] and {} are atoms but [, ], {, and } are not.

However, when they are used as functors (see below) the form {X } is allowed as an alternative to {}(X ). The form [X ] is the normal notation for lists, as an alternative to .(X,[]).

3.1.1.4 Variables

Variables may be written as any sequence of alphanumeric characters (including _) starting with either a capital letter or _; e.g.

X Value A A1 _3 _RESULT

If a variable is only referred to once in a clause, it does not need to be named and may be written as an anonymous variable, indicated by the underline character _. A clause may contain several anonymous variables; they are all read and treated as distinct variables.

A variable should be thought of as standing for some definite but unidentified object. This is analogous to the use of a pronoun in natural language. Note that a variable is not simply a writable storage location as in most programming languages; rather it is a local name for some data object, cf. the variable of pure LISP and identity declarations in Algol68.

3.1.1.5 Compound Terms

The structured data objects of the language are the compound terms. A compound term comprises a functor (called the principal functor of the term) and a sequence of one or more terms called arguments. A functor is characterized by its name, which is an atom, and its arity or number of arguments. For example the compound term whose functor is named point of arity 3, with arguments X, Y and Z, is written point(X, Y, Z)

Chapter 3: The Prolog Language 41

Note that an atom is considered to be a functor of arity 0.

Functors are generally analogous to common nouns in natural language. One may think of a functor as a record type and the arguments of a compound term as the fields of a record.

Compound terms are usefully pictured as trees. For example, the term s(np(john),vp(v(likes),np(mary))) would be pictured as the compound term np

| john s

/ \ vp

/ \ v

| np

| likes mary

Sometimes it is convenient to write certain functors as operators—2-ary functors may be declared as infix operators and 1-ary functors as prefix or postfix operators. Thus it is possible to write, e.g.

X+Y (P;Q) as optional alternatives to

X<Y +X P;

+(X,Y) ;(P,Q) <(X,Y) +(X) ;(P)

The use of operators is described fully below (see

Section 3.6 [Operators], page 49

).

Lists form an important class of data structures in Prolog. They are essentially the same as the lists of LISP: a list either is the atom [] representing the empty list, or is a compound term with functor . and two arguments which are respectively the head and tail of the list.

Thus a list of the first three natural numbers is the compound term

.

1

/ \

.

/ \

2 .

/ \

3 [] which could be written, using the standard syntax, as

.(1,.(2,.(3,[]))) but which is normally written, in a special list notation, as

[1,2,3]

42 SICStus Prolog

The special list notation in the case when the tail of a list is a variable is exemplified by

[X|L] [a,b|L] representing

X

.

/ \

L a

.

/ \

.

/ \ b L respectively.

Note that this notation does not add any new power to the language; it simply makes it more readable. e.g. the above examples could equally be written

.(X,L) .(a,.(b,L))

For convenience, a further notational variant is allowed for lists of integers which correspond to character codes or one-char atoms. Lists written in this notation are called strings. E.g.

"SICStus" which, by default, represents exactly the same list as

[83,73,67,83,116,117,115]

The Prolog flag double_quotes can be used to change the way strings are interpreted. The default value of the flag is codes, which implies the above interpretation. If the flag is set to chars, a string is transformed to a list of one-char atoms. E.g. with this setting the above string represents the list:

[’S’,’I’,’C’,’S’,t,u,s]

Finally if double_quotes has the value atom, then the string is made equivalent to the atom formed from its characters: the above sample string is then the same as the atom

’SICStus’.

As for quoted atoms, if a double quote character is included in the sequence it must be escaped, e.g. "can\’t". Backslashes in the sequence denote escape sequences (see

[Escape

Sequences], page 578 ).

3.1.2 Programs

A fundamental unit of a logic program is the goal or procedure call. e.g.

gives(tom, apple, teacher) reverse([1,2,3], L) X<Y

A goal is merely a special kind of term, distinguished only by the context in which it appears in the program. The (principal) functor of a goal identifies what predicate the goal is for. It

Chapter 3: The Prolog Language 43 corresponds roughly to a verb in natural language, or to a procedure name in a conventional programming language.

A logic program consists simply of a sequence of statements called sentences, which are analogous to sentences of natural language. A sentence comprises a head and a body. The head either consists of a single goal or is empty. The body consists of a sequence of zero or more goals (i.e. it too may be empty). If the head is not empty, the sentence is called a clause.

If the body of a clause is empty, the clause is called a unit clause, and is written in the form

P.

where P is the head goal. We interpret this declaratively as

Goals matching P are true.

and procedurally as

Goals matching P are satisfied.

If the body of a clause is non-empty, the clause is called a rule, and is written in the form

P :- Q, R, S.

where P is the head goal and Q, R and S are the goals which make up the body. We can read such a clause either declaratively as

P is true if Q and R and S are true.

or procedurally as

To satisfy goal P, satisfy goals Q, R and S.

A sentence with an empty head is called a directive (see

Section 2.4.2 [Directives], page 24

), and is written in the form

:- P, Q.

where P and Q are the goals of the body. Such a query is read declaratively as

Are P and Q true?

and procedurally as

Satisfy goals P and Q.

Sentences generally contain variables. Note that variables in different sentences are completely independent, even if they have the same name—i.e. the lexical scope of a variable is limited to a single sentence. Each distinct variable in a sentence should be interpreted as standing for an arbitrary entity, or value. To illustrate this, here are some examples of sentences containing variables, with possible declarative and procedural readings:

44 SICStus Prolog

1. employed(X ) :- employs(Y,X ).

“Any X is employed if any Y employs X.”

“To find whether a person X is employed, find whether any Y employs X.”

2. derivative(X,X,1).

“For any X, the derivative of X with respect to X is 1.”

“The goal of finding a derivative for the expression X with respect to X itself is satisfied by the result 1.”

3. ?- ungulate(X ), aquatic(X ).

“Is it true, for any X, that X is an ungulate and X is aquatic?”

“Find an X which is both an ungulate and aquatic.”

In any program, the predicate for a particular (principal) functor is the sequence of clauses in the program whose head goals have that principal functor. For example, the predicate for a 3-ary functor concatenate/3 might well consist of the two clauses concatenate([], L, L).

concatenate([X|L1], L2, [X|L3]) :- concatenate(L1, L2, L3).

where concatenate(L1,L2,L3) means “the list L1 concatenated with the list L2 is the list L3”. Note that for predicates with clauses corresponding to a base case and a recursive case, the preferred style is to write the base case clause first.

In Prolog, several predicates may have the same name but different arities. Therefore, when it is important to specify a predicate unambiguously, the form name/arity is used; e.g. concatenate/3.

Certain predicates are predefined by built-in predicates supplied by the Prolog system. Such predicates are called built-in predicates.

As we have seen, the goals in the body of a sentence are linked by the operator ‘,’ which can be interpreted as conjunction (“and”). It is sometimes convenient to use an additional operator ‘;’, standing for disjunction (“or”). (The precedence of ‘;’ is such that it dominates

‘,’ but is dominated by ‘:-’.) An example is the clause grandfather(X, Z) :-

(mother(X, Y); father(X, Y)), father(Y, Z).

which can be read as

For any X, Y and Z, X has Z as a grandfather if either the mother of X is Y or the father of X is Y, and the father of Y is Z.

Such uses of disjunction can always be eliminated by defining an extra predicate—for instance the previous example is equivalent to grandfather(X,Z) :- parent(X,Y), father(Y,Z).

Chapter 3: The Prolog Language 45 parent(X,Y) :- mother(X,Y).

parent(X,Y) :- father(X,Y).

—and so disjunction will not be mentioned further in the following, more formal, description of the semantics of clauses.

The token ‘|’, when used outside a list, is an alias for ‘;’. The aliasing is performed when terms are read in, so that a :- b | c.

is read as if it were a :- b ; c.

Note the double use of the ‘.’ character. On the one hand it is used as a sentence terminator, while on the other it may be used in a string of symbols which make up an atom (e.g. the list functor ./2). The rule used to disambiguate terms is that a ‘.’ followed by layout-text is regarded as a sentence terminator (see

[Token String], page 574

).

3.2 Declarative Semantics

The semantics of definite clauses should be fairly clear from the informal interpretations already given. However it is useful to have a precise definition. The declarative semantics of definite clauses tells us which goals can be considered true according to a given program, and is defined recursively as follows.

A goal is true if it is the head of some clause instance and each of the goals (if any) in the body of that clause instance is true, where an instance of a clause

(or term) is obtained by substituting, for each of zero or more of its variables, a new term for all occurrences of the variable.

For example, if a program contains the preceding procedure for concatenate/3, then the declarative semantics tells us that

?- concatenate([a], [b], [a,b]).

is true, because this goal is the head of a certain instance of the first clause for concatenate/3, namely, concatenate([a], [b], [a,b]) :- concatenate([], [b], [b]).

and we know that the only goal in the body of this clause instance is true, since it is an instance of the unit clause which is the second clause for concatenate/3.

3.3 Procedural Semantics

Note that the declarative semantics makes no reference to the sequencing of goals within the body of a clause, nor to the sequencing of clauses within a program. This sequencing

46 SICStus Prolog information is, however, very relevant for the procedural semantics which Prolog gives to definite clauses. The procedural semantics defines exactly how the Prolog system will execute a goal, and the sequencing information is the means by which the Prolog programmer directs the system to execute the program in a sensible way. The effect of executing a goal is to enumerate, one by one, its true instances. Here then is an informal definition of the procedural semantics. We first illustrate the semantics by the simple query

?- concatenate(X, Y, [a,b]).

We find that it matches the head of the first clause for concatenate/3, with X instantiated to [a|X1]. The new variable X1 is constrained by the new query produced, which contains a single recursive procedure call:

?- concatenate(X1, Y, [b]).

Again this goal matches the first clause, instantiating X1 to [b|X2], and yielding the new query:

?- concatenate(X2, Y, [])

Now the single goal will only match the second clause, instantiating both X2 and Y to [].

Since there are no further goals to be executed, we have a solution

X = [a,b]

Y = [] i.e. a true instance of the original goal is concatenate([a,b], [], [a,b])

If this solution is rejected, backtracking will generate the further solutions

X = [a]

Y = [b]

X = []

Y = [a,b] in that order, by re-matching, against the second clause for concatenate, goals already solved once using the first clause.

Thus, in the procedural semantics, the set of clauses

H :- B1, ..., Bm.

H’ :- B1’, ..., Bm’.

...

are regarded as a procedure definition for some predicate H, and in a query

?- G1, ..., Gn.

Chapter 3: The Prolog Language 47 each Gi is regarded as a procedure call. To execute a query, the system selects by its computation rule a goal, Gj say, and searches by its search rule a clause whose head matches

Gj. Matching is done by the unification algorithm (see [Robinson 65] which computes the most general unifier, mgu, of Gj and H. The mgu is unique if it exists. If a match is found, the current query is reduced to a new query

?- (G1, ..., Gj-1, B1, ..., Bm, Gj+1, ..., Gn)mgu.

and a new cycle is started. The execution terminates when the empty query has been produced.

If there is no matching head for a goal, the execution backtracks to the most recent successful match in an attempt to find an alternative match. If such a match is found, an alternative new query is produced, and a new cycle is started.

In SICStus Prolog, as in other Prolog systems, the search rule is simple: “search forward from the beginning of the program”.

The computation rule in traditional Prolog systems is also simple: “pick the leftmost goal of the current query”. However, SICStus Prolog and other modern implementations have a somewhat more complex computation rule “pick the leftmost unblocked goal of the current query”.

A goal can be blocked on one ore more uninstantiated variables, and a variable may block several goals. Thus binding a variable can cause blocked goals to become unblocked, and backtracking can cause currently unblocked goals to become blocked again. Moreover, if the current query is

?- G1, ..., Gj-1, Gj, Gj+1, ..., Gn.

where Gj is the first unblocked goal, and matching Gj against a clause head causes several blocked goals in G1, ..., Gj-1 to become unblocked, then these goals may become reordered.

The internal order of any two goals that were blocked on the same variable is retained, however.

Another consequence is that a query may be derived consisting entirely of blocked goals.

Such a query is said to have floundered. The top-level checks for this condition. If detected, the outstanding blocked subgoals are printed on the standard error stream along with the answer substitution, to notify the user that the answer (s)he has got is really a speculative one, since it is only valid if the blocked goals can be satisfied.

A goal is blocked if certain arguments are uninstantiated and its predicate definition is annotated with a matching block declaration (see

Section 5.2.5 [Block Declarations], page 64

).

Goals of certain built-in may also be blocked if their arguments are not sufficiently instantiated.

When this mechanism is used, the control structure resembles that of coroutines, suspending and resuming different threads of control. When a computation has left blocked goals behind, the situation is analogous to spawning a new suspended thread. When a blocked

48 SICStus Prolog goal becomes unblocked, the situation is analogous to temporarily suspending the current thread and resuming the thread to which the blocked goal belongs.

3.4 Occurs-Check

It is possible, and sometimes useful, to write programs which unify a variable to a term in which that variable occurs, thus creating a cyclic term. The usual mathematical theory behind Logic Programming forbids the creation of cyclic terms, dictating that an occurscheck should be done each time a variable is unified with a term. Unfortunately, an occurscheck would be so expensive as to render Prolog impractical as a programming language.

Thus cyclic terms may be created and may cause loops trying to print them.

SICStus Prolog mitigates the problem by its ability to unify, compare (see

Section 7.3

[Term Compare], page 129

), assert, and copy cyclic terms without looping. The write_ term/[2,3] built-in predicate can optionally handle cyclic terms; see

Section 7.1.3 [Term

I/O], page 108

. Unification with occurs-check is available as a built-in predicate; see

Section 7.16 [Misc Pred], page 160 . Predicates testing (a)cyclicity are available in a library

package; see

Chapter 18 [Term Utilities], page 265 . Other predicates usually do not handle

cyclic terms well.

3.5 The Cut Symbol

Besides the sequencing of goals and clauses, Prolog provides one other very important facility for specifying control information. This is the cut symbol, written !. It is inserted in the program just like a goal, but is not to be regarded as part of the logic of the program and should be ignored as far as the declarative semantics is concerned.

The effect of the cut symbol is as follows. When first encountered as a goal, cut succeeds immediately. If backtracking should later return to the cut, the effect is to fail the parent goal, i.e. that goal which matched the head of the clause containing the cut, and caused the clause to be activated. In other words, the cut operation commits the system to all choices made since the parent goal was invoked, and causes other alternatives to be discarded. The goals thus rendered determinate are the parent goal itself, any goals occurring before the cut in the clause containing the cut, and any subgoals which were executed during the execution of those preceding goals.

For example: member(X, [X|_]).

member(X, [_|L]) :- member(X, L).

This predicate can be used to test whether a given term is in a list. E.g.

| ?- member(b, [a,b,c]).

returns the answer ‘yes’. The predicate can also be used to extract elements from a list, as in

Chapter 3: The Prolog Language 49

| ?- member(X, [d,e,f]).

With backtracking this will successively return each element of the list. Now suppose that the first clause had been written instead: member(X, [X|_]) :- !.

In this case, the above call would extract only the first element of the list (d). On backtracking, the cut would immediately fail the whole predicate.

x :- p, !, q.

x :- r.

This is equivalent to x := if p then q else r; in an Algol-like language.

It should be noticed that a cut discards all the alternatives since the parent goal, even when the cut appears within a disjunction. This means that the normal method for eliminating a disjunction by defining an extra predicate cannot be applied to a disjunction containing a cut.

A proper use of the cut is usually a major difficulty for new Prolog programmers. The usual mistakes are to over-use cut, and to let cuts destroy the logic. A cut that doesn’t destroy the logic is called a green cut; a cut that does is called a red cut. We would like to advise all users to follow these general rules. Also see

Chapter 11 [Example Intro], page 239

.

• Write each clause as a self-contained logic rule which just defines the truth of goals which match its head. Then add cuts to remove any fruitless alternative computation paths that may tie up memory.

• Cuts are usually placed right after the head, sometimes preceded by simple tests.

• Cuts are hardly ever needed in the last clause of a predicate.

3.6 Operators

Operators in Prolog are simply a notational convenience. For example, the expression 2+1 could also be written +(2,1). This expression represents the compound term

2

+

/ \

1 and not the number 3. The addition would only be performed if the term were passed as an argument to an appropriate predicate such as is/2 (see

Section 7.2 [Arithmetic], page 125

).

50 SICStus Prolog

The Prolog syntax caters for operators of three main kinds—infix, prefix and postfix. An infix operator appears between its two arguments, while a prefix operator precedes its single argument and a postfix operator is written after its single argument.

Each operator has a precedence, which is a number from 1 to 1200. The precedence is used to disambiguate expressions where the structure of the term denoted is not made explicit through the use of parentheses. The general rule is that it is the operator with the highest precedence that is the principal functor. Thus if ‘+’ has a higher precedence than ‘/’, then a+b/c a+(b/c) are equivalent and denote the term +(a,/(b,c)). Note that the infix form of the term

/(+(a,b),c) must be written with explicit parentheses, i.e.

(a+b)/c

If there are two operators in the subexpression having the same highest precedence, the ambiguity must be resolved from the types of the operators. The possible types for an infix operator are xfx xfy yfx

Operators of type xfx are not associative: it is a requirement that both of the two subexpressions which are the arguments of the operator must be of lower precedence than the operator itself, i.e. their principal functors must be of lower precedence, unless the subexpression is explicitly parenthesized (which gives it zero precedence).

Operators of type xfy are right-associative: only the first (left-hand) subexpression must be of lower precedence; the right-hand subexpression can be of the same precedence as the main operator. Left-associative operators (type yfx) are the other way around.

A functor named name is declared as an operator of type Type and precedence precedence by the directive:

:- op(precedence, type, name).

The argument name can also be a list of names of operators of the same type and precedence.

It is possible to have more than one operator of the same name, so long as they are of different kinds, i.e. infix, prefix or postfix. Note that the ISO Prolog standard contains a limitation that there should be no infix and postfix operators with the same name, however,

SICStus Prolog lifts this restriction.

An operator of any kind may be redefined by a new declaration of the same kind. This applies equally to operators which are provided as standard, except for the ’,’ operator.

Declarations of all the standard operators can be found elsewhere (see

[Standard Operators], page 581 ).

For example, the standard operators + and - are declared by

Chapter 3: The Prolog Language 51

:- op(500, yfx, [ +, - ]).

so that a-b+c is valid syntax, and means

(a-b)+c i.e.

-

+

/ \ c

/ \ a b

The list functor . is not a standard operator, but if we declare it thus:

:- op(900, xfy, .).

then a.b.c would represent the compound term

.

/ \ a .

/ \ b c

Contrasting this with the diagram above for a-b+c shows the difference between yfx operators where the tree grows to the left, and xfy operators where it grows to the right.

The tree cannot grow at all for xfx operators; it is simply illegal to combine xfx operators having equal precedences in this way.

The possible types for a prefix operator are fx fy and for a postfix operator they are xf yf

The meaning of the types should be clear by analogy with those for infix operators. As an example, if not were declared as a prefix operator of type fy, then not not P would be a permissible way to write not(not(P)). If the type were fx, the preceding expression would not be legal, although not P

52 SICStus Prolog would still be a permissible form for not(P).

If these precedence and associativity rules seem rather complex, remember that you can always use parentheses when in any doubt.

Note that the arguments of a compound term written in standard syntax must be expressions of precedence below 1000. Thus it is necessary to parenthesize the expression P :- Q in

| ?- assert((P :- Q)).

3.7 Syntax Restrictions

Note carefully the following syntax restrictions, which serve to remove potential ambiguity associated with prefix operators.

1. In a term written in standard syntax, the principal functor and its following ( must not be separated by any intervening layout-text. Thus point (X,Y,Z) is invalid syntax.

2. If the argument of a prefix operator starts with a (, this ( must be separated from the operator by at least one layout-char. Thus

:-(p;q),r.

(where ‘:-’ is the prefix operator) is invalid syntax. The system would try to interpret it as the compound term:

,

:-

/ \

|

;

/ \ p q r

That is, it would take ‘:-’ to be a functor of arity 1. However, since the arguments of a functor are required to be expressions of precedence below 1000, this interpretation would fail as soon as the ‘;’ (precedence 1100) was encountered.

In contrast, the term:

:- (p;q),r.

is valid syntax and represents the following compound term:

:-

|

,

/ \

; r

/ \ p q

Chapter 3: The Prolog Language 53

3.8 Comments

Comments have no effect on the execution of a program, but they are very useful for making programs more readily comprehensible. Two forms of comment are allowed in Prolog:

1. The character % followed by any sequence of characters up to end of line.

2. The symbol /* followed by any sequence of characters (including new lines) up to */.

54 SICStus Prolog

Chapter 4: The Module System 55

4 The Module System

By making use of the module systems facilities, programs can be divided into different modules. Each module has its own independent predicate name space. This is an important feature for the development of larger programs. The module system of SICStus Prolog is procedure based. This means that only the predicates are local to a module, whereas terms are global. The module system is flat, not hierarchical, so all modules are visible to one another. It is non-strict, i.e. the normal visibility rules can be overridden by special syntax.

No overhead is incurred on compiled calls to predicates in other modules. It is modeled after and compatible with the Quintus Prolog module system. Finally, using the module system is optional, and SICStus Prolog may be used without the user being aware of the module system at all.

Modules in SICStus Prolog can also be used for object-oriented programming. See

Chapter 34 [Obj Intro], page 395 , for details.

4.1 Basic Concepts

Each predicate in the Prolog system, whether built-in or user defined, belongs to a module.

A predicate is generally only visible in the module where it is defined. However a predicate may be imported by another module. It is thereby made visible in that module too. Built-in predicates are visible in every module. Predicates declared as public in a module declaration

(see below) are exported. Normally only public predicates may be imported by another module.

For any given goal, the source module is the module in which the corresponding predicate must be visible. Similarly, for any given clause, the source module of its head is the module into which the clause is loaded.

For goals occurring in a source file with a module declaration, the source module is the declared module. For goals occurring in a source file without a module declaration, the source module is the module that the file is being loaded into. For goals typed at the top level, the source module is the type-in module. The type-in module is by default the user module but may be changed by the built-in predicate module/1.

The other predefined module is the prolog module where all the built-in predicates reside.

The exported built-in predicates are automatically imported into each new module as it is created.

4.2 Module Prefixing

Notwithstanding the visibility rules, any predicate can be called from any other module by prefixing the goal with the module name and the colon operator, thus overriding the source module of the goal:

| ?- foo:bar(X).

56 SICStus Prolog

This feature is intended mainly for debugging purposes, since it defies the purposes of the module system. If the prefixed goal is a meta-predicate, however, the prefixed module name may affect the module name expansion of the goal (see

Section 4.5 [Meta Exp], page 57

).

If multiple module prefixes are used, the innermost one has priority.

It is also possible to override the source module of clauses and directives by module prefixing.

For example,

:- dynamic mod:p/1.

p(X) :- mod:(q(X), r(X)).

mod:(q(X) :- r(X)).

mod:s(X) :- t(X).

declares mod:p/1 as dynamic, whatever the source module is; defines p/1 in the source module as calling mod:q/1 and mod:r/1; defines mod:q/1 as calling mod:r/1; and defines mod:s/1 as calling t/1 in the source module. The latter technique is particularly useful when the prefix is user and the predicate is a hook predicate such as user:portray/1 which must be defined in the user module, but the rest of the file consists of predicates belonging to some other module.

4.3 Defining Modules

A module is normally defined by putting a module declaration in a source file. A module declaration has the form:

:- module(ModuleName, ExportList[, Options]).

and should precede all other clauses and directives of that file.

When the file is loaded, all predicates in the file go into ModuleName and the predicates of the ExportList are exported. When a module declaration is processed, all existing predicates in the module are erased before the new ones are loaded. A file which contains a module declaration is henceforth called a module-file.

Options is an optional argument, and should be a list.

The only available option is hidden(Boolean), where Boolean is false (the default) or true. In the latter case, tracing of the predicates of the module is disabled (although spypoints can be set), and no source information is generated at compile time.

A module can also be defined dynamically by asserting or loading predicates to it:

| ?- assert(m:p(x)).

creates the module m, if it does not already exists, and asserts p(x) to it.

| ?- compile(m:f).

creates the module m and loads f into m.

Dynamically created modules have no public predicates.

Chapter 4: The Module System 57

4.4 Importation

When a module-file is loaded by load_files/[1,2] or one of its shorthands (see

Section 7.1.1 [Read In], page 100 ), by default all the public predicates of the module-file are

imported by the receiving module. An explicit list of predicates to import may also be specified.

Clashes with already existing predicates, local or imported from other modules, are handled in two different ways: If the receiving module is the user module, the user is asked for redefinition of the predicate. For other receiving modules, a warning is issued and the importation is canceled. In the first case redefinition silently takes place if the flag redefine_warnings has the value off (see prolog_flag/3). The binding of an imported predicate remains, even if the origin is reloaded or deleted. However, abolish/[1,2] break up the importation binding. When a module-file is reloaded, a check is made that the predicates imported by other modules are still in the public list. If that is not the case, a warning is issued. Note that an imported predicate may be re-exported.

4.5 Module Name Expansion

Some predicates take goals as arguments (i.e. meta-predicates). These arguments must include a module specification stating which module the goal refers. Some other predicates also need module information i.e. compile/1. The property of needing module information is declared with a meta-predicate declaration (see

Section 4.6 [Meta Decl], page 58 ). Goals

for these predicates are module name expanded to ensure the module information. Goals appearing in queries and meta-calls are expanded prior to execution while goals in the bodies of clauses and directives are expanded at compile time. The expansion is made by preceding the relevant argument with ‘Module:’. If the goal is prefixed by ‘Module:’, Module is used for the expansion, otherwise the source/type-in module is used. An argument is not expanded if:

• It already has a module prefix, or

• It is a variable which appears in an expandable position in the head of the clause.

Some examples:

| ?- [user].

| :- meta_predicate p(:), q(:).

| r(X) :- p(X).

| q(X) :- p(X).

| ^D

{user consulted, 40 msec 1088 bytes} yes

| ?- listing.

r(A) :p(user:A).

58 SICStus Prolog q(A) :p(A).

yes

Here, p/1 and q/1 are declared as meta-predicates while r/1 is not. Thus the clause r(X)

:- p(X) will be transformed to r(X) :- p(M :X), by item 2 above, where M is the type-in module, whereas q(X) :- p(X) will not.

| ?- m:assert(f(1)).

Here, assert/1 is called in the module m. However, this does not ensure that f(1) is asserted into m. The fact that assert/1 is a meta-predicate makes the system module name expand the goal, transforming it to m:assert(m:f(1)) before execution. This way, assert/1 is supplied the correct module information.

4.6 Meta-Predicate Declarations

The fact that a predicate needs module name expansion is declared in a meta-predicate declaration:

:- meta_predicate MetaPredSpec, ..., MetaPredSpec.

where each MetaPredSpec is a mode spec. E.g.

:- meta_predicate p(:, +).

which means that the first argument of p/2 shall be module name expanded. The arguments in the mode spec are interpreted as:

:

An integer

This argument, in any call to the declared predicate, shall be expanded. (Integers are allowed for compatibility reasons).

Anything else e.g. +, - or ?

This argument shall not be expanded

A number of built-in predicates have predefined meta-predicate declarations, as indicated by the mode specs in this manual, e.g. call(:Term).

Chapter 5: Loading Programs 59

5 Loading Programs

Programs can be loaded in three different ways: consulted or compiled from source file, or loaded from object files.

The latter is the fastest way of loading programs, but of course requires that the programs have been compiled to object files first. Object files may be handy when developing large applications consisting of many source files, but are not strictly necessary since it is possible to save and restore entire execution states (see

Section 7.16 [Misc Pred], page 160

).

Consulted, or interpreted, predicates are equivalent to, but slower than, compiled ones.

Although they use different representations, the two types of predicates can call each other freely.

The SICStus Prolog compiler produces compact and efficient code, running about 8 times faster than consulted code, and requiring much less runtime storage. Compiled Prolog programs are comparable in efficiency with LISP programs for the same task. However, against this, compilation itself takes about twice as long as consulting, and tracing of goals that compile in-line are not available in compiled code.

The compiler operates in four different modes, controlled by the “Compilation mode” flag

(see prolog_flag/3). The possible states of the flag are: compactcode

Compilation produces byte-coded abstract instructions.

This is the default unless SICStus Prolog has been installed with support for fastcode compilation.

fastcode

Compilation produces native machine instructions. Currently only available for

Sparc platforms. Fastcode runs about 3 times faster than compactcode. This is the default if SICStus Prolog has been installed with support for fastcode compilation.

profiledcode

Compilation produces byte-coded abstract instructions instrumented to produce execution profiling data. See

Section 7.15 [Profiling], page 158 . Profiling

is not available in runtime systems.

debugcode

Compilation produces interpreted code, i.e. compiling is replaced by consulting.

The compilation mode can be changed by issuing the query:

| ?- prolog_flag(compiling, OldValue, NewValue).

A Prolog program consists of a sequence of sentences (see

[Sentence], page 571

). Directives encountered among the sentences are executed immediately as they are encountered, unless they can be interpreted as declarations (see

Section 5.2 [Declarations], page 62 ), which

affect the treatment of forthcoming clauses, or as initializations, which build up a set of goals to be executed after the program has been loaded. Clauses are loaded as they are encountered.

60 SICStus Prolog

A Prolog program may also contain a list of sentences (including the empty list). This is treated as equivalent to those sentences occurring in place of the list. This feature makes it possible to have user:term_expansion/[2,4] (see

Section 7.1.2 [Definite], page 103 )

“return” a list of sentences, instead of a single sentence.

5.1 Predicates which Load Code

This section contains a summary of the relevant predicates. For a more precise description, see

Section 7.1.1 [Read In], page 100 .

To consult a program, issue the query:

| ?- consult(Files).

where Files is either a filename or a list of filenames, instructs the processor to read in the program which is in the files. For example:

| ?- consult([dbase,’extras.pl’,user]).

When a directive is read it is immediately executed. Any predicate defined in the files erases any clauses for that predicate already present. If the old clauses were loaded from a different file than the present one, the user will be queried first whether (s)he really wants the new definition. However, if a multifile declaration (see

Section 5.2 [Declarations], page 62

) is read and the corresponding predicate exists and has previously been declared as multifile, new clauses will be added to the predicate, rather than replacing the old clauses. If clauses for some predicate appear in more than one file, the later set will effectively overwrite the earlier set. The division of the program into separate files does not imply any module structure—any predicate can call any other (see

Chapter 4 [Module Intro], page 55 ).

consult/1, used in conjunction with save_program/[1,2] and restore/1, makes it possible to amend a program without having to restart from scratch and consult all the files which make up the program. The consulted file is normally a temporary “patch” file containing only the amended predicate(s). Note that it is possible to call consult(user) and then enter a patch directly on the terminal (ending with ^D). This is only recommended for small, tentative patches.

| ?- [File|Files].

This is a shorthand way of consulting a list of files. (The case where there is just one filename in the list was described earlier (see

Section 2.2 [Reading In], page 21

).

To compile a program in-core, use the built-in predicate:

| ?- compile(Files).

where Files is specified just as for consult/1.

Chapter 5: Loading Programs 61

The effect of compile/1 is very much like that of consult/1, except all new procedures will be stored in compiled rather than consulted form. However, predicates declared as dynamic

(see below) will be stored in consulted form, even though compile/1 is used.

Programs can be compiled into an intermediate representation known as ‘.ql’ (for Quick

Load file). As of SICStus Prolog 3.8, this feature is obsolescent with the introduction of partial saved states (‘.po’ files, see

Section 2.10 [Saving], page 27

), which can be handled much more efficiently.

To compile a program into a ‘.ql’ file, use the built-in predicate:

| ?- fcompile(Files).

where Files is specified just as for consult/1. For each filename in the list, the compiler will append the suffix ‘.pl’ to it and try to locate a source file with that name and compile it to a ‘.ql’ file. The filename is formed by appending the suffix ‘.ql’ to the specified name. The internal state of SICStus Prolog is not changed as result of the compilation. See

Section 5.4 [Considerations], page 66 .

To load a program from a set of source or object files, use the built-in predicates load_ files/[1,2] (the latter is controlled by an options list):

| ?- load_files(Files).

where Files is either a single filename or a list of filenames, optionally with ‘.pl’ or ‘.po’ or ‘.ql’ extensions. This predicate takes the following action for each File in the list of filenames:

1. If the File is user, compile(user) or [user] is performed;

2. If File cannot be found, not even with an extension, an existence error is signaled;

3. If an ‘.po’ file is found, the file is loaded;

4. If an ‘.ql’ file is found, the file is loaded;

5. If a source file is found, the file is compiled or consulted.

6. If more than one file is found for File, item 3 or 4 or 5 applies depending on which file was modified most recently.

7. If File cannot be found, not even with an extension, an existence error is signaled.

8. Source files are compiled, unless load_files/1 was called from a directive of a file being consulted.

Finally, to ensure that some files have been loaded, use the built-in predicate:

| ?- ensure_loaded(Files).

Same as load_files(Files), except if the file to be loaded has already been loaded and has not been modified since that time, in which case the file is not loaded again. If a source file has been modified, ensure_loaded/1 does not cause any object file to become recompiled.

62 SICStus Prolog

5.2 Declarations

When a program is to be loaded, it is sometimes necessary to tell the system to treat some of the predicates specially. This information is supplied by including declarations about such predicates in the source file, preceding any clauses for the predicates which they concern.

A declaration is written just as a directive, beginning with ‘:-’. A declaration is effective from its occurrence through the end of file.

Although declarations that affect more than one predicate may be collapsed into a single declaration, the recommended style is to write the declarations for a predicate immediately before its first clause.

Operator declarations are not declarations proper, but rather directives that modify the global table of syntax operators. Operator declarations are executed as they are encountered while loading programs.

The rest of this section details the available forms of predicate declarations.

5.2.1 Multifile Declarations

A declaration

:- multifile PredSpec, ..., PredSpec.

[ISO] where each PredSpec is a predicate spec, causes the specified predicates to become multifile.

This means that if more clauses are subsequently loaded from other files for the same predicate, then the new clauses will not replace the old ones, but will be added at the end instead. As of release 3, multifile declarations are required in all files from where clauses to a multifile predicate are loaded.

An example when multifile declarations are particularly useful is in defining hook predicates.

A hook predicate is a user-defined predicate that somehow alters or customizes the behavior of SICStus Prolog. A number of such hook predicates are described in this manual. Often, an application needs to combine the functionality of several software modules, some of which define clauses for such hook predicates. By simply declaring every hook predicates as multifile, the functionality of the clauses for the hook predicates is automatically combined.

If this is not done, the last software module to define clauses for a particular hook predicate will effectively supersede any clauses defined for the same hook predicate in a previous module. By default, hook predicates must be defined in the user module, and only their first solution is relevant.

If a file containing clauses for a multifile predicate is reloaded, the old clauses from the same file are removed. The new clauses are added at the end.

If a multifile predicate is loaded from a file with no multifile declaration for it, the predicate is redefined as if it were an ordinary predicate (i.e. the user is asked for confirmation).

Clauses of multifile predicates are (currently) always loaded in interpreted form, even if they were processed by the compiler. If performance is an issue, define the multifile predicates as

Chapter 5: Loading Programs 63 unit clauses or as clauses with a single goal that just calls an auxiliary compiled predicate to perform any time-critical computation.

If a multifile predicate is declared dynamic in one file, it must also be done so in the other files from where it is loaded. Hook predicates should always be declared as multifile and dynamic, as this is the convention followed in the library modules.

Multifile declarations must precede any other declarations for the same predicate(s)!

5.2.2 Dynamic Declarations

A declaration

:- dynamic PredSpec, ..., PredSpec.

[ISO] where each PredSpec is a predicate spec, causes the specified predicates to become dynamic, which means that other predicates may inspect and modify them, adding or deleting individual clauses. Dynamic predicates are always stored in consulted form even if a compilation is in progress. This declaration is meaningful even if the file contains no clauses for a specified predicate—the effect is then to define a dynamic predicate with no clauses.

5.2.3 Volatile Declarations

A declaration

:- volatile PredSpec, ..., PredSpec.

where each PredSpec is a predicate spec, causes the specified predicates to become volatile.

A predicate should be declared as volatile if it refers to data that cannot or should not be saved in a saved state. In most cases a volatile predicate will be dynamic, and it will be used to keep facts about streams or memory references. When a program state is saved at runtime, the clauses of all volatile predicates will be left unsaved. The predicate definitions will be saved though, which means that the predicates will keep all properties, that is volatile and maybe dynamic or multifile, when the saved state is restored.

5.2.4 Discontiguous Declarations

A declaration

:- discontiguous PredSpec, ..., PredSpec.

[ISO] where each PredSpec is a predicate spec, disables warnings about clauses not being together for the specified predicates. By default, such warnings are issued in development systems unless disabled selectively for specific predicates, or globally by setting the discontiguous_ warnings flag to off.

64 SICStus Prolog

5.2.5 Block Declarations

The declaration

:- block BlockSpec, ..., BlockSpec.

where each BlockSpec is a mode spec, specifies conditions for blocking goals of the predicate referred to by the mode spec (f/3 say). When a goal for f/3 is to be executed, the mode specs are interpreted as conditions for blocking the goal, and if at least one condition evaluates to true, the goal is blocked.

A block condition evaluates to true iff all arguments specified as ‘-’ are uninstantiated, in which case the goal is blocked until at least one of those variables is instantiated. If several conditions evaluate to true, the implementation picks one of them and blocks the goal accordingly.

The recommended style is to write the block declarations in front of the source code of the predicate they refer to. Indeed, they are part of the source code of the predicate, and must precede the first clause. For example, with the definition:

:- block merge(-,?,-), merge(?,-,-).

merge([], Y, Y).

merge(X, [], X).

merge([H|X], [E|Y], [H|Z]) :- H @< E, merge(X, [E|Y], Z).

merge([H|X], [E|Y], [E|Z]) :- H @>= E, merge([H|X], Y, Z).

calls to merge/3 having uninstantiated arguments in the first and third position or in the second and third position will suspend.

The behavior of blocking goals for a given predicate on uninstantiated arguments cannot be switched off, except by abolishing or redefining the predicate.

Block declarations generalize the "wait declarations" of earlier versions of SICStus Prolog.

A declaration ‘:- wait f/3’ in the old syntax corresponds to ‘:- block f(-,?,?)’ in the current syntax. See

Section 11.5.6 [Use Of Term Exp], page 243

, for a simple way to extend the system to accept the old syntax.

5.2.6 Meta-Predicate Declarations

A declaration

:- meta_predicate MetaPredSpec, ..., MetaPredSpec.

where each MetaPredSpec is a mode spec, informs the compiler that certain arguments of the declared predicates are used for passing goals. To ensure the correct semantics in the context of multiple modules, clauses or directives containing goals for the declared predicates may need to have those arguments module name expanded. See

Section 4.5 [Meta Exp], page 57 , for details.

Chapter 5: Loading Programs 65

5.2.7 Module Declarations

A declaration

:- module(ModuleName, ExportList[, Options]).

where ExportList is a list of predicate specs, declares that the forthcoming predicates should go into the module named ModuleName and that the predicates listed should be exported.

See

Section 4.3 [Def Modules], page 56 , for details.

5.2.8 Public Declarations

A declaration

:- public PredSpec, ..., PredSpec.

where each PredSpec is a predicate spec, has no effect whatsoever, but is accepted for compatibility reasons. In some Prologs, this declaration is necessary for making compiled predicates visible. In SICStus Prolog, predicate visibility is handled by the module system.

This declaration is obsolescent. See

Chapter 4 [Module Intro], page 55 .

5.2.9 Mode Declarations

A declaration

:- mode ModeSpec, ..., ModeSpec.

where each ModeSpec is a mode spec, has no effect whatsoever, but is accepted for compatibility reasons. In some Prologs, this declaration helps reduce the size of the compiled code for a predicate, and may speed up its execution. Unfortunately, writing mode declarations can be error-prone, and since errors in mode declaration do not show up while running the predicates interpretively, new bugs may show up when predicates are compiled. However, mode declarations may be used as a commenting device, as they express the programmer’s intention of data flow in predicates.

5.2.10 Include Declarations

A declaration

:- include(Files).

[ISO] where Files is a file name or a list of file names, instructs the processor to literally embed the Prolog clauses and directives in Files into the file being loaded. This means that the effect of the include directive is such as if the include directive itself was replaced by the text in the Files. Including some files is thus different from loading them in several respects:

• The embedding file counts as the source file of the predicates loaded, e.g. with respect to the built-in predicate source_file/2; see

Section 7.1.1 [Read In], page 100

.

66 SICStus Prolog

• Some clauses of a predicate can come from the embedding file, and some from included files.

• When including a file twice, all the clauses in it will be entered twice into the program

(although this is not very meaningful).

SICStus Prolog uses the included file name (as opposed to the embedding file name) only in source level debugging and error reporting. Note that source level debugging information is not kept for included files which are compiled to ‘.ql’ format; in such cases the debugger will show the include directive itself as the source information.

5.3 Initializations

A directive

:- initialization :Goal.

[ISO] in a file includes Goal to the set of goals which shall be executed after that file has been loaded.

initialization/1 is actually callable at any point during loading of a file. Initializations are saved by save_modules/2 and save_program/[1,2], and so are executed after loading or restoring such files too.

Goal is associated with the file loaded, and with a module, if applicable. When a file, or module, is going to be reloaded, all goals earlier installed by that file, or in that module, are removed first.

5.4 Considerations for File-To-File Compilation

When compiling a source file to a ‘.ql’ file, remember that clauses are loaded and directives are executed at run time, not at compile time. Only predicate declarations are processed at compile time. For instance, it does not work to include operator declarations or clauses of user:term_expansion/[2,4] or user:goal_expansion/3 or any auxiliary predicates that they might need, and rely on the new transformations to be effective for subsequent clauses of the same file or subsequent files of the same compilation.

Any directives or clauses that affect the compile-time environment must be loaded prior to compiling source files to ‘.ql’ files. This also holds for meta-predicates called by the source files but defined elsewhere, for module name expansion to work correctly. If this separation into files is unnatural or inconvenient, one can easily ensure that the compiletime environment is up to date by doing:

| ?- ensure_loaded(Files), fcompile(Files).

Since module name expansion takes place at compile time, the module into which the file is to be loaded must be known when compiling to ‘.ql’ files. This is no problem for module-files because the module name is picked from the module declaration. When non-

Chapter 5: Loading Programs 67 module-files are compiled, the file name may be prefixed with the module name that is to be used for expansion:

| ?- fcompile(Module:Files).

If an ‘.ql’ file is loaded into a different module from which it was compiled for, a warning is issued.

68 SICStus Prolog

Chapter 6: Debugging 69

6 Debugging

This chapter describes the debugging facilities that are available in development systems.

The purpose of these facilities is to provide information concerning the control flow of your program.

The main features of the debugging package are as follows:

• The Procedure Box model of Prolog execution which provides a simple way of visualizing control flow, especially during backtracking. Control flow is viewed at the predicate level, rather than at the level of individual clauses.

• The ability to exhaustively trace your program or to selectively set spypoints. Spypoints allow you to nominate interesting predicates at which, for example, the program is to pause so that you can interact.

• The ability to set advice-points. An advice-point allows you to carry out some actions at certain points of execution, independently of the tracing activity. Advice-points can be used, e.g., for checking certain program invariants (cf. the assert facility of the

C programming language), or for gathering profiling or branch coverage information.

Spypoints and advice-points are collectively called breakpoints.

• The wide choice of control and information options available during debugging.

The Procedure Box model of execution is also called the Byrd Box model after its inventor,

Lawrence Byrd.

Much of the information in this chapter is also in Chapter eight of [Clocksin & Mellish 81] which is recommended as an introduction.

Unless otherwise stated, the debugger prints goals using write_term/3 with the value of the Prolog flag debugger_print_options (see

Section 7.6 [State Info], page 137 ).

The debugger is not available in runtime systems and the predicates defined in this chapter are undefined; see

Section 8.7.1 [Runtime Systems], page 190 .

6.1 The Procedure Box Control Flow Model

During debugging, the debugger prints out a sequence of goals in various states of instantiation in order to show the state the program has reached in its execution. However, in order to understand what is occurring it is necessary to understand when and why the debugger prints out goals. As in other programming languages, key points of interest are predicate entry and return, but in Prolog there is the additional complexity of backtracking. One of the major confusions that novice Prolog programmers have to face is the question of what actually happens when a goal fails and the system suddenly starts backtracking. The

Procedure Box model of Prolog execution views program control flow in terms of movement about the program text. This model provides a basis for the debugging mechanism in development systems, and enables the user to view the behavior of the program in a consistent way.

70 SICStus Prolog

Let us look at an example Prolog predicate :

Call

*--------------------------------------*

| | Exit

---------> + descendant(X,Y) :- offspring(X,Y).

+ --------->

| |

| descendant(X,Z) :-

<--------- +

| offspring(X,Y), descendant(Y,Z). + <---------

Fail Redo | |

*-------------------+------------------*

|

<------------------------------+

Exception

The first clause states that Y is a descendant of X if Y is an offspring of X, and the second clause states that Z is a descendant of X if Y is an offspring of X and if Z is a descendant of Y. In the diagram a box has been drawn around the whole predicate and labeled arrows indicate the control flow in and out of this box. There are five such arrows which we shall look at in turn.

Call

Exit

Redo

Fail

This arrow represents initial invocation of the predicate. When a goal of the form descendant(X,Y) is required to be satisfied, control passes through the

Call port of the descendant box with the intention of matching a component clause and then satisfying the subgoals in the body of that clause. Note that this is independent of whether such a match is possible; i.e. first the box is called, and then the attempt to match takes place. Textually we can imagine moving to the code for descendant when meeting a call to descendant in some other part of the code.

This arrow represents a successful return from the predicate. This occurs when the initial goal has been unified with one of the component clauses and the subgoals have been satisfied. Control now passes out of the Exit port of the descendant box. Textually we stop following the code for descendant and go back to the place we came from.

This arrow indicates that a subsequent goal has failed and that the system is backtracking in an attempt to find alternatives to previous solutions. Control passes through the Redo port of the descendant box. An attempt will now be made to resatisfy one of the component subgoals in the body of the clause that last succeeded; or, if that fails, to completely rematch the original goal with an alternative clause and then try to satisfy any subgoals in the body of this new clause. Textually we follow the code backwards up the way we came looking for new ways of succeeding, possibly dropping down on to another clause and following that if necessary.

This arrow represents a failure of the initial goal, which might occur if no clause is matched, or if subgoals are never satisfied, or if any solution produced is always rejected by later processing. Control now passes out of the Fail port of the descendant box and the system continues to backtrack. Textually we

Chapter 6: Debugging 71 move back to the code which called this predicate and keep moving backwards up the code looking for choice points.

Exception This arrow represents an exception which was raised in the initial goal, either by a call to raise_exception/1 or by an error in a built-in predicate. See

Section 7.5 [Exception], page 134 . Control now passes out of the Exception

port of the descendant box and the system continues to pass the exception to outer levels. Textually we move back to the code which called this predicate and keep moving backwards up the code looking for a call to on_exception/3.

In terms of this model, the information we get about the procedure box is only the control flow through these five ports. This means that at this level we are not concerned with which clause matches, and how any subgoals are satisfied, but rather we only wish to know the initial goal and the final outcome. However, it can be seen that whenever we are trying to satisfy subgoals, what we are actually doing is passing through the ports of their respective boxes. If we were to follow this, then we would have complete information about the control flow inside the procedure box.

Note that the box we have drawn round the predicate should really be seen as an invocation box. That is, there will be a different box for each different invocation of the predicate.

Obviously, with something like a recursive predicate, there will be many different Calls and

Exits in the control flow, but these will be for different invocations. Since this might get confusing each invocation box is given a unique integer identifier.

In addition to the five basic ports discussed above, there are two more ports for invocations involving a blocked goal:

Block This port is passed through when a goal is blocked.

Unblock This port is passed through when a previously blocked goal is unblocked.

6.2 Basic Debugging Predicates

Development systems provide a range of built-in predicates for control of the debugging facilities. The most basic predicates are as follows: debug

Switches the debugger on, and ensures that the next time control reaches a spypoint, it will be activated. In basic usage this means that a message will be produced and you will be prompted for a command. In order for the full range of control flow information to be available it is necessary to have the debugger on from the start. When it is off the system does not remember invocations that are being executed. (This is because it is expensive and not required for normal running of programs.) You can switch Debug Mode on in the middle of execution, either from within your program or after a ^C (see trace/0 below), but information prior to this will just be unavailable.

zip

72 SICStus Prolog

Same as debug/0, except no debugging information is being collected, and so is almost as fast as running with the debugger switched off.

trace

Switches the debugger on, and ensures that the next time control enters an invocation box, a message will be produced and you will be prompted for a command. The effect of trace can also be achieved by typing t after a ^C interruption of a program.

At this point you have a number of options. See

Section 6.6 [Debug Commands], page 78

. In particular, you can just type h

RET i to creep (or single-step) into your program. If you continue to creep through your program you will see every entry and exit to/from every invocation box, including compiled code, except for code belonging to hidden modules (see

Section 4.3 [Def Modules], page 56

).

You will notice that the debugger stops at all ports. However, if this is not what you want, the following built-in predicate gives full control over the ports at which you are prompted: leash(+Mode)

Leashing Mode is set to Mode. Leashing Mode determines the ports of invocation boxes at which you are to be prompted when you Creep through your program. At unleashed ports a tracing message is still output, but program execution does not stop to allow user interaction. Note that leash/1 does not apply to spypoints, the leashing mode of these can be set using the advanced debugger features; see

Section 6.5 [Breakpoints], page 75

. Mode can be a subset of the following, specified as a list: call

Prompt on Call.

exit redo

Prompt on Exit.

Prompt on Redo.

fail

Prompt on Fail.

exception

Prompt on Exception.

The initial value of Leashing Mode is [call,exit,redo,fail,exception] (full leashing).

nodebug notrace nozip

Switches the debugger off. If there are any spypoints set then they will be kept but will never be activated.

debugging

Prints information about the current debugging state. This will show:

1. Whether undefined predicates are being trapped.

2. What breakpoints have been set (see below).

3. What mode of leashing is in force (see above).

Chapter 6: Debugging 73

6.3 Plain Spypoints

For programs of any size, it is clearly impractical to creep through the entire program.

Spypoints make it possible to stop the program whenever it gets to a particular predicate which is of interest. Once there, one can set further spypoints in order to catch the control flow a bit further on, or one can start creeping.

In this section we discuss the simplest form of spypoints, the plain spypoints. The more advanced forms, the conditional and generic spypoints will be discussed later; see

Section 6.5

[Breakpoints], page 75

.

Setting a plain spypoint on a predicate indicates that you wish to see all control flow through the various ports of its invocation boxes, except during skips. When control passes through any port of an invocation box with a spypoint set on it, a message is output and the user is asked to interact. Note that the current mode of leashing does not affect plain spypoints: user interaction is requested on every port.

Spypoints are set and removed by the following built-in predicates. The first two are also standard operators: spy :Spec

Sets plain spypoints on all the predicates given by the generalized predicate spec Spec.

Examples:

| ?- spy [user:p, m:q/[2,3]].

| ?- spy m:[p/1, q/1].

If you set some spypoints when the debugger is switched off then it will be automatically switched on, entering zip mode.

nospy :Spec

Similar to spy Spec except that all the predicates given by Spec will have all previously set spypoints removed from them (including conditional spypoints; see

Section 6.5 [Breakpoints], page 75 ).

nospyall

Removes all the spypoints, including the conditional ones, that have been set.

The commands available when you arrive at a spypoint are described later. See

Section 6.6

[Debug Commands], page 78

.

6.4 Format of Debugging Messages

We shall now look at the exact format of the message output by the system at a port.

All trace messages are output to the standard error stream, using the print_message/2 predicate; see

Section 7.5 [Exception], page 134

. This allows you to trace programs while they are performing file I/O. The basic format is as follows:

N S

23 6 Call: T foo(hello,there,_123) ?

74 SICStus Prolog

N is only used at Exit ports and indicates whether the invocation could backtrack and find alternative solutions. Unintended non-determinism is a source of inefficiency, and this annotation can help spot such efficiency bugs. It is printed as ‘?’, indicating that foo/3 could backtrack and find alternative solutions, or ‘ ’ otherwise.

S is a spypoint indicator. If there is a plain spypoint on foo/3, it is printed as ‘+’. In case of conditional and generic spypoints it takes the form ‘*’ and ‘#’, respectively. Finally, it is printed as ‘ ’, if there is no spypoint on the predicate being traced.

The first number is the unique invocation identifier. It is nondecreasing regardless of whether or not you are actually seeing the invocations (provided that the debugger is switched on).

This number can be used to cross correlate the trace messages for the various ports, since it is unique for every invocation. It will also give an indication of the number of procedure calls made since the start of the execution. The invocation counter starts again for every fresh execution of a command, and it is also reset when retries (see later) are performed.

The number following this is the current depth; i.e. the number of direct ancestors this goal has.

The next word specifies the particular port (Call, Exit, Redo, Fail, or Exception).

T is a subterm trace. This is used in conjunction with the ‘^’ command (set subterm), described below. If a subterm has been selected, T is printed as the sequence of commands used to select the subterm. Normally, however, T is printed as ‘ ’, indicating that no subterm has been selected.

The goal is then printed so that you can inspect its current instantiation state.

The final ‘?’ is the prompt indicating that you should type in one of the commands allowed

(see

Section 6.6 [Debug Commands], page 78 ). If this particular port is unleashed then you

will obviously not get this prompt since you have specified that you do not wish to interact at this point.

At Exception ports, the trace message is preceded by a message about the pending exception, formatted as if it would arrive uncaught at the top level.

Note that calls that are compiled in-line and built-in predicates that are called directly from the top level are not traced.

Block and unblock ports are exceptions to the above debugger message format. A message

S -

Block: p(_133) indicates that the debugger has encountered a blocked goal, i.e. one which is temporarily suspended due to insufficiently instantiated arguments (see

Section 3.3 [Procedural], page 45

). By default, no interaction takes place at this point, and the debugger simply proceeds to the next goal in the execution stream. The suspended goal will be eligible for execution once the blocking condition ceases to exist, at which time a message

S -

Unblock: p(_133)

Chapter 6: Debugging 75 is printed. Although Block and Unblock ports are unleashed by default in trace mode, you can make the debugger interact at these ports by using conditional spypoints.

6.5 Breakpoints

This section gives an overview of the advanced debugger features. These center around the notion of breakpoint, a generalization of the plain spypoint introduced earlier. The details of built-in predicates dealing with breakpoints are given in

Section 6.7 [Breakpoint

Predicates], page 84

and in

Section 6.9 [Breakpoint Conditions], page 87 .

A breakpoint is specified by providing the conditions under which the breakpoint applies and its effect on the execution. Syntactically, the breakpoint is given as a pair of two lists of conditions. The first list, the tests, contains the tests for the applicability of the breakpoint, the second, the actions describes the effects of the breakpoint on the execution. Example:

| ?- add_breakpoint([goal(foo(1,_)),port(call)]-

[show(display),command(proceed)],

BID).

This invocation of the add_breakpoint/2 built-in predicate creates a breakpoint which will apply to goals which unify with foo(1,_) and only to their call ports — this is the test part. The second list, the action part specifies that the debugger should show the debugged goal using display/1 and should proceed without stopping, whenever the breakpoint is applicable. The add_breakpoint/2 built-in returns a BID, a numeric breakpoint identifier.

There are two types of breakpoints, advice and debugger.

These are distinguished by the presence of conditions advice and debugger, the latter being the default. Debugger breakpoints are also called spypoints. Spypoints apply only when the debugger is switched on. Advice breakpoints, or advice-points, apply irrespectively of the debugger state. Advice is checked even inside hook predicates.

The most important test condition is goal(M :Goal). This makes the breakpoint applicable only to invocations whose goal and module matches Goal and M. A breakpoint with a goal condition where both the module and the goal are non-variable is called a specific breakpoint. If any of these is a variable (or there is no goal condition), the breakpoint is generic. The presence of a generic breakpoint slows down the execution, as its conditions have to be examined at every predicate invocation. Specific breakpoints, on the other hand, are handled efficiently, as only the execution of the specified predicate is affected.

The following example shows the creation of a generic breakpoint.

| ?- add_breakpoint([advice,port(call)]-

[line(L),true(assert(line_reached(L)))],

BID).

This advice-point will apply to all predicates (or more precisely to all predicates in the module in question — as the conditions argument is subject to module name expansion; see

Section 6.9.1 [Goal Tests], page 87

). Whenever a call port is reached, the action part of

76 SICStus Prolog the breakpoint gets hold of the source line number (if available) and asserts a fact with this number. Here the condition true(PrologCondition) executes PrologCondition as a Prolog goal and succeeds accordingly. An advice breakpoint of this kind can be used for profiling or branch-coverage analysis.

The pred condition is a variant of the goal condition: pred(F/N ) is equivalent to goal(Goal), where Goal is the most general term with the functor F/N. A specific spypoint whose conditions consist of a single pred condition (or an equivalent goal condition) is the same as the plain spypoint introduced earlier; see

Section 6.3 [Plain Spypoint], page 73

.

Specific spypoints which are not plain, are called conditional spypoints. For example, the following three goals have the same effect, creating a plain spypoint for predicate foo/2:

| ?- add_breakpoint(pred(foo/2), _).

| ?- add_breakpoint(goal(foo(_,_)), _).

| ?- spy foo/2.

The effect of the breakpoint on the execution is specified by the actions list. In the case of spypoints, the action part may specify values for the following three debugger action variables:

• show, i.e. whether the debugger should display anything, and if so, in what format.

Examples: print, display, silent, etc.

• command, i.e. what should the debugger do. Examples: ask (the normal query to the user), proceed (continue the execution without stopping, as for unleashed ports), etc.

• mode, i.e. in what mode should the debugger continue the execution: trace (creeping), debug (leaping), zip (zipping), etc.

If no actions are specified at a spypoint, the default [command(ask),show(print)] is assumed.

In the case of advice-points only the command action can be specified, its meaning will be explained later. Here the default is not to apply any actions.

The action part can contain the other conditions allowed in the test part, too. The advicepoint example above uses the line(L) condition in the action part, as the applicability of the breakpoint does not depend on the line number. This can be contrasted with the the following example, where the success of the conditions does depend on the line number, and so the line condition appears in the test part: add_breakpoint([line(Line),true((Line>=20,Line=<40))], _).

This example introduces a generic spypoint which applies to invocations appearing between lines 20–40.

We have seen the true condition being used in both the test and the action part. If it is used for testing (i.e. it can fail), it should be placed in the test part. On the other hand, it should appear in the action part if it has a side effect, e.g. asserts some clauses or writes out a message. This is because the test part of spypoint conditions may be evaluated multiple times for a single port.

Chapter 6: Debugging 77

Note that this distinction between the test part and the action part is not strictly enforced.

If a condition in the action part fails, the breakpoint will not be applied, but, of course, the side effects preceding the failed condition will have already taken place.

Having created some breakpoints, you can use the built-in predicate current_ breakpoint/4 to enumerate all breakpoints together with their properties. By reference to their identifiers breakpoints can be removed, temporarily disabled, or enabled.

Naturally, there can be multiple breakpoints at any given time of execution. They are handled in a stack-like discipline, i.e. the most recent breakpoints (the ones with higher breakpoint identifiers) are tried first. At each port the debugger first looks for enabled advice-points, the first (most recent) one found applicable is then activated, i.e. its action part is executed. Subsequently the debugger scans the breakpoint stack again, looking for an enabled spypoint applicable for the current invocation, and again the first such spypoint is activated. For the details of how the breakpoints are processed, see

Section 6.8 [Breakpoint

Processing], page 85 .

As explained earlier, when the system is in Debug Mode, the debugger collects control flow information about the goals being executed. This collection of information, the backtrace, includes the invocations that were called but not exited yet, as well as those that exited non-deterministically. For each invocation, the main data items present in the backtrace are the following: the goal, the module, the invocation number, the depth and the source information, if any. There is also a field in the backtrace reserved for the user, initially an empty variable. The goal_private(_) condition unifies its argument with this field.

We will show an example on how to use the goal_private field to transfer instantiation information from the call port to the exit port; see

Section 6.12 [Breakpoint Example], page 93 .

When an invocation is at the call port, the user can decide not to build a procedure box for the invocation and not to put it on the backtrace. This can be done by supplying the action command(flit). In this case the execution continues without building the procedure box, and with much smaller space overhead then otherwise.

1

This also means, of course, that the execution will not stop at the remaining ports of this invocation. The command(flit) is the only meaningful execution control action that can be used for advice-points.

The backtrace can be accessed outside the debugger, too, via the built-in predicates execution_state/[1,2]. The predicate execution_state(Tests) accesses the current execution state. Arbitrary tests can be used in this predicate, if it is called from within a true condition in an add_breakpoint/2. For example the advice breakpoint shown earlier can be simplified to:

[advice,call]-true(assert_line)

The current line number then can be accessed from within assert_line: assert_line :-

1

Note that a flit command will reserve some space on the heap and trail, but most of this will be garbage collected.

78 SICStus Prolog execution_state(line(L)), assert(line_reached(L)).

Note that we applied some syntactic simplifications in the above texts. First, we simplified port(call) to call: for certain tests, which have a pre-defined range of possible arguments, the name of the test is automatically inserted if a concrete argument is given. Second, we omitted the square brackets around the action part: this is applicable to both tests and actions, if they contain a single condition only.

Invoking execution_state/1 can also be useful inside the user:debugger_command_ hook/2 predicate, which serves for defining new interactive debugger commands.

If execution_state/1 is called outside the debugger, only some tests are available, e.g.

those querying to data stored in the backtrace.

The execution_state/2 predicate lets us examine past elements of the backtrace. For example: execution_state(inv(N ), goal(Goal)) retrieves the Goal stored in the backtrace under invocation number N.

The debugger is also able to keep track of multiple backtraces associated with different break levels. We can access data stored in previous break levels, using execution_state/2, as e.g. in: execution_state([break_level(0),inv(N )], goal(Goal))

This example accesses the N th invocation of the outermost break level. Such use can be helpful if one enters a break during tracing, and wants to access the outer backtrace from within the break.

6.6 Commands Available during Debugging

This section describes the particular commands that are available when the system prompts you after printing out a debugging message.

All the commands are one or two letter mnemonics, some of which can be optionally followed by a decimal integer. They are read from the standard input stream with any blanks being completely ignored up to the end of the line ( h

RET i ). Some commands only actually require the terminator; e.g. the creep command, as we have already seen, only requires h

RET i

.

The only command which you really have to remember is ‘h’ (followed by h

RET i

). This provides help in the form of the following list of available commands.

Chapter 6: Debugging 79

<cr> creep l leap s o skip out q r f q-skip retry fail j<p> jump to port d display p print c z creep zip s <i> skip i o <n> out n q <i> q-skip i r <i> retry i f <i> fail i j<p><i>jump to port i w write p <i> print partial n

+

g t

& ancestors backtrace blocked goals nodebug spy this nospy this g <n> ancestors n t <n> backtrace n

& <n> nth blocked goal

=

* debugging spy conditionally

\ <i> remove brkpoint

^

?

e

<

D <i> disable brkpoint E <i> enable brkpoint a abort b break

@ command u unify raise exception .

reset printdepth < <n> set printdepth reset subterm help

^ <n> h find this set subterm help z s c h

RET i l creep causes the debugger to single-step to the very next port and print a message. Then if the port is leashed (see

Section 6.2 [Basic Debug], page 71

), the user is prompted for further interaction. Otherwise, it continues creeping.

If leashing is off, creep is the same as leap (see below) except that a complete trace is printed on the standard error stream.

leap causes the debugger to resume running your program, only stopping when a spypoint is reached (or when the program terminates). Leaping can thus be used to follow the execution at a higher level than exhaustive tracing. All you need to do is to set spypoints on an evenly spread set of pertinent predicates, and then follow the control flow through these by leaping from one to the other.

Debugging information is collected while leaping, so when a spypoint is reached, it is possible to inspect the ancestor goals, or creep into them upon entry to

Redo ports.

zip is like leap, except no debugging information is being collected while zipping, resulting in significant savings in memory and execution time.

skip is only valid for Call and Redo ports. It skips over the entire execution of the predicate. That is, you will not see anything until control comes back to this predicate (at either the Exit port or the Fail port). Skip is particularly useful while creeping since it guarantees that control will be returned after the

(possibly complex) execution within the box. If you skip then no message at all will appear until control returns. This includes calls to predicates with spy-

80 SICStus Prolog o q r f j<p> points set; they will be masked out during the skip. No debugging information is being collected while skipping.

If you supply an integer argument, then this should denote an invocation number of an ancestral goal. The system tries to get you to the Exit or Fail port of the invocation box you have specified.

out is a shorthand for skipping to the Exit or Fail port of the immediate ancestor goal. If you supply an integer argument n, it denotes skipping to the Exit or

Fail port of the nth ancestor goal.

quasi-skip is like a combination of zip and skip: execution stops when either control comes back to this predicate, or a spypoint is reached. No debugging information is being collected while quasi-skipping.

An integer argument can be supplied as for skip.

retry can be used at any of the four ports (although at the Call port it has no effect). It transfers control back to the Call port of the box. This allows you to restart an invocation when, for example, you find yourself leaving with some weird result. The state of execution is exactly the same as when you originally called, (unless you use side effects in your program; i.e. asserts etc. will not be undone). When a retry is performed the invocation counter is reset so that counting will continue from the current invocation number regardless of what happened before the retry. This is in accord with the fact that you have, in executional terms, returned to the state before anything else was called.

If you supply an integer argument, then this should denote an invocation number of an ancestral goal. The system tries to get you to the Call port of the box you have specified. It does this by continuously failing until it reaches the right place. Unfortunately this process cannot be guaranteed: it may be the case that the invocation you are looking for has been cut out of the search space by cuts (!) in your program. In this case the system fails to the latest surviving

Call port before the correct one.

fail can be used at any of the four ports (although at the Fail port it has no effect). It transfers control to the Fail port of the box, forcing the invocation to fail prematurely.

If you supply an integer after the command, then this is taken as specifying an invocation number and the system tries to get you to the Fail port of the invocation box you have specified. It does this by continuously failing until it reaches the right place. Unfortunately this process cannot be guaranteed: it may be the case that the invocation you are looking for has been cut out of the search space by cuts (!) in your program. In this case the system fails to the latest surviving Fail port before the correct one.

jump to port transfers control back to the prescribed port <p>. Here <p> is one of: ‘c’, ‘e’, ‘r’, ‘f’, standing for Call, Exit, Redo and Fail ports. Takes an optional integer argument, an invocation number.

Jumping to a call port is the same as retrying it, i.e. ‘jc’ is the same as the ‘r’ debugger command; and similarly ‘jf’ is the same as ‘f’.

w g d p

Chapter 6: Debugging t

&

81

The ‘je’ jump to Exit port command transfers control back to the Exit port of the box. It can be used at a Redo or an Exit port (although at the latter it has no effect). This allows you to restart a computation following an Exit port, which you first leapt over, but because of its unexpected failure you arrived at the Redo port. When you supply an integer argument, then this should denote an exact invocation number of an exited invocation present in the backtrace, and then the system will get you to the specified Exit port. The debugger requires here an exact invocation number so that it does not jump too far back in the execution (if an Exit port is not present in the backtrace, it may be be a better choice to jump to the preceding Call port, rather than to continue looking for another Exit port).

The ‘jr’ jump to Redo port command transfers control back to the Redo port of the box. It can be used at an Exit or a Redo port (although at the latter it has no effect). This allows you to force the goal in question to try to deliver another solution. When you supply an integer argument, then this should denote an exact invocation number of an exited invocation present in the backtrace, and then the system will get you to the specified Redo port.

display goal displays the current goal using display/1. See Write (below).

print goal re-prints the current goal. An argument will override the default printdepth, treating 0 as infinity.

write goal writes the current goal using write/1.

print ancestor goals provides you with a list of ancestors to the current goal, i.e.

all goals that are hierarchically above the current goal in the calling sequence.

You can always be sure of jumping to the Call or Fail port of any goal in the ancestor list (by using retry etc). If you supply an integer n, then only that number of ancestors will be printed. That is to say, the last n ancestors will be printed counting back from the current goal. Each entry is displayed just as they would be in a trace message.

print backtrace is the same as the above, but also shows any goals that have exited non-deterministically and their ancestors. This information shows where there are outstanding choices that the program could backtrack to.

If you supply an integer n, then only that number of goals will be printed.

Ancestors to the current goal are annotated with the ‘Call:’ port, as they have not yet exited, whereas goals that have exited are annotated with the ‘Exit:’ port. You can always be sure of jumping to the Exit or Redo port of any goal shown to be exited in the backtrace listing.

The backtrace is a tree rather than a stack: to find the parent of a given goal with depth indicator d, look for the closest goal above it with depth indicator d-1.

print blocked goals prints a list of the goals which are currently blocked in the current debugging session together with the variable that each such goal is blocked on (see

Section 3.3 [Procedural], page 45 ). The goals are enumerated

from 1 and up. If you supply an integer n, then only that goal will be printed.

Each entry is preceded by the goal number followed by the variable name.

-

\

E

.

a b

82 n

=

+

*

D

@ u

SICStus Prolog nodebug switches the debugger off. Note that this is the correct way to switch debugging off at a trace point. You cannot use the @ or b commands because they always return to the debugger.

debugging outputs information concerning the status of the debugging package.

See

Section 7.14 [Debug Pred], page 157 , the built-in debugging/0.

spy this sets a plain spypoint on the current goal.

spy this conditionally sets a conditional spypoint on the current goal. Prompts for the Conditions, and issues a spy(Func, Conditions) command, where Func is the functor of the current invocation.

nospy this removes all spypoints applicable to the current goal. Equivalent to nospy Func, where Func is the functor of the current invocation.

remove this removes the spypoint which caused the debugger to interact at the current port. With an argument n, it removes the breakpoint with identifier n. Equivalent to remove_breakpoints(BID), where BID is the current breakpoint identifier, or the supplied argument.

disable this disables the spypoint which caused the debugger to interact at the current port. With an argument n, it disables the breakpoint with identifier n.

Equivalent to disable_breakpoints(BID), where BID is the current breakpoint identifier, or the supplied argument.

enable this enables all specific spypoints for the predicate at the current port.

With an argument n, it enables the breakpoint with identifier n.

find this outputs information about where the predicate being called is defined.

abort causes an abort of the current execution. All the execution states built so far are destroyed and you are put right back at the top-level. (This is the same as the built-in predicate abort/0.) break calls the built-in predicate break/0, thus putting you at a recursive toplevel with the execution so far sitting underneath you. When you end the break

(^D) you will be reprompted at the port at which you broke. The new execution is completely separate from the suspended one; the invocation numbers will start again from 1 during the break. The debugger is temporarily switched off as you call the break and will be re-switched on when you finish the break and go back to the old execution. However, any changes to the leashing or to spypoints will remain in effect.

command gives you the ability to call arbitrary Prolog goals. It is effectively a one-off break (see above). The initial message ‘| :- ’ will be output on the standard error stream, and a command is then read from the standard input stream and executed as if you were at top level.

unify is available at the Call port and gives you the option of providing a solution to the goal from the standard input stream rather than executing the

Chapter 6: Debugging 83 e

<

^ goal. This is convenient e.g. for providing a “stub” for a predicate that has not yet been written. A prompt will be output on the standard error stream, and the solution is then read from the standard input stream and unified with the goal. If the term read in is of the form Head :- Body, then Head will be unified with the current goal, and Body will be executed in its place.

raise exception is available at all ports. A prompt will be output on the standard error stream, and an exception term is then read from the standard input stream and raised in the program being debugged.

This command, without arguments, resets the printdepth to 10. With an argument of n, the printdepth is set to n, treating 0 as infinity.

While at a particular port, a current subterm of the current goal is maintained.

It is the current subterm which is displayed, printed, or written when prompting for a debugger command. Used in combination with the printdepth, this provides a means for navigating in the current goal for focusing on the part which is of interest. The current subterm is set to the current goal when arriving at a new port. This command, without arguments, resets the current subterm to the current goal. With an argument of n (> 0), the current subterm is replaced by its n:th subterm. With an argument of 0, the current subterm is replaced by its parent term. With a list of arguments, the arguments are applied from left to right.

?

h help displays the table of commands given above.

The following hook predicate can be used to customize the behavior of the interactive debugger.

debugger_command_hook(+Char,?Actions)

[Hook] user:debugger_command_hook(+Char,?Actions)

This predicate is called each time the debugger has read the first character

Char of a debugger command. If it succeeds, Actions is taken as the list of actions (see

Section 6.9.6 [Action Conditions], page 90

) to be done for the given debugger command. If it fails, further arguments are read in, and the debugger command is interpreted in the standard way.

The above hook makes it possible to extend the interactive debugger with user-defined commands. The following example defines the ‘S’ interactive debugger command to behave as skip at Call and Redo ports, and as creep otherwise: debugger_command_hook(0’S, Actions) :ttyskip(0’\n), % skip till end of line execution_state([port(P),inv(I)]),

Actions = [Mode,proceed,silent],

( P = call -> Mode = skip(I)

; P = redo -> Mode = skip(I)

; Mode = trace

).

84 SICStus Prolog

Note that the silent action is needed above, otherwise the debugger message will be printed a second time, before continuing the execution.

6.7 Breakpoint Handling Predicates

This section describes the advanced built-in predicates for creating and removing breakpoints.

add_breakpoint(:Conditions, ?BID)

Adds a breakpoint with conditions Conditions, the breakpoint identifier assigned is unified with BID. Conditions is one of the following:

Tests-Actions

Tests standing for Tests-[]

-Actions standing for []-Actions

Here Tests and Actions are lists of Conditions or a single Condition, see

Section 6.9 [Breakpoint Conditions], page 87

.

The add_breakpoint/2 predicate performs some transformations on the Conditions before adding the breakpoint. The goal and pred conditions are extracted from both the test and the action part and their consistency is checked.

A goal condition is then inserted as the first element of the tests list, encapsulating all supplied goal conditions as well as those pred conditions which can be transformed to a goal condition. Furthermore the debugger condition is removed, and the advice condition is moved to the second element of the tests list. Finally a pred condition is inserted in front of the remaining tests, in the rare cases when it can not be made part of the preceding goal test. The rest of the test part and the action part is the same as supplied, with the extracted conditions removed.

There can only be a single plain spypoint for each predicate. If a plain spypoint is added, and there is already a plain spypoint for the given functor, then: a. the old spypoint is deleted and a new added as the most recent breakpoint, if this change affects the breakpoint selection mechanism.

b. otherwise the old spypoint is kept and enabled if needed.

spy(:PredSpec, :Conditions)

Adds a conditional spypoint with conditions [pred(Pred)|Conditions], for each predicate Pred designated by the generalized predicate spec PredSpec.

current_breakpoint(:Conditions, ?BID, ?Status, ?Kind)

There is a breakpoint with conditions Conditions, breakpoint identifier BID, status Status and kind Kind. Status is one of on or off, referring to enabled and disabled breakpoints. Kind is one of plain, conditional or generic.

current_breakpoint/4 enumerates all breakpoints on backtracking.

The Conditions as returned by current_breakpoint/4 may not be exactly the same as supplied at the creation of the breakpoint, because of the transformations done at creation, see the description of add_breakpoint/2 above.

Chapter 6: Debugging 85 remove_breakpoints(+BIDs) disable_breakpoints(+BIDs) enable_breakpoints(+BIDs)

Removes, disables or enables the breakpoints with identifiers specified by BIDs.

BIDs can be a number, a list of numbers or one of the atoms: all, debugger, advice.

The atoms specify all breakpoints, debugger type breakpoints and advice type breakpoints respectively.

execution_state(:Tests)

Tests are satisfied in the current state of the execution.

execution_state(+FocusConditions, :Tests)

Tests are satisfied in the state of the execution pointed to by FocusConditions.

Note that the built-in predicate arguments holding breakpoint conditions (Conditions or

Tests above) are subject to module name expansion. The primitive conditions goal(_), pred(_), ancestor(_), and true(_) will inherit the module name from the (module name expanded) conditions argument, in the absence of explicit module qualification within the primitive condition.

6.8 The Processing of Breakpoints

This section describes in detail how the debugger handles the breakpoints. For the purpose of this section disabled breakpoints are not taken into account: whenever we refer to the existence of some breakpoint(s), we always mean the existence of enabled breakpoint(s).

The Prolog engine can be in one of the following three states with respect to the debugger: no debugging if there are no advice-points and the debugger is either switched off, or doing a skip; full debugging if the debugger is in trace or debug mode (creeping or leaping), or there are any generic breakpoints; selective debugging in all other cases.

In the selective debugging state only those predicate invocations are examined, for which there exists a specific breakpoint. In the full debugging state all invocations are examined, except those calling a non-exported predicate of a hidden module (but even these will be examined, if there is a specific breakpoint for them). In the no debugging state the debugger is not entered at predicate invocations.

Now we describe what the debugger does when examining an invocation of a predicate, i.e.

executing its Call port.

First the debugger tries to select an applicable advice-point. It does this by considering all advice-points, most recent first, and evaluating the conditions in its tests and actions, in

86 SICStus Prolog the order as stored by add_breakpoint/2; see

Section 6.7 [Breakpoint Predicates], page 84

.

The first advice-point, for which this process succeeds, is selected. If there was no advicepoint selected, or the variable command (initialized to proceed) was re-set to flit in the course of the selection process, then the debugger notes that the advice facility does not require the creation of a procedure box.

Second, the search for spypoints takes place. This is started by initializing the debugger action variables. The mode variable is set to the current debugger mode. The values for show and command depend on the hiddenness of the predicate being invoked, the debugger mode and the leashing status of the port. If the predicate is both defined in, and called from a hidden module, then the defaults will be silent and flit. An example of this is when a built-in predicate is called from a hidden module, e.g. from a library. Otherwise, in trace mode, the default values are print and ask for leashed ports, and print and proceed for unleashed ports. In debug mode, the variables default to silent and proceed, while in zip mode to silent and flit. These default values reflect the behaviour expected for the given debugger mode, e.g. in zip mode the debugger does not print debugging information and does not build procedure boxes.

Having initialized the debugger action variables, the system does the spypoint search, unless the debugger mode is off or skip. Finding an applicable spypoint is done in the same way as described for advice points. The only difference is that, when the test part of a spypoint succeeds and there is no action part, the [print,ask] actions are executed.

The third stage is the interactive part. First, the goal in question is displayed according to the value of show. Next, the value of command is checked: if it is other than ask the interactive stage ends. Otherwise, if it is ask, the debugger prompts the user for a command which is interpreted either in the standard way, or through user:debugger_command_hook/2. In both cases the debugger action variables are modified as requested, and the interactive part is repeated.

After the debugger went through all the three stages, it decides whether to build a procedure box. This will happen if either the advice-point stage or the other two stages require it.

The latter is decided by checking the functor of command: if that is flit, then no procedure box is required by the spypoint part. In such a case, if the advice-point does require the building of a procedure box, the command variable is modified, by changing its functor to proceed.

At the end of the process the value of mode will be the new debugging mode, and command will determine what the debugger will do; see

Section 6.9.9 [Action Variables], page 91 .

A similar three-stage process is carried out when the debugger arrives at a non-Call port of a predicate. The only difference is that the building of a procedure box is not considered

(flit is equivalent to proceed), and the hiddenness of the predicate is not taken into account.

While the Prolog system is executing the above three-stage process for any of the ports, it is said to be inside the debugger. This is relevant, because some of the conditions can only be evaluated in this context.

Chapter 6: Debugging 87

6.9 Breakpoint Conditions

This section describes the format of primitive breakpoint conditions.

We first list the tests that can be used to enquire the state of execution. We then proceed to describe the conditions usable in the actions part and the options for focusing on past execution states.

Finally we describe some simple condition macros and the valid values for the debugger action variables.

Unless noted otherwise, the tests are usable both inside the debugger, and outside it. Most of the tests can also be used in queries about past execution states, in execution_state/2. For the latter type of usage, in the following descriptions the term current should be interpreted as referring to the execution state focused on.

The test will fail if the given query is not meaningful in the given context, e.g. if execution_ state(goal(G)) is queried before any breakpoints were encountered.

6.9.1 Tests Related to the Current Goal

The following tests give access to basic information about the current invocation.

inv(Inv )

The invocation number of the current goal is Inv.

depth(Depth)

The current execution depth is Depth.

goal(MGoal)

The module name expanded MGoal template matches the current goal. The unification required for matching is carried out.

pred(MFunc)

The module name expanded MFunc template matches the functor (M :F/N ) of the current goal. The unification required for matching is carried out.

module(Module)

The module of the current goal is Module.

goal_private(GoalPriv )

The private information associated with the current goal is GoalPriv.

last_port(LastPort)

LastPort is the last completed port of the invocation present on the backtrace.

Practically, this is only useful when looking at past execution states. LastPort will be exit(nondet) if the invocation has been exited, and call otherwise.

parent_inv(Inv )

The invocation number of the debugger-parent of the current goal is Inv.

ancestor(AncGoal,Inv )

The youngest debugger-ancestor of the current goal, which matches the module name expanded AncGoal template, is at invocation number Inv. The unification required for matching is carried out.

88 SICStus Prolog

Notes:

The debugger-parent of a goal is the youngest ancestor of the goal present on the backtrace.

This will differ from the ordinary parent if not all goals are traced, e.g. if the goal in question is reached in zip mode. A debugger-ancestor of a goal is any of its ancestors on the backtrace.

In the goal and ancestor tests above, there is a given module qualified goal template, say

ModT:GoalT, and it is matched against a concrete goal term Mod:Goal in the execution state. This matching is carried out as follows: a. For the match to succeed, Goal and GoalT have to be unifiable and are unified.

b. Mod and ModT are either unifiable (and are unified), or name such modules in which

Goal has the same meaning, i.e. either one of Mod:Goal and ModT:Goal is an exported variant of the other, or both are imported from the same module.

The above matching rules also apply for predicate functors, in the pred condition.

6.9.2 Tests Related to Source Information

These tests provide access to source related information. The file and line tests will fail if no source information is present. The parent_clause and parent_pred tests are available for interpreted code only.

file(File)

The current goal is invoked from file File.

line(File,Line)

The current goal is invoked from file File, line Line.

line(Line)

The current goal is invoked from line Line.

parent_clause(Cl)

The current goal is invoked from clause Cl.

parent_clause(Cl,Sel)

The current goal is invoked from clause Cl and within its body it is pointed to by the subterm selector Sel.

parent_clause(Cl,Sel,I )

The current goal is invoked from clause Cl, it is pointed to by the subterm selector Sel within its body, and it is the I th goal within it. The goals in the body are counted following their textual occurrence.

parent_pred(Pred)

The current goal is invoked from predicate Pred.

parent_pred(Pred,N )

The current goal is invoked from predicate Pred, clause number N.

Chapter 6: Debugging 89

6.9.3 Tests Related to the Break Level

These tests can be used both inside and outside the condition evaluation process, and also can be used in queries about past break levels.

break_level(N )

We are at (or focused on) break level N (N = 0 for the outermost break level).

max_inv(MaxInv )

The last invocation number in use within the break level is MaxInv.

private(Priv )

The private information associated with the break level is Priv. Similarly to goal_private/1, this condition refers initially to an uninstantiated variable and can be used to store an arbitrary Prolog term.

6.9.4 Tests Related to the Current Port

These tests can only be used inside the debugger and only when focused on the current invocation. If they appear in execution_state/2 or in execution_state/1 called outside the debugger, an exception will be raised.

The notion of port in breakpoint handling is more general than outlined earlier in

Section 6.1

[Procedure Box], page 69 . Here the following terms are used to describe a port:

call, exit(nondet), exit(det), redo, fail, exception(Exception), block, unblock

Furthermore, the atoms exit and exception can be used in the port condition (see below), to denote any of the two exit ports and an arbitrary exception port, respectively.

port(Port)

The current execution port matches Port in the following sense: either Port and the current port unify, or Port is the functor of the current port (e.g.

port(exit) holds for both exit(det) and exit(nondet) ports).

bid(BID) The breakpoint being examined has a breakpoint identifier BID. (BID = none if no breakpoint was selected.) mode(Mode)

The debugger mode is Mode.

command(Command)

Command is the command to be executed if the breakpoint is selected.

show(Show )

The current show method (the goal display method) is Show.

The last three of the above tests access the debugger action variables. For example, the condition mode(trace), if it occurs in the tests, checks if the current debugger mode is trace. On the other hand, if the same term occurs within the action part, it sets the debugger mode to trace.

90 SICStus Prolog

For the port, mode, command and show conditions, the condition can be replaced by its argument, if that is not a variable. For example the condition call can be used instead of port(call). Conditions matching the templates listed above as valid port values will be converted to a port condition. Similarly, any valid value for the three debugger action variables is converted to an appropriate condition. These valid values are described below; see

Section 6.9.9 [Action Variables], page 91

.

6.9.5 Other Conditions

The following conditions are for prescribing or checking the breakpoint type. They are only meaningful inside the debugger and only for the current invocation.

advice

The breakpoint in question is of advice type.

debugger

The breakpoint in question is of debugger type.

The following construct converts an arbitrary Prolog goal into a condition.

true(Cond)

The Prolog goal Cond is true, (Cond is executed and the condition is satisfied iff the goal completes successfully). The substitutions done on executing Cond are carried out. Cond is subject to module name expansion. If used in the test part of spypoint conditions, the goal should not have any side effects, as the test part may be evaluated several times.

6.9.6 Conditions Usable in the Action Part mode(Mode)

Set the debugger mode to Mode.

command(Command)

Set the command to be executed to Command.

show(Show )

Set the show method to Show.

The values admissible for Mode, Command and Show are described below; see

Section 6.9.9

[Action Variables], page 91

.

Furthermore, any other condition can be used in the action part, except for the ones specifying the type (advice or debugger).

6.9.7 Options for Focusing on a Past State

The following terms can be used in the first argument of execution_state/2 (see

Section 6.7

[Breakpoint Predicates], page 84

).

break_level(I )

Focus on the current invocation of break level I.

Chapter 6: Debugging 91 inv(Inv )

Focus on the invocation number Inv of the currently focused break level.

6.9.8 Condition Macros

There are a few condition macros expanding to a list of other conditions: unleash

Expands to [show(print),command(proceed)] hide

Expands to [show(silent),command(proceed)] leash

Expands to [show(print),command(ask)]

6.9.9 The Action Variables

We first list the possible values of the debugger action variables, and their meaning. We then discuss how these variables are initialized and changed.

Values allowed in the show condition: print silent

Write using options stored in the debugger_print_options Prolog flag.

Display nothing.

display

Write using display.

write

Write using writeq.

write_term(Options)

Write using options Options.

Method-Sel

Display only the subterm selected by Sel, using Method. Here Method is one of the methods above, and Sel is a subterm selector.

Values allowed in the command condition: ask

Ask the user what to do next.

proceed

Continue the execution without interacting with the user (cf. unleashing).

flit

Continue the execution without building a procedure box for the current goal

(and consequently not encountering any other ports for this invocation). Only meaningful at call ports, at other ports it is equivalent to proceed.

proceed(Goal,New )

Unify the current goal with Goal and execute the goal New in its place. Goal and New are module name expanded only at execution time with the current type-in module as the default. Only available at call ports. This construct is used by the ‘u’ (unify) interactive debugger command.

flit(Goal,New )

Unify the current goal with Goal and execute the goal New in its place, without creating a procedure box for Goal (and consequently not encountering any other

92 SICStus Prolog ports for this invocation). Only available at call ports. Also see note on module name expansion for proceed/2.

exception(E)

Raise the exception E.

abort

Abort the execution.

retry(Inv )

Retry the the most recent goal with an invocation number less or equal to Inv

(go back to the call port of the goal). This is used by the interactive debugger command ‘r’, retry; see

Section 6.6 [Debug Commands], page 78

.

reexit(Inv )

Re-exit the the invocation with number Inv (go back to the exit port of the goal). Inv must be an exact reference to an exited invocation present in the backtrace (exited nondeterministically, or currently being exited). This is used by the interactive debugger command ‘je’, jump to exit port; see

Section 6.6

[Debug Commands], page 78 .

redo(Inv )

Redo the the invocation with number Inv (go back to the redo port of the goal).

Inv must be an exact reference to an exited invocation present in the backtrace.

This is used by the interactive debugger command ‘jr’, jump to redo port; see

Section 6.6 [Debug Commands], page 78

.

fail(Inv )

Fail the the most recent goal with an invocation number less or equal to Inv

(transfer control back to the fail port of the goal). This is used by the interactive debugger command ‘f’, fail; see

Section 6.6 [Debug Commands], page 78

.

Values allowed in the mode condition: qskip(Inv )

Quasi-skip until the first port with invocation number less or equal to Inv is reached. Valid only if Inv >= 1 and furthermore Inv =< CurrInv for entry ports (call, redo), and Inv < CurrInv for all other ports, where CurrInv is the invocation number of the current port.

skip(Inv ) trace debug zip off

Skip until the first port with invocation number less or equal to Inv is reached.

Inv should obey the same rules as for qskip.

Creep.

Leap.

Zip.

Continue without debugging.

Chapter 6: Debugging 93

6.10 Consulting during Debugging

It is possible, and sometimes useful, to consult a file whilst in the middle of program execution. Predicates, which have been successfully executed and are subsequently redefined by a consult and are later reactivated by backtracking, will not notice the change of their definitions. In other words, it is as if every predicate, when called, creates a virtual copy of its definition for backtracking purposes.

If SICStus Prolog is run via the Emacs interface, the commands for loading code (such as Cc C-p, consulting the current predicate) are not directly available when the system prompts you after printing out a debugging message. Press b followed by h

RET i to get a recursive top-level, ready to accept the Emacs commands. Type ^D to return to the debugging port.

6.11 Catching Exceptions

Usually, exceptions that occur during debugging sessions are displayed only in trace mode and for invocation boxes for predicates with spypoints on them, and not during skips.

However, it is sometimes useful to make exceptions trap to the debugger at the earliest opportunity instead. The hook predicate user:error_exception/1 provides such a possibility: error_exception(+Exception) user:error_exception(+Exception)

[Hook]

This predicate is called at all exception ports. If it succeeds, the debugger enters trace mode and prints an exception port message. Otherwise, the debugger mode is unchanged and a message is printed only in trace mode or if a spypoint is reached, and not during skips.

6.12 Advanced Debugger Examples

We will show two examples using the advanced features of the debugger.

The first example defines a hide_exit(Pred) predicate, which will hide the exit port for

Pred (i.e. it will silently proceed), provided the current goal was already ground at the call port, and nothing was traced inside the given invocation. The hide_exit(Pred) creates two spypoints for predicate Pred:

:- meta_predicate hide_exit(:).

hide_exit(Pred) :add_breakpoint([pred(Pred),call]true(save_groundness), _), add_breakpoint([pred(Pred),exit,true(hide_exit)]-hide, _).

The first spypoint is applicable at the call port, and it calls save_groundness to check if the given invocation was ground, and if so, it sets the goal_private attribute of the invocation to ground.

94 SICStus Prolog save_groundness :execution_state([goal(_:G),goal_private(Priv)]), ground(G), !, Priv = ground.

save_groundness.

The second spypoint created by hide_exit is applicable at the exit port and it checks whether the hide_exit condition is true. If so, it issues a hide action, which is an abbreviation of [show(silent),command(proceed)].

hide_exit :execution_state([inv(I),max_inv(I),goal_private(Priv)]),

Priv == ground.

Here hide_exit encapsulates the tests that the invocation number is the same as the last invocation number used (max_inv), and that the goal_private attribute of the invocation is identical to ground. The first test ensures that nothing was traced inside the current invocation.

If we load the above code, as well as the small example below, then the following interaction can take place. Note that the hide_exit is called with the _:_ argument, resulting in generic spypoints being created.

| ?- [user].

| cnt(0) :- !.

| cnt(N) :-

| N > 0, N1 is N-1, cnt(N1).

| {user consulted, 0 msec 424 bytes} yes

| ?- hide_exit(_:_), trace, cnt(1).

{The debugger will first zip -- showing spypoints (zip)}

{Generic spypoint added, BID=1}

{Generic spypoint added, BID=2}

{The debugger will first creep -- showing everything (trace)}

#

#

#

#

3

4

1

1

2

3

1 Call: cnt(1) ?

2 Call: 1>0 ?

2 Call: _2019 is 1-1 ?

2 Exit: 0 is 1-1 ?

2 Call: cnt(0) ?

1 Exit: cnt(1) ?

yes

{trace}

| ?-

Our second example defines a predicate call_backtrace(Goal, BTrace), which will execute Goal and build a backtrace showing the successful invocations executed during the solution of Goal.

Chapter 6: Debugging 95

The advantages of such a special backtrace over the one incorporated in the debugger are the following:

• it has much lower space consumption;

• the user can control what is put on and removed from the backtrace (e.g. in this example all goals are kept, even the ones that exited deterministically);

• the interactive debugger can be switched on and off without affecting the "private" backtrace being built.

The call_backtrace predicate is based on the advice facility. It uses the variable accessible via the private(_) condition to store a mutable holding the backtrace. Outside the call_ backtrace predicate the mutable will have the value off.

The example is a module-file, so that internal invocations can be identified by the modulename.

:- module(backtrace, [call_backtrace/2]).

call_backtrace/2 is a meta-predicate, which first sets up an appropriate advice-point for building the backtrace. This step is skipped if there already exists a breakpoint of this kind. Note the careful formulation of the breakpoint condition Cond: a fully spelled out form is used (e.g. port(call) instead of call), so that Cond is good both for checking the presence of a breakpoint with the given conditions in current_breakpoint/4, and for creating a new breakpoint in add_breakpoint/2.

Having ensured the appropriate advice-point exists, call_backtrace/2 picks up the private field of the execution state and calls call_backtrace/3 with a cleanup operation ensuring that the breakpoint added is removed at the end.

:- meta_predicate call_backtrace(:, ?).

call_backtrace(Goal, BTrace) :-

Cond = [goal(M:G),advice,port(call),private(Priv), true(backtrace:store_goal(M,G,Priv))]

-[command(flit)],

( current_breakpoint(Cond, _, on, _) -> B = []

; add_breakpoint(Cond, B)

), execution_state(private(Priv)), call_cleanup(call_backtrace(Goal, BTrace, Priv), remove_breakpoints(B)).

The predicate call_backtrace/3 receives the private field of the execution state in its Priv argument. It assumes that this is either a mutable or a yet uninstantiated variable. In the latter case the variable is set to a newly created mutable. In both cases the mutable is initialized to [], and the Goal is called. In the course of the execution of the Goal the debugger will accumulate the backtrace in the mutable. Finally, the mutable is read, its value returned in BTrace and it is restored to its old value (or off).

:- meta_predicate call_backtrace(:, ?, ?).

96 SICStus Prolog call_backtrace(Goal, BTrace, Priv) :-

( is_mutable(Priv) -> get_mutable(Old, Priv), update_mutable([], Priv)

; create_mutable([], Priv), Old = off

), call(Goal), get_mutable(BTrace, Priv), update_mutable(Old, Priv).

store_goal/3 is the predicate called by the advice-point, with the module, the goal and the private mutable as arguments.

The first clause ensures that calls from within the backtrace module get ignored. The second clause prepends the module qualified goal term to the private mutable, provided the latter exists and its value is not off.

store_goal(backtrace, _, _) :- !, fail.

store_goal(M, G, Priv) :is_mutable(Priv), get_mutable(BTrace, Priv),

BTrace \== off, update_mutable([M:G|BTrace], Priv).

Below is an example run, using a small program:

| ?- [user].

| cnt(N):- N =< 0, !.

| cnt(N) :-

N > 0, N1 is N-1, cnt(N1).

| {consulted user in module user, 0 msec 224 bytes} yes

| ?- call_backtrace(cnt(1), B).

{Generic advice point added, BID=1}

{Generic advice point, BID=1, removed (last)}

B = [user:(0=<0),user:cnt(0),user:(0 is 1-1),user:(1>0),user:cnt(1)] ?

yes

| ?-

Chapter 7: Built-In Predicates 97

7 Built-In Predicates

It is not possible to redefine built-in predicates. An attempt to do so will give an error message. See

[Pred Summary], page 553

.

SICStus Prolog provides a wide range of built-in predicates to perform the following tasks:

Input / Output

Reading-in Programs

Term and Goal Expansion

Input and Output of Terms

Character I/O

Stream I/O

Dec-10 Prolog File I/O

Arithmetic

Comparison of Terms

Control

Error and Exception Handling

Information about the State of the Program

Meta-Logic

Modification of Terms

Modification of the Program

Internal Database

Blackboard Primitives

All Solutions

Coroutining

Debugging

Execution Profiling

Miscellaneous

When introducing a built-in predicate, we shall present its usage with a mode spec, and optionally with an annotation containing one or more of:

ISO The predicate complies with the ISO Prolog Standard.

ISO only The predicate variant described complies with the ISO Prolog Standard and is valid in the iso execution mode only.

SICStus only

The predicate variant described is valid in the sicstus execution mode only.

declaration

A declaration that can’t be redefined as a predicate.

hook The predicate is a hook predicate.

hookable The predicate is a hookable predicate.

obsolescent

The predicate is obsolescent and should be avoided in new code.

reserved A reserved construct that can’t be defined as a predicate.

98 SICStus Prolog

The following descriptions of the built-in predicates are grouped according to the above categorization of their tasks.

7.1 Input / Output

There are two sets of file manipulation predicates in SICStus Prolog. One set is inherited from DEC-10 Prolog. These predicates always refer to a file by name. The other set of predicates is modeled after Quintus Prolog and refer to files as streams. Streams correspond to the file pointers used at the operating system level.

This second set of file manipulation predicates, the one involving streams, is supported by the ISO Prolog standard. Note that the notion of file is used here in a generalized sense; it may refer to a name file, the user’s terminal, or some other device. The ISO Prolog standard refers to this generalized notion of file using the term source/sink.

A stream can be opened and connected to a filename or file descriptor for input or output by calling the predicates open/[3,4]. These predicates will return a reference to a stream which may then be passed as an argument to various I/O predicates. Alternatively, a stream can be assigned an alias at the time of opening, and referred to by this alias afterwards.

The predicate close/1 is used for closing a stream.

There are two types of streams, binary or text. Binary streams are seen as a sequence of bytes, i.e. integers in the range 0–255. Text streams, on the other hand, are considered a sequence of characters, represented by their character codes. SICStus Prolog handles wide characters, i.e. characters with codes larger than 255. The WCX (Wide Character eXtension) component of SICStus Prolog allows selecting various encoding schemes via environment variables or hook procedures; see

Chapter 10 [Handling Wide Characters], page 221 .

The predicates current_stream/3 and stream_property/2 are used for retrieving information about a stream, and for finding the currently existing streams.

Prolog streams can be accessed from C functions as well. See

Section 8.5 [SICStus Streams], page 184 , for details.

The possible formats of a stream are:

’$stream’(X )

A stream connected to some file. X is an integer.

Atom A stream alias. Aliases can be associated with streams using the alias(Atom) option of open/4. There are also three predefined aliases: user_input

An alias initially referring to the UNIX stdin stream. The alias can be changed with prolog_flag/3 and accessed by the C variable

SP_stdin.

Chapter 7: Built-In Predicates 99 user_output

An alias initially referring to the UNIX stdout stream. The alias can be changed with prolog_flag/3 and accessed by the C variable

SP_stdout.

user_error

An alias initially referring to the UNIX stderr stream. The alias can be changed with prolog_flag/3 and accessed by the C variable

SP_stderr.

This stream is used by the Prolog top level and debugger, and for system messages.

Certain I/O predicates manipulate streams implicitly, by maintaining the notion of a current input stream and a current output stream. The current input and output streams are set to the user_input and user_output initially and for every new break (see

Section 2.9 [Nested], page 27

). The predicate see/1 (tell/1) can be used for setting the current input (output) stream to newly opened streams for particular files. The predicate seen/0 (told/0) closes the current input (output) stream, and resets it to the standard input (output) stream.

The predicate seeing/1 (telling/1) is used for retrieving the filename associated with the current input (output) streams.

The possible formats of a filename are:

The filename user stands for the standard input or output stream, depending on context.

Terminal output is only guaranteed to be displayed if the output stream is explicitly flushed.

A filename other than user must be an atom or a compound term. It is subject to four phases of rewriting:

1. Rewriting to an atomic filename (see below).

2. Expansion of any leading ‘~’ or ‘$’ (see below).

3. If the filename is still not absolute, the name of the current working directory is prepended. Note that while loading code, the current working directory is temporarily changed to the directory containing the file being read in.

4. An optional extension is added, depending on the operation to be performed on the file.

General filenames are rewritten to atomic filenames as follows:

• A filename which is an atom is rewritten to itself.

• A filename F of the form Alias(File) is rewritten by first looking up a clause of the hook predicate user:file_search_path(Alias,Expansion). If such a clause is found, and

Expansion can be rewritten to the atomic filename FirstPart, and File can be rewritten to the atomic filename SecondPart, then F is rewritten to FirstPart/SecondPart.

Since it is possible to have multiple definitions for the same alias, predicates such as load_files/1 may have to explore several alternative expansions before they locate the file to load, depending on the operation to be performed on the file.

100 SICStus Prolog

• A filename of any other form is invalid.

Once an atomic filename has been obtained, it is subject to another rewriting step if it begins with ‘~’ or ‘$’. For example,

‘~/sample.pl’ is equivalent to ‘/home/sics/al/sample.pl’, if ‘/home/sics/al’ is the user’s home directory. (This is also equivalent to ‘$HOME/sample.pl’ as explained below.)

‘~clyde/sample.pl’ is equivalent to ‘/home/sics/clyde/sample.pl’, if ‘/home/sics/clyde’ is

Clyde’s home directory.

‘$UTIL/sample.pl’ is equivalent to ‘/usr/local/src/utilities/sample.pl’, provided the value of the environment variable UTIL is ‘/usr/local/src/utilities’.

For example, given the clauses: file_search_path(home, ’$HOME’).

file_search_path(demo, home(prolog(demo))).

file_search_path(prolog, prolog).

the filename demo(mydemo) would be rewritten to ’$HOME/prolog/demo/mydemo’, where

’$HOME’ is interpreted as an environment variable (the user’s home directory).

Failure to open a file normally causes an exception to be raised. This behavior can be turned off and on by of the built-in predicates nofileerrors/0 and fileerrors/0 described below.

7.1.1 Reading-in Programs

When the predicates discussed in this section are invoked, filenames are relative to the current working directory. During the load, the current working directory is temporarily changed to the directory containing the file being read in. This has the effect that if one of these predicates is invoked recursively, the filename of the recursive load is relative to the directory of the enclosing load. See

Chapter 5 [Load Intro], page 59 , for an introduction to

these predicates.

Directives will be executed in order of occurrence. Be aware of the changed current working directory as it could have an effect on the semantics of directives. Only the first solution of directives is produced, and variable bindings are not displayed. Directives that fail or raise exceptions give rise to warning or error messages, but do not terminate the load. However, these warning or error messages can be intercepted by the hook user:portray_message/2 which can call abort/0 to terminate the load, if that is the desired behavior.

Predicates loading source code are affected by the character-conversion mapping, cf. char_ conversion/2; see

Section 7.1.3 [Term I/O], page 108 .

Chapter 7: Built-In Predicates 101

Most of the predicates listed below take an argument Files which is a single file name or a list of file names. Source, object and QL files usually end with a ‘.pl’, ‘.po’ and ‘.ql’ suffix respectively. These suffixes are optional. Each file name may optionally be prefixed by a module name. The module name specifies where to import the exported predicates of a module-file, or where to store the predicates of a non-module-file. The module is created if it doesn’t exist already.

absolute_file_name/2 (see

Section 7.1.5 [Stream Pred], page 118 ) is used to look up the

files. The file name user is reserved and denotes the standard input stream.

These predicates are available in runtime systems with the following limitations:

• The compiler is not available, so compiling is replaced by consulting.

• The Prolog flags discontiguous_warnings, redefine_warnings and single_var_ warnings have no effect.

• Informational messages are suppressed; other messages are printed unformatted.

• The user is not prompted in the event of name clashes etc.

load_files(:Files) load_files(:Files, +Options)

A generic predicate for loading files with a list of options to provide extra control. This predicate in fact subsumes the other predicates except use_module/3 which also returns the name of the loaded module, or imports a set of predicates from an existing module. Options is a list of zero or more of the following: if(X ) true (the default) to always load, or changed to load only if the file has not yet been loaded or if it has been modified since it was last loaded. A non-module-file is not considered to have been previously loaded if it was loaded into a different module. The file user is never considered to have been previously loaded.

when(When) always (the default) to always load, or compile_time to load only if the goal is not in the scope of another load_files/[1,2] directive occurring in a ‘.po’ or ‘.ql’ file.

The latter is intended for use when the file only defines predicates that are needed for proper term or goal expansion during compilation of other files.

load_type(LoadType) source to load source files only, object to load object (‘.po’) files only, ql (obsolescent) to load ‘.ql’ files only, or latest (the default) to load any type of file, whichever is newest. If the file is user, source is forced.

imports(Imports) all (the default) to import all exported predicates if the file is a module-file, or a list of predicates to import.

102 SICStus Prolog compilation_mode(Mode) compile to translate into compiled code, consult to translate into static, interpreted code, or assert_all to translate into dynamic, interpreted code.

The default is the compilation mode of any ancestor load_ files/[1,2] goal, or compile otherwise.

Note that Mode has no effect when an ‘.po’ or ‘.ql’ file is loaded, and that it is recommended to use assert_all in conjunction with load_ type(source), to ensure that the source file will be loaded even in the presence of a ‘.po’ or ‘.ql’ file.

wcx(Wcx)

To pass the term Wcx to the wide character extension component; see

Section 10.3 [Prolog Level WCX Features], page 223

.

consult(:Files) reconsult(:Files)

[Obsolescent]

[]

[:File|+Files]

Consults the source file or list of files specified by File and Files. Same as load_files(Files, [load_type(source),compilation_mode(consult)]).

compile(:Files)

Compiles the source file or list of files specified by Files. The compiled code is placed in-core, i.e. is added incrementally to the Prolog database. Same as load_files(Files, [load_type(source),compilation_mode(compile)]).

load(:Files)

[Obsolescent]

Loads the ‘.ql’ file or list of files specified by Files. Same as load_files(Files,

[load_type(ql)]).

ensure_loaded(:Files)

[ISO]

Compiles or loads the file or files specified by Files that have been modified after the file was last loaded, or that have not yet been loaded. The recommended style is to use this predicate for non-module-files only, but if any module-files are encountered, their public predicates are imported. Same as load_files(Files,

[if(changed)]).

use_module(:File)

Compiles or loads the module-file specified by File if it has been modified after it was last loaded, or not yet been loaded. Its public predicates are imported.

The recommended style is to use this predicate for module-files only, but any non-module-files encountered are simply compiled or loaded. Same as load_ files(File, [if(changed)]).

use_module(:File, +Imports)

Loads the module-file File like ensure_loaded/1 and imports the predicates in

Imports. If any of these are not public, a warning is issued. Imports may also be set to the atom all in which case all public predicates are imported. Same as load_files(File, [if(changed),imports(Imports)]).

Chapter 7: Built-In Predicates 103 use_module(-Module, :File, +Imports) use_module(+Module, :File, +Imports)

If used with +Module, and that module already exists, this merely imports

Imports from that module. Otherwise, this is equivalent to use_module(File,

Imports) with the addition that Module is unified with the loaded module.

fcompile(:Files)

[Obsolescent]

Compiles the source file or list of files specified by Files. If Files are prefixed by a module name, that module name will be used for module name expansion during the compilation (see

Section 5.4 [Considerations], page 66

). The suffix

‘.pl’ is added to the given filenames to yield the real source filenames. The compiled code is placed on the ‘.ql’ file or list of files formed by adding the suffix ‘.ql’ to the given filenames. (This predicate is not available in runtime systems.) source_file(?File)

File is the absolute name of a source file currently in the system.

source_file(:Head,?File) source_file(-Head,?File)

Head is the most general goal for a predicate loaded from File.

require(:PredSpecOrSpecs)

PredSpecOrSpecs is a predicate spec or a list or a conjunction of such. The predicate will check if the specified predicates are loaded and if not, will try to load or import them using use_module/2. The file containing the predicate definitions will be located in the following way:

• The directories specified with user:library_directory/1 are searched for a file ‘INDEX.pl’. This file is taken to contain relations between all exported predicates of the module-files in the library directory and its subdirectories.

If an ‘INDEX.pl’ is not found, require/1 will try to create one by loading the library package mkindex and calling make_index:make_library_ index(Directory ) (see

Chapter 12 [The Prolog Library], page 247 ).

• The first index entry for the requested predicate will be used to determine the file to load. An exception is raised if the predicate can’t be located.

• Once an ‘INDEX.pl’ is read, it is cached internally for use in subsequent calls to require/1.

• Not available in runtime systems.

7.1.2 Term and Goal Expansion

When a program is being read in, SICStus Prolog provides hooks that enable the terms being read in to be source-to-source transformed before the usual processing of clauses or directives. The hooks consist in user-defined predicates that define the transformations.

One transformation is always available, however: definite clause grammars, a convenient notation for expressing grammar rules. See [Colmerauer 75] and [Pereira & Warren 80].

Definite clause grammars are an extension of the well-known context-free grammars. A grammar rule in Prolog takes the general form

104 SICStus Prolog head --> body.

meaning “a possible form for head is body”. Both body and head are sequences of one or more items linked by the standard Prolog conjunction operator ‘,’.

Definite clause grammars extend context-free grammars in the following ways:

1. A non-terminal symbol may be any Prolog term (other than a variable or number).

2. A terminal symbol may be any Prolog term.

To distinguish terminals from nonterminals, a sequence of one or more terminal symbols is written within a grammar rule as a Prolog list. An empty sequence is written as the empty list ‘[]’. If the terminal symbols are character codes, such lists can be written (as elsewhere) as strings. An empty sequence is written as the empty list, ‘[]’ or ‘""’.

3. Extra conditions, in the form of Prolog procedure calls, may be included in the righthand side of a grammar rule. Such procedure calls are written enclosed in ‘{}’ brackets.

4. The left-hand side of a grammar rule consists of a non-terminal, optionally followed by a sequence of terminals (again written as a Prolog list).

5. Disjunction, if-then, if-then-else, and not-provable may be stated explicitly in the righthand side of a grammar rule, using the operators ‘;’ (‘|’), ‘->’, and ‘\+’ as in a Prolog clause.

6. The cut symbol may be included in the right-hand side of a grammar rule, as in a

Prolog clause. The cut symbol does not need to be enclosed in ‘{}’ brackets.

As an example, here is a simple grammar which parses an arithmetic expression (made up of digits and operators) and computes its value.

expr(Z) --> term(X), "+", expr(Y), {Z is X + Y}.

expr(Z) --> term(X), "-", expr(Y), {Z is X - Y}.

expr(X) --> term(X).

term(Z) --> number(X), "*", term(Y), {Z is X * Y}.

term(Z) --> number(X), "/", term(Y), {Z is X / Y}.

term(Z) --> number(Z).

number(C) --> "+", number(C).

number(C) --> "-", number(X), {C is -X}.

number(X) --> [C], {"0"=<C, C=<"9", X is C - "0"}.

In the last rule, C is the character code of some digit.

The query

| ?- expr(Z, "-2+3*5+1", []).

will compute Z=14. The two extra arguments are explained below.

Now, in fact, grammar rules are merely a convenient “syntactic sugar” for ordinary Prolog clauses. Each grammar rule takes an input string, analyses some initial portion, and produces the remaining portion (possibly enlarged) as output for further analysis. The ar-

Chapter 7: Built-In Predicates 105 guments required for the input and output strings are not written explicitly in a grammar rule, but the syntax implicitly defines them. We now show how to translate grammar rules into ordinary clauses by making explicit the extra arguments.

A rule such as p(X) --> q(X).

translates into p(X, S0, S) :- q(X, S0, S).

If there is more than one non-terminal on the right-hand side, as in p(X, Y) --> q(X), r(X, Y), s(Y).

then corresponding input and output arguments are identified, as in p(X, Y, S0, S) :q(X, S0, S1), r(X, Y, S1, S2), r(Y, S2, S).

Terminals are translated using the built-in predicate ’C’(S1, X, S2), read as “point S1 is connected by terminal X to point S2”, and defined by the single clause

’C’([X|S], X, S).

(This predicate is not normally useful in itself; it has been given the name upper-case c simply to avoid using up a more useful name.) Then, for instance p(X) --> [go,to], q(X), [stop].

is translated by p(X, S0, S) :-

’C’(S0, go, S1),

’C’(S1, to, S2), q(X, S2, S3),

’C’(S3, stop, S).

Extra conditions expressed as explicit procedure calls naturally translate as themselves, e.g.

p(X) --> [X], {integer(X), X>0}, q(X).

translates to p(X, S0, S) :-

’C’(S0, X, S1),

106 SICStus Prolog integer(X),

X>0, q(X, S1, S).

Similarly, a cut is translated literally.

Terminals are translated using the built-in predicate ’C’(S1, X, S2), read as “point S1 is connected by terminal X to point S2”, and defined by the single clause

Terminals on the left-hand side of a rule are also translated using ’C’/3, connecting them to the output argument of the head non-terminal, e.g.

is(N), [not] --> [aint].

becomes is(N, S0, S) :-

’C’(S0, aint, S1),

’C’(S, not, S1).

Disjunction has a fairly obvious translation, e.g.

args(X, Y) -->

( dir(X), [to], indir(Y)

; indir(Y), dir(X)

).

translates to args(X, Y, S0, S) :-

( dir(X, S0, S1),

’C’(S1, to, S2), indir(Y, S2, S)

; indir(Y, S0, S1), dir(X, S1, S)

).

Similarly for if-then, if-then-else, and not-provable.

The built-in predicates which are concerned with grammar rules and other compile/consult time transformations are as follows: expand_term(+Term1,?Term2)

If Term1 is a term that can be transformed, Term2 is the result.

Otherwise Term2 is just Term1 unchanged.

This transformation takes place automatically when grammar rules are read in, but sometimes it is useful to be able to perform it explicitly.

Grammar rule expansion is not the only transformation available; the user may define clauses for the predicate user:term_expansion/[2,4] to perform other transformations. user:term_

Chapter 7: Built-In Predicates 107 expansion(Term1[,Layout1],Term2[,Layout2]) is called first, and only if it fails is the standard expansion used.

term_expansion(+Term1,?TermOrTerms) term_expansion(+Term1,+Layout1,?TermOrTerms,?Layout2)

[Hook]

[Hook] user:term_expansion(+Term1,?TermOrTerms) user:term_expansion(+Term1,+Layout1,?TermOrTerms,?Layout2)

Defines transformations on terms read while a program is consulted or compiled.

It is called for every Term1 read, including at end of file, represented as the term end_of_file. If it succeeds, TermOrTerms is used for further processing, otherwise the default grammar rule expansion is attempted. It is often useful to let a term expand to a list of directives and clauses, which will then be processed sequentially.

The 4 arguments version also defines transformations on the layout of the term read, so that the source-linked debugger can display accurate source code lines if the transformed code needs debugging. Layout1 is the layout corresponding to

Term1, and Layout2 should be a valid layout of TermOrTerms (see

Section 7.1.3

[Term I/O], page 108

).

For accessing aspects of the load context, e.g. the name of the file being compiled, the predicate prolog_load_context/2 (see

Section 7.6 [State Info], page 137

) can be used.

user:term_expansion/[2,4] may also be used to transform queries entered at the terminal in response to the ‘| ?- ’ prompt. In this case, it will be called with Term1 = ?-(Query) and should succeed with TermOrTerms =

?-(ExpandedQuery).

goal_expansion(+Goal,+Module,?NewGoal) user:goal_expansion(+Goal,+Module,?NewGoal)

[Hook]

Defines transformations on goals while clauses are being consulted, compiled or asserted, after any processing by user:term_expansion/[2,4] of the terms being read in. It is called for every simple Goal encountered in the calling context Module. If it succeeds, Goal is replaced by NewGoal, otherwise Goal is left unchanged. NewGoal may be an arbitrarily complex goal, and user:goal_ expansion/3 is recursively applied to its subgoals.

This predicate is also used to resolve meta-calls to Goal at runtime via the same mechanism. If the transformation succeeds, NewGoal is simply called instead of Goal. Otherwise, if Goal is a goal of an existing predicate, that predicate is invoked. Otherwise, error recovery is attempted by user:unknown_predicate_ handler/3 as described below.

user:goal_expansion/3 can be regarded as a macro expansion facility.

It is used for this purpose to support the interface to attributed variables in library(atts), which defines the predicates M :get_atts/2 and M :put_ atts/2 to access module-specific variable attributes. These “predicates” are actually implemented via the user:goal_expansion/3 mechanism. This has the effect that calls to the interface predicates are expanded at compile time to efficient code.

108 SICStus Prolog

For accessing aspects of the load context, e.g. the name of the file being compiled, the predicate prolog_load_context/2 (see

Section 7.6 [State Info], page 137

) can be used.

phrase(:Phrase,?List) phrase(:Phrase,?List,+Remainder)

The list List is a phrase of type Phrase (according to the current grammar rules), where Phrase is either a non-terminal or more generally a grammar rule body. Remainder is what remains of the list after a phrase has been found. If called with 2 arguments, the remainder has to be the empty list.

’C’(?S1,?Terminal,?S2)

Not normally of direct use to the user, this built-in predicate is used in the expansion of grammar rules (see above).

It is defined as if by the clause

’C’([X|S], X, S).

7.1.3 Input and Output of Terms

Most of the following predicates come in two versions, with or without a stream argument.

Predicates without a stream argument operate on the current input or output stream, depending on context. Predicates with a stream argument can take stream reference or an alias in this argument position, the alias being replaced by the stream it was associated with.

Some of these predicates support a notation for terms containing multiple occurrences of the same subterm (cycles and DAGs). The notation is @(Template,Substitution) where

Substitution is a list of Var=Term pairs where the Var occurs in Template or in one of the Terms. This notation stands for the instance of Template obtained by binding each

Var to its corresponding Term. The purpose of this notation is to provide a finite printed representation of cyclic terms. This notation is not used by default, and @/2 has no special meaning except in this context.

read(?Term) read(+Stream,?Term)

[ISO]

[ISO]

The next term, delimited by a full-stop (i.e. a ., possibly followed by layout text), is read from Stream and is unified with Term. The syntax of the term must agree with current operator declarations. If a call read(Stream, Term) causes the end of Stream to be reached, Term is unified with the term end_of_ file. Further calls to read/2 for the same stream will then raise an exception, unless the stream is connected to the terminal. The characters read are subject to character-conversion, see below.

read_term(?Term,+Options) read_term(+Stream,?Term,+Options)

[ISO]

[ISO]

Same as read/[1,2] with a list of options to provide extra control or information about the term. Options is a list of zero or more of:

Chapter 7: Built-In Predicates 109 syntax_errors(+Val)

Controls what action to take on syntax errors. Val must be one of the values allowed for the syntax_errors Prolog flag. The default is set by that flag.

variables(?Vars)

Vars is bound to the list of variables in the term input, in left-toright traversal order.

variable_names(?Names)

Names is bound to a list of Name=Var pairs, where each Name is an atom indicating the name of a non-anonymous variable in the term, and Var is the corresponding variable.

singletons(?Names)

Names is bound to a list of Name=Var pairs, one for each variable appearing only once in the term and whose name does not begin with _.

cycles(+Boolean)

Boolean must be true or false. If selected, any occurrences of

@/2 in the term read in are replaced by the potentially cyclic terms they denote as described above. Otherwise (the default), Term is just unified with the term read in.

layout(?Layout)

Layout is bound to a layout term corresponding to Term. The layout Y of a term X is one of:

• If X is a variable or atomic term, Y is the number of the line where X occurs.

• If X is a compound term, Y is a list whose head is the number of the line where the first token of X occurs, and whose remaining elements are the layouts of the arguments of X.

• [], if no line number information is available for X.

| ?- read_term(T, [layout(L), variable_names(Va), singletons(S)]).

|: [ foo(X),

X = Y

].

L = [35,[36,36],[36,[37,37,37],38]],

S = [’Y’=_A],

T = [foo(_B),_B=_A],

Va = [’X’=_B,’Y’=_A] char_conversion(+InChar, +OutChar)

[ISO]

InChar and OutChar should be one-char atoms. If they are not the same, then the mapping of InChar to OutChar is added to the character-conversion mapping. This means that in all subsequent term and program input operations any unquoted occurrence of InChar will be replaced by OutChar. The rationale for

110 SICStus Prolog providing this facility is that in some extended character sets (such as Japanese

JIS character sets) the same character can appear several times and thus have several codes, which the users normally expect to be equivalent. It is advisable to always quote the arguments of char_conversion/2.

If InChar and OutChar are the same, the effect of char_conversion/2 is to remove any mapping of InChar from the character-conversion mapping.

current_char_conversion(?InChar, ?OutChar)

[ISO]

The character of one-char atom InChar is mapped to that of the one-char atom

OutChar in the current character-conversion mapping. Enumerates all such pairs on backtracking.

write(?Term) write(+Stream,?Term)

[ISO]

[ISO]

The term Term is written onto Stream according to current operator declarations. Same as write_term([Stream,] Term, [numbervars(true)]).

display(?Term)

The term Term is displayed onto the standard output stream (which is not necessarily the current output stream) in standard parenthesized prefix notation.

Same as write_term(user, Term, [ignore_ops(true)]).

write_canonical(?Term) write_canonical(+Stream,?Term)

[ISO]

[ISO]

Similar to write(Stream,Term).

The term will be written according to the standard syntax.

The output from write_canonical/2 can be parsed by read/2 even if the term contains special characters or if operator declarations have changed.

Same as write_term([Stream,] Term,

[quoted(true),ignore_ops(true)]).

writeq(?Term) writeq(+Stream,?Term)

[ISO]

[ISO]

Similar to write(Stream,Term), but the names of atoms and functors are quoted where necessary to make the result acceptable as input to read/2, provided the same operator declarations are in effect.

Same as write_ term([Stream,] Term, [quoted(true),numbervars(true)]).

print(?Term) print(+Stream,?Term)

[Hookable]

[Hookable]

Prints Term onto Stream. This predicate provides a handle for user defined pretty printing:

• If Term is a variable then it is output using write(Stream,Term).

• If Term is non-variable then a call is made to the user defined predicate user:portray/1. If this succeeds then it is assumed that Term has been output.

• Otherwise print/2 is called recursively on the components of Term, unless

Term is atomic in which case it is written via write/2.

In particular, the debugging package prints the goals in the tracing messages, and the top-level prints the final values of variables. Thus you can vary the forms of these messages if you wish.

Chapter 7: Built-In Predicates 111

Note that on lists ([_|_]), print/2 will first give the whole list to user:portray/1, but if this fails it will only give each of the (top level) elements to user:portray/1. That is, user:portray/1 will not be called on all the tails of the list.

Same as

[portrayed(true),numbervars(true)]).

write_term([Stream,] Term, portray(+Term) user:portray(+Term)

[Hook]

This should either print the Term and succeed, or do nothing and fail. In the latter case, the default printer (write/1) will print the Term.

portray_clause(?Clause) portray_clause(+Stream,?Clause)

Writes the clause Clause onto Stream exactly as listing/[0,1] would have written it.

Same as write_term([Stream,] Term,

[quoted(true),numbervars(true),indented(true)]) followed by a period and a newline, removing redundant module prefixes and binding variables to terms of the form ’$VAR’(N ) yielding friendlier variable names.

write_term(+Term,+Options) write_term(+Stream,+Term,+Options)

[ISO]

[ISO]

Same as write/[1,2] etc. with a list of options to provide extra control.

This predicate in fact subsumes the above output predicates except portray_ clause/[1,2] which additionally prints a period and a newline, and removes module prefixes that are redundant wrt. the current type-in module. Options is a list of zero or more of the following, where Boolean must be true or false

(false is the default).

quoted(+Boolean)

If selected, functors are quoted where necessary to make the result acceptable as input to read/1. write_canonical/1, writeq/1, and portray_clause/1 select this.

ignore_ops(+Boolean)

If selected, Term is written in standard parenthesized notation instead of using operators. write_canonical/1 and display/1 select this.

portrayed(+Boolean)

If selected, user:portray/1 is called for each subterm. print/1 selects this.

numbervars(+Boolean)

If selected, occurrences of ’$VAR’(N ) where N is an integer >=

0 are treated specially (see numbervars/3). print/1, write/1, writeq/1, and portray_clause/1 select this.

cycles(+Boolean)

If selected, the potentially cyclic term is printed in finite @/2 notation, as discussed above.

112 SICStus Prolog indented(+Boolean)

If selected, the term is printed with the same indentation as is used by portray_clause/1 and listing/[0,1].

max_depth(N )

Depth limit on printing. N is an integer. 0 (the default) means no limit.

format(+Format,:Arguments) format(+Stream,+Format,:Arguments)

Prints Arguments onto Stream according to format Format. Format is a list of formatting characters or character codes. If Format is an atom then is will be used to translate it into a list of character codes. Thus:

| ?- format("Hello world!", []).

has the same effect as

| ?- format(’Hello world!’, []).

no matter which value the double_quotes Prolog flag has.

format/2 and format/3 is a Prolog interface to the C stdio function printf().

It is modeled after and compatible with Quintus Prolog.

Arguments is a list of items to be printed. If there are no items then an empty list should be supplied.

The default action on a format character is to print it. The character ~ introduces a control sequence. To print a ~ repeat it:

| ?- format("Hello ~~world!", []).

will result in

Hello ~world!

The escape sequence (see

[Escape Sequences], page 578

) \c (c for continue) is useful when formatting a string for readability. It causes all characters up to, but not including, the next non-layout character to be ignored.

| ?- format("Hello \c world!", []).

will result in

Hello world!

The general format of a control sequence is ‘~N C’. The character C determines the type of the control sequence. N is an optional numeric argument. An alternative form of N is ‘*’. ‘*’ implies that the next argument in Arguments should be used as a numeric argument in the control sequence. Example:

| ?- format("Hello~4cworld!", [0’x]).

and

| ?- format("Hello~*cworld!", [4,0’x]).

both produce

Helloxxxxworld!

The following control sequences are available.

‘~a’ The argument is an atom. The atom is printed without quoting.

Chapter 7: Built-In Predicates 113

‘~N c’ (Print character.) The argument is a number that will be interpreted as a character code. N defaults to one and is interpreted as the number of times to print the character.

‘~N e’

‘~N E’

‘~N f’

‘~N g’

‘~N G’

‘~N d’

‘~N D’

‘~N r’

‘~N R’

(Print float). The argument is a float. The float and N will be passed to the C printf() function as printf("%.N e", Arg ) printf("%.N E", Arg ) printf("%.N f", Arg ) printf("%.N g", Arg ) printf("%.N G", Arg ) respectively.

If N is not supplied the action defaults to printf("%e", Arg ) printf("%E", Arg ) printf("%f", Arg ) printf("%g", Arg ) printf("%G", Arg ) respectively.

(Print decimal.) The argument is an integer. N is interpreted as the number of digits after the decimal point. If N is 0 or missing, no decimal point will be printed. Example:

| ?- format("Hello ~1d world!", [42]).

Hello 4.2 world!

| ?- format("Hello ~d world!", [42]).

Hello 42 world!

(Print decimal.) The argument is an integer. Identical to ‘~N d’ except that ‘,’ will separate groups of three digits to the left of the decimal point. Example:

| ?- format("Hello ~1D world!", [12345]).

Hello 1,234.5 world!

(Print radix.) The argument is an integer. N is interpreted as a radix, 2 =< N =< 36. If N is missing the radix defaults to 8. The letters ‘a-z’ will denote digits larger than 9. Example:

| ?- format("Hello ~2r world!", [15]).

Hello 1111 world!

| ?- format("Hello ~16r world!", [15]).

Hello f world!

(Print radix.) The argument is an integer. Identical to ‘~N r’ except that the letters ‘A-Z’ will denote digits larger than 9. Example:

114 SICStus Prolog

‘~N s’

‘~i’

‘~k’

‘~p’

‘~q’

‘~w’

[email protected]

‘~~’

| ?- format("Hello ~16R world!", [15]).

Hello F world!

(Print string.) The argument is a list of character codes. Exactly

N characters will be printed. N defaults to the length of the string.

Example:

| ?- format("Hello ~4s ~4s!", ["new","world"]).

Hello new worl!

| ?- format("Hello ~s world!", ["new"]).

Hello new world!

(Ignore.) The argument, which may be of any type, is ignored.

Example:

| ?- format("Hello ~i~s world!", ["old","new"]).

Hello new world!

(Print canonical.) The argument may be of any type. The argument will be passed to write_canonical/1 (see

Section 7.1.3 [Term

I/O], page 108 ). Example:

| ?- format("Hello ~k world!", [[a,b,c]]).

Hello .(a,.(b,.(c,[]))) world!

(Print.) The argument may be of any type. The argument will be passed to print/1 (see

Section 7.1.3 [Term I/O], page 108

).

Example:

| ?- assert((portray([X|Y]) :- print(cons(X,Y)))).

| ?- format("Hello ~p world!", [[a,b,c]]).

Hello cons(a,cons(b,cons(c,[]))) world!

(Print quoted.) The argument may be of any type. The argument will be passed to writeq/1 (see

Section 7.1.3 [Term I/O], page 108

).

Example:

| ?- format("Hello ~q world!", [[’A’,’B’]]).

Hello [’A’,’B’] world!

(Write.) The argument may be of any type. The argument will be passed to write/1 (see

Section 7.1.3 [Term I/O], page 108

).

Example:

| ?- format("Hello ~w world!", [[’A’,’B’]]).

Hello [A,B] world!

(Call.) The argument is a goal, which will be called and expected to print on the current output stream. If the goal performs other side-effects or does not succeed deterministically, the behavior is undefined. Example:

| ?- format("Hello [email protected] world!", [write(new)]).

Hello new world!

(Print tilde.) Takes no argument. Prints ‘~’. Example:

Chapter 7: Built-In Predicates 115

‘~N n’

| ?- format("Hello ~~ world!", []).

Hello ~ world!

(Print newline.) Takes no argument. Prints N newlines. N defaults to 1. Example:

| ?- format("Hello ~n world!", []).

Hello world!

‘~N’ (Print Newline.) Prints a newline if not at the beginning of a line.

The following control sequences set column boundaries and specify padding.

A column is defined as the available space between two consecutive column boundaries on the same line. A boundary is initially assumed at line position

0. The specifications only apply to the line currently being written.

When a column boundary is set (‘~|’ or ‘~+’) and there are fewer characters written in the column than its specified width, the remaining space is divided equally amongst the pad sequences (‘~t’) in the column. If there are no pad sequences, the column is space padded at the end.

If ‘~|’ or ‘~+’ specifies a position preceding the current position, the boundary is set at the current position.

‘~N |’ Set a column boundary at line position N. N defaults to the current position.

‘~N +’

‘~N t’

Set a column boundary at N positions past the previous column boundary. N defaults to 8.

Specify padding in a column. N is the fill character code. N may also be specified as ‘C where C is the fill character. The default fill character is h

SPC i

. Any (‘~t’) after the last column boundary on a line is ignored.

Example:

| ?format("~‘*t NICE TABLE ~‘*t~61|~n", []), format("*~t*~61|~n", []), format("*~t~a~20|~t~a~t~20+~a~t~20+~t*~61|~n",

[’Right aligned’,’Centered’,’Left aligned’]), format("*~t~d~20|~t~d~t~20+~d~t~20+~t*~61|~n",

[123,45,678]), format("*~t~d~20|~t~d~t~20+~d~t~20+~t*~61|~n",

[1,2345,6789]), format("~‘*t~61|~n", []).

************************ NICE TABLE *************************

* *

* Right aligned Centered Left aligned *

*

*

123

1

45

2345

678

6789

*************************************************************

*

*

116 SICStus Prolog

7.1.4 Character Input/Output

Most of character I/O predicates have several variants: bytes vs. characters

There are separate predicates for binary I/O, which work on bytes, and for text I/O, which work on characters. The former have the suffix _byte, e.g.

put_byte.

character codes vs. one-char atoms

The text I/O predicates come in two variants, those which use character codes

(suffix _code, e.g. put_code), and those using one-char atoms (suffix _char, e.g. put_char).

SICStus compatibility predicates

The SICStus compatibility predicates work on both binary and text streams and use character codes or bytes, depending on the stream type. They normally have no suffix (e.g. put), with the exception of peek_char.

explicit vs. implicit stream

Each of the above predicates comes in two variants: with an explicit first argument, which is the stream or alias to which the predicate applies (e.g.

put_byte(Stream, Byte)), or without the stream argument, in which case the current input or output stream is used, depending on the context (e.g. put_ byte(Byte)).

I/O on standard streams

These are variants of SICStus compatibility predicates which always work on the standard input or output.

These predicates have the prefix tty, e.g.

ttyput(Code).

nl nl(+Stream)

[ISO]

[ISO]

A new line is started on the text stream Stream by printing an h

LFD i . If Stream is connected to the terminal, its buffer is flushed.

get_code(?Code) get_code(+Stream,?Code)

[ISO]

[ISO]

Code is the character code of the next character read from text stream Stream.

If all characters of Stream have been read, Code is -1, and further calls to get_code/2 for the same stream will normally raise an exception, unless the stream is connected to the terminal (but see the eof_action option of open/4; see

Section 7.1.5 [Stream Pred], page 118

).

get_char(?Char)

[ISO] get_char(+Stream,?Char)

[ISO]

Char is the one-char atom naming the next character read from text stream

Stream. If all characters of Stream have been read, Char is end_of_file, and further calls to get_char/2 for the same stream will normally raise an exception, unless the stream is connected to the terminal (but see the eof_ action option of open/4; see

Section 7.1.5 [Stream Pred], page 118

).

Chapter 7: Built-In Predicates 117 get_byte(?Byte)

[ISO] get_byte(+Stream,?Byte)

[ISO]

Byte is the next byte read from the binary stream Stream. It has the same behavior at the end of stream as get_code.

get0(?Code) get0(+Stream,?Code)

[Obsolescent]

[Obsolescent]

A combination of get_code and get_byte: Code is the next character code or byte read from the arbitrary stream Stream.

get(?N ) get(+Stream,?N )

[Obsolescent]

[Obsolescent]

Same as get0/2, except N is the character code of the next character that is not a layout-char (see

[Token String], page 574 ) read from Stream.

peek_code(?Code) peek_code(+Stream,?Code)

[ISO]

[ISO]

Code is the character code of the next character from text stream Stream, or

-1, if all characters of Stream have been read. The character is not actually read, it is only looked at and is still available for subsequent input.

peek_char(?Char) peek_char(+Stream,?Char)

[ISO only]

[ISO only]

Char is the one-char atom naming the next character from text stream Stream, or end_of_file, if all characters of Stream have been read. The character is not actually read.

peek_char(?Code) peek_char(+Stream,?Code)

Identical to peek_code.

[SICStus only]

[SICStus only] peek_byte(?Byte) peek_byte(+Stream,?Byte)

[ISO]

[ISO]

Byte is the next byte from binary stream Stream, or -1, if all bytes of Stream have been read. The byte is not actually read.

skip(+Code) skip(+Stream,+Code)

[Obsolescent]

[Obsolescent]

Skips just past the next character code Code from Stream. Code may be an arithmetic expression.

skip_line skip_line(+Stream)

Skips just past the next h

LFD i from the text stream Stream.

[Obsolescent]

[Obsolescent] put_code(+Code) put_code(+Stream,+Code)

[ISO]

[ISO]

Character code Code is output onto text stream Stream. Code may be an arithmetic expression.

118 SICStus Prolog put_char(+Char)

[ISO] put_char(+Stream,+Char)

[ISO]

The character named by the one-char atom Char is output onto text stream

Stream.

put_byte(+Byte) put_byte(+Stream,+Byte)

[ISO]

[ISO]

Byte Byte is output onto binary stream Stream. Byte may be an arithmetic expression.

put(+Code) put(+Stream,+Code)

[Obsolescent]

[Obsolescent]

A combination of put_code and put_byte: Code is output onto (an arbitrary stream) Stream. Code may be an arithmetic expression.

tab(+N ) tab(+Stream,+N )

[Obsolescent]

[Obsolescent]

N spaces are output onto text stream Stream. N may be an arithmetic expression.

The above predicates are the ones which are the most commonly used, as they can refer to any streams. The predicates listed below always refer to the standard input and output streams. They are provided for compatibility with DEC-10 character I/O, and are actually redundant and easily recoded in terms of the above predicates.

[Obsolescent] ttynl

Same as nl(user_output).

ttyflush

Same as flush_output(user_output).

ttyget0(?N )

Same as get0(user_input, N ).

ttyget(?N )

Same as get(user_input, N ).

ttyput(+N )

Same as put(user_output, N ).

ttyskip(+N )

Same as skip(user_input, N ).

ttytab(+N )

Same as tab(user_output, N ).

[Obsolescent]

[Obsolescent]

[Obsolescent]

[Obsolescent]

[Obsolescent]

[Obsolescent]

7.1.5 Stream I/O

The following predicates manipulate streams. Character, byte and line counts are maintained per stream. All streams connected to the terminal, however, share the same set of counts. For example, writing to user_output will advance the counts for user_input, if

Chapter 7: Built-In Predicates 119 both are connected to the terminal. Bidirectional streams use the same counters for input and output.

Wherever a stream argument appears as input (+Stream), an alias can be used instead.

open(+FileName,+Mode,-Stream)

[ISO] open(+FileName,+Mode,-Stream,+Options)

[ISO]

If FileName is a valid file name, the file is opened in mode Mode (invoking the

UNIX function fopen) and the resulting stream is unified with Stream. Mode is one of: read write

Open the file for input.

Open the file for output. The file is created if it does not already exist, the file will otherwise be truncated.

append

Open the file for output. The file is created if it does not already exist, the file will otherwise be appended to.

If FileName is an integer, it is assumed to be a file descriptor passed to Prolog from C. The file descriptor is connected to a Prolog stream (invoking the POSIX function fdopen) which is unified with Stream.

Options is a list of zero or more of: type(+T) Specifies whether the stream is a text or binary stream. Default is text.

reposition(+Boolean)

Specifies whether repositioning is required for the stream (true), or not (false). The latter is the default.

alias(+A)

Specifies that the atom A is to be an alias for the stream.

eof_action(+Action)

Specifies what action is to be taken when the end of stream has already been reported (by returning -1 or end_of_file), and a further attempt to input is made. Action can have the following values: error

An exception is raised. This is the default.

eof_code

An end of stream indicator (-1 or end_of_file) is returned again.

reset

The stream is considered not to be at end of stream and another attempt is made to input from it.

close(+X ) wcx(Wcx)

Specifies to pass the term Wcx to the wide character extension component; see

Section 10.3 [Prolog Level WCX Features], page 223

.

[ISO]

120 SICStus Prolog close(+X, +Options)

[ISO]

If X is a stream or alias, the stream is closed. If X is the name of a file opened by see/1 or tell/1, the corresponding stream is closed. Options is a list possibly containing the following element: force(Boolean)

Specifies whether SICStus Prolog is to close the stream forcefully, even in the presence of errors (true), or not (false). The latter is the default. Currently this option has no effect.

absolute_file_name(+RelativeName,-AbsoluteName)

True if RelativeName can be expanded to an absolute file name (an atom)

AbsoluteName, according to the filename syntax rules (see

Section 7.1 [Input

Output], page 98 ). This predicate will first search for a file with the suffix ‘.pl’

added to the name given as an argument. If this fails, it will look for a file with no extra suffix added. If a file is found AbsoluteName is its absolute file name is returned. Otherwise, AbsoluteName is a valid expansion of RelativeName.

absolute_file_name/2 does not produce alternative expansions via backtracking.

If RelativeName is user, then AbsoluteName is also unified with user; this

“filename” stands for the standard input or output stream, depending on context.

Variants of this predicate are used by all predicates that refer to filenames for resolving these. Predicates that load code require that the specified file exist, possibly with an extension.

file_search_path(+Alias,-Expansion)

[Hook] user:file_search_path(+Alias,-Expansion)

Specifies how to rewrite compound filenames to atomic ones, as described in

Section 7.1 [Input Output], page 98 . Alias should be an atom and Expansions

a filename. The predicate may succeed non-deterministically in this search for an atomic filename.

The predicate exists as a dynamic, multifile predicate at startup with the following clause, defining an expansion for the library and system aliases. See

Section 7.6 [State Info], page 137

for more info on the Prolog flag host_type.

file_search_path(library, Path) :library_directory(Path).

file_search_path(system, Platform) :prolog_flag(host_type, Platform).

library_directory(-Directory )

[Hook] user:library_directory(-Directory )

Specifies a directory to be searched when a filename of the form library(Name) is used. The predicate exists as a dynamic, multifile predicate at startup with a single clause defining the location of the Prolog library.

It may succeed non-deterministically in this search for a library directory.

Chapter 7: Built-In Predicates 121 current_input(?Stream)

[ISO]

Stream is the current input stream. The current input stream is also accessed by the C variable SP_curin.

current_output(?Stream)

[ISO]

Stream is the current output stream. The current output stream is also accessed by the C variable SP_curout.

current_stream(?FileName,?Mode,?Stream)

Stream is a stream which was opened in mode Mode and which is connected to the absolute file name Filename (an atom) or to the file descriptor Filename

(an integer). This predicate can be used for enumerating all currently open streams through backtracking.

set_input(+Stream)

Sets the current input stream to Stream.

[ISO] set_output(+Stream)

Sets the current output stream to Stream.

[ISO] flush_output

[ISO] flush_output(+Stream)

[ISO]

Flushes all internally buffered characters or bytes for Stream to the operating system.

open_null_stream(-Stream)

Opens a text output stream. Everything written to this stream will be thrown away.

character_count(+Stream,?N )

N is the number of characters read/written on text stream Stream.

byte_count(+Stream,?N )

N is the number of bytes read/written on stream Stream. Meaningful for both binary and text streams.

In the latter case it will differ from the number returned by character_count/2 in the presence of wide characters.

line_count(+Stream,?N )

N is the number of lines read/written on text stream Stream.

line_position(+Stream,?N )

N is the number of characters read/written on the current line of text stream

Stream.

stream_position(+Stream,?Position)

Position is a term representing the current position of Stream. The relative order of stream position terms can be tested with standard term comparison predicates such as compare/3, but you should not otherwise rely on their internal representation. This operation is available for any Prolog stream.

stream_property(?Stream, ?Property ))

[ISO]

Stream Stream has property Property. Enumerates through backtracking all currently open streams, including the standard input/output/error streams, and all their properties.

122 SICStus Prolog

Property can be one of the following: file_name(?F)

F is the file name associated with the Stream.

mode(?M ) input output

Stream has been opened in mode M.

Stream is an input stream.

Stream is an output stream.

alias(?A)

Stream has an alias A.

position(?P)

P is a term representing the current position of Stream. Same as stream_position(Stream, P).

end_of_stream(?E)

E describes the position of the input stream Stream, with respect to the end of stream. If not all characters have been read, then E is unified with not, otherwise (all characters read) but no end of stream indicator (-1 or end_of_file) was reported yet, then E is unified with at, otherwise E is unified with past.

eof_action(?A)

A is the end-of-file action applicable to Stream, cf. the eof_action option of open/4.

type(?T)

Stream is of type T.

wcx(?Wcx)

Wide character extension information Wcx was supplied at opening

Stream; see

Section 10.3 [Prolog Level WCX Features], page 223

.

set_stream_position(+Stream,+Position)

[ISO]

Position is a term representing a new position of Stream, which is then set to the new position. This operation is only available for Prolog streams connected to “seekable devices” (disk files, usually). If the option reposition(true) was supplied at the successful opening of the stream, then set_stream_position/2 is guaranteed to be successful.

seek(+Stream,+Offset,+Method,-NewLocation)

True if the stream Stream can be set to the byte offset Offset relative to Method, and NewLocation is the new byte offset from the beginning of the file after the operation. Method must be one of: bof

Seek from the beginning of the file stream.

current

Seek from the current position of the file stream.

eof

Seek from the end of the file stream.

This operation is only available for Prolog streams connected to “seekable devices” (disk files, usually) and is an interface to the stdio functions fseek and ftell.

Chapter 7: Built-In Predicates 123 at_end_of_stream

[ISO] at_end_of_stream(+Stream)

[ISO]

The end of stream has been reached for the input stream Stream. An input stream reaches end of stream when all characters (except ‘EOF’, i.e., -1) of the stream have been read. These predicates peek ahead for next input character if there is no character available on the buffer of Stream. Unless the stream is to be treated as connected to the terminal (see SP_force_interactive,

Section 8.7.4.1 [Initializing the Prolog Engine], page 198 ), a stream remains at

end of stream after ‘EOF’ has been read, and any further attempt to read from the stream will raise an existence error (see

Section 7.5 [Exception], page 134

).

at_end_of_line at_end_of_line(+Stream)

The end of stream or end of line has been reached for the input stream Stream.

An input stream reaches end of line when all the characters except h

LFD i of the current line have been read. These predicates peek ahead for next input character if there is no character available on the buffer of Stream.

fileerrors

Undoes the effect of nofileerrors/0.

nofileerrors

After a call to this predicate, failure to locate or open a file will cause the operation to fail instead of the default action, which is to raise an exception with an error message.

stream_select(+Streams,+TimeOut,-ReadStreams)

The list of streams in Streams is checked for readable characters. A stream can be any stream associated with an I/O descriptor. The list ReadStreams returns the streams with readable data. If TimeOut is instantiated to off, the predicate waits until something is available. If TimeOut is S:U the predicate waits at most S seconds and U microseconds. Both S and U must be integers

>=0. If there is a timeout, ReadStreams is [].

Not available in operating systems that do not support the system() system call.

stream_interrupt(+Stream,?OldHandler,?NewHandler)

Installs NewHandler as an interrupt-handler which is invoked when something is readable on Stream. OldHandler is the current interrupt handler Stream must be associated with an I/O descriptor. Interrupt handlers are specified as atoms. The atom off indicates that the interrupt mechanism is turned off for

Stream. Any other atom is the name of a predicate invoked when something is readable on Stream. The handler predicate has one argument, the stream that is readable. For example, stream_interrupt(Stream, _, int_handler).

will enable the interrupt mechanism. Given the predicate int_handler(Stream) :read(Stream, Data), write(Data), nl.

124 SICStus Prolog the term read from Stream will be written to the current output. NOTE: there is no guarantee that a complete Prolog term is available yet. If not, read/2 will suspend as usual.

Not available in operating systems that do not provide the ability to generate signals when new data becomes available on a file descriptor.

7.1.6 DEC-10 Prolog File I/O

The following predicates manipulate files.

see(+File)

The file File becomes the current input stream. File may be a stream previously opened by see/1 or a filename. If it is a filename, the following action is taken:

If there is a stream opened by see/1 associated with the same file already, then it becomes the current input stream. Otherwise, the file File is opened for input and made the current input stream.

seeing(?FileName)

FileName is unified with the name of the current input file, if it was opened by see/1, with the current input stream, if it is not user_input, otherwise with user.

seen

Closes the current input stream, and resets it to user_input.

tell(+File)

The file File becomes the current output stream. File may be a stream previously opened by tell/1 or a filename. If it is a filename, the following action is taken: If there is a stream opened by tell/1 associated with the same file already, then it becomes the current output stream. Otherwise, the file File is opened for output and made the current output stream.

telling(?FileName)

FileName is unified with the name of the current output file, if it was opened by tell/1, with the current output stream, if it is not user_output, otherwise with user.

told

Closes the current output stream, and resets it to user_output.

7.1.7 An Example

Here is an example of a common form of file processing:

Chapter 7: Built-In Predicates 125 process_file(F) :seeing(OldInput), see(F), repeat, read(T), process_term(T),

T == end_of_file,

!, seen, see(OldInput).

% Open file F

% Read a term

% Process it

% Loop back if not at end of file

% Close the file

The above is an example of a repeat loop. Nearly all sensible uses of repeat/0 follow the above pattern. Note the use of a cut to terminate the loop.

7.2 Arithmetic

Arithmetic is performed by built-in predicates which take as arguments arithmetic expressions and evaluate them. An arithmetic expression is a term built from numbers, variables, and functors that represent arithmetic functions. At the time of evaluation, each variable in an arithmetic expression must be bound to a non-variable expression. An expression evaluates to a number, which may be an integer or a float.

The range of integers is [-2^2147483616, 2^2147483616). Thus for all practical purposes, the range of integers can be considered infinite.

The range of floats is the one provided by the C double type, typically [4.9e-324,

1.8e+308] (plus or minus). In case of overflow or division by zero, iso execution mode will raise an evaluation error exception. In sicstus execution mode no exceptions will be raised, instead appropriate infinity values, as defined by the IEEE standard, will be used.

Only certain functors are permitted in an arithmetic expression. These are listed below, together with an indication of the functions they represent. X and Y are assumed to be arithmetic expressions. Unless stated otherwise, the arguments of an expression may be any numbers and its value is a float if any of its arguments is a float, otherwise the value is an integer. Any implicit coercions are performed with the integer/1 and float/1 functions.

The arithmetic functors are annotated with [ISO], [ISO only], or [SICStus only], with the same meaning as for the built-in predicates; see

[ISO Compliance], page 6 .

The value is X.

+(X )

-X

X +Y

[ISO] The value is the negative of X.

[ISO]

The value is the sum of X and Y.

X-Y [ISO]

The value is the difference of X and Y.

X *Y [ISO]

The value is the product of X and Y.

126 SICStus Prolog

X /Y

X //Y

X rem Y

X mod Y

X mod Y

[ISO]

The value is the float quotient of X and Y.

[ISO]

The value is the integer quotient of X and Y. The result is always truncated towards zero. In iso execution mode X and Y have to be integers.

[ISO]

The value is the integer remainder after dividing X by Y, i.e. integer(X )integer(Y )*(X //Y ). The sign of a nonzero remainder will thus be the same as that of the dividend. In iso execution mode X and Y have to be integers.

[ISO only]

The value is X modulo Y, i.e. integer(X )-integer(Y )*floor(X /Y ). The sign of a nonzero remainder will thus be the same as that of the divisor. X and

Y have to be integers.

[SICStus only]

The value is the same as that of X rem Y.

integer(X )

The value is the closest integer between X and 0, if X is a float, otherwise to

X itself.

float_integer_part(X )

The same as integer(X). In iso execution mode X has to be a float.

[ISO] float_fractional_part(X )

[ISO]

The value is the fractional part of X, i.e. X - float_integer_part(X). In iso execution mode X has to be a float.

float(X )

[ISO]

The value is the float equivalent of X, if X is an integer, otherwise to X itself.

X /\Y

X \/Y

X #Y

\(X )

X <<Y

X >>Y

[ISO]

The value is the bitwise conjunction of the integers X and Y. In iso execution mode X and Y have to be integers.

[ISO]

The value is the bitwise disjunction of the integers X and Y. In iso execution mode X and Y have to be integers.

The value is the bitwise exclusive or of the integers X and Y.

[ISO]

The value is the bitwise negation of the integer X. In iso execution mode X has to be an integer.

[ISO]

The value is the integer X shifted left by Y places. In iso execution mode X and Y have to be integers.

[ISO]

The value is the integer X shifted right by Y places. In iso execution mode X and Y have to be integers.

Chapter 7: Built-In Predicates 127

[X ]

A list of just one number X evaluates to X. Since a quoted string is just a list of integers, this allows a quoted character to be used in place of its character code; e.g. "A" behaves within arithmetic expressions as the integer 65.

SICStus Prolog also includes an extra set of functions listed below. These may not be supported by other Prologs.

All trigonometric and transcendental functions take float arguments and deliver float values. The trigonometric functions take arguments or deliver values in radians.

abs(X ) sign(X )

[ISO]

The value is the absolute value of X.

[ISO]

The value is the sign of X, i.e. -1, if X is negative, 0, if X is zero, and 1, if X is positive, coerced into the same type as X (i.e. the result is an integer, iff X is an integer).

gcd(X,Y )

The value is the greatest common divisor of the two integers X and Y. In iso execution mode X and Y have to be integers.

min(X,Y )

The value is the lesser value of X and Y.

max(X,Y )

The value is the greater value of X and Y.

msb(X )

The value is the most significant bit position of the integer X. It is equivalent to, but more efficient than, integer(log(2,X)). In iso execution mode X has to be an integer.

round(X )

[ISO only]

The value is the closest integer to X. X has to be a float. If X is exactly halfway between two integers, it is rounded up (i.e. the value is the least integer greater than X ).

round(X )

[SICStus only]

The value is the float that is the closest integral value to X. If X is exactly half-way between two integers, it is rounded to the closest even integral value.

truncate(X )

[ISO only]

The value is the closest integer between X and 0. X has to be a float.

truncate(X )

[SICStus only]

The value is the float that is the closest integer between X and 0.

floor(X )

[ISO only]

The value is the greatest integer less or equal to X. X has to be a float.

floor(X )

[SICStus only]

The value is the float that is the greatest integral value less or equal to X.

ceiling(X )

[ISO only]

The value is the least integer greater or equal to X. X has to be a float.

128 SICStus Prolog ceiling(X )

[SICStus only]

The value is the float that is the least integral value greater or equal to X.

sin(X )

[ISO]

The value is the sine of X.

cos(X )

[ISO]

The value is the cosine of X.

tan(X )

The value is the tangent of X.

cot(X )

The value is the cotangent of X.

sinh(X )

The value is the hyperbolic sine of X.

cosh(X )

The value is the hyperbolic cosine of X.

tanh(X )

The value is the hyperbolic tangent of X.

coth(X )

The value is the hyperbolic cotangent of X.

asin(X )

The value is the arc sine of X.

acos(X )

The value is the arc cosine of X.

atan(X )

[ISO]

The value is the arc tangent of X.

atan2(X,Y )

The value is the four-quadrant arc tangent of X and Y.

acot(X )

The value is the arc cotangent of X.

acot2(X,Y )

The value is the four-quadrant arc cotangent of X and Y.

asinh(X ) The value is the hyperbolic arc sine of X.

acosh(X ) The value is the hyperbolic arc cosine of X.

atanh(X ) The value is the hyperbolic arc tangent of X.

acoth(X ) The value is the hyperbolic arc cotangent of X.

sqrt(X )

The value is the square root of X.

log(X )

The value is the natural logarithm of X.

log(Base,X )

The value is the logarithm of X in the base Base.

exp(X )

The value is the natural exponent of X.

X ** Y exp(X,Y )

The value is X raised to the power of Y.

[ISO]

[ISO]

[ISO]

[ISO]

Chapter 7: Built-In Predicates 129 inf

[SICStus only]

The value is infinity as defined in the IEEE standard.

nan

[SICStus only]

The value is not-a-number as defined in the IEEE standard.

Variables in an arithmetic expression which is to be evaluated may be bound to other arithmetic expressions rather than just numbers, e.g.

evaluate(Expression, Answer) :- Answer is Expression.

| ?- evaluate(24*9, Ans).

Ans = 216 ?

yes

Arithmetic expressions, as described above, are just data structures. If you want one evaluated you must pass it as an argument to one of the built-in predicates listed below. Note that is/2 only evaluates one of its arguments, whereas all the comparison predicates evaluate both of theirs. In the following, X and Y stand for arithmetic expressions, and Z for some term.

Z is X

X =:= Y

X =\= Y

X < Y

X > Y

X =< Y

X >= Y

[ISO]

X, which must be an arithmetic expression, is evaluated and the result is unified with Z.

[ISO]

The numeric values of X and Y are equal.

[ISO]

The numeric values of X and Y are not equal.

[ISO]

The numeric value of X is less than the numeric value of Y.

[ISO]

The numeric value of X is greater than the numeric value of Y.

[ISO]

The numeric value of X is less than or equal to the numeric value of Y.

[ISO]

The numeric value of X is greater than or equal to the numeric value of Y.

7.3 Comparison of Terms

These built-in predicates are meta-logical. They treat uninstantiated variables as objects with values which may be compared, and they never instantiate those variables. They should not be used when what you really want is arithmetic comparison (see

Section 7.2

[Arithmetic], page 125 ) or unification.

130 SICStus Prolog

The predicates make reference to a standard total ordering of terms, which is as follows:

• Variables, by age (oldest first—the order is not related to the names of variables).

• Floats, in numeric order (e.g. -1.0 is put before 1.0).

• Integers, in numeric order (e.g. -1 is put before 1).

• Atoms, in alphabetical (i.e. character code) order.

• Compound terms, ordered first by arity, then by the name of the principal functor, then by age for mutables and by the arguments in left-to-right order for other terms.

Recall that lists are equivalent to compound terms with principal functor ./2.

For example, here is a list of terms in standard order:

[ X, -1.0, -9, 1, fie, foe, X = Y, foe(0,2), fie(1,1,1) ]

NOTE: the standard order is only well-defined for finite (acyclic) terms. There are infinite

(cyclic) terms for which no order relation holds. Furthermore, blocking goals (see

Section 3.3 [Procedural], page 45 ) on variables or modifying their attributes (see

Chapter 15

[Attributes], page 253 ) does not preserve their order.

These are the basic predicates for comparison of arbitrary terms:

Term1 == Term2 [ISO]

The terms currently instantiating Term1 and Term2 are literally identical (in particular, variables in equivalent positions in the two terms must be identical).

For example, the query

| ?- X == Y.

fails (answers ‘no’) because X and Y are distinct uninstantiated variables. However, the query

| ?- X = Y, X == Y.

succeeds because the first goal unifies the two variables (see

Section 7.16 [Misc

Pred], page 160 ).

Term1 \== Term2 [ISO]

The terms currently instantiating Term1 and Term2 are not literally identical.

Term1 @< Term2

The term Term1 is before the term Term2 in the standard order.

[ISO]

[ISO] Term1 @> Term2

The term Term1 is after the term Term2 in the standard order.

Term1 @=< Term2

The term Term1 is not after the term Term2 in the standard order.

Term1 @>= Term2

The term Term1 is not before the term Term2 in the standard order.

[ISO]

[ISO]

Some further predicates involving comparison of terms are:

Chapter 7: Built-In Predicates 131

?=(?X,?Y )

X and Y are either syntactically identical or syntactically non-unifiable.

compare(?Op,?Term1,?Term2)

The result of comparing terms Term1 and Term2 is Op, where the possible values for Op are:

= if Term1 is identical to Term2,

<

> if Term1 is before Term2 in the standard order, if Term1 is after Term2 in the standard order.

Thus compare(=,Term1,Term2) is equivalent to Term1 == Term2.

sort(+List1,?List2)

The elements of the list List1 are sorted into the standard order (see

Section 7.3

[Term Compare], page 129 ) and any identical elements are merged, yielding the

list List2. (The time and space complexity of this operation is at worst O(N lg

N) where N is the length of List1.) keysort(+List1,?List2)

The list List1 must consist of pairs of the form Key-Value. These items are sorted into order according to the value of Key, yielding the list List2. No merging takes place. This predicate is stable, i.e. if K-A occurs before K-B in the input, then K-A will occur before K-B in the output. (The time and space complexity of this operation is at worst O(N lg N) where N is the length of

List1.)

7.4 Control

+P , +Q

+P ; +Q

!

\+ +P

[ISO]

P and Q.

[ISO]

P or Q.

[ISO]

See

Section 3.5 [Cut], page 48

.

[ISO]

Fails if the goal P has a solution, and succeeds otherwise. This is not real negation (“P is false”), but a kind of pseudo-negation meaning “P is not provable”.

It is defined as if by

\+(P) :- P, !, fail.

\+(_).

In sicstus execution mode no cuts are allowed in P. In iso execution mode cuts are allowed in P and their scope is the goal P.

Remember that with prefix operators such as this one it is necessary to be careful about spaces if the argument starts with a (. For example:

132 SICStus Prolog

+P -> +Q

| ?- \+ (P,Q).

is this operator applied to the conjunction of P and Q, but

| ?- \+(P,Q).

would require a predicate \+ /2 for its solution. The prefix operator can however be written as a functor of one argument; thus

| ?- \+((P,Q)).

is also correct.

+P -> +Q ; +R

Analogous to if P then Q else R and defined as if by

[ISO]

(P -> Q; R) :- P, !, Q.

(P -> Q; R) :- R.

except the scope of any cut in Q or R extends beyond the if-then-else construct.

In sicstus execution mode no cuts are allowed in P. In iso execution mode cuts are allowed in P and their scope is the goal P.

Note that this form of if-then-else only explores the first solution to the goal P.

Note also that the ; is not read as a disjunction operator in this case; instead, it is part of the if-then-else construction.

The precedence of -> is less than that of ; (see

Section 3.6 [Operators], page 49

), so the expression is read as

;(->(P,Q),R)

[ISO]

When occurring as a goal, this construction is read as equivalent to

(P -> Q; fail) if(+P,+Q,+R)

Analogous to if P then Q else R but differs from P -> Q ; R in that if(P, Q, R) explores all solutions to the goal P. There is a small time penalty for this—if P is known to have only one solution of interest, the form P -> Q ; R should be preferred.

In sicstus execution mode no cuts are allowed in P. In iso execution mode cuts are allowed in P and their scope is the goal P.

once(+P)

[ISO]

Finds the first solution, if any, of goal P. Fails if no solutions are found. Will not explore further solutions on backtracking. Equivalent to

(P -> true; fail) otherwise true

[ISO]

These always succeed. Use of otherwise/0 is discouraged, because it is not as portable as true/0, and because the former may suggest a completely different semantics than the latter.

Chapter 7: Built-In Predicates 133 false fail repeat

[ISO]

These always fail. Use of false/0 is discouraged, because it is not as portable as fail/0, and because the latter has a more procedural flavor to it.

[ISO]

Generates an infinite sequence of backtracking choices.

In sensible code, repeat/0 is hardly ever used except in repeat loops. A repeat loop has the structure

Head :-

...

save(OldState), repeat, generate(Datum), action(Datum), test(Datum),

!, restore(OldState),

...

The purpose is to repeatedly perform some action on elements which are somehow generated, e.g. by reading them from a stream, until some test becomes true. Usually, generate, action, and test are all determinate. Repeat loops cannot contribute to the logic of the program. They are only meaningful if the action involves side-effects.

The only reason for using repeat loops instead of a more natural tail-recursive formulation is efficiency: when the test fails back, the Prolog engine immediately reclaims any working storage consumed since the call to repeat/0.

call(:Term) incore(:Term)

:Term

[ISO]

[Obsolescent]

If Term is instantiated to a term which would be acceptable as the body of a clause, then the goal call(Term) is executed exactly as if that term appeared textually in its place, except that any cut (!) occurring in Term only cuts alternatives in the execution of Term. Use of incore/1 is not recommended.

If Term is not instantiated as described above, an error message is printed and the call fails.

call_cleanup(:Goal,:Cleanup)

This construction can be used to ensure that Cleanup is executed as soon as

Goal has completed execution, no matter how it finishes. In more detail:

When call_cleanup/2 with a continuation C is called or backtracked into, first Goal is called or backtracked into. Then there are four possibilities:

1. Goal succeeds deterministically, possibly leaving some blocked subgoals.

Cleanup is executed with continuation C.

2. Goal succeeds with some alternatives outstanding.

Execution proceeds to C. If a cut that removes the outstanding alternatives is encountered,

134 SICStus Prolog

Cleanup is executed with continuation to proceed after the cut. Also, if an exception E that will be caught by an ancestor of the call_cleanup/2 Goal is raised, Cleanup is executed with continuation raise_exception(E).

3. Goal fails. Cleanup is executed with continuation fail.

4. Goal raises an exception E. Cleanup is executed with continuation raise_ exception(E).

In a typical use of call_cleanup/2, Cleanup succeeds deterministically after performing some side-effect; otherwise, unexpected behavior may result.

Note that the Prolog top level operates as a read-execute-fail loop, which backtracks into or cuts the query when the user types ; or h

RET i respectively.

Also, the predicates halt/0, abort/0, and reinitialise/0 are implemented in terms of exceptions. All of these circumstances can trigger the execution of

Cleanup.

7.5 Error and Exception Handling

The built-in predicates described in this section are used to alter the control flow to meet exception and error conditions. The equivalent of a raise_exception/1 is also executed by the built-in predicates when errors occur.

catch(:ProtectedGoal,?Pattern,:Handler) on_exception(?Pattern,:ProtectedGoal,:Handler) throw(+Exception) raise_exception(+Exception)

[ISO]

[ISO] catch/3 is the same as on_exception/3 (but note different argument order), and throw/1 is the same as raise_exception/1. on_exception/3 calls ProtectedGoal.

If this succeeds or fails, so does the call to on_exception/3.

If however, during the execution of ProtectedGoal, there is a call to raise_ exception(Exception), then Exception is copied and the stack is unwound back to the call to on_exception/3, whereupon the copy of Exception is unified with Pattern. If this unification succeeds, then on_exception/3 calls the goal Handler in order to determine the success or failure of on_exception/3.

Otherwise, the stack keeps unwinding, looking for an earlier invocation of on_ exception/3. Exception may be any term.

In a development system, any previously uncaught exception is caught and an appropriate error message is printed before returning to the top level. In recursive calls to Prolog from

C, uncaught exceptions are returned back to C instead. The printing of these and other messages in a development system is handled by the predicate print_message/2. The behavior of this predicate can be overridden by defining user:portray_message/2, so as to suppress or alter the format of certain messages. These predicates work as follows: print_message(+Severity, +Message)

[Hookable]

Most messages from the system are printed by calling this predicate. Before anything is printed, however, print_message/2 calls user:portray_message/2

Chapter 7: Built-In Predicates 135 with the same arguments, so as to give the user a means of intercepting the message before it is actually printed. If user:portray_message/2 succeeds, nothing is printed, otherwise Message is formatted and printed using the default method. In runtime systems, the message is printed unformatted.

Message is a term that encodes the message to be printed. The format of message terms is subject to change, but can be inspected in the file ‘Bips/msgs.pl’ of the SICStus Prolog distribution. Severity is a term denoting the severity of the message, and is one of: force(Severity )

Message should be printed without calling the user:portray_ message/2 hook. This is useful if user:portray_message/2 has intercepted the message, and now wants to print a reformatted version of it using print_message/2.

error

Message is an uncaught exception. The execution will normally be aborted and return to the top level. Syntax errors and exceptions that occur while loading files do not necessarily abort the execution, however.

warning

Message is a warning (e.g. singleton variables).

informational

Message provides information e.g. about files being loaded. Such messages are suppressed in runtime systems, but can be intercepted by user:portray_message/2.

help

Message is normally a response to a query.

portray_message(+Severity, +Message)

[Hook] user:portray_message(+Severity, +Message)

Called by print_message/2 before printing the message. If this succeeds, the default message for printing Message is overridden, and nothing more is printed.

The format of the exception raised by the built-in predicates depends on the execution mode. In iso execution mode the format is error(ISO Error, SICStus Error) where ISO Error is the error term prescribed by the ISO Prolog standard, while SICStus Error is the part defined by the standard to be implementation dependent. In case of

SICStus Prolog this is the SICStus error term, which normally contains additional information, such as the goal and the argument number causing the error.

In sicstus execution mode, the SICStus error term is used when raising an exception in a built-in predicate.

The list below itemizes the error terms, showing the ISO Error and SICStus Error form of each one, in that order. Note that the SICStus and ISO error terms do not always belong to the same error class, and that the context and consistency error classes are extensions to the ISO Prolog standard.

136 SICStus Prolog

The goal part of the error term may optionally have the form [email protected](Callable,PC) where PC is an internal encoding of the line of code containing the culprit goal or one of its ancestors.

instantiation_error instantiation_error(Goal,ArgNo)

Goal was called with insufficiently instantiated variables.

type_error(TypeName,Culprit) type_error(Goal,ArgNo,TypeName,Culprit)

Goal was called with the wrong type of argument(s). TypeName is the expected type and Culprit what was actually found.

domain_error(Domain,Culprit) domain_error(Goal,ArgNo,Domain,Culprit)

Goal was called with argument(s) of the right type but with illegal value(s).

Domain is the expected domain and Culprit what was actually found.

existence_error(ObjectType,Culprit) existence_error(Goal,ArgNo,ObjectType,Culprit,Reserved)

Something does not exist as indicated by the arguments. If the unknown-flag

(see prolog_flag/3) is set to error, this error is raised with ArgNo set to 0 when an undefined predicate is called.

permission_error(Operation,ObjectType,Culprit) permission_error(Goal,Operation,ObjectType,Culprit,Reserved)

The Operation is not permitted on Culprit of the ObjectType.

context_error(ContextType,CommandType) context_error(Goal,ContextType,CommandType)

The CommandType is not permitted in ContextType.

syntax_error(Message) syntax_error(Goal,Position,Message,Tokens,AfterError)

A syntax error was found when reading a term with read/[1,2] or assembling a number from its characters with number_chars/2. In the former case this error is raised only if the syntax_errors flag (see prolog_flag/3) is set to error.

evaluation_error(ErrorType,Culprit) evaluation_error(Goal,ArgNo,ErrorType,Culprit)

An incorrect arithmetic expression was evaluated. Only occurs in iso execution mode.

representation_error(ErrorType) representation_error(Goal,ArgNo,ErrorType)

A representation error occurs when the program tries to compute some welldefined value which cannot be represented, such as a compound term with arity

> 255.

consistency_error(Culprit1,Culprit2,Message) consistency_error(Goal,Culprit1,Culprit2,Message)

A consistency error occurs when two otherwise valid values or operations have been specified which are inconsistent with each other.

Chapter 7: Built-In Predicates 137 resource_error(ResourceType) resource_error(Goal,ResourceType)

A resource error occurs when SICStus Prolog has insufficient resources to complete execution. Currently no such error is raised.

system_error system_error(Message)

An error occurred while dealing with the operating system.

It is possible to handle a particular kind of existence errors locally: calls to undefined predicates. This can be done by defining clauses for: unknown_predicate_handler(+Goal,+Module,-NewGoal)

[Hook] user:unknown_predicate_handler(+Goal,+Module,-NewGoal)

Called as a result of a call to an undefined predicate. Goal is bound to the goal of the undefined predicate and Module to the module where the call was made. If this predicate succeeds, Module:NewGoal is called; otherwise, the action taken is governed by the unknown Prolog flag.

The following example shows an auto-loader for library packages: user:unknown_predicate_handler(Goal, Module, Goal) :functor(Goal, Name, Arity), require(Module:(Name/Arity)).

7.6 Information about the State of the Program

listing

Lists onto the current output stream all the clauses in the current interpreted program (in the type-in module; see

Section 4.2 [Module Spec], page 55

).

Clauses listed onto a file can be consulted back.

listing(:Spec)

Lists all interpreted predicates covered by the generalized predicate spec Spec.

For example:

| ?- listing([concatenate/3, reverse, m:go/[2-3], bar:_]).

current_atom(?Atom)

Atom is an atom known to SICStus Prolog. Can be used to enumerate (through backtracking) all currently known atoms, and return each one as Atom.

current_predicate(?Name,:Head) current_predicate(?Name,-Head)

Name is the name of a user defined or library predicate, and Head is the most general goal for that predicate, possibly prefixed by a module name.

This predicate can be used to enumerate all user defined or library predicates through backtracking.

138 SICStus Prolog current_predicate(?Name/?Arity )

[ISO]

Name is the name of a user defined or library predicate, possibly prefixed by a module name and Arity is its arity. This predicate can be used to enumerate all user defined or library predicates through backtracking.

predicate_property(:Head,?Property ) predicate_property(-Head,?Property )

Head is the most general goal for an existing predicate, possibly prefixed by a module name, and Property is a property of that predicate, where the possible properties are

• one of the atoms built_in (for built-in predicates) or compiled or interpreted (for user defined predicates) or fd_constraint for FD predicates see

Section 32.9 [Defining Primitive Constraints], page 358

.

• the atom dynamic for predicates that have been declared dynamic (see

Section 5.2.2 [Dynamic Declarations], page 63

),

• the atom multifile for predicates that have been declared multifile (see

Section 5.2.1 [Multifile Declarations], page 62 ),

• the atom volatile for predicates that have been declared volatile (see

Section 5.2.3 [Volatile Declarations], page 63 ),

• one or more terms (block Term) for predicates that have block declarations (see

Section 5.2.5 [Block Declarations], page 64

),

• the atom exported or terms imported_from(ModuleFrom) for predicates exported or imported from modules (see

Chapter 4 [Module Intro], page 55

),

• the term (meta_predicate Term) for predicates that have meta-predicate declarations (see

Section 4.6 [Meta Decl], page 58 ).

This predicate can be used to enumerate all existing predicates and their properties through backtracking.

current_module(?Module)

Module is a module in the system. It can be used to backtrack through all modules present in the system.

current_module(?Module, ?File)

Module is the module defined in File.

module(+Module)

The type-in module is set to Module.

set_prolog_flag(+FlagName,+NewValue)

[ISO] prolog_flag(+FlagName,?OldValue,?NewValue)

OldValue is the value of the Prolog flag FlagName, and the new value of

FlagName is set to NewValue.

The possible Prolog flag names and values are: agc_margin

An integer Margin. The atoms will be garbage collected when Margin new atoms have been created since the last atom garbage collection. Initially 10000.

Chapter 7: Built-In Predicates 139 argv

A read-only flag. The value is a list of atoms of the program arguments supplied when the current SICStus Prolog process was started. For example, if SICStus Prolog were invoked with:

% sicstus -a hello world 2001 then the value will be [hello,world,’2001’].

bounded

[ISO]

A read-only flag, one of the flags defining the integer type. For

SICStus, its value is false, indicating that the domain of integers is practically unbounded.

char_conversion

[ISO]

If this flag is on, unquoted characters in terms and programs read in will be converted, as specified by previous invocations of char_ conversion/2. If the flag is off no conversion will take place. The default value is on.

compiling

Governs the mode in which compile/1 and fcompile/1 operate

(see

Chapter 5 [Load Intro], page 59

).

compactcode

Compilation produces byte-coded abstract instructions

(the default).

fastcode

Compilation produces native machine instructions.

Currently only available for Sparc platforms.

profiledcode

Compilation produces byte-coded abstract instructions instrumented to produce execution profiling data.

debugcode

Compiling is replaced by consulting.

debugging debug

Corresponds to the predicates debug/0, nodebug/0, trace/0, notrace/0, zip/0, nozip/0 (see

Section 7.14 [Debug Pred], page 157

).

The flag describes the mode the debugger is in, or is required to be switched to: trace

Trace mode (the debugger is creeping).

debug

Debug mode (the debugger is leaping).

zip off

Zip mode (the debugger is zipping).

The debugger is switched off (the default).

[ISO]

The flag debug, prescribed by the ISO Prolog standard, is a simplified form of the debugging flag: off

The debugger is switched off (the default).

140 SICStus Prolog on

The debugger is switched on (to trace mode, if previously switched off).

(The flags debugging and debug are not available in runtime systems.) double_quotes

[ISO]

Governs the interpretation of double quoted strings (see

Section 3.1.1.5 [Compound Terms], page 40

): codes chars atom

List of character codes comprising the string.

List of one-char atoms comprising the string.

The atom composed of the same characters as the string.

character_escapes on or off.

If this flag is on, a backslash occurring inside integers in ‘0’’ notation or inside quoted atoms or strings has special meaning, and indicates the start of an escape sequence (see

[Escape

Sequences], page 578

). This flag is relevant when reading as well as when writing terms, and is initially on.

debugger_print_options

The value is a list of options for write_term/3 (see

Section 7.1.3

[Term I/O], page 108

), to be used in the debugger’s messages.

Not available in runtime systems.

The initial value is

[quoted(true),numbervars(true),portrayed(true),max_ depth(10)].

discontiguous_warnings on or off. Enable or disable warning messages when clauses are not together in source files. Initially on. (This warning is always disabled in runtime systems.) fileerrors on or off.

Enables or disables raising of file error exceptions.

Equivalent to fileerrors/0 and nofileerrors/0, respectively

(see

Section 7.1.5 [Stream Pred], page 118 ). Initially on (enabled).

gc on or off.

Enables or disables garbage collection of the global stack. Initially on (enabled).

gc_margin

Margin: At least Margin kilobytes of free global stack space are guaranteed to exist after a garbage collection. Also, no garbage collection is attempted unless the global stack is at least Margin kilobytes. Initially 1000.

gc_trace

Governs global stack garbage collection trace messages.

verbose

Turn on verbose tracing of garbage collection.

Chapter 7: Built-In Predicates 141 terse off

Turn on terse tracing of garbage collection.

Turn off tracing of garbage collection (the default).

host_type

A read-only flag. The value is an atom identifying the platform on which SICStus was compiled, such as ’x86-linux-glibc2.1’ or

’sparc-solaris-5.6’.

integer_rounding_function

[ISO]

A read-only flag, one of the flags defining the integer type.

In

SICStus Prolog its value is toward_zero, indicating that the integer division ((//)/2) and integer remainder (rem/2) arithmetic functions use rounding toward zero; see

Section 7.2 [Arithmetic], page 125 .

language iso or sicstus. Selects the execution mode specified.

max_arity

[ISO]

A read-only flag, specifying the maximum arity allowed for a compound term. In SICStus Prolog this is 255.

max_integer

[ISO]

A read-only flag, specifying the largest possible integer value. As in SICStus Prolog the range of integers in not bounded, prolog_ flag/3 and current_prolog_flag/2 will fail, when accessing this flag.

min_integer

[ISO]

A read-only flag, specifying the smallest possible integer value. As in SICStus Prolog the range of integers in not bounded, prolog_ flag/3 and current_prolog_flag/2 will fail, when accessing this flag.

redefine_warnings on or off. Enable or disable warning messages when :

• a module or predicate is being redefined from a different file than its previous definition. Such warnings are currently not issued when a ‘.po’ file is being loaded.

• a predicate is being imported while it was locally defined already.

• a predicate is being redefined locally while it was imported already.

• a predicate is being imported while it was imported from another module already.

Initially on. (This warning is always disabled in runtime systems.) single_var_warnings on or off. Enable or disable warning messages when a clause containing variables not beginning with _ occurring once only is compiled or consulted. Initially on.

142 SICStus Prolog source_info emacs or on or off. If not off while source code is being loaded, information about line numbers and filenames are stored with the loaded code. If the value is on while debugging, this information is used to print the source code location while prompting for a debugger command. If the value is on while printing an uncaught error exception message, the information is used to print the source code location of the culprit goal or one of its ancestors, as far as it can be determined. If the value is emacs in any of these cases, the appropriate line of code is instead highlighted, and no extra text is printed. The value is off initially, and that is its only available value in runtime systems.

syntax_errors

Controls what action is taken upon syntax errors in read/[1,2].

dec10

The syntax error is reported and the read is repeated.

error fail quiet

An exception is raised. See

Section 7.5 [Exception], page 134 . (the default).

The syntax error is reported and the read fails.

The read quietly fails.

system_type

A read-only flag. The value is development in development systems and runtime in runtime systems.

toplevel_print_options

The value is a list of options for write_term/3 (see

Section 7.1.3

[Term I/O], page 108 ), to be used when the top level dis-

plays variable bindings, answer constraints, and uncaught exceptions.

Not available in runtime systems.

The initial value is

[quoted(true),numbervars(true),portrayed(true),max_ depth(10)].

typein_module

Permitted values are atoms. Controls the current type-in module

(see

Section 4.2 [Module Spec], page 55 ). Corresponds to the pred-

icate module/1.

unknown

[ISO]

Corresponds to the predicate unknown/2 (see

Section 7.14 [Debug

Pred], page 157 ).

trace

Causes calls to undefined predicates to be reported and the debugger to be entered at the earliest opportunity.

(This setting is not possible in runtime systems.) fail

Causes calls to such predicates to fail.

warning

Causes calls to such predicates to display a warning message and then fail.

Chapter 7: Built-In Predicates 143 error

Causes calls to such predicates to raise an exception

(the default). See

Section 7.5 [Exception], page 134 .

user_input

Permitted values are any stream opened for reading.

Controls which stream is referenced by user_input and SP_stdin. It is initially set to a stream connected to UNIX stdin.

user_output

Permitted values are any stream opened for writing. Controls which stream is referenced by user_output and SP_stdout. It is initially set to a stream connected to UNIX stdout.

user_error

Permitted values are any stream opened for writing. Controls which stream is referenced by user_error and SP_stderr. It is initially set to a stream connected to UNIX stderr.

version

A read-only flag. The value is an atom containing the banner text displayed on startup and reinitialization, such as ’SICStus 3 #0:

Wed Mar 15 12:29:29 MET 1995’.

wcx

The value of the flag is the default term to be passed to the wide character extension component; see

Section 10.3 [Prolog Level

WCX Features], page 223

.

prolog_flag(?FlagName,?Value) current_prolog_flag(?FlagName,?Value)

[ISO]

Value is the current value of the Prolog flag FlagName. Can be used to enumerate all Prolog flags and their values by backtracking.

prolog_load_context(?Key,?Value)

This predicate gives access to context variables during compilation and loading of Prolog files. It unifies Value with the value of the variable identified by Key.

Possible keys are: source

The absolute path name of the file being compiled. During loading of a ‘.po’ or ‘.ql’ file, the corresponding source file name is returned.

file

Outside included files (see

Section 5.2.10 [Include Declarations], page 65 ) this is the same as the source key. In included files this

is the absolute path name of the file being included.

directory module

The absolute path name of the directory of the file being compiled/loaded. In included files this is the directory of the file being included.

The source module (see

Section 4.5 [Meta Exp], page 57

). This is useful for example if you are defining clauses for user:term_ expansion/[2,4] and need to access the source module at compile time.

144 SICStus Prolog stream

The stream being compiled or loaded from.

term_position

A term representing the position of the last clause read (see

Section 7.1.5 [Stream Pred], page 118 ).

statistics

Displays on the standard error stream statistics relating to memory usage, run time, garbage collection of the global stack and stack shifts.

statistics(?Key,?Value)

This allows a program to gather various execution statistics. For each of the possible keys Key, Value is unified with a list of values, as follows: global_stack

[size used,free]

This refers to the global stack, where compound terms are stored.

The values are gathered before the list holding the answers is allocated.

local_stack

[size used,free]

This refers to the local stack, where recursive predicate environments are stored.

trail choice

[size used,free]

This refers to the trail stack, where conditional variable bindings are recorded.

[size used,free]

This refers to the choicepoint stack, where partial states are stored for backtracking purposes.

core memory [size used,0]

These refer to the amount of memory actually allocated by the process.

heap program [size used,0]

These refer to the amount of memory allocated for compiled and interpreted clauses, symbol tables, and the like.

runtime [since start of Prolog,since previous statistics]

These refer to

CPU time used while executing, excluding time spent garbage collecting, stack shifting, or in system calls.

walltime [since start of Prolog,since previous statistics] These refer to absolute time elapsed.

garbage_collection

[no. of GCs,bytes freed,time spent] stack_shifts

[no. of local shifts,no. of trail shifts,time spent]

Chapter 7: Built-In Predicates 145 atoms [no. of atoms,bytes used,bytes free] atom_garbage_collection

[no. of AGCs,bytes freed,time spent]

Times are in milliseconds, sizes of areas in bytes.

trimcore

Reclaims any dead clauses and predicates, defragmentizes Prolog’s memory, and attempts to return any unused memory to the operating system. It is called automatically at every top level query.

7.7 Meta-Logic

The predicates in this section are meta-logical and perform operations that require reasoning about the current instantiation of terms or decomposing terms into their constituents. Such operations cannot be expressed using predicate definitions with a finite number of clauses.

var(?X )

[ISO]

Tests whether X is currently uninstantiated (var is short for variable). An uninstantiated variable is one which has not been bound to anything, except possibly another uninstantiated variable. Note that a compound term with some components which are uninstantiated is not itself considered to be uninstantiated. Thus the query

| ?- var(foo(X, Y )).

always fails, despite the fact that X and Y are uninstantiated.

nonvar(?X )

Tests whether X is currently instantiated. This is the opposite of var/1.

[ISO] ground(?X )

Tests whether X is completely instantiated, i.e. free of unbound variables. In this context, mutable terms are treated as nonground, so as to make ground/1 a monotone predicate.

atom(?X ) float(?X )

[ISO]

Checks that X is currently instantiated to an atom (i.e. a non-variable term of arity 0, other than a number).

[ISO]

Checks that X is currently instantiated to a float.

integer(?X )

Checks that X is currently instantiated to an integer.

[ISO] number(?X )

Checks that X is currently instantiated to a number.

[ISO] atomic(?X )

Checks that X is currently instantiated to an atom or number.

[ISO] simple(?X )

Checks that X is currently uninstantiated or instantiated to an atom or number.

146 SICStus Prolog compound(?X )

Checks that X is currently instantiated to a compound term.

[ISO] callable(?X )

Checks that X is currently instantiated to a term valid as a goal i.e. a compound term or an atom.

is_mutable(?X )

Checks that X is currently instantiated to a mutable term (see

Section 7.8

[Modify Term], page 149

).

functor(+Term,?Name,?Arity ) functor(?Term,+Name,+Arity )

[ISO]

[ISO]

The principal functor of term Term has name Name and arity Arity, where

Name is either an atom or, provided Arity is 0, a number. Initially, either Term must be instantiated, or Name and Arity must be instantiated to, respectively, either an atom and an integer in [0,255] or an atomic term and 0. In the case where Term is initially uninstantiated, the result of the call is to instantiate

Term to the most general term having the principal functor indicated.

arg(+ArgNo,+Term,?Arg )

[ISO]

Arg is the argument ArgNo of the compound term Term. The arguments are numbered from 1 upwards, ArgNo must be instantiated to a positive integer and Term to a compound term.

+Term =.. ?List

[ISO]

?Term =.. +List [ISO]

List is a list whose head is the atom corresponding to the principal functor of

Term, and whose tail is a list of the arguments of Term. e.g.

| ?- product(0, n, n-1) =.. L.

L = [product,0,n,n-1]

| ?- n-1 =.. L.

L = [-,n,1]

| ?- product =.. L.

L = [product]

If Term is uninstantiated, then List must be instantiated either to a list of determinate length whose head is an atom, or to a list of length 1 whose head is a number. Note that this predicate is not strictly necessary, since its functionality can be provided by arg/3 and functor/3, and using the latter two is usually more efficient.

name(+Const,?CharList) name(?Const,+CharList)

[Obsolescent]

[Obsolescent]

If Const is an atom or number, CharList is a list of the character codes of the characters comprising the name of Const. e.g.

Chapter 7: Built-In Predicates 147

| ?- name(product, L).

L = [112,114,111,100,117,99,116]

| ?- name(product, "product").

| ?- name(1976, L).

L = [49,57,55,54]

| ?- name(’1976’, L).

L = [49,57,55,54]

| ?- name((:-), L).

L = [58,45]

If Const is uninstantiated, CharList must be instantiated to a list of character codes. If CharList can be interpreted as a number, Const is unified with that number, otherwise with the atom whose name is CharList. E.g.

| ?- name(X, [58,45]).

X = :-

| ?- name(X, ":-").

X = :-

| ?- name(X, [49,50,51]).

X = 123

Note that there atoms are for which name(Const,CharList) is true, but which will not be constructed if name/2 is called with Const uninstantiated. One such atom is the atom ’1976’. It is recommended that new programs use atom_ codes/2 or number_codes/2, as these predicates do not have this inconsistency.

atom_codes(+Const,?CodeList) atom_codes(?Const,+CodeList)

[ISO]

[ISO]

The same as name(Const,CodeList), but Const is constrained to be an atom.

number_codes(+Const,?CodeList)

[ISO] number_codes(?Const,+CodeList)

[ISO]

The same as name(Const,CodeList), but Const is constrained to be a number.

atom_chars(+Const,?CharList)

[ISO only] atom_chars(?Const,+CharList)

[ISO only]

Analogous to atom_codes/2, but CharList is a list of one-char atoms, rather than of character codes.

148 SICStus Prolog atom_chars(+Const,?CodeList) atom_chars(?Const,+CodeList)

The same as atom_codes(Const,CharList).

char_code(+Char,?Code) char_code(?Char,+Code)

Code is the character code of the one-char atom Char.

[SICStus only]

[SICStus only] number_chars(+Const,?CharList) number_chars(?Const,+CharList)

[ISO only]

[ISO only]

Analogous to number_codes/2, but CharList is a list of one-char atoms, rather than of character codes.

number_chars(+Const,?CodeList) number_chars(?Const,+CodeList)

The same as number_codes(Const,CharList).

[SICStus only]

[SICStus only]

[ISO]

[ISO] atom_length(+Atom,?Length)

Length is the number of characters of the atom Atom.

[ISO] atom_concat(+Atom1,+Atom2,?Atom12)

[ISO] atom_concat(?Atom1,?Atom2,+Atom12)

[ISO]

The characters of the atom Atom1 concatenated with those of Atom2 are the same as the characters of atom Atom12. If the last argument is instantiated, nondeterministically enumerates all possible atom-pairs that concatenate to the given atom, e.g.

| ?- atom_concat(A, B, ’ab’).

A = ’’,

B = ab ? ;

A = a,

B = b ? ;

A = ab,

B = ’’ ? ; no sub_atom(+Atom,?Before,?Length,?After,?SubAtom)

[ISO]

The characters of SubAtom form a sublist of the characters of Atom, such that the number of characters preceding SubAtom is Before, the number of characters after SubAtom is After, and the length of SubAtom is Length. Capable of nondeterministically enumerating all sub-atoms and their all possible placements, e.g.

| ?- sub_atom(abrakadabra, Before, _, After, ab).

After = 9,

Before = 0 ? ;

After = 2,

Chapter 7: Built-In Predicates 149

Before = 7 ? ; no copy_term(?Term,?CopyOfTerm)

[ISO]

CopyOfTerm is a renaming of Term, such that brand new variables have been substituted for all variables in Term. If any of the variables of Term have goals blocked on them, the copied variables will have copies of the goals blocked on them as well. Similarly, independent copies are substituted for any mutable terms in term. It behaves as if defined by: copy_term(X, Y) :assert(’copy of’(X)), retract(’copy of’(Y)).

The implementation of copy_term/2 conserves space by not copying ground subterms.

7.8 Modification of Terms

One of the tenets of logic programming is that terms are immutable objects of the Herbrand universe, and the only sense in which they can be modified is by means of instantiating non-ground parts. There are, however, algorithms where destructive assignment is essential for performance. Although alien to the ideals of logic programming, this feature can be defended on practical grounds.

SICStus Prolog provides an abstract datatype and three operations for efficient backtrackable destructive assignment.

In other words, any destructive assignments are transparently undone on backtracking. Modifications that are intended to survive backtracking must be done by asserting or retracting dynamic program clauses instead. Unlike previous releases of SICStus Prolog, destructive assignment of arbitrary terms is not allowed.

A mutable term is represented as a compound terms with a reserved functor:

’$mutable’(Value,Timestamp) where Value is the current value and Timestamp is reserved for bookkeeping purposes [Aggoun & Beldiceanu 90].

Any copy of a mutable term created by copy_term/2, assert, retract, an internal database predicate, or an all solutions predicate, is an independent copy of the original mutable term.

Any destructive assignment done to one of the copies will not affect the other copy.

The following operations are provided: create_mutable(+Datum,-Mutable)

Mutable is a new mutable term with initial value Datum. Datum must not be an unbound variable.

get_mutable(?Datum,+Mutable)

Datum is the current value of the mutable term Mutable.

150 SICStus Prolog update_mutable(+Datum,+Mutable)

Updates the current value of the mutable term Mutable to become Datum.

Datum must not be an unbound variable.

is_mutable(?Mutable)

Checks that Mutable is currently instantiated to a mutable term.

NOTE: the effect of unifying two mutables is undefined.

7.9 Modification of the Program

The predicates defined in this section allow modification of dynamic predicates. Dynamic clauses can be added (asserted) or removed from the program (retracted).

For these predicates, the argument Head must be instantiated to an atom or a compound term, with an optional module prefix. The argument Clause must be instantiated either to a term Head :- Body or, if the body part is empty, to Head, with an optional module prefix. An empty body part is represented as true.

Note that a term Head :- Body must be enclosed in parentheses when it occurs as an argument of a compound term, as ‘:-’ is a standard infix operator with precedence greater than 1000 (see

Section 3.6 [Operators], page 49 ), e.g.:

| ?- assert((Head :- Body )).

Like recorded terms (see

Section 7.10 [Database], page 152 ), the clauses of dynamic predi-

cates have a unique implementation-defined identifier. Some of the predicates below have an additional argument which is this identifier. This identifier makes it possible to access clauses directly instead of requiring a normal database (hash-table) lookup.

assert(:Clause) assert(:Clause,-Ref )

The current instance of Clause is interpreted as a clause and is added to the current interpreted program. The predicate concerned must currently be dynamic or undefined and the position of the new clause within it is implementationdefined. Ref is a database reference to the asserted clause. Any uninstantiated variables in the Clause will be replaced by new private variables, along with copies of any subgoals blocked on these variables (see

Section 3.3 [Procedural], page 45 ).

asserta(:Clause)

[ISO] asserta(:Clause,-Ref )

Like assert/2, except that the new clause becomes the first clause for the predicate concerned.

assertz(:Clause)

[ISO] assertz(:Clause,-Ref )

Like assert/2, except that the new clause becomes the last clause for the predicate concerned.

Chapter 7: Built-In Predicates 151 clause(:Head,?Body )

[ISO] clause(:Head,?Body,?Ref ) clause(?Head,?Body,+Ref )

The clause (Head :- Body) exists in the current interpreted program, and its database reference is Ref. The predicate concerned must currently be dynamic.

At the time of call, either Ref must be instantiated, or Head must be instantiated to an atom or a compound term. Thus clause/3 can have two different modes of use.

retract(:Clause)

[ISO]

The first clause in the current interpreted program that matches Clause is erased. The predicate concerned must currently be dynamic. retract/1 may be used in a non-determinate fashion, i.e. it will successively retract clauses matching the argument through backtracking. If reactivated by backtracking, invocations of the predicate whose clauses are being retracted will proceed unaffected by the retracts. This is also true for invocations of clause/[2,3] for the same predicate. The space occupied by a retracted clause will be recovered when instances of the clause are no longer in use.

retractall(:Head)

Erases all clauses whose head matches Head, where Head must be instantiated to an atom or a compound term. The predicate concerned must currently be dynamic. The predicate definition is retained.

NOTE: all predicates mentioned above first look for a predicate that is visible in the module in which the call textually appears. If no predicate is found, a new dynamic predicate (with no clauses) is created automatically. It is recommended to declare as dynamic predicates for which clauses will be asserted.

abolish(:Spec) abolish(:Name,+Arity )

[ISO]

Abolishes the procedures specified by the generalized predicate spec Spec or

Name/Arity. Name may be prefixed by a module name (see

Section 4.2 [Module Spec], page 55 ). In iso execution mode only dynamic predicates can be

abolished. In sicstus execution mode only built-in predicates cannot be abolished, the user-defined ones always can be, even when static.

erase(+Ref )

The dynamic clause or recorded term (see

Section 7.10 [Database], page 152 )

whose database reference is Ref is effectively erased from the internal database or interpreted program.

instance(+Ref,?Term)

A (most general) instance of the dynamic clause or recorded term whose database reference is Ref is unified with Term.

152 SICStus Prolog

7.10 Internal Database

The predicates described in this section were introduced in early implementations of Prolog to provide efficient means of performing operations on large quantities of data. The introduction of indexed dynamic predicates have rendered these predicates obsolete, and the sole purpose of providing them is to support existing code. There is no reason whatsoever to use them in new code.

These predicates store arbitrary terms in the database without interfering with the clauses which make up the program. The terms which are stored in this way can subsequently be retrieved via the key on which they were stored. Many terms may be stored on the same key, and they can be individually accessed by pattern matching. Alternatively, access can be achieved via a special identifier which uniquely identifies each recorded term and which is returned when the term is stored.

recorded(?Key,?Term,?Ref )

[Obsolescent]

The internal database is searched for terms recorded under the key Key. These terms are successively unified with Term in the order they occur in the database.

At the same time, Ref is unified with the database reference to the recorded item. If the key is instantiated to a compound term, only its principal functor is significant. If the key is uninstantiated, all terms in the database are successively unified with Term in the order they occur.

recorda(+Key,?Term,-Ref )

[Obsolescent]

The term Term is recorded in the internal database as the first item for the key

Key, where Ref is its database reference. The key must be given, and only its principal functor is significant. Any uninstantiated variables in the Term will be replaced by new private variables, along with copies of any subgoals blocked on these variables (see

Section 3.3 [Procedural], page 45

).

recordz(+Key,?Term,-Ref )

[Obsolescent]

Like recorda/3, except that the new term becomes the last item for the key

Key.

current_key(?KeyName,?KeyTerm)

[Obsolescent]

KeyTerm is the most general form of the key for a currently recorded term, and

KeyName is the name of that key. This predicate can be used to enumerate in undefined order all keys for currently recorded terms through backtracking.

7.11 Blackboard Primitives

The predicates described in this section store arbitrary terms in a per-module repository known as the “blackboard”. The main purpose of the blackboard was initially to provide a means for communication between branches executing in parallel, but the blackboard works equally well during sequential execution. The blackboard implements a mapping from keys to values. Keys are restricted to being atoms or integers in the range [-33554432,

33554431], whereas values are arbitrary terms. In contrast to the predicates described in the previous sections, a given key can map to at most a single term.

Chapter 7: Built-In Predicates 153

Each Prolog module maintains its own blackboard, so as to avoid name clashes if different modules happen to use the same keys. The “key” arguments of these predicates are subject to module name expansion, so the module name does not have to be explicitly given unless multiple Prolog modules are supposed to share a single blackboard.

The predicates below implement atomic blackboard actions.

bb_put(:Key, +Term)

A copy of Term is stored under Key. Any previous term stored under the same

Key is simply deleted.

bb_get(:Key, ?Term)

If a term is currently stored under Key, a copy of it is unified with Term.

Otherwise, bb_get/2 silently fails.

bb_delete(:Key, ?Term)

If a term is currently stored under Key, the term is deleted, and a copy of it is unified with Term. Otherwise, bb_delete/2 silently fails.

bb_update(:Key, ?OldTerm, ?NewTerm)

If a term is currently stored under Key and unifies with OldTerm, the term is replaced by a copy of NewTerm. Otherwise, bb_update/3 silently fails. This predicate provides an atomic swap operation.

The following example illustrates how these primitives may be used to implement a “maxof” predicate that finds the maximum value computed by some non-determinate goal, which may execute in parallel. We use a single key max. Note the technique of using bb_update/3 in a repeat-fail loop, since other execution branches may be competing for updating the value, and we only want to store a new value if it is greater than the old value.

We assume that Goal does not produce any “false” solutions that would be eliminated by cuts in a sequential execution. Thus, Goal may need to include redundant checks to ensure that its solutions are valid, as discussed above.

maxof(Value, Goal, _) :bb_put(max, -1), call(Goal), update_max(Value), fail.

maxof(_, _, Max) :bb_delete(max, Max),

Max > 1.

% initialize max-so-far update_max(New):repeat, bb_get(max, Old), compare(C, Old, New), update_max(C, Old, New), !.

update_max(<, Old, New) :- bb_update(max, Old, New).

154 SICStus Prolog update_max(=, _, _).

update_max(>, _, _).

7.12 All Solutions

When there are many solutions to a problem, and when all those solutions are required to be collected together, this can be achieved by repeatedly backtracking and gradually building up a list of the solutions. The following built-in predicates are provided to automate this process.

Note that the Goal argument to the predicates listed below is called as if by call/1 at runtime. Thus if Goal is complex and if performance is an issue, define an auxiliary predicate which can then be compiled, and let Goal call it.

setof(?Template,:Goal,?Set)

[ISO]

Read this as “Set is the set of all instances of Template such that Goal is satisfied, where that set is non-empty”. The term Goal specifies a goal or goals as in call(Goal) (see

Section 7.4 [Control], page 131

). Set is a set of terms represented as a list of those terms, without duplicates, in the standard order for terms (see

Section 7.3 [Term Compare], page 129

). If there are no instances of Template such that Goal is satisfied then the predicate fails.

The variables appearing in the term Template should not appear anywhere else in the clause except within the term Goal. Obviously, the set to be enumerated should be finite, and should be enumerable by Prolog in finite time. It is possible for the provable instances to contain variables, but in this case the list Set will only provide an imperfect representation of what is in reality an infinite set.

If there are uninstantiated variables in Goal which do not also appear in Template, then a call to this built-in predicate may backtrack, generating alternative values for Set corresponding to different instantiations of the free variables of

Goal. (It is to cater for such usage that the set Set is constrained to be nonempty.) Two instantiations are different iff no renaming of variables can make them literally identical. For example, given the clauses: likes(bill, cider).

likes(dick, beer).

likes(harry, beer).

likes(jan, cider).

likes(tom, beer).

likes(tom, cider).

the query

| ?- setof(X, likes(X,Y), S).

might produce two alternative solutions via backtracking:

S = [dick,harry,tom],

Y = beer ? ;

S = [bill,jan,tom],

Y = cider ? ;

Chapter 7: Built-In Predicates 155

The query:

| ?- setof((Y,S), setof(X, likes(X,Y), S), SS).

would then produce:

SS = [(beer,[dick,harry,tom]),(cider,[bill,jan,tom])]

Variables occurring in Goal will not be treated as free if they are explicitly bound within Goal by an existential quantifier. An existential quantification is written:

Y ^Q meaning “there exists a Y such that Q is true”, where Y is some Prolog variable.

For example:

| ?- setof(X, Y^(likes(X,Y)), S).

would produce the single result:

S = [bill,dick,harry,jan,tom] in contrast to the earlier example.

Note that in iso execution mode, only outermost existential quantification is accepted, i.e. if the Goal argument is of form V1 ^ ... ^ N ^ SubGoal. In sicstus execution mode existential quantification is handled also deeper inside

Goal.

bagof(?Template,:Goal,?Bag )

[ISO]

This is exactly the same as setof/3 except that the list (or alternative lists) returned will not be ordered, and may contain duplicates. The effect of this relaxation is to save a call to sort/2, which is invoked by setof/3 to return an ordered list.

?X ^:P

The all solution predicates recognize this as meaning “there exists an X such that P is true”, and treats it as equivalent to P (see

Section 7.4 [Control], page 131

). The use of this explicit existential quantifier outside the setof/3 and bagof/3 constructs is superfluous and discouraged.

findall(?Template,:Goal,?Bag )

[ISO]

Bag is a list of instances of Template in all proofs of Goal found by Prolog. The order of the list corresponds to the order in which the proofs are found. The list may be empty and all variables are taken as being existentially quantified. This means that each invocation of findall/3 succeeds exactly once, and that no variables in Goal get bound. Avoiding the management of universally quantified variables can save considerable time and space.

findall(?Template,:Goal,?Bag,?Remainder)

Same as findall/3, except Bag is the list of solution instances appended with

Remainder, which is typically unbound.

7.13 Coroutining

The coroutining facility can be accessed by a number of built-in predicates. This makes it possible to use coroutines in a dynamic way, without having to rely on block declarations:

156 SICStus Prolog when(+Condition,:Goal)

Blocks Goal until the Condition is true, where Condition is a Prolog goal with the restricted syntax: nonvar(X ) ground(X )

?=(X,Y )

Condition,Condition

Condition;Condition

For example:

| ?- when(((nonvar(X);?=(X,Y)),ground(T)), process(X,Y,T)).

freeze(?X,:Goal)

Blocks Goal until nonvar(X ) (see

Section 7.7 [Meta Logic], page 145

) holds.

This is defined as if by: freeze(X, Goal) :- when(nonvar(X), Goal).

or

:- block freeze(-, ?).

freeze(_, Goal) :- Goal.

frozen(-Var,?Goal)

If some goal is blocked on the variable Var, or Var has attributes that can be interpreted as a goal (see

Chapter 15 [Attributes], page 253

), then that goal is unified with Goal. If no goals are blocked, Goal is unified with the atom true.

If more than one goal is blocked, a conjunction is unified with Goal.

dif(?X,?Y )

Constrains X and Y to represent different terms i.e. to be non-unifiable. Calls to dif/2 either succeed, fail, or are blocked depending on whether X and Y are sufficiently instantiated. It is defined as if by: dif(X, Y) :- when(?=(X,Y), X\==Y).

call_residue(:Goal,?Residue)

The Goal is executed as if by call/1. If after the execution there are still some subgoals of Goal that are blocked on some variables, then Residue is unified with a list of VariableSet-Goal pairs, and those subgoals are no longer blocked on any variables. Otherwise, Residue is unified with the empty list [].

VariableSet is a set of variables such that when any of the variables is bound,

Goal gets unblocked. Usually, a goal is blocked on a single variable, in which case VariableSet is a singleton.

Goal is an ordinary goal, sometimes module prefixed. For example:

| ?- call_residue((dif(X,f(Y)), X=f(Z)), Res).

X = f(Z),

Res = [[Y,Z]-(prolog:dif(f(Z),f(Y)))]

Chapter 7: Built-In Predicates 157

7.14 Debugging

Debugging predicates are not available in runtime systems.

unknown(?OldState,?NewState)

OldState is the current state of the “Action on unknown predicates” flag, and sets the flag to NewState. This flag determines whether or not the system is to catch calls to undefined predicates (see

Section 2.6 [Undefined Predicates], page 25 ), when user:unknown_predicate_handler/3 cannot handle the goal.

The possible states of the flag are: trace

Causes calls to undefined predicates to be reported and the debugger to be entered at the earliest opportunity. Not available in runtime systems.

fail

Causes calls to such predicates to fail.

warning

Causes calls to such predicates to display a warning message and then fail.

error

Causes calls to such predicates to raise an exception (the default).

See

Section 7.5 [Exception], page 134 .

debug

The debugger is switched on in leap mode. See

Section 6.2 [Basic Debug], page 71 .

trace

The debugger is switched on in creep mode. See

Section 6.2 [Basic Debug], page 71 .

zip

The debugger is switched on in zip mode.

See

Section 6.2 [Basic Debug], page 71 .

nodebug notrace nozip

The debugger is switched off. See

Section 6.2 [Basic Debug], page 71 .

leash(+Mode)

Leashing Mode is set to Mode. See

Section 6.2 [Basic Debug], page 71

.

spy :Spec

Plain spypoints are placed on all the predicates given by Spec. See

Section 6.3

[Plain Spypoint], page 73 .

spy(:Spec, :Conditions)

Spypoints with condition Conditions are placed on all the predicates given by

Spec. See

Section 6.7 [Breakpoint Predicates], page 84 .

nospy :Spec

All spypoints (plain and conditional) are removed from all the predicates given by Spec. See

Section 6.3 [Plain Spypoint], page 73

.

158 SICStus Prolog nospyall

Removes all the spypoints (including the generic ones) that have been set.

debugging

Displays information about the debugger.

See

Section 6.2 [Basic Debug], page 71 .

add_breakpoint(:Conditions, ?BID)

Creates a breakpoint with Conditions and with identifier BID. See

Section 6.7

[Breakpoint Predicates], page 84 .

current_breakpoint(:Conditions, ?BID, ?Status, ?Kind)

There is a breakpoint with conditions Conditions, identifier BID, enabledness

Status, and kind Kind. See

Section 6.7 [Breakpoint Predicates], page 84

.

remove_breakpoints(+BIDs) disable_breakpoints(+BIDs) enable_breakpoints(+BIDs)

Removes, disables or enables the breakpoints specified by BIDs. See

Section 6.7

[Breakpoint Predicates], page 84 .

execution_state(:Tests)

Tests are satisfied in the current state of the execution.

execution_state(+FocusConditions, :Tests)

Tests are satisfied in the state of the execution pointed to by FocusConditions.

debugger_command_hook(+Char,?Actions)

[Hook] user:debugger_command_hook(+Char,?Actions)

Allows the interactive debugger to be extended with user-defined commands.

See

Section 6.6 [Debug Commands], page 78

.

error_exception(+Exception)

[Hook] user:error_exception(+Exception)

Tells the debugger to enter trace mode on certain exceptions. See

Section 6.5

[Breakpoints], page 75 .

7.15 Execution Profiling

Execution profiling is a common aid for improving software performance. The SICStus

Prolog compiler has the capability of instrumenting compiled code with counters which are initially zero and incremented whenever the flow of control passes a given point in the compiled code. This way the number of calls, backtracks, choicepoints created, etc., can be counted for the instrumented predicates, and an estimate of the time spent in individual clauses and disjuncts can be calculated.

Gauge is a graphical user interface for inspecting execution profiles. It is available as a library module (see

Chapter 36 [Gauge Intro], page 521 ).

The original version of the profiling package was written by M.M. Gorlick and C.F. Kesselman at the Aerospace Corporation [Gorlick & Kesselman 87].

Chapter 7: Built-In Predicates 159

Only compiled code can be instrumented. To get an execution profile of a program, the compiler must first be told to produce instrumented code. This is done by issuing the query:

| ?- prolog_flag(compiling,_,profiledcode).

after which the program to be analyzed can be compiled as usual. Any new compiled code will be instrumented while the compilation mode flag has the value profiledcode.

The profiling data is generated by simply running the program. The predicate profile_ data/4 (see below) makes available a selection of the data as a Prolog term. The predicate profile_reset/1 zeroes the profiling counters for a selection of the currently instrumented predicates.

profile_data(:Spec,?Selection,?Resolution,-Data)

Data is profiling data collected from the predicates covered by the generalized predicate spec Spec.

The Selection argument determines the kind of profiling data to be collected. If uninstantiated, the predicate will backtrack over its possible values, which are: calls

All instances of entering a clause by a procedure call are counted.

This is equivalent to counting all procedure calls that have not been determined to fail by indexing on the first argument.

backtracks

All instances of entering a clause by backtracking are counted.

choice_points

All instances of creating a choicepoint are counted. This occurs, roughly, when the implementation determines that there are more than one possibly matching clauses for a procedure call, and when a disjunction is entered.

shallow_fails

Failures in the “if” part of if-then-else statements, and in the

“guard” part of guarded clauses, are counted as shallow failures.

See

Section 11.4 [If-Then-Else], page 240

.

deep_fails

Any failures that do not classify as shallow as above are counted as deep failures. The reason for distinguishing shallow and deep failures is that the former are considerably cheaper to execute than the latter.

execution_time

The execution time for the selected predicates, clauses, or disjuncts is estimated in artificial units.

The Resolution argument determines the level of resolution of the profiling data to be collected. If uninstantiated, the predicate will backtrack over its possible values, which are:

160 SICStus Prolog predicate clause

Data is a list of Module:PredName-Count, where Count is a sum of the corresponding counts per clause.

Data is a list of Module:ClauseName-Count, where Count includes counts for any disjunctions occurring inside that clause. Note, however, that the selections calls and backtracks do not include counts for disjunctions.

all

Data is a list of Module:InternalName-Count. This is the finest resolution level, counting individual clauses and disjuncts.

Above, PredName is a predicate spec, ClauseName is a compound term Pred-

Name/ClauseNumber, and InternalName is either

ClauseName—corresponding to a clause, or

(ClauseName-DisjNo)/Arity /AltNo—corresponding to a disjunct.

profile_reset(:Spec)

Zeroes all counters for predicates covered by the generalized predicate spec

Spec.

7.16 Miscellaneous

?X = ?Y

[ISO]

Defined as if by the clause Z=Z.; i.e. X and Y are unified.

?X \= ?Y

[ISO]

The same as \+ X = Y ; i.e. X and Y are not unifiable.

unify_with_occurs_check(?X, ?Y )

[ISO]

True if X and Y unify to a finite (acyclic) term. Runs in almost linear time.

length(?List,?Length)

If List is instantiated to a list of determinate length, then Length will be unified with this length.

If List is of indeterminate length and Length is instantiated to an integer, then

List will be unified with a list of length Length. The list elements are unique variables.

If Length is unbound then Length will be unified with all possible lengths of

List.

numbervars(?Term,+N,?M )

Unifies each of the variables in term Term with a special term, so that write(Term) (or writeq(Term)) (see

Section 7.1.3 [Term I/O], page 108 )

prints those variables as (A + (i mod 26))(i/26) where i ranges from N to M-1.

N must be instantiated to an integer. If it is 0 you get the variable names A, B,

. . . , Z, A1, B1, etc. This predicate is used by listing/[0,1] (see

Section 7.6

[State Info], page 137

).

halt

[ISO]

Causes Prolog to exit back to the shell. (In recursive calls to Prolog from C, this predicate will return back to C instead.)

Chapter 7: Built-In Predicates 161 halt(+Code)

[ISO]

Causes the Prolog process to immediately exit back to the shell with the integer return code Code, even if it occurs in a recursive call from C.

op(+Precedence,+Type,+Name)

[ISO]

Declares the atom Name to be an operator of the stated Type and Precedence

(see

Section 3.6 [Operators], page 49

). Name may also be a list of atoms in which case all of them are declared to be operators. If Precedence is 0 then the operator properties of Name (if any) are cancelled.

current_op(?Precedence,?Type,?Op)

[ISO]

The atom Op is currently an operator of type Type and precedence Precedence.

Neither Op nor the other arguments need be instantiated at the time of the call; i.e. this predicate can be used to generate as well as to test.

break

Invokes a recursive top-level. See

Section 2.9 [Nested], page 27

. (This predicate is not available in runtime systems.) abort

Aborts the current execution. See

Section 2.9 [Nested], page 27 . (In recursive

calls to Prolog from C, this predicate will return back to C instead.) save_files(+SourceFiles, +File)

Any module declarations, predicates, multifile clauses, or directives encountered in SourceFiles are saved in object format into the file File. Source file information as provided by source_file/[1,2] for the relevant predicates and modules is also saved.

If File does not have an explicit suffix, the suffix ‘.po’ will be appended to it.

SourceFiles should denote a single file or a list of files. File can later be loaded by load_files/[1,2], at which time any saved directives will be re-executed.

If any of the SourceFiles declares a module, File too will behave as a module-file and export the predicates listed in the first module declaration encountered in

SourceFiles. See

Section 2.10 [Saving], page 27 .

save_modules(+Modules, +File)

The module declarations, predicates, multifile clauses and initializations belonging to Modules are saved in object format into the file File. Source file information and arbitrary directives are not saved.

If File does not have an explicit suffix, the suffix ‘.po’ will be appended to it.

Modules should denote a single module or a list of modules. File can later be loaded by load_files/[1,2] and will behave as a module-file and export any predicates exported by the first module in Modules. See

Section 2.10 [Saving], page 27 .

save_predicates(:Spec, +File)

The predicates specified by the generalized predicate spec Spec are saved in object format into the file File. Source file information and directives are not saved. Thus, this predicate is intended for saving data represented as tables of dynamic facts, not for saving static code.

162 SICStus Prolog

If File does not have an explicit suffix, the suffix ‘.po’ will be appended to it. File can later be loaded by load_files/[1,2]. See

Section 2.10 [Saving], page 27 .

save_program(+File) save_program(+File, :Goal)

The system saves the program state into file File. When the program state is restored, Goal is executed. Goal defaults to true. See

Section 2.10 [Saving], page 27 .

restore(+File)

The system is returned to the program state previously saved to file File with start-up goal Goal. restore/1 may succeed, fail or raise an exception depending on Goal. See

Section 2.10 [Saving], page 27 .

reinitialise

This predicate can be used to force the reinitialization behavior to take place at any time. When SICStus Prolog is reinitialized it:

• calls any user defined C function pointed at by SP_reinit_hook (see

Section 8.6 [Hooks], page 190 ).

• runs any initializations.

• calls version/0 to write banners.

(In recursive calls to Prolog from C, this predicate will return back to C instead.) garbage_collect

Performs a garbage collection of the global stack immediately.

garbage_collect_atoms

Performs a garbage collection of the atoms immediately.

gc

Enables garbage collection of the global stack (the default).

nogc

Disables garbage collection of the global stack.

prompt(?Old,?New )

The sequence of characters (prompt) which indicates that the system is waiting for user input is represented as an atom, and unified with Old; the atom bound to New specifies the new prompt. In particular, the goal prompt(X, X) unifies the current prompt with X, without changing it. Note that this predicate only affects the prompt given when a user’s program is trying to read from the standard input stream (e.g. by calling read/1). Note also that the prompt is reset to the default ‘|: ’ on return to top-level.

version

Displays the introductory messages for all the component parts of the current system.

Prolog will display its own introductory message when initially run and on reinitialization by calling version/0. If this message is required at some other

Chapter 7: Built-In Predicates 163 time it can be obtained using this predicate which displays a list of introductory messages; initially this list comprises only one message (Prolog’s), but you can add more messages using version/1. (This predicate is not available in runtime systems.) version(+Message)

Appends Message to the end of the message list which is output by version/0.

Message must be an atom. (This predicate is not available in runtime systems.)

The idea of this message list is that, as systems are constructed on top of other systems, each can add its own identification to the message list. Thus version/0 should always indicate which modules make up a particular package.

It is not possible to remove messages from the list.

help

[Hookable]

Displays basic information, or a user defined help message.

It first calls user:user_help/0, and only if that call fails is a default help message printed on the current output stream. (This predicate is not available in runtime systems.) user_help

[Hook] user:user_help

This may be defined by the user to print a help message on the current output stream.

164 SICStus Prolog

Chapter 8: Mixing C and Prolog 165

8 Mixing C and Prolog

SICStus Prolog provides a bi-directional, procedural interface for program parts written in C and Prolog. The C side of the interface defines a number of functions and macros for various operations. On the Prolog side, you have to supply declarations specifying the names and argument/value types of C functions being called as Prolog predicates. These declarations are used by the predicate load_foreign_resource/1, which performs the actual binding of C functions to Prolog predicates.

In most cases, the argument/value type declaration suffice for making the necessary conversions of data automatically as they are passed between C and Prolog. However, it is possible to declare the type of an argument to be a Prolog term, in which case the receiving function will see it as a “handle” object, called an SP term ref, for which access functions are provided.

The C support routines are available in a development system as well as in runtime systems.

The support routines include:

• Static and dynamic linking of C code into the Prolog environment.

• Automatic conversion between Prolog terms and C data with foreign/[2,3] declarations.

• Functions for accessing and creating Prolog terms, and for creating and manipulating

SP term refs.

• The Prolog system may call C predicates which may call Prolog back without limits on recursion.

• Support for creating stand-alone executables.

• Support for creating user defined Prolog streams.

• Functions to read and write on Prolog streams from C.

• Functions to install interrupt handlers that can safely call Prolog.

• User hooks that can be used to perform user defined actions on a number of occasions e.g. before reading a character from the standard input stream, upon reinitialization, etc.

8.1 Notes

ANSI Conformance

Throughout this chapter, void * in the function definitions may be changed to char * on non ANSI conforming C compilers.

The SP PATH variable

It is normally not necessary to set this environment variable, but its value will be used at runtime if no explicit boot path is given when initializing a runtime or development System. In this chapter, the environment variable SP_PATH is used as a shorthand for the SICStus Prolog installation directory, whose default UNIX location is ‘/usr/local/lib/sicstus-3.8’). See

Section 2.1.1

[Environment Variables], page 20 .

166 SICStus Prolog

Definitions and declarations

Type definitions and function declarations for the interface are found in the header file ‘<sicstus/sicstus.h>’.

Error Codes

The value of many support functions is a return code which is one of SP_SUCCESS for success, SP_FAILURE for failure, SP_ERROR if an error condition occurred. In particular, uncaught exceptions resulting from calls from C to Prolog raise an error condition. In error situations, the macro SP_errno will return a value describing the error condition: int SP_errno

The function SP_error_message returns a pointer to the diagnostic message corresponding to a specified error number: char *SP_error_message(int errno)

Wide Characters

The foreign interface supports wide characters. Whenever a sequence of possibly wide character codes is to be passed to or from a C function it is encoded as a sequence of bytes, using the so called internal encoding of SICStus Prolog, the UTF-8 encoding; see

Section 10.2 [WCX Concepts], page 221 . Unless noted

otherwise the encoded form is terminated by a NULL byte. This sequence of bytes will be called an encoded string, representing the given sequence of character codes. Note that it is a property of the UTF-8 encoding that it does not change ASCII character code sequences.

If a foreign function is specified to return an encoded string, an exception will be raised if, on return to Prolog, the actual string is ill-formed (is not a valid sequence of UTF-8 encoded characters).

The exception raised is representation_error(...,...,mis_encoded_string).

8.2 Calling C from Prolog

Functions written in the C language may be called from Prolog using an interface in which automatic type conversions between Prolog terms and common C types are declared as

Prolog facts.

Calling without type conversion can also be specified, in which case the arguments and values are passed as SP term refs. This interface is partly modeled after

Quintus Prolog.

The functions installed using this foreign language interface may invoke Prolog code and use the support functions described in the other sections of this chapter.

Functions, or their equivalent, in any other language having C compatible calling conventions may also be interfaced using this interface. When referring to C functions in the following, we also include such other language functions. Note however that a C compiler is needed since a small amount of glue code (in C) must be generated for interfacing purposes.

Chapter 8: Mixing C and Prolog 167

8.2.1 Foreign Resources

A foreign resource is a set of C functions, defined in one or more files, installed as an atomic operation. The name of a foreign resource, the resource name, is an atom, which should uniquely identify the resource. Thus, two foreign resources with the same name cannot be installed at the same time.

For each foreign resource, a foreign_resource/2 fact is used to declare the interfaced functions. For each of these functions, a foreign/[2,3] fact is used to specify conversions between predicate arguments and C-types. These conversion declarations are used for creating the necessary interface between Prolog and C.

The functions making up the foreign resource, the automatically generated glue code, and any libraries, are compiled and linked, using the program splfr (see

Section 8.2.5 [The splfr utility], page 172 ) or link_foreign_resource/6 (see

Section 8.2.4 [Interface Predicates], page 171 ), to form a linked foreign resource. A linked foreign resource exists in two different

flavors, static and dynamic. A static resource is simply a relocatable object file containing the foreign code. A dynamic resource is a shared library (.so under most UNIX dialects,

.dll under Windows) which is loaded into the Prolog executable at runtime.

Foreign resources can be linked into the Prolog executable either when the executable is built (pre-linked), or at runtime. Pre-linking can be done using static or dynamic resources.

Runtime-linking can only be done using dynamic resources. Dynamic resources can also be unlinked.

In all cases, the declared predicates are installed by the built-in predicate load_foreign_ resource/1. The resource name of a linked foreign resource is derived from its filename by deleting any leading path and the suffix. If the resource was pre-linked, only the predicate names are bound, otherwise runtime-linking is attempted (using dlopen(), LoadLibrary(), or similar).

Static and pre-linked resources are not supported under Windows. See section “Windows

Notes” in SICStus Prolog Release Notes , for more information.

8.2.2 Conversion Declarations

Conversion declaration predicates: foreign_resource(+ResourceName,+Functions)

[Hook]

Specifies that a set of foreign functions, to be called from Prolog, are to be found in the resource named by ResourceName. Functions is a list of functions exported by the resource. Only functions that are to be called from Prolog and optionally one init function and one deinit function should be listed. The init and deinit functions are specified as init(Function) and deinit(Function) respectively (see

Section 8.2.6 [Init and Deinit Functions], page 173 ). This

predicate should be defined entirely in terms of facts (unit clauses) and will be called in the relevant module, i.e. not necessarily in the user module. For example:

168 SICStus Prolog foreign_resource(’terminal’, [scroll,pos_cursor,ask]).

specifies that functions scroll(), pos_cursor() and ask() are to be found in the resource ‘terminal’.

foreign(+CFunctionName, +Predicate) foreign(+CFunctionName, +Language, +Predicate)

[Hook]

[Hook]

Specify the Prolog interface to a C function. Language is at present constrained to the atoms c and java. CFunctionName is the name of a C function. Predicate specifies the name of the Prolog predicate that will be used to call CFunction(). Predicate also specifies how the predicate arguments are to be translated into the corresponding C arguments. These predicates should be defined entirely in terms of facts (unit clauses) and will be called in the relevant module, i.e. not necessarily in the user module. For example: foreign(pos_cursor, c, move_cursor(+integer, +integer)).

The above example says that the C function pos_cursor() has two integer value arguments and that we will use the predicate move_cursor/2 to call this function. A goal move_cursor(5, 23) would translate into the C call pos_ cursor(5,23);.

The third argument of the predicate foreign/3 specifies how to translate between Prolog arguments and C arguments. A call to a foreign predicate will raise an exception if an input arguments is uninstantiated (instantiation_ error/2) or has the wrong type (type_error/4) or domain (domain_error/4).

The call will fail upon return from the function if the output arguments do not unify with the actual arguments.

The available conversions are listed in the next subsection.

8.2.3 Conversions between Prolog Arguments and C Types

The following table lists the possible values for the arguments in the predicate specification of foreign/[2,3]. The value declares which conversion between corresponding Prolog argument and C type will take place. Note that the term chars below refers to a list of character codes, rather than to one-char atoms.

Prolog: +integer

C: long

The argument should be a number. It is converted to a C long and passed to the C function.

Prolog: +float

C: double The argument should be a number. It is converted to a C double and passed to the C function.

Prolog: +atom

C: unsigned long

The argument should be an atom. Its canonical representation is passed to the

C function.

Prolog: +chars

Chapter 8: Mixing C and Prolog 169

C: char *

The argument should be a list of character codes. The C function will be passed the address of an array with the encoded string representation of these characters. The array is subject to reuse by other support functions, so if the value is going to be used on a more than temporary basis, it must be moved elsewhere.

Prolog: +string

C: char *

The argument should be an atom. The C function will be passed the address of an encoded string representing the characters of the atom. The C function should not overwrite the string.

Prolog: +string(N )

C: char *

The argument should be an atom. The encoded string representing the atom will be copied into a newly allocated buffer. The string will be truncated (at wide character boundary) if it is longer than N bytes. The string will be blank padded on the right if it is shorter than N bytes. The C function will be passed the address of the buffer. The C function may overwrite the buffer, but should not assume that it remains valid after returning.

Prolog: +address

C: void *

The argument should be an integer which should be a valid second argument to

SP_put_address() (see

Section 8.3.2 [Creating Prolog Terms], page 176 ). The

value passed will be a void * pointer.

Prolog: +address(TypeName)

C: TypeName *

The argument should be an integer which should be a valid second argument to

SP_put_address() (see

Section 8.3.2 [Creating Prolog Terms], page 176 ). The

value passed will be a TypeName * pointer.

Prolog: +term

C: SP_term_ref

The argument could be any term. The value passed will be the internal representation of the term.

Prolog: -integer

C: long *

The C function is passed a reference to an uninitialized long. The value returned will be converted to a Prolog integer.

Prolog: -float

C: double *

The C function is passed a reference to an uninitialized double. The value returned will be converted to a Prolog float.

Prolog: -atom

C: unsigned long *

The C function is passed a reference to an uninitialized unsigned long. The value returned should be the canonical representation of a Prolog atom.

Prolog: -chars

170 SICStus Prolog

C: char **

The C function is passed the address of an uninitialized char *. The returned encoded string will be converted to a Prolog list of character codes.

Prolog: -string

C: char **

The C function is passed the address of an uninitialized char *. The returned encoded string will be converted to a Prolog atom. Prolog will copy the string to a safe place, so the memory occupied by the returned string may be reused during subsequent calls to foreign code.

Prolog: -string(N )

C: char *

The C function is passed a reference to a character buffer large enough to store an N bytes. The C function is expected to fill the buffer with an encoded string of N bytes (not NULL-terminated). This encoded string will be stripped of trailing blanks and converted to a Prolog atom.

Prolog: -address

C: void **

The C function is passed the address of an uninitialized void *. The returned value, which should be a valid second argument to SP_put_address() (see

Section 8.3.2 [Creating Prolog Terms], page 176

), will be converted to a Prolog integer.

Prolog: -address(TypeName)

C: TypeName **

The C function is passed the address of an uninitialized TypeName *. The returned value, which should be a valid second argument to SP_put_address()

(see

Section 8.3.2 [Creating Prolog Terms], page 176 ), will be converted to a

Prolog integer.

Prolog: -term

C: SP_term_ref

The C function is passed a new SP term ref, and is expected to set its value to a suitable Prolog term. Prolog will try to unify the value with the actual argument.

Prolog: [-integer]

C: long F()

The C function should return a long. The value returned will be converted to a Prolog integer.

Prolog: [-float]

C: double F()

The C function should return a double. The value returned will be converted to a Prolog float.

Prolog: [-atom]

C: unsigned long F()

The C function should return an unsigned long. The value returned must be the canonical representation of a Prolog atom.

Chapter 8: Mixing C and Prolog 171

Prolog: [-chars]

C: char *F()

The C function should return a char *. The returned encoded string will be converted to a Prolog list of character codes.

Prolog: [-string]

C: char *F()

The C function should return a char *. The returned encoded string will be converted to a Prolog atom. Prolog will copy the string to a safe place, so the memory occupied by the returned string may be reused during subsequent calls to foreign code.

Prolog: [-string(N )]

C: char *F()

The C function should return a char *. The first N bytes of the encoded string

(not necessarily NULL-terminated) will be copied and the copied string will be stripped of trailing blanks. The stripped string will be converted to a Prolog atom. C may reuse or destroy the string buffer during later calls.

Prolog: [-address]

C: void *F()

The C function should return a void *. The returned value, which should be a valid second argument to SP_put_address() (see

Section 8.3.2 [Creating

Prolog Terms], page 176 ), will be converted to a Prolog integer.

Prolog: [-address(TypeName)]

C: TypeName *F()

The C function should return a TypeName *. The returned value, which should be a valid second argument to SP_put_address() (see

Section 8.3.2 [Creating

Prolog Terms], page 176 ), will be converted to a Prolog integer.

Prolog: [-term]

C: SP_term_ref F()

The C function should return an SP term ref. Prolog will try to unify its value with the actual argument.

8.2.4 Interface Predicates link_foreign_ resource(+Resource,+SourceFile,+Option,+CFiles,+ObjectFiles,+Libraries)

Builds a linked foreign resource, using the splfr utility. Option can be either dynamic or static as described in the alternative form shown below.

The

Resource argument is an unsuffixed filename of a linked foreign resource. This predicate is not available in runtime systems.

load_foreign_resource(:Resource)

Unless a foreign resource with the same name as Resource has been statically linked, the linked foreign resource specified by Resource is linked into the Prolog

172 SICStus Prolog load image. In both cases, the predicates defined by Resource are installed, and any init function is called. Dynamic linking is not possible if the foreign resource was linked using the +static option.

If a resource with the same name has been previously loaded, it will be unloaded as if unload_foreign_resource(Resource) was called, before Resource is loaded.

unload_foreign_resource(:Resource)

Any deinit function associated with Resource is called, and the predicates defined by Resource are uninstalled. If Resource has been dynamically linked, it is unlinked from the Prolog load image.

The following predicates are provided for backwards compatibility and should be avoided in new code: foreign_file(+File,+Functions)

[Hook,Obsolescent]

Specifies that a set of foreign functions, to be called from Prolog, are to be found in File. This predicate is only called from load_foreign_files/2.

load_foreign_files(:ObjectFiles,+Libraries)

[Hookable,Obsolescent]

A resource name is derived from the first file name in ObjectFiles by stripping off the suffix. If this resource has been statically linked, the predicates defined by it are installed; otherwise, a linked foreign resource containing the declared functions is created and loaded. Not available in runtime systems.

8.2.5 The splfr utility

The splfr utility is used to create foreign resources (see

Section 8.2.1 [Foreign Resources], page 167

). splfr reads terms from a Prolog file, applying op declarations and extracting any foreign_resource/2 fact with first argument matching the resource name and all foreign/[2,3] facts. Based on this information, it generates the necessary glue code, and combines it with any additional C or object files provided by the user into a linked foreign resource. The output filename will be the resource name with a suitable extension.

splfr is invoked as

% splfr [ Option | InputFile ] ...

The input to splfr can be divided into Options and InputFiles and they can be arbitrarily mixed on the command line. Anything not interpreted as an option will be interpreted as an input file. Exactly one of the input files should be a Prolog file. The following options are available:

--help

Prints out a summary of all options.

-v

--verbose

Print detailed information about each step in the compilation/linking sequence.

-version

Prints out the version number of splfr.

Chapter 8: Mixing C and Prolog 173

--resource=ResourceName

Specify the resource’s name. This defaults to the basename of the Prolog source file found on the command line.

--manual

Do not generate any glue code. This option can only be used when the interface code is generated manually as described in

Chapter 40 [Runtime Utilities], page 547

.

-S

--static

Create a statically linked foreign resource instead of a dynamically linked shared object (which is the default). A statically linked foreign resource is a single object file which can be pre-linked into a Prolog system. Only available under

UNIX. See also the spld utility,

Section 8.7.3 [The spld utility], page 191

.

--keep

Keep temporary files and interface code and rename them to human-readable names. Not intended for the casual user, but useful if you want to know exactly which glue code is generated.

--import

Creates a import library for this resource. Necessary if you want to link external code against the resource. (Win32 only.)

--cflag=CFlags

CFlags is a comma-separated list of C-compiler options which will be separated at the commas and sent to the C-compiler passes.

The key input to splfr is the SourceFile. The contents of this file determines how the foreign resource’s interface will look like. When the source-file is read in, foreign_resource/2 facts with first argument matching the name of this resource (i.e. ResourceName) is extracted together with all foreign/[2,3] facts.

8.2.6 Init and Deinit Functions

An init function and/or a deinit function can be declared by foreign_resource/2. If this is the case, these functions should have the prototype: void FunctionName (int when)

The init function is called by load_foreign_resource/1 after the resource has been loaded and the interfaced predicates have been installed.

The deinit function is called by unload_foreign_resource/1 before the interfaced predicates have been uninstalled and the resource has been unloaded.

The init and deinit functions may use the C-interface to call Prolog etc.

Foreign resources are unloaded before saving states, and reloaded afterwards or when the saved state is restored; see

Section 2.10 [Saving], page 27 . Foreign resources are also un-

loaded when exiting Prolog execution. The parameter when reflects the context of the

(un)load_foreign_resource/1 and is set as follows for init functions:

174 SICStus Prolog

SP_WHEN_EXPLICIT

Explicit call to load_foreign_resource/1.

SP_WHEN_RESTORE

Resource is reloaded after save or restore.

For deinit functions:

SP_WHEN_EXPLICIT

Explicit call to unload_foreign_resource/1.

SP_WHEN_SAVE

Resource is unloaded before save.

SP_WHEN_EXIT

Resource is unloaded before exiting Prolog.

8.2.7 Creating the Linked Foreign Resource

Suppose we have a Prolog source file ex.pl containing: foreign(f1, p1(+integer,[-integer])).

foreign(f2, p2(+integer,[-integer])).

foreign_resource(ex, [f1,f2]).

:- load_foreign_resource(ex).

and a C source file ex.c with definitions of the functions f1 and f2, both returning long and having a long as only parameter. The conversion declarations in ‘ex.pl’ state that these functions form the foreign resource ex.

To create the linked foreign resource, simply type (to Prolog):

| ?- link_foreign_resource(ex,’ex.pl’,dynamic,[’ex.c’],[],[]).

or alternatively (to the Shell):

% splfr ex.pl ex.c

The linked foreign resource ‘ex.so’ (file suffix ‘.so’ is system dependent) has been created.

It will be dynamically linked by the directive :- load_foreign_resource(ex). when the file ‘ex.pl’ is loaded. Linked foreign resources can also be created manually (see

Chapter 40

[Runtime Utilities], page 547 ).

Dynamic linking of foreign resources can also be used by Runtime Systems.

On some platforms, however, the executable must not be stripped for dynamic linking to work, i.e.

its symbol table must remain.

Chapter 8: Mixing C and Prolog 175

8.3 Support Functions

The support functions include functions to manipulate SP term refs, functions to convert data between the basic C types and Prolog terms, functions to test whether a term can be converted to a specific C type, and functions to unify or compare two terms.

8.3.1 Creating and Manipulating SP term refs

Normally, C functions only have indirect access to Prolog terms via SP term refs. C functions may receive arguments as unconverted Prolog terms, in which case the actual arguments received will have the type SP_term_ref. Also, a C function may return an unconverted Prolog term, in which case it must create an SP term ref. Finally, any temporary

Prolog terms created by C code must be handled as SP term refs.

SP term refs are motivated by the fact that SICStus Prolog’s memory manager must have a means of reaching all live Prolog terms for memory management purposes, including such terms that are being manipulated by the user’s C code. Previous releases of SICStus

Prolog provided direct access to Prolog terms and the ability to tell the memory manager that a given memory address points to a Prolog term, but this approach was too low level and highly error-prone. The current design is modeled after and largely compatible with

Quintus Prolog release 3.

SP term refs are created dynamically. At any given time, an SP term ref has a value (a

Prolog term). This value can be examined, accessed, and updated by the support functions described in this section.

It is important to understand the rules governing the scope of SP term refs in conjunction with calls from Prolog to C and vice versa:

• When a C function called from Prolog returns, all SP term refs passed to the function or dynamically created by the function become invalid.

• When terms are passed to C as a result of calling Prolog, those terms and any

SP term refs created since the start of the query are only valid until backtracking into the query or an enclosing one.

A new SP term ref whose value is [] is created by calling:

SP_term_ref SP_new_term_ref(void)

The value of the SP term ref to is set to the value of the SP term ref from by calling

SP_put_term(to,from). The previous value of to is lost: void SP_put_term(SP_term_ref to, SP_term_ref from)

Each Prolog atom is represented internally by a unique integer, represented in C as an unsigned long. This mapping between atoms and integers depends on the execution history. Certain functions require this representation as opposed to an SP term ref. It can be obtained by a special argument type declaration when calling C from Prolog, by calling

176 SICStus Prolog

SP_get_atom(), or by looking up an encoded string s in the Prolog symbol table by calling

SP_atom_from_string(s): unsigned long SP_atom_from_string(char *s)

The encoded string containing the characters of a Prolog atom a can be obtained by calling: char *SP_string_from_atom(unsigned long a)

The length of the encoded string representing a Prolog atom a can be obtained by calling: int SP_atom_length(unsigned long a)

Same as strlen(SP_string_from_atom(a)) but runs in O(1) time.

Prolog atoms, and the space occupied by their print names, are subject to garbage collection when the number of atoms has reached a certain threshold, under the control of the agc_margin Prolog flag (see

Section 7.6 [State Info], page 137

), or when the atom garbage collector is called explicitly. The atom garbage collector will find all references to atoms from the Prolog specific memory areas, including SP term refs and arguments passed from

Prolog to foreign language functions. However, atoms created by SP_atom_from_string and merely stored in a local variable are endangered by garbage collection. The following functions make it possible to protect an atom while it is in use. The operations are implemented using reference counters to cater for multiple, independent use of the same atom in different foreign resources: int SP_register_atom(unsigned long a)

Registers the atom a with the Prolog memory manager by incrementing its reference counter.

Returns a nonzero value if the operation succeeds.

int SP_unregister_atom(unsigned long a)

Unregisters the atom a with the Prolog memory manager by decrementing its reference counter. Returns a nonzero value if the operation succeeds.

8.3.2 Creating Prolog Terms

These functions create a term and store it as the value of an SP term ref, which must exist prior to the call. They return zero if the conversion fails (as far as failure can be detected), and a nonzero value otherwise, assigning to t the converted value. Note that the term chars here refers to a list of character codes, rather than to one-char atoms: int SP_put_variable(SP_term_ref t)

Assigns to t a new Prolog variable.

int SP_put_integer(SP_term_ref t, long l)

Assigns to t a Prolog integer from a C long integer.

int SP_put_float(SP_term_ref t, double d)

Assigns to t a Prolog float from a C double.

Chapter 8: Mixing C and Prolog 177 int SP_put_atom(SP_term_ref t, unsigned long a)

Assigns to t a Prolog atom from a, which must be the canonical representation of a Prolog atom. (see

Section 8.2 [Calling C], page 166

).

int SP_put_string(SP_term_ref t, char *name)

Assigns to t a Prolog atom from a encoded C string.

int SP_put_address(SP_term_ref t, void *pointer)

Assigns to t a Prolog integer from a C pointer.

The pointer must be NULL or an address having the four most significant bits consistent with the smallest non-NULL value returned by the function malloc(). In particular, under Linux, these bits must be zero. Furthermore, the address must be aligned on a four bytes boundary.

NULL is converted to the integer 0.

int SP_put_list_chars(SP_term_ref t, SP_term_ref tail, char *s)

Assigns to t a Prolog list of the character codes represented by the encoded string s, prepended to the value of tail.

int SP_put_list_n_chars(SP_term_ref t, SP_term_ref tail, long n, char *s)

Assigns to t a Prolog list of the character codes represented by the first n bytes in encoded string s, prepended in front of the value of tail.

int SP_put_number_chars(SP_term_ref t, char *s)

Assigns to t a Prolog number by parsing the string in s.

int SP_put_functor(SP_term_ref t, unsigned long name, int arity)

Assigns to t a Prolog compound term with all the arguments unbound variables.

If arity is 0, assigns the Prolog atom whose canonical representation is name to t. This is similar to calling functor/3 with the first argument unbound and the second and third arguments bound to an atom and an integer, respectively.

int SP_put_list(SP_term_ref t)

Assigns to t a Prolog list whose head and tail are both unbound variables.

int SP_cons_functor(SP_term_ref t, unsigned long name, int arity, SP_term_ref arg, ...)

Assigns to t a Prolog compound term whose arguments are the values of arg...

If arity is 0, assigns the Prolog atom whose canonical representation is name to t. This is similar to calling =../2 with the first argument unbound and the second argument bound.

int SP_cons_list(SP_term_ref t, SP_term_ref head, SP_term_ref tail)

Assigns to t a Prolog list whose head and tail are the values of head and tail.

8.3.3 Accessing Prolog Terms

These functions will take an SP term ref and convert it to C data. They return zero if the conversion fails, and a nonzero value otherwise, and (except the last one) store the C data in output arguments. Note that the term chars here refers to a list of character codes, rather than to one-char atoms:

178 SICStus Prolog int SP_get_integer(SP_term_ref t, long *l)

Assigns to *l the C long corresponding to a Prolog number. The value must fit in *l for the operation to succeed.

int SP_get_float(SP_term_ref t, double *d)

Assigns to *d the C double corresponding to a Prolog number.

int SP_get_atom(SP_term_ref t, unsigned long *a)

Assigns to *a the canonical representation of a Prolog atom.

int SP_get_string(SP_term_ref t, char **name)

Assigns to *name a pointer to the encoded string representing the name of a

Prolog atom. This string must not be modified.

int SP_get_address(SP_term_ref t, void **pointer)

Assigns to *pointer a C pointer from a Prolog term. The term should be an integer whose value should be a valid second argument to SP_put_address()

(see

Section 8.3.2 [Creating Prolog Terms], page 176

).

int SP_get_list_chars(SP_term_ref t, char **s)

Assigns to *s a zero-terminated array containing an encoded string which corresponds to the given Prolog list of character codes. The array is subject to reuse by other support functions, so if the value is going to be used on a more than temporary basis, it must be moved elsewhere.

int SP_get_list_n_chars(SP_term_ref t, SP_term_ref tail, long n, long *w, char

*s)

Copies into s the encoded string representing the character codes in the initial elements of list t, so that at most n bytes are used. The number of bytes actually written is assigned to *w. tail is set to the remainder of the list. The array s must have room for at least n bytes.

int SP_get_number_chars(SP_term_ref t, char **s)

Assigns to *s a zero-terminated array of characters corresponding to the printed representation of a Prolog number. The array is subject to reuse by other support functions, so if the value is going to be used on a more than temporary basis, it must be moved elsewhere.

int SP_get_functor(SP_term_ref t, unsigned long *name, int *arity)

Assigns to *name and *arity the canonical representation and arity of the principal functor of a Prolog compound term. If the value of t is an atom, then that atom is assigned to *name and 0 is assigned to *arity. This is similar to calling functor/3 with the first argument bound to a compound term or an atom and the second and third arguments unbound.

int SP_get_list(SP_term_ref t, SP_term_ref head, SP_term_ref tail)

Assigns to head and tail the head and tail of a Prolog list.

int SP_get_arg(int i, SP_term_ref t, SP_term_ref arg)

Assigns to arg the i:th argument of a Prolog compound term. This is similar to calling arg/3 with the third argument unbound.

Chapter 8: Mixing C and Prolog 179

8.3.4 Testing Prolog Terms

There is one general function for type testing of Prolog terms and a set of specialized, more efficient, functions, one for each term type: int SP_term_type(SP_term_ref t)

Depending on the type of the term t, one of SP_TYPE_VARIABLE, SP_TYPE_

INTEGER, SP_TYPE_FLOAT, SP_TYPE_ATOM, or SP_TYPE_COMPOUND is returned.

int SP_is_variable(SP_term_ref t)

Returns nonzero if the term is a Prolog variable, zero otherwise.

int SP_is_integer(SP_term_ref t)

Returns nonzero if the term is a Prolog integer, zero otherwise.

int SP_is_float(SP_term_ref t)

Returns nonzero if the term is a Prolog float, zero otherwise.

int SP_is_atom(SP_term_ref t)

Returns nonzero if the term is a Prolog atom, zero otherwise.

int SP_is_compound(SP_term_ref t)

Returns nonzero if the term is a Prolog compound term, zero otherwise.

int SP_is_list(SP_term_ref t)

Returns nonzero if the term is a Prolog list, zero otherwise.

int SP_is_atomic(SP_term_ref t)

Returns nonzero if the term is an atomic Prolog term, zero otherwise.

int SP_is_number(SP_term_ref t)

Returns nonzero if the term is a Prolog number, zero otherwise.

8.3.5 Unifying and Comparing Terms int SP_unify(SP_term_ref x, SP_term_ref y)

Unifies two terms, returning zero on failure and nonzero on success.

int SP_compare(SP_term_ref x, SP_term_ref y)

Returns -1 if x @< y, 0 if x == y and 1 if x @> y

8.3.6 Operating System Services

The usual C library memory allocation functions (malloc, realloc, and free) may not work properly in foreign code. The following functions provide these services from SICStus

Prolog’s memory manager: void *SP_malloc(unsigned int size)

Returns a properly aligned pointer to a block of at least size bytes.

void *SP_realloc(void *ptr, unsigned int size)

Changes the size of the block referenced by ptr to size bytes and returns a pointer to the (possibly moved) block. The contents will be unchanged up to

180 SICStus Prolog the lesser of the new and old sizes. The block referenced by ptr must have been obtained by a call to SP_malloc or SP_realloc, and must not have been released by a call to SP_free or SP_realloc.

void SP_free(void *ptr)

Releases the block referenced by ptr, which must have been obtained by a call to SP_malloc or SP_realloc, and must not have been released by a call to

SP_free or SP_realloc.

SICStus Prolog caches the name of the current working directory. To take advantage of the cache and to keep it consistent, foreign code should call the following interface functions instead of calling chdir() and getcwd() directly: int SP_chdir(char *path)

Cause a directory pointed to by path to become the current working directory.

Returns 0 upon successful completion. Otherwise, a value of -1 is returned and errno is set to indicate the error.

char *SP_getcwd(char *buf, unsigned int size);

Returns a pointer to the current directory pathname. If buf is not NULL, the pathname will be stored in the space pointed to by buf. If buf is a NULL pointer, size bytes of space will be obtained using SP_malloc(). In this case, the pointer returned may be used as the argument in a subsequent call to SP_ free(). Returns NULL with errno set if size is not large enough to store the pathname.

8.4 Calling Prolog from C

In development and runtime systems alike, Prolog and C code may call each other to arbitrary depths.

Before calling a predicate from C you must look up the predicate definition by module, name, and arity. The function SP_predicate() will return a pointer to this definition or return NULL if the predicate is not visible in the module. This definition could be used in more than one call to the same predicate. The module specification is optional. If NULL or "" (the empty string) is given then the default type-in module (see

Section 4.2 [Module

Spec], page 55

) is assumed:

SP_pred_ref SP_predicate(char *name_string, long arity, char *module_string)

Note that the first and third arguments point to encoded strings, representing the characters of the predicate and module name.

The function SP_pred() may be used as an alternative to the above. The only difference is that the name and module arguments are passed as Prolog atoms rather than strings, and the module argument is mandatory. This saves the cost of looking up the two arguments in the Prolog symbol table. This cost dominates the cost of SP_predicate():

Chapter 8: Mixing C and Prolog 181

SP_pred_ref SP_pred(unsigned long name_atom, long arity, unsigned long module_atom)

8.4.1 Finding One Solution of a Call

The easiest way to call a predicate if you are only interested in the first solution is to call the function SP_query(). It will create a goal from the predicate definition and the arguments, call it, and commit to the first solution found, if any.

Returns SP_SUCCESS if the goal succeeded, SP_FAILURE if it failed, and SP_ERROR if an error condition occurred. Only when the return value is SP_SUCCESS are the values in the query arguments valid, and will remain so until backtracking into any enclosing query: int SP_query(SP_pred_ref predicate, SP_term_ref arg1, ...)

If you are only interested in the side effects of a predicate you can call SP_query_cut_ fail(). It will try to prove the predicate, cut away the rest of the solutions, and finally fail. This will reclaim the storage used after the call, and throw away any solution found: int SP_query_cut_fail(SP_pred_ref predicate, SP_term_ref arg1, ...)

8.4.2 Finding Multiple Solutions of a Call

If you are interested in more than one solution a more complicated scheme is used. You find the predicate definition as above but you don’t call the predicate directly.

1. Set up a call with SP_open_query()

2. Call SP_next_solution() to find a solution. Call this predicate again to find more solutions if there are any.

3. Terminate the call with SP_close_query() or SP_cut_query()

The function SP_open_query() will return an identifier of type SP_qid that you use in successive calls, or NULL, if given an invalid predicate reference. Note that if a new query is opened while another is already open, the new query must be terminated before exploring the solutions of the old one. That is, queries must be strictly nested:

SP_qid SP_open_query(SP_pred_ref predicate, SP_term_ref arg1, ...)

The function SP_next_solution() will cause the Prolog engine to backtrack over any current solution of an open query and look for a new one. The given argument must be the innermost query that is still open, i.e. it must not have been terminated explicitly by

SP_close_query() or SP_cut_query() or implicitly by an unsuccessful call to SP_next_ solution(). Returns SP_SUCCESS for success, SP_FAILURE for failure, SP_ERROR if an error condition occurred. Only when the return value is SP_SUCCESS are the values in the query arguments valid, and will remain so until backtracking into this query or an enclosing one: int SP_next_solution(SP_qid query)

182 SICStus Prolog

A query must be terminated in either of two ways. The function SP_cut_query() will discard the choices created since the corresponding SP_open_query(), like the goal !. The current solution is retained in the arguments until backtracking into any enclosing query.

The given argument does not have to be the innermost open query; any open queries in its scope will also be cut. Returns SP_SUCCESS for success and SP_ERROR for invalid usage: int SP_cut_query(SP_qid query)

Alternatively, the function SP_close_query() will discard the choices created since the corresponding SP_open_query(), and then backtrack into the query, throwing away any current solution, like the goal !, fail. The given argument does not have to be the innermost open query; any open queries in its scope will also be closed. Returns SP_SUCCESS for success and SP_ERROR for invalid usage: int SP_close_query(SP_qid query)

8.4.3 Calling Prolog Asynchronously

A Prolog execution may be interrupted by signals or similar asynchronous events. If you wish to call Prolog back from a signal handler you cannot use SP_query() etc. directly.

The call to Prolog has to be delayed until a time when the Prolog execution can accept an interrupt. The function SP_event() serves this purpose, and installs the function func to be called from Prolog when the execution can accept a callback. Returns non-zero iff installation succeeded. func is called with arg as first argument.

A queue of functions, with corresponding arguments, is maintained; that is, if several calls to SP_event() occur before Prolog can a accept an interrupt, the functions are queued and executed in turn at the next possible opportunity. Note that the queuing facility is only safe for signal handlers installed using SP_signal() (see below).

Depending on the value returned from func, the interrupted Prolog execution will just continue (SP_SUCCESS) or backtrack (SP_FAILURE or SP_ERROR). An exception raised by func will be processed in the interrupted Prolog execution. In case of fail or exception the event queue is flushed: int SP_event(int (*func)(), void *arg)

A signal handler having called SP_event() should call SP_continue() as its last action, to ensure that the interrupt is processed as early as possible: void SP_continue()

To install a function, func, as a handler for the signal sig, call: void (*SP_signal (int sig, void (*func)()))()

SP_signal() will also, if permitted by the operating system, add sig to a set of signals which are all blocked during the handling of the event queue. Some operating systems require that:

Chapter 8: Mixing C and Prolog 183 void (*SP_reinstall_signal (int sig, void (*func)()))() be called from a signal handler to unblock or reinstall the handler. This function should be called before SP_continue().

The following piece of C code illustrates these facilities.

The function signal_init() installs the function signal_handler() as the primary signal handler for the signals USR1 and USR2. That function invokes the predicate prolog_handler/1 as the actual signal handler, passing the signal number as an argument to the predicate.

SP_pred_ref event_pred; static int signal_event(signal_no) void *signal_no;

{

SP_term_ref x=SP_new_term_ref(); int rc;

}

SP_put_integer(x, (int)signal_no); rc = SP_query(event_pred, x); if (rc == SP_ERROR && SP_exception_term(x))

SP_raise_exception(x); return rc;

/* Propagate any raised exception */ static void signal_handler(sig) int sig;

{

SP_event(signal_event, (void *)sig);

SP_reinstall_signal(sig, signal_handler);

SP_continue();

} void signal_init()

{ event_pred = SP_predicate("prolog_handler",1,"");

}

SP_signal(SIGUSR1, signal_handler);

SP_signal(SIGUSR2, signal_handler);

8.4.4 Exception Handling in C

When an exception has been raised, the functions SP_query(), SP_query_cut_fail() and

SP_next_solution() return SP_ERROR. To access the exception term (the argument of the call to raise_exception/1), which is asserted when the exception is raised, the function SP_ exception_term() is used. As a side effect, the exception term is retracted, so if your code wants to pass the exception term back to Prolog, it must use the SP_raise_exception()

184 SICStus Prolog function below. If an exception term exists, SP_exception_term() retracts it and stores it as the value of an SP term ref which must exist prior to the call and returns nonzero.

Otherwise, it returns zero: int SP_exception_term(SP_term_ref t)

To raise an exception from a C function called from Prolog, just call SP_raise_ exception(t) where t is the SP term ref whose value is the exception term. The glue code will detect that an exception has been raised, any value returned from the function will be ignored, and the exception will be passed back to Prolog: void SP_raise_exception(SP_term_ref t)

8.5 SICStus Streams

With the SICStus Prolog C interface, the user can define his/her own streams as well as from C read or write on the predefined streams. The stream interface is modeled after

Quintus Prolog release 2. It provides:

• C functions to perform I/O on Prolog streams. This way you can use the same stream from Prolog and C code.

• User defined streams. You can define your own Prolog streams in C.

• Bidirectional streams. A SICStus stream supports reading or writing or both.

• Hookable standard input/output/error streams.

8.5.1 Prolog Streams

From the Prolog level there is a unique number that identifies a stream. This identifier can be converted from/to a Prolog stream: stream_code(+Stream,?StreamCode) stream_code(?Stream,+StreamCode)

StreamCode is the C stream identifier (an integer) corresponding to the Prolog stream Stream. This predicate is only useful when streams are passed between

Prolog and C. Note that StreamCode no longer has any relation to the file descriptor.

The StreamCode is a Prolog integer representing a SP_stream * pointer whose value should be a valid second argument to SP_put_address() (see

Section 8.3.2 [Creating Prolog

Terms], page 176

).

To read or write on a Prolog stream from C, special versions of the most common standard

C I/O functions are used: int SP_getc(void) int SP_fgetc(SP_stream *s)

Chapter 8: Mixing C and Prolog 185 void SP_putc(int c) void SP_fputc(int c, SP_stream *s)

The above functions deliver or accept wide character codes.

void SP_puts(char *string) void SP_fputs(char *string, SP_stream *s) int SP_printf(char *format, ...) int SP_fprintf(SP_stream *s, char *format, ...) int SP_fflush(SP_stream *s) int SP_fclose(SP_stream *s)

The above functions expect and deliver encoded strings in their char * and char ** arguments. Specifically, in the SP_printf() and SP_fprintf() functions, first the formatting operation will be performed. The resulting string will be assumed to be in internal encoding, and will be then output using the SP_puts() or SP_fputs() function (see below).

This means, e.g., that the %c printf conversion specification can only be used for ASCII characters, and the strings included using a %s specification should also be ASCII strings, or already transformed to the encoded form.

The SP_puts() and SP_fputs() functions first convert their encoded string argument into a sequence of wide character codes, and then output these on the required stream according to the external encoding; see

Section 10.6 [WCX Foreign Interface], page 231

.

There are three predefined streams accessible from C:

SP_stdin

Standard input. Refers to the same stream as user_input in Prolog. Which stream is referenced by user_input is controlled by the flag user_input (see prolog_flag/3) .

SP_stdout

Standard output. Refers to the same stream as user_output in Prolog. Which stream is referenced by user_output is controlled by the flag user_output (see prolog_flag/3).

SP_stderr

Standard error. Refers to the same stream as user_error in Prolog. Which stream is referenced by user_error is controlled by the flag user_error (see prolog_flag/3).

SP_curin

Current input. It is initially set equal to SP_stdin. It can be changed with the predicates see/1 and set_input/1.

SP_curout

Current output. It is initially set equal to SP_stdout. It can be changed with the predicates tell/1 and set_output/1.

Note that these variables are read only. They are set but never read by the stream handling.

186 SICStus Prolog

8.5.2 Defining a New Stream

The following steps are required to define a new stream in C:

• Define low level functions (character reading, writing etc).

• Initialize and open your stream.

• Allocate memory needed for your particular stream.

• Initialize and install a Prolog stream with SP_make_stream().

• Initialize additional fields. Some streams may require additional changes to the fields in the SP_stream structure than the default values set by SP_make_stream().

8.5.2.1 Low Level I/O Functions

For each new stream the appropriate low level I/O functions have to be defined. Error handling, prompt handling and character counting is handled in a layer above these functions.

They all operate on a user defined private data structure pointed out by user_handle in

SP_stream.

User defined low level I/O functions may invoke Prolog code and use the support functions described in the other sections of this chapter.

int my fgetc(void *handle)

Should return the character read or -1 on end of file.

int my fputc(char c, int handle)

Should write the character c and return the character written.

int my flush(void *handle)

Should flush the stream and return 0 on success, EOF on error.

int my eof (void *handle)

Should return 1 on end of file, else 0.

void my clrerr(void *handle)

Should reset the stream’s error and EOF indicators.

int my close(void *handle)

Should close the stream and return zero.

8.5.2.2 Installing a New Stream

A new stream is made accessible to Prolog using one of the functions: int SP_make_stream( void *handle, int (*sgetc)(), int (*sputc)(), int (*sflush)(), int (*seof)(),

Chapter 8: Mixing C and Prolog 187 void (*sclrerr)(), int (*sclose)(),

SP_stream **stream) int SP_make_stream_context( void *handle, int (*sgetc)(), int (*sputc)(), int (*sflush)(), int (*seof)(), void (*sclrerr)(), int (*sclose)(),

SP_stream **stream, unsigned long option, int context)

The functions return SP_SUCCESS on success and SP_ERROR for invalid usage, and will:

• Allocate a SP_stream structure

• Install your low level I/O functions. For those not supplied default functions are installed.

• Determine if the stream is for input or output or both from the functions supplied.

• Fill in fields in the SP_stream structure with default values

The handle pointer will be supplied as the handle argument in the calls to the low level functions.

A stream without a close function will be treated as not closable i.e. close/1 will not have any effect on it.

The SP_make_stream_context function has two additional arguments supplying information related to the handling of wide characters; see

Section 10.6 [WCX Foreign Interface], page 231 .

8.5.2.3 Internal Representation

For most streams you don’t have to know anything about the internal representation but there may be occasions when you have to set some fields manually or do some processing on all streams of a particular type. SICStus Prolog maintain a circular list of stream objects of type SP_stream.

SP_stream *backward;

SP_stream *forward;

Used for linking streams together. The insertion is done by SP_make_stream() and the deletion is done from the Prolog predicate close/1.

188 SICStus Prolog char *filename;

This field is set to the empty string, "", by SP_make_stream(). May be set to a suitable encoded string, provided the string will not be overwritten until the stream is closed.

unsigned long mode;

A bit vector that contains information about the access modes supported, if the stream is a TTY stream etc. It is not available to the user but the TTY property can be set by the function: void SP_set_tty(SP_stream *s) int fd;

The I/O descriptor if the stream is associated with a file, socket etc. Otherwise a negative number.

void *user_handle;

This is the pointer to the user supplied private data for the stream. In the case of SICStus Prolog predefined file streams the user_handle could be a pointer to the standard I/O FILE.

There is no standard way to tell if a stream is user defined. You have to save pointers to the streams created or check if one of the stream functions installed is user defined, i.e: int is_my_stream(SP_stream *s)

{ return (s->sclose == my_close);

}

8.5.3 Hookable Standard Streams

As of release 3.7, the standard I/O streams (input, output, and error) are hookable, i.e. the streams can be redefined by the user.

SP_UserStreamHook *SP_set_user_stream_hook(SP_UserStreamHook *hook)

Sets the user-stream hook to hook.

SP_UserStreamPostHook *SP_set_user_stream_post_hook(SP_UserStreamPostHook

*hook)

Sets the user-stream post-hook to hook.

These hook functions must be called before SP_initialize() (see

Section 8.7.4.1 [Initializing the Prolog Engine], page 198

). In custom built development systems, they may be called in the hook function SU_initialize(). See

Section 8.7.3 [The spld utility], page 191

.

8.5.3.1 Writing User-stream Hooks

The user-stream hook is, if defined, called during SP_initialize(). It has the following prototype:

SP_stream *user_stream_hook(int which)

Chapter 8: Mixing C and Prolog 189

If the hook is not defined, SICStus will attempt to open the standard TTY/console versions of these streams. If they are unavailable (such as for windowed executables under Windows), the result is undefined.

It is called three times, one for each stream. The which argument indicates which stream it is called for. The value of which is one of:

SP_STREAMHOOK_STDIN

Create stream for standard input.

SP_STREAMHOOK_STDOUT

Create stream for standard output.

SP_STREAMHOOK_STDERR

Create stream for standard error.

The hook should return a standard SICStus I/O stream, as described in

Section 8.5.2

[Defining a New Stream], page 186

.

8.5.3.2 Writing User-stream Post-hooks

The user-stream post-hook is, if defined, called after all the streams have been defined, once for each of the three standard streams. It has a slightly different prototype: void user_stream_post_hook(int which, SP_stream *str) where str is a pointer to the corresponding SP_stream structure. There are no requirements as to what this hook must do; the default behavior is to do nothing at all.

The post-hook is intended to be used to do things which may require that all streams have been created.

8.5.3.3 User-stream Hook Example

This section contains an example of how to create and install a set of user-streams.

The hook is set by calling SP_set_user_stream_hook() in the main program like this:

SP_set_user_stream_hook((SP_UserStreamHook *)user_strhook);

Remember: SP_set_user_stream_hook() and SP_set_user_stream_post_hook() must be called before SP_initialize().

The hook user_strhook() is defined like this:

SP_stream *user_strhook(int std_fd)

{

SP_stream *s;

SP_make_stream(NULL, my_getc, my_putc, my_flush, my_eof, my_clrerr, NULL, &s);

190 SICStus Prolog

} return s;

See

Section 8.5.2.2 [Installing a New Stream], page 186

for a description on the parameters to SP_make_stream().

8.6 Hooks

The user may define functions to be called at certain occasions by the Prolog system. This is accomplished by passing the functions as arguments to the following set-hook-functions.

The functions can be removed by passing a NULL.

typedef int (SP_ReadHookProc) (int fd)

SP_ReadHookProc SP_set_read_hook (SP_ReadHookProc *)

The installed function is called before reading a character from fd provided it is associated with a terminal device. This function shall return nonzero when there is input available at fd. It is called repeatedly until it returns nonzero.

typedef void (SP_VoidFun) (void)

SP_VoidFun * SP_set_reinit_hook (SP_VoidFun *)

The installed function is called upon abort and reinitialization. The call is made after SICStus Prolog’s signal handler installation but before any initializations are run and the version banners are displayed. Calling Prolog from functions invoked through this hook is not supported. (This hook is not available in runtime systems.) typedef void (SP_VoidFun) (void);

SP_VoidFun * SP_set_interrupt_hook (SP_VoidFun *)

The installed function is called on occasions like expansion of stacks, garbage collection and printouts, in order to yield control to special consoles etc. for interrupt checking. Calling Prolog from functions invoked through this hook is not supported. (This hook is not available in runtime systems.)

8.7 Stand-alone Executables

So far we have only discussed foreign code as pieces of code loaded into a Prolog executable.

This is often not the desired situation. Instead, people often want to create stand-alone executables. i.e. an application where Prolog is used as a component, accessed through the

API described in the previous sections.

8.7.1 Runtime Systems

Stand-alone applications containing debugged Prolog code and destined for end-users are typically packaged as runtime systems. No SICStus license is needed by a runtime system.

A runtime system has the following limitations:

Chapter 8: Mixing C and Prolog 191

• No top level. You have to supply a main program and explicitly initialize the Prolog engine with SP_initialize().

• No debugger

• No compiler

• No informational messages

• No profiling

• No OR-parallelism

• No redefinition warnings

• No signal handling except as installed by SP_signal()

• Uncaught exceptions are returned to C

• The predicates halt/0, abort/0 and reinitialise/0 will return to C

8.7.2 Runtime Systems on Target Machines

When a runtime system is delivered to the end user, chances are that the user does not have an existing SICStus installation. To deliver such an executable, you need: the executable

This is your executable program, usually created by spld (see

Section 8.7.3

[The spld utility], page 191

).

the runtime kernel

This is a shared object or a DLL, usually ‘$SP_PATH/../libsprt38.so’ under

UNIX, or ‘%SP_PATH%\..\sprt38.dll’ under Windows.

the runtime library

The saved state ‘$SP_PATH/bin/sprt.sav’ contains the built-in predicates written in Prolog. It is restored into the program at runtime by the function SP_ initialize().

your Prolog code

As a saved state, ‘.po’ files, ‘.ql’, or source code (‘.pl’ files). They must be explicitly loaded by the program at runtime (see

Section 8.7.4.2 [Loading Prolog

Code], page 201

).

your linked foreign resources

Any linked foreign resources which are not pre-linked with the executable, including any linked foreign resources for library modules which are located in

‘$SP_PATH/library’.

See section “Launching Runtime Systems on Target Machines” in SICStus Prolog Release

Notes , for more information about runtime systems on Target Machines.

8.7.3 The spld utility

A stand-alone executable is created using the spld utility. This utility replaces the scripts spmkrs and spmkds in previous versions of SICStus. It is invoked as:

192 SICStus Prolog

% spld [ Option | InputFile ] ...

spld takes the files specified on the command line and combines them into an executable file, much like the UNIX ld or the Windows link.

The input to spld can be divided into Options and Files and they can be arbitrarily mixed on the command line. Anything not interpreted as an option will be interpreted as an input file. The following options are available:

--help

Prints out a summary of all options.

-v

--verbose

Print detailed information about each step in the compilation/linking sequence.

--version

Prints out the version number of spld.

-o

--output

Specify output filename. The default depends on the linker (i.e. "a.out" on

UNIX systems).

-D

--development

Create a development system (with debugger, compiler, etc.). The default is to create a runtime system.

--main=type

Specify what to do when running the executable, or, in other words, what kind of user_main() program to use. The possible values are prolog, restore, load and user.

prolog

Implies -D. The executable will start the Prolog toplevel interpreter. This is the default if -D is specified and no ‘.sav’, ‘.pl’,

‘.po’, or ‘.ql’ files are specified.

user

The user supplies his/her own main-program by including C-code

(object file or source) which defines a function user_main().

int user_main(int argc, char *argv[]) user_main() is responsible for initializing the Prolog engine, loading code, and starting any Prolog-queries. This option is not compatible with -D.

restore

The executable will restore a saved-state. This is the default if a

‘.sav’ file is found among Files. It is only meaningful to specify one ‘.sav’ file.

The saved-state should be created using save_ program/2, otherwise the executable will simply restore the state and then terminate. Not compatible with -D.

load

The executable will load any Prolog code specified on the command line, i.e. files with extension ‘.pl’, ‘.po’ or ‘.ql’. It will then try to prove the goal runtime_entry(start). This is the default if

Chapter 8: Mixing C and Prolog 193 there are ‘.pl’, ‘.po’ or ‘.ql’ but no ‘.sav’ files among Files. Not compatible with -D.

--window

Win32 only. Create a windowed executable. A console window will be opened and connected to the Prolog standard streams. If --main=user is specified, user_main() should not set the user-stream hooks. C/C++ source code files specified on the command-line will be compiled with -DSP_WIN=1 if this option is given.

--moveable

Under UNIX, paths are normally hardcoded into executables in order for them to find the SICStus libraries and bootfiles. Two paths are normally hardcoded; the value of SP_PATH and, where possible, the runtime library search path using the -R linker option (or equivalent). If the linker does not support the -R flag (or an equivalent), a wrapper script is generated instead which sets LD_LIBRARY_

PATH (or equivalent).

The --moveable option turns of this behavior, so the executable is not dependent on SICStus being installed in a specific place. If this flag is given, the executable will rely on environment variables (SP_PATH (see

Section 2.1.1 [Environment Variables], page 20 ) and LD_LIBRARY_PATH, etc.) to find all relevant

files.

Under Windows, this option is always on, since Windows applications do no need to hardcode paths in order for them to find out where they’re installed. See section “Launching Runtime Systems on Target Machines” in SICStus Prolog

Release Notes , for more information on how SICStus locates its libraries and bootfiles.

-S

--static

Prefer static linking over dynamic. When --static is specified, spld will try to use static (not shared) versions of libraries and object-files. This option can be used to minimize runtime dependencies on shared libraries/object-files.

Under UNIX, even if there is a static version of a library (such as for example Tcl/Tk (libtcl8.0.a), spld may still use the dynamic version

(libtcl8.0.so). This is because there is no portable way of telling the linker to prefer static libraries over shared, it is usually only possible to link statically or dynamically; the modes are usually exclusive. In this case, spld will go with the linker’s default, which is usually dynamic. If you are in a situation where you would want spld to use a static library instead of a dynamic one, you will have to hack into spld’s configuration file ‘spld.config’ (normally located in

‘<installdir>/bin’). We recommend that you make a copy of ‘spld.config’ and specify the new configuration file using the --config=<file>. A typical modification of spld.config for this purpose may look like:

[...]

TCLLIB=-Bstatic -L/usr/local/lib -ltk8.0 -ltcl8.0 -Bdynamic

[...]

Use the new spld.config by typing

194 SICStus Prolog

% spld [...] -S --config=/home/joe/hacked_spld.config [...]

Note: these remarks are not applicable to Win32.

--resources=ResourceList

ResourceList is a comma-separated list of resource names, describing which resources should be pre-linked with the executable. Names can be either simple resource names, for example tcltk, or they can be complete paths to a foreign resource (with or without extensions). Example

% spld [...] --resources=tcltk,clpfd,/home/joe/foobar.so

This will cause library(tcltk), library(clpfd), and ‘/home/joe/foobar.so’ to be pre-linked with the executable. See also the --respath below. Not supported under Windows.

--respath=Path

Specify additional paths used for searching for resources.

Path is a colonseparated list of search-paths. spld will always search the default library directory as a last resort, so if this option is not specified, only the default resources will be found. See also the --resources option above.

--config=ConfigFile

Specify another configuration file. This option is not intended for normal use.

--cflag=CFlag

CFlag is a comma-separated list of options to send to the C-compiler. Any commas in the list will be replaced by spaces.

-LD

Do not process the rest of the command-line, but send it directly to the compiler/linker. Syntactic sugar for --cflag.

--sicstus=Executable spld relies on using SICStus during some stages of its execution. The default is the SICStus-executable installed with the distribution. Executable can be used to override this, in case the user wants to use another SICStus executable.

--interactive

-i

Only applicable with --main=load or --main=restore.

Calls SP_force_ interactive() (see

Section 8.7.4.1 [Initializing the Prolog Engine], page 198 )

before initializing SICStus.

--userhook

This option enables you to define your own version of the SU_initialize() function.

SU_initialize() is called by the main program before SP_ initialize(). Its purpose is to call interface functions which must be called before SP_initialize(), such as SP_set_memalloc_hooks(). It is not meaningful to specify this option if --main=user is given.

SU_initialize() should be defined as: int SU_initialize(int argc, char *argv[])

The contents of argv should not be modified. SU_initialize() should return

0 for success and non-zero for failure.

If a non-zero value is returned, the development system exits with the return value as error code.

Chapter 8: Mixing C and Prolog 195

Arguments to spld which are not recognized as options are assumed to be input-files and are handled as follows:

‘*.pl’

‘*.po’

‘*.ql’

‘*.so’

‘*.sl’

‘*.s.o’

‘*.o’

‘*.obj’

‘*.dll’

‘*.lib’

These are interpreted as Prolog code and will be loaded by SP_load().

These files are assumed to be input-files to the linker and will be passed on unmodified.

‘*.c’

‘*.cc’

‘*.C’

‘*.cpp’

‘*.c++’ These files are assumed to be C/C++ source code and will be compiled by the

C/C++-compiler before being passed to the linker.

If an argument is still not recognized, it will be passed unmodified to the linker.

8.7.3.1 Examples

1. The character-based SICStus development system executable (sicstus) can be created using

% spld --main=prolog -o sicstus

This will create a development system which is dynamically linked and has no pre-linked foreign resources.

2.

% spld --static -D --resources=random -o main -ltk8.0 -ltcl8.0

This will create a statically linked executable called main which has the resource random pre-linked (statically). The linker will receive -ltk8.0 -ltcl8.0 which will work under

UNIX (if Tcl/Tk is installed correctly) but will probably fail under Windows.

3. The follow is a little more advanced example demonstrating two things. One is how to use the --userhook option to perform initializations in development systems before

SP_initialize() is called. It also demonstrates how to use this mechanism to redefine the memory manager bottom layer.

196 SICStus Prolog

/* --------------------------------------------------------------

* userhook.c - an example of how to use SU_initialize() to

*

* define your own memory manager bottom layer.

* The following commands create a sicstus-executable ’msicstus’

* which uses malloc() as its own memory manager bottom layer.

* This is more or less equivalent to specifying "-m" to the

* regular ’sicstus’ executable, except that these memory hooks

* print out messages when they are called.

* --------------------------------------------------------------

*/

#include <stdio.h>

#include <stdlib.h>

#include <sicstus/sicstus.h>

/* for mallopt() */

#ifdef linux

#include <malloc.h>

#endif void * my_alloc_hook (unsigned int size, unsigned int align, unsigned int *actual_size)

{ void *mem; printf ("Inside my_alloc_hook(%d,%d,%p) -> ", size, align, actual_size); if (align > sizeof (double)) size += align; mem = malloc (size);

*actual_size = size; if (mem) printf ("%p\n", mem); else printf ("NULL (malloc failed to allocate memory)\n");

} return mem;

Chapter 8: Mixing C and Prolog void * my_init_alloc_hook (unsigned int size, unsigned int align, unsigned int *actual_size)

{ printf ("Inside my_init_alloc_hook(%d,%d,%p)\n", size, align, actual_size);

/* Do not use MMAP on Linux. */

#ifdef linux mallopt (M_MMAP_MAX, 0);

#endif

} return my_alloc_hook (size, align, actual_size); void * my_realloc_hook (void *ptr, unsigned int oldsize, unsigned int newsize, unsigned int align, unsigned int *actual_size)

{ void *mem; printf ("Inside my_realloc_hook(%p,%d,%d,%d,%p) -> ", ptr, oldsize, newsize, align, actual_size); if (align > sizeof (double)) newsize += align; mem = realloc (ptr, newsize);

*actual_size = newsize; if (mem) printf ("%p\n", mem); else printf ("NULL (realloc() failed to re-allocate memory)\n"); return mem;

} int my_free_hook (void *ptr, unsigned int size)

{ printf ("Inside my_free_hook(%p,%d)\n", ptr, size);

} free (ptr); return 1;

197

198 SICStus Prolog

/* Entry point for initializations to be done before SP_initialize() */ int

SU_initialize (int argc, char **argv)

{

SP_set_memalloc_hooks (MM_USE_OTHER, my_init_alloc_hook, my_alloc_hook, my_realloc_hook, my_free_hook); return 0;

}

Compile userhook.c like this:

% spld -D --userhook userhook.c -o ./msicstus

Created "./msicstus"

% ./msicstus

Inside my_init_alloc_hook(524304,8,0xbfffc318)

Inside my_alloc_hook(524304,8,0xbfffc318) -> 0x804b920

Inside my_alloc_hook(65536,8,0xbfff019c) -> 0x80cba38

Inside my_alloc_hook(65536,8,0xbfff019c) -> 0x80dba40

Inside my_alloc_hook(65536,8,0xbfff00d0) -> 0x80eba48

Inside my_alloc_hook(65544,8,0xbfff0110) -> 0x80fba50

Inside my_alloc_hook(65536,8,0xbfff019c) -> 0x810ba60

Inside my_alloc_hook(65536,8,0xbfff00f4) -> 0x811ba68

Inside my_alloc_hook(65536,8,0xbfff019c) -> 0x812ba70

Inside my_alloc_hook(65536,8,0xbfff019c) -> 0x813ba78

Inside my_alloc_hook(65536,8,0xbfff019c) -> 0x814ba80

Inside my_alloc_hook(65536,8,0xbfff019c) -> 0x815ba88

Inside my_alloc_hook(65536,8,0xbfff019c) -> 0x816ba90

SICStus 3.8b1 (x86-linux-glibc2.1): Fri Oct 29 11:24:24 CEST 1999

Licensed to SICS

Inside my_free_hook(0x80fba50,65544)

| ?-

8.7.4 Instead of a Top-level Interpreter

Stand-alone executables may or may not have a top-level interpreter. This is in general controlled by the --main to spld (see

Section 8.7.3 [The spld utility], page 191 ). When

a stand-alone executable does not have a top-level interpreter, the Prolog engine must be explicitly initialized by the user and the Prolog code must be explicitly loaded.

8.7.4.1 Initializing the Prolog Engine

The Prolog Engine is initialized by calling SP_initialize().

This must be done before any interface functions are called, except SP_force_interactive, SP_set_memalloc_ hooks, SP_set_wcx_hooks, SP_set_user_stream_post_hook and SP_set_user_stream_ hook. The function will allocate data areas used by Prolog, initialize command line argu-

Chapter 8: Mixing C and Prolog 199 ments so that they can be accessed by the argv Prolog flag, and load the Runtime Library.

It is called like this: int SP_initialize(int argc, char **argv, char *boot_path) boot_path should be the name of a directory, equivalent to ‘$SP_PATH/bin’. If boot_path is NULL, SP_initialize() will look up the value of the environment variable SP_PATH and look for the file ‘$SP_PATH/bin/sprt.sav’ which contains the Runtime Library.

It returns SP_SUCCESS if initialization was successful, and SP_ERROR otherwise. If initialization was successful, further calls to SP_initialize() will be no-ops (and return SP_

SUCCESS).

To unload the SICStus emulator, SP_deinitalize() can be called.

void SP_deinitialize(void)

SP_deinitialize() will make a best effort to restore the system to the state it was in at the time of calling SP_initialize(). This involves unloading foreign resources, shutting down the emulator by calling halt/0, and deallocate memory used by Prolog. SP_ deinitialize() is idempotent as well, i.e. it is a no-op unless SICStus has actually been initialized.

You may also call SP_force_interactive() before calling SP_initialize(). This will force the I/O built-in predicates to treat the standard input stream as a terminal, even if it does not appear to be a terminal. Same as the ‘-i’ option in development systems. (see

Section 2.1 [Start], page 19 ).

void SP_force_interactive(void)

You may also call SP_set_memalloc_hooks() before calling SP_initialize(). This will define the bottom layer of Prolog’s memory manager, in case your application has special requirements.

typedef void *(SP_AllocHook)(unsigned int size, unsigned int align, unsigned int *actual_sizep); typedef void *(SP_ReAllocHook)(void *ptr, unsigned int oldsize, unsigned int newsize, unsigned int align, unsigned int *actual_sizep); typedef int (SP_FreeHook)(void *ptr, unsigned int size); void SP_set_memalloc_hooks(int usage,

SP_AllocHook *init_alloc_hook,

SP_AllocHook *alloc_hook,

SP_ReAllocHook *realloc_hook,

SP_FreeHook *free_hook)

200 SICStus Prolog

The effect of SP_set_memalloc_hooks is controlled by the value of usage, which should be one of:

MM_USE_MALLOC

The bottom layer will be based on malloc()/free(). The other arguments are ignored. Same as the ‘-m’ option in development systems. (see

Section 2.1

[Start], page 19

).

MM_USE_SBRK

The bottom layer will be based on sbrk(). The default for UNIX; not available for Windows. The other arguments are ignored.

MM_USE_SPARSE

The bottom layer will be based on VirtualAlloc()/VirtualFree(). The default for Windows; not available for UNIX. The other arguments are ignored.

MM_USE_OTHER

The bottom layer will be based on the other arguments. Their meaning is explained below. See

Section 8.7.3 [The spld utility], page 191

, for an example.

In the latter case, the other arguments should be functions as specified below. Pointer arguments and values should be valid second arguments to SP_put_address() (see

Section 8.3.2

[Creating Prolog Terms], page 176

).

alloc_hook must allocate and return a pointer to a piece of memory that has at least size bytes aligned at align in it. align is guaranteed to be a power of 2.

The actual size of the piece of memory should be returned in *actual_sizep.

Should return NULL if it cannot allocate any more memory.

init_alloc_hook is a special case of alloc_hook. It will be called initially whereas alloc_hook will be called subsequently. It can do whatever initialization that this layer of memory management wants to do.

realloc_hook is called with a piece of memory to be resized and possibly moved. ptr is the pointer, oldsize its current size. The function must allocate and return a pointer to a piece of memory that has at least newsize bytes aligned at align in it, and that has the same contents as the old block up to the lesser of oldsize and newsize. align is guaranteed to be a power of 2. The actual size of the piece of memory should be returned in *actual_sizep. Should return NULL if it cannot allocate any more memory, or if it cannot reclaim the old block in a meaningful way. In that case, Prolog will use the other functions.

free_hook is called with a pointer to the piece of memory to be freed and its size. Should return non-zero iff the function was able to free this piece of memory. Otherwise,

Prolog will keep using the memory as if it were not freed.

Chapter 8: Mixing C and Prolog 201

The default bottom layers look at the environment variables

PROLOGINITSIZE,

PROLOGINCSIZE, PROLOGKEEPSIZE and PROLOGMAXSIZE. They are useful to customize the default memory manager. If users redefine the bottom layer, they can choose to ignore these environment variables. See

Section 2.1.1 [Environment Variables], page 20 .

8.7.4.2 Loading Prolog Code

You can load your Prolog code with the call SP_load(). This is the C equivalent of the

Prolog predicate load_files/1: int SP_load(char *filename)

Alternatively, you can restore a saved state with the call SP_restore(), which is the C equivalent of the Prolog predicate restore/1: int SP_restore(char *filename)

SP_load() and SP_restore() return SP_SUCCESS for success or SP_ERROR if an error condition occurred. The filename arguments in both functions are encoded strings.

Prolog error handling is mostly done by raising and catching exceptions. However, some faults are of a nature such that when they occur, the internal program state may be corrupted, and it is not safe to merely raise an exception. Memory allocation failures are examples of faults. In runtime systems, the following C macro provides an environment for handling faults: int SP_on_fault(Stmt, Message, Cleanup) which should occur in the scope of a char *Message declaration. Stmt is run, and if a fault occurs, Stmt is aborted, Message gets assigned a value explaining the fault, the Prolog internal state is cleaned, all queries and SP term refs become invalid, and Cleanup run. If

Stmt terminates normally, Message is left unchanged. For example, a “fault-proof” runtime system could have the structure: int main(int argc, char **argv)

{ char *message;

SP_initialize(argc, argv, "/usr/local/lib/sicstus38/bin"); loop:

SP_on_fault(main_loop(), message,

{printf("ERROR: %s\n",message); goto loop;}); exit(0);

} main_loop()

{...}

202 SICStus Prolog

Faults that occur outside the scope of SP_on_fault() cause the runtime system to halt with an error message.

The following function can be used to raise a fault. For example, it can be used in a signal handler for SIGSEGV to prevent the program from dumping core in the event of a segmentation violation (runtime systems have no predefined signal handling): void SP_raise_fault(char *message)

8.8 Examples

8.8.1 Train Example (connections)

This is an example of how to create a runtime system. The Prolog program ‘train.pl’ will display a route from one train station to another. The C program ‘train.c’ calls the

Prolog code and writes out all the routes found between two stations:

% train.pl

connected(From, From, [From], _):- !.

connected(From, To, [From| Way], Been):-

( no_stop(From, Through)

; no_stop(Through, From)

), not_been_before(Been, Through), connected(Through, To, Way, Been).

no_stop(’Stockholm’, ’Katrineholm’).

no_stop(’Stockholm’, ’Vasteras’).

no_stop(’Katrineholm’, ’Hallsberg’).

no_stop(’Katrineholm’, ’Linkoping’).

no_stop(’Hallsberg’, ’Kumla’).

no_stop(’Hallsberg’, ’Goteborg’).

no_stop(’Orebro’, ’Vasteras’).

no_stop(’Orebro’, ’Kumla’).

not_been_before(Way, _) :- var(Way),!.

not_been_before([Been| Way], Am) :-

Been \== Am, not_been_before(Way, Am).

/* train.c */

#include <stdio.h>

#include <sicstus/sicstus.h> void write_path(SP_term_ref path)

Chapter 8: Mixing C and Prolog 203

{ char *text = NULL;

SP_term_ref tail = SP_new_term_ref(), via = SP_new_term_ref();

SP_put_term(tail,path); while (SP_get_list(tail,via,tail))

{ if (text) printf(" -> ");

}

SP_get_string(via, &text); printf("%s",text);

} printf("\n"); int user_main(int argc, char **argv)

{ int rval;

SP_pred_ref pred;

SP_qid goal;

SP_term_ref from, to, path;

/* Initialize Prolog engine. This call looks up SP_PATH in order to

* find the Runtime Library.

*/ if (SP_FAILURE == SP_initialize(argc, argv, NULL))

{ fprintf(stderr, "SP_initialize failed: %s\n", SP_error_message(SP_errno)); exit(1);

} rval = SP_restore("train.sav"); if (rval == SP_ERROR || rval == SP_FAILURE)

{ fprintf(stderr, "Could not restore \"train.sav\".\n"); exit(1);

}

/* Look up connected/3. */ if (!(pred = SP_predicate("connected",4,"")))

{ fprintf(stderr, "Could not find connected/4.\n"); exit(1);

}

204 SICStus Prolog

/* Create the three arguments to connected/4. */

SP_put_string(from = SP_new_term_ref(), "Stockholm");

SP_put_string(to = SP_new_term_ref(), "Orebro");

SP_put_variable(path = SP_new_term_ref());

/* Open the query. In a development system, the query would look like:

*

* | ?- connected(’Stockholm’,’Orebro’,X).

*/ if (!(goal = SP_open_query(pred,from,to,path,path)))

{ fprintf(stderr, "Failed to open query.\n"); exit(1);

}

/*

* Loop through all the solutions.

*/ while (SP_next_solution(goal))

{ printf("Path: "); write_path(path);

}

SP_close_query(goal);

} exit(0);

Create the saved-state containing the Prolog code:

% sicstus

SICStus 3.8 (sparc-solaris-5.5.1): Thu Sep 30 15:20:42 MET DST 1999

Licensed to SICS

| ?- compile(train),save_program(’train.sav’).

{compiling [...]/train.pl...}

{compiled [...]/train.pl in module user, 10 msec 2848 bytes}

{[...]/train.sav created in 0 msec} yes

| ?- halt.

Create the executable using spld:

% spld --main=user train.c -o train.exe

And finally, run the executable:

% ./train

Path: Stockholm -> Katrineholm -> Hallsberg -> Kumla -> Orebro

Chapter 8: Mixing C and Prolog 205

Path: Stockholm -> Vasteras -> Orebro

8.8.2 I/O on Lists of Character Codes

This example is taken from the SICStus Prolog library (simplified, but operational). A stream for writing is opened where the written characters are placed in a buffer. When the stream is closed a list of character codes is made from the contents of the buffer. The example illustrates the use of user definable streams.

The open_buf_stream() function opens a stream where the characters are put in a buffer.

The stream is closed by stream_to_chars() which returns the list constructed on the heap.

The Prolog code (simplified): foreign(open_buf_stream, ’$open_buf_stream’(-address(’SP_stream’))).

foreign(stream_to_chars, ’$stream_to_chars’(+address(’SP_stream’),

-term)).

foreign_resource(example, [open_buf_stream,stream_to_chars]).

:- load_foreign_resource(example).

%% with_output_to_chars(+Goal, -Chars)

%% runs Goal with current_output set to a list of characters with_output_to_chars(Goal, Chars) :-

’$open_buf_stream’(StreamCode), stream_code(Stream, StreamCode), current_output(CurrOut), set_output(Stream), call_and_reset(Goal, Stream, CurrOut, StreamCode, Chars).

call_and_reset(Goal, Stream, CurrOut, StreamCode, Chars) :call(Goal), !, put(0),

’$stream_to_chars’(StreamCode, Chars), reset_stream(Stream, CurrOut).

call_and_reset(_, Stream, CurrOut, _, _) :reset_stream(Stream, CurrOut).

reset_stream(Stream, CurrOut) :set_output(CurrOut), close(Stream).

The C code:

#include <sicstus/sicstus.h> struct open_chars {

206 SICStus Prolog char *chars; int index;

}; int size;

/* character buffer */

/* current insertion point */

#define INIT_BUFSIZE 512 static int lputc(c, buf) int c; struct open_chars *buf;

{ if (buf->index == buf->size) /* grow buffer if necessary */

{ buf->size *= 2; buf->chars = (char *)realloc(buf->chars, buf->size);

} return (buf->chars[buf->index++] = c);

} static int lwclose(buf) struct open_chars *buf;

{ free(buf->chars); free(buf); return 0;

} void open_buf_stream(streamp)

SP_stream **streamp;

{ struct open_chars *buf;

/* Allocate buffer, create stream & return stream code */ buf = (struct open_chars *)malloc(sizeof(struct open_chars));

SP_make_stream(buf, NULL, lputc, NULL, NULL, NULL, lwclose, streamp);

} buf->chars = (char *)malloc(INIT_BUFSIZE); buf->size = INIT_BUFSIZE; buf->index = 0; void stream_to_chars(streamp, head)

SP_stream *streamp;

SP_term_ref head;

{

SP_term_ref tail = SP_new_term_ref(); struct open_chars *buf = (struct open_chars *)streamp->user_handle;

Chapter 8: Mixing C and Prolog 207

/* Make a list of character codes out of the buffer */

}

SP_put_string(tail, "[]");

SP_put_list_chars(head, tail, buf->chars);

8.8.3 Exceptions from C

Consider, for example, a function which returns the square root of its argument after checking that the argument is valid. If the argument is invalid, the function should raise an exception instead.

/* math.c */

#include <math.h>

#include <stdio.h>

#include <sicstus/sicstus.h> double sqrt_check(d) double d;

{ if (d < 0.0)

{ /* build a domain_error/4 exception term */

SP_term_ref culprit=SP_new_term_ref();

SP_term_ref argno=SP_new_term_ref();

SP_term_ref expdomain=SP_new_term_ref();

SP_term_ref t1=SP_new_term_ref();

}

SP_put_float(culprit, d);

SP_put_integer(argno, 1);

SP_put_string(expdomain, ">=0.0");

SP_cons_functor(t1, SP_atom_from_string("sqrt"), 1, culprit);

SP_cons_functor(t1, SP_atom_from_string("domain_error"), 4,

SP_raise_exception(t1); return 0.0; t1, argno, expdomain, culprit);

/* raise the exception */

} return sqrt(d);

The Prolog interface to this function is defined in a file ‘math.pl’. The function uses the sqrt() library function, and so the math library ‘-lm’ has to be included:

/* math.pl */ foreign_resource(math, [sqrt_check]).

foreign(sqrt_check, c, sqrt(+float, [-float])).

208 SICStus Prolog

:- load_foreign_resource(math).

A linked foreign resource is created:

% splfr math math.pl +c math.c +l -lm

A simple session using this function could be:

% sicstus

SICStus 3.8 (SunOS-5.5.1-sparc): Thu Aug 19 16:25:28 MET DST 1999

Licensed to SICS

| ?- [math].

{consulting /home/san/pl/math.pl...}

{/home/san/pl/math.pl consulted, 10 msec 816 bytes} yes

| ?- sqrt(5.0,X).

X = 2.23606797749979 ?

yes

| ?- sqrt(a,X).

{TYPE ERROR: sqrt(a,_30) - arg 1: expected number, found a}

| ?- sqrt(-5,X).

{DOMAIN ERROR: sqrt(-5.0) - arg 1: expected ’>=0.0’, found -5.0}

The above example used the foreign language interface with dynamic linking. To statically link ‘math.o’ with the Prolog emulator, the following steps would have been taken:

% splfr math math.pl +static +c math.c +l -lm

SICStus 3.8 (SunOS-5.5.1-sparc): Thu Aug 19 16:25:28 MET DST 1999

Licensed to SICS

{/tmp/00249aaa.c generated, 10 msec} yes

% spmkds mathsp math

SICStus 3.8 (SunOS-5.5.1-sparc): Thu Aug 19 16:25:28 MET DST 1999

Licensed to SICS

{/tmp/00274aaa.c generated, 0 msec} yes

% sicstus -base ./mathsp

SICStus 3.8 (SunOS-5.5.1-sparc): Thu Aug 19 16:25:28 MET DST 1999

Licensed to SICS

| ?- [math].

{consulting /home/san/pl/math.pl...}

{/home/san/pl/math.pl consulted, 0 msec 864 bytes} yes

Chapter 8: Mixing C and Prolog 209

| ?- sqrt(5.0,X).

X = 2.23606797749979 ?

yes

8.8.4 Stream Example

This is a small example how to initialize a bidirectional socket stream (error handling omitted): typedef struct { int fd;

FILE *r_stream;

FILE *w_stream;

} SocketData;

/* socket number */

/* For reading */

/* For writing */ int socket_sgetc(SocketData *socket)

{ return fgetc(socket->r_stream);

} int socket_sputc(char c, SocketData *socket)

{ return fputc(c, socket->w_stream);

} int socket_sflush(SocketData *socket)

{ return fflush(socket->w_stream);

} int socket_seof(SocketData *socket)

{ return feof(socket->r_stream);

} void socket_sclrerr(SocketData *socket)

{ clearerr(socket->r_stream); clearerr(socket->w_stream);

} int socket_sclose(SocketData *socket)

{ fclose(socket->r_stream); fclose(socket->w_stream); close(socket->fd);

210

} free(socket); return 0;

SP_stream *new_socket_stream(int fd)

{

SP_stream *stream;

SocketData *socket;

/* Allocate and initialize data local to socket */ socket = (SocketData *)malloc(sizeof(SocketData)); socket->fd = fd; socket->r_stream = fdopen(fd,"r"); socket->w_stream = fdopen(fd,"w");

/* Allocate and initialize Prolog stream */

SP_make_stream( socket, socket_sgetc, socket_sputc, socket_sflush, socket_seof, socket_sclrerr, socket_sclose,

&stream);

/* Allocate and copy string */ stream->filename = "socket"; stream->fd = fd;

} return stream;

SICStus Prolog

Chapter 9: Mixing Java and Prolog 211

9 Mixing Java and Prolog

Jasper is a bi-directional interface between Java and SICStus. The Java-side of the interface constists of a Java package (se.sics.jasper) containing classes representing the

SICStus emulator (SICStus, SPTerm, etc). The Prolog part is designed as library module

(library(jasper)) and an extension to the foreign language interface.

The foreign language interface extensions enables Java-methods to be called as Prolog predicates using foreign/3 declarations, much like the C-Prolog facilities described in

Chapter 8

[Mixing C and Prolog], page 165 .

The library module library(jasper) (see

Chapter 38 [Jasper], page 527 ) provides function-

ality for controlling the loading and unloading the JVM, meta-call functionality (jasper_ call_instance/6, jasper_call_static/6), and predicates for managing global and local references. These are provided in order to make it easy to call Java methods on-the-fly from

Prolog without having to create a foreign resource first.

Jasper can be used in two modes, depending on which system acts as Parent Application.

If Java is the parent application, the SICStus runtime kernel will be loaded into the JVM using the System.loadLibrary() method (this is done indirectly when instantiating the

SICStus object). In this mode, SICStus is loaded as a runtime system (see

Section 8.7.1

[Runtime Systems], page 190 ).

If SICStus is the parent application, Java will be loaded as a foreign resource using the query use_module(library(jasper)). The Java engine is initialized using jasper_ initialize/[1-2].

• Some of the information in this chapter is a recapitulation of the information in the chapter

Chapter 8 [Mixing C and Prolog], page 165

. The intention is that this chapter should be possible to read fairly independently.

• Before proceeding, section “Jasper Notes” in SICStus Prolog Release Notes . It contains important information about requirements, availability, installation tips, limitations, and how to access other (online) Jasper/Java resources.

9.1 Getting Started

See section “Getting Started” in SICStus Prolog Release Notes , for a detailed description of how to get started using the interface. It addresses issues such as finding SICStus from within Java and vice versa, setting the classpath correctly, etc. If you have trouble in getting started with Jasper, read that chapter before contacting SICStus Support.

9.2 Calling Java from Prolog

Java methods can be called from Prolog in two ways. One way is to use the meta-call predicates provided in library(jasper).

See

Chapter 38 [Jasper], page 527

for more information. Another, more efficient, method will be described here.

212 SICStus Prolog

Java methods can be called much in the same way as C functions are called (see

Section 8.2 [Calling C], page 166

); by creating a foreign resource. When loaded, this resource installs a set of predicates which are mapped onto Java-methods such that invoking a Java method looks like any other Prolog predicate call. Such methods are sometimes called direct mapped.

In fact, a foreign resource (as defined in

Section 8.2.1 [Foreign Resources], page 167

) is not language specific itself. The language is instead specified in the second argument to the foreign/3 fact and it is possible to mix foreign C functions with foreign Java methods.

How a foreign resource is created in general is described in detail in

Section 8.2.7 [Creating the Linked Foreign Resource], page 174

. The following section(s) will focus on the Javaspecific parts of foreign resources.

9.2.1 Static and Dynamic Linking

There is only limited support for static foreign resources (see

Section 8.2.1 [Foreign Resources], page 167

) (mainly due to the fact the Java does not support statically linked code). Even though Jasper allows static foreign resources may be used to some extent, it is recommended that only dynamic foreign resources are used.

9.2.2 Declarating Java-methods

Java-methods are declared similarly to C-functions. There are two major differences. The first is how methods are identified. It is not enough to simply use an atom as the C interface does. Instead, a term method/3 is used: method(+ClassName,+MethodName,+Flags)

Used as first argument to foreign/3 when declaring Java methods. The first argument is an atom containing the Fully Qualified Classname of the class

(for example, java/lang/String) The second argument is the method name.

The third argument is a list of flags. Possible flags are instance or static, indicating whether or not the method is static or non-static. Non-static methods must have an object-reference as their first argument. This is a reference to the object on which the method will be invoked.

This term is then used to identify the method in the foreign_resource/2 predicate. So, to define a foreign resource exporting the non-static Java method getFactors in the class PrimeNumber in the package numbers, the method/3 term would look like method(’numbers/PrimeNumber’,’getFactors’,[instance])

The syntax for foreign/3 is the basically the same as for C-functions: foreign(+MethodIdentifier, java, +Predicate)

A hook predicate, specifies the Prolog interface to a Java method. MethodIdentifier is method/3 term as described above. Predicate specifies the name of the

Chapter 9: Mixing Java and Prolog 213

Prolog predicate that will be used to call MethodIdentifier. Predicate also specifies how the predicate arguments are to be translated into the corresponding

Java arguments.

9.3 Conversions between Prolog Arguments and Java Types

The following table lists the possible values of arguments of the predicate specification to foreign/3. The value declares which conversion between corresponding Prolog argument and Java type will take place.

NOTE: The conversion declarations (composed of the declarators specified below) together with the method/3 term are used by the glue code generator to create the method’s type signature, i.e. a string which can uniquely identify a method within a class. This means that unlike the C interface, the conversion declarations for a Java method will affect the lookup of the method-name (in the C interface, only the function name is relevant). So, if a method is declared as foo(+integer), there must be a method which has the name foo and takes one argument of type int, or an argument which can be automatically converted to an int (a short, for example).

Prolog: +integer

Java: int

The argument should be a number. It is converted to a Java int.

Prolog: +byte

Java: byte

The argument should be a number. It is converted to a Java byte.

Prolog: +short

Java: short

The argument should be a number. It is converted to a Java short.

Prolog: +long

Java: long

The argument should be a number. It is converted to a Java long.

NOTE: Since Java’s long type is 64 bits wide and there is no standardized support for 64 bits integers in C, the value will be truncated. So, this declaration is really only useful in order to indicate which method should be used. For example: class Bar

{ void foo(int x)

{ ... }

} void foo(long x)

{ ... }

214 SICStus Prolog

In order to be able to indicate that the latter of the foo methods should be called, a +long declaration must be used, even if the value itself will be truncated in the call.

Prolog: +float

Java: float

The argument should be a number. It is converted to a Java float.

Prolog: +double

Java: double

The argument should be a number. It is converted to a Java double.

Prolog: +term

Java: SPTerm

The argument can be any term. It is passed to Java as an object of the class

SPTerm.

Prolog: +object(Class)

Java: SPTerm

The argument should be the Prolog representation of a Java object of class

Class. Unless it is the first argument in a non-static method (in which case is it treated as the object on which the method should be invoked), it is passed to the Java method as an object of class Class.

Prolog: +atom

Java: SPTerm

The argument should be an atom. The Java method will be passed an object of class SPTerm.

Prolog: +boolean

Java: boolean

The argument should be an atom in {true,false}. The Java method will receive a boolean.

Prolog: +chars

Java: String

The argument should be a list of character codes. The Java method will receive an object of class String.

Prolog: +string

Java: String

The argument should be an atom. The Java method will receive an object of class String.

Prolog: -atom

Java: SPTerm

The argument should be an unbound variable. The Java method will receive an atom of class SPTerm which can be modified. The argument will be bound to the value of the atom when the method returns.

Prolog: -chars

Chapter 9: Mixing Java and Prolog 215

Java: StringBuffer

The argument should be an unbound variable. The Java method will receive an object of type StringBuffer which can be modified. The argument will be bound to a list of the character codes of the StringBuffer object.

Prolog: -string

Java: StringBuffer

The argument should be an unbound variable. The Java method will receive an object of type StringBuffer which can be modified. The argument will be bound to an atom converted from the StringBuffer object.

Prolog: [-integer]

Java: int M ()

The Java method should return an int. The value will be converted to a Prolog integer.

Prolog: [-byte]

Java: byte M ()

The Java method should return a byte. The value will be converted to a Prolog integer.

Prolog: [-short]

Java: short M ()

The Java method should return a short. The value will be converted to a

Prolog integer.

Prolog: [-long]

Java: long M ()

The Java method should return a long. The value will be converted and possibly truncated to a Prolog integer.

Prolog: [-float]

Java: float M ()

The Java method should return a float. The value will be converted to a

Prolog float.

Prolog: [-double]

Java: double M ()

The Java method should return a double. The value will be converted to a

Prolog float.

Prolog: [-term]

Java: SPTerm M ()

The Java method should return an object of class SPTerm which will be converted to a Prolog term.

Prolog: [-object(Class)]

Java: SPTerm M ()

The Java method should return an object of class Class which will be converted to the internal Prolog representation of the Java object.

216 SICStus Prolog

Prolog: [-atom]

Java: SPTerm M ()

The Java method should return an object of class SPTerm which will be converted to a Prolog atom.

Prolog: [-boolean]

Java: boolean M ()

The Java should return a boolean. The value will be converted to a Prolog atom in {true,false}.

Prolog: [-chars]

Java: String M ()

The Java method should return an object of class String which will be converted to a list of character codes.

Prolog: [-string]

Java: String M ()

The Java method should return an object of class String which will be converted to an atom.

9.3.1 Calling Java from Prolog: An Example

The following is an simple, but fairly complete example of how a Java method can be called from Prolog.

First, we must write the resource file. Let us call it ‘simple.pl’.

% File: simple.pl

:- module(simple, [simple/2]).

:- use_module(library(jasper)).

:- load_foreign_resource(simple).

foreign(method(’Simple’, ’simpleMethod’, [static]), java, simple(+integer,[-integer])).

foreign_resource(simple,

[ method(’Simple’, ’simpleMethod’, [static])

]).

This file is the processed with the script splfr (see

Section 8.2.4 [Interface Predicates], page 171 ) to produce a foreign resource:

% splfr simple simple.pl

SICStus 3.8 (SunOS-5.5.1-sparc): Thu Aug 19 16:25:28 MET DST 1999

Licensed to SICS

{/var/tmp/aaaa004Cd.c generated, 20 msec}

Chapter 9: Mixing Java and Prolog 217 yes

Note that we do not specify any Java files to splfr as we would specify C files when building foreign resources for C code. This is because the C code can be compiled into the resource itself, while the Java code must be loaded at runtime into the JVM. This means that the resource will only contain the glue code for calling the JVM, and no actual Java code.

Hence, these resources are usually quite small.

Now, we need some Java code to call:

Simple.java: public class Simple

{ static int simpleMethod(int value)

{ return value*42;

}

}

This Java code must now be compiled. Refer to the documentation of your Java implementation exactly how to do this. On Solaris, this might look like:

% javac Simple.java

Now we are ready to call the method simple/2 from inside SICStus.

% sicstus

SICStus 3.8 (SunOS-5.5.1-sparc): Thu Aug 19 16:25:28 MET DST 1999

Licensed to SICS

| ?- compile(simple).

{compiling ...}

[...]

{compiled ... simple.pl in module simple, 160 msec 48640 bytes} yes

| ?- simple(17,X).

X = 714 ?

yes

| ?-

What has happened is that the predicate simple/2 has been installed as a predicate defined in Java (this is not exactly true; the predicate is defined as a C-function which calls the Java method). When we load the simple module, we will first load the jasper module (and thereby the JVM) and then load the simple foreign resource, which defines the simple/2 predicate.

218 SICStus Prolog

9.4 Calling Prolog from Java

Calling Prolog from Java is done by using the Java package jasper. This package contains a set of Java classes which can be used to create and manipulate terms, ask queries and request one or more solutions. The functionality provided by this set of classes is basically the same as the functionality provided by the C-Prolog interface (see

Chapter 8 [Mixing C and Prolog], page 165 ).

The usage is easiest described by an example. The following is a Java version of the train example. See

Section 8.8.1 [Train], page 202

.

import se.sics.jasper.*; public class Simple

{ public static void main(String argv[]) {

SICStus sp;

SPPredicate pred;

SPTerm from, to, way;

SPQuery query; int i; try

{ sp = new SICStus(argv,null); sp.load("train.ql"); pred = new SPPredicate(sp, "connected", 4, ""); to = new SPTerm(sp, "Orebro"); from = new SPTerm(sp, "Stockholm"); way = new SPTerm(sp).putVariable(); query = sp.openQuery(pred, new SPTerm[] { from, to, way, way });

}

} catch ( Exception e )

{ e.printStackTrace();

}

} while (query.nextSolution())

{

System.out.println(way.toString());

}

Chapter 9: Mixing Java and Prolog 219

It is assumed that the reader has read the section on

Section 9.1 [Getting Started], page 211

, which describes how to get the basics up and running.

This is how the example works:

1. Before any predicates can be called, the SICStus emulator must be initialized. This is done by instantiating the SICStus class. NOTE: This class must only be instantiated once per Java process. Multiple SICStus-objects are not supported.

In this example, we have specified null as the second argument to SICStus. This instructs SICStus to search for sprt.sav using its own internal methods, or by passing

-Dsicstus.path=[...] to the JVM.

Most methods take a reference to the SICStus object as their first argument. This is implicit in the rest of this chapter, unless otherwise stated.

2. The next step is to load the Prolog code. This is done by the method load. Corresponds to SP_load() in the C-interface. See

Section 8.7.4.2 [Loading Prolog Code], page 201

.

3. Now, everything is set up to start making queries. In order to make a query, the actual query term must be created. This is done by creating an object of the SPPredicate class:

SPPredicate pred = new SPPredicate(sp, "connected", 4, "");

4. At this point, we have created a predicate object for the predicate connected/4. It is now time to create the arguments for the query. The arguments are placed in an array which is passed to a suitable method to make the query.

The arguments consist of objects of the class SPTerm. For example, if we need two atoms and a variable for the query

| ?- connected(’Stockholm’, ’Orebro’, X, X).

the following Java code will do it for us: to = new SPTerm(sp, "Orebro"); from = new SPTerm(sp, "Stockholm"); way = new SPTerm(sp).putVariable();

5. Now it is time to make the query. As in the C-Prolog interface, there are three ways of making a query.

query(pred, args)

This method is useful if you are only interested in finding the first solution to a goal. In the case of connected/4 this is not the case; there are more than one solution.

queryCutFail(pred, args)

This method is useful if you are only interested in the side-effects of the query. As query() it only finds the first solution, and then it cuts away all other solutions and fails.

openQuery(pred, args)

This method is useful when you are interested in some or all solutions to the query. Since the connected/4 may give us multiple solution, this is what we will use.

220 SICStus Prolog

SPQuery query; query = sp.openQuery(pred, new SPTerm[] { from, to, way, way }); while (query.nextSolution())

System.out.println(way.toString()); query.close();

The openQuery method returns a reference to the query, an object of the SPQuery class. To obtain solutions, the method nextSolution() is called with no arguments. nextSolution returns true as long as there are more solutions, the example above will print the value of way until there are no more solutions. Observe that the query must be closed, even if nextSolution has indicated that there are no more solutions.

9.5 Jasper Package Class Reference

Detailed documentation of the classes in the jasper package can be found in the HTML documentation installed with SICStus and also at the SICStus documantation page http://www.sics.se/sicstus/docs/

9.6 Exception Handling

Exceptions are handled seamlessly between Java and Prolog. This means that exceptions can be thrown in Prolog and caught in Java and the other way around. For example, if a predicate called from Java raises an exception by raise_exception/1 and the predicate itself does not catch the exception, the Java-method which performed the query, queryCutFail() for example, will throw an exception containing the exception term. Symmetrically, a Java-exception thrown (and not caught) in a method called from Prolog will cause the corresponding predicate (simple/2 in the example above) to raise an exception containing the exception object (in the internal Prolog representation of a Java object).

Chapter 10: Handling Wide Characters 221

10 Handling Wide Characters

The chapter describes the SICStus Prolog features for handling wide characters. We will refer to these capabilities as Wide Character eXtensions, and will use the abbreviation

WCX.

10.1 Introduction

SICStus Prolog supports character codes up to 31 bits wide. It has a set of hooks for specifying how the character codes should be read in and written out to streams, how they should be classified (as letters, symbol-chars, etc.), and how strings of wide characters should be exchanged with the operating system. There are three sets of predefined hook functions supporting ISO 8859/1, UNICODE/UTF-8 and EUC external encodings, selectable using an environment variable. Alternatively, users may plug in their own definition of hook functions and implement arbitrary encodings.

Section 10.2 [WCX Concepts], page 221

, introduces the basic WCX concepts and presents their implementation in SICStus Prolog.

Section 10.3 [Prolog Level WCX Features], page 223 , gives an overview of those Prolog language features which are affected by wide

character handling.

Section 10.4 [WCX Environment Variables], page 223

, and

Section 10.5

[WCX Hooks], page 225

, describe the options for customization of SICStus Prolog WCX through environment variables and through the hook functions, respectively.

Section 10.6

[WCX Foreign Interface], page 231

, and

Section 10.7 [WCX Features in Libraries], page 233

, summarize the WCX extensions in the foreign language interface and in the libraries.

Section 10.8 [WCX Utility Functions], page 233 , describes the utility functions provided by

SICStus Prolog to help in writing the WCX hook functions, while

Section 10.9 [Representation of EUC Wide Characters], page 235

, presents the custom-made internal code-set for the EUC encoding. Finally

Section 10.10 [A Sample WCX Box], page 236 , describes an

example implementation of the WCX hook functions, which supports a composite character code set and four external encodings. The code for this example is included in the distribution as library(wcx_example).

10.2 Concepts

First let us introduce some notions concerning wide characters.

(Wide) character code an integer, possibly outside the 0..255 range.

SICStus Prolog allows character codes in the range 0..2147483647 (= 2^31-

1). Consequently, the built-in predicates for building and decomposing atoms from/into character codes (e.g., atom_codes/2, name/2, etc.) accept and produce lists of integers in the above range (excluding the 0 code).

Wide characters can be used in all contexts: in atoms (single quoted, or unquoted, depending on the character-type mapping), strings, character code notation (0’char), etc.

222 SICStus Prolog

External (stream) encoding a way of encoding sequences of wide characters as sequences of (8-bit) bytes, used in stream input and output.

SICStus Prolog has three different external stream encoding schemes built-in, selectable through an environment variable.

Furthermore it provides hooks for users to plug in their own external stream encoding functions. The builtin predicates put_code/1, get_code/1, etc. accept and return wide character codes, converting the bytes written or read using the external encoding in force.

Note that an encoding need not be able to handle the whole range of character codes allowed by SICStus Prolog.

Character code set a subset of the set {0, ..., 2^31-1} that can be handled by an external encoding.

SICStus Prolog assumes that the character code set is an extension of the

ASCII code set, i.e. it includes codes 0..127, and these codes are interpreted as ASCII characters. Note that ASCII characters can still have an arbitrary external encoding, cf. the usage flag WCX_CHANGES_ASCII, see

Section 10.5

[WCX Hooks], page 225

.

Character type mapping a function mapping each element of the character code set to one of the character categories (layout, small-letter, symbol-char, etc. see

[Token String], page 574

).

This is required for parsing tokens. The character-type mapping for non-ASCII characters is hookable in SICStus Prolog and has three built-in defaults, depending on the external encoding selected.

System encoding a way of encoding wide character strings, used or required by the operating system environment in various contexts (e.g. file names in open/3, command line options, as returned by prolog_flag(argv, Flags), etc.). The system encoding is hookable in SICStus Prolog and has two built-in defaults.

Internal encoding a way of encoding wide character strings internally within the SICStus Prolog system. This is of interest to the user only if the foreign language interface is used in the program, or a system encoding hook function needs to be written.

SICStus Prolog has a fixed internal encoding, which is UTF-8.

As discussed above there are several points where the users can influence the behavior of

SICStus Prolog. The user can decide on

• the character code set,

• the character-type mapping,

• the external encoding, and

• the system encoding.

Let us call WCX mode a particular setting of these parameters.

Chapter 10: Handling Wide Characters 223

Note that the selection of the character code set is conceptual only and need not be communicated to SICStus Prolog, as the decision materializes in the functions for the mapping and encodings.

10.3 Summary of Prolog level WCX features

SICStus Prolog has a Prolog flag, called wcx, whose value can be an arbitrary atom, and which is initialized to []. This flag is used at opening a stream, its value is normally passed to a user-defined hook function. This can be used to pass some information from Prolog to the hook function. In the example of

Section 10.10 [A Sample WCX Box], page 236 , which

supports the selection of external encodings on a stream-by-stream basis, the value of the wcx flag is used to specify the encoding to be used for the newly opened stream.

The value of the wcx flag can be overridden by supplying a wcx(Value) option to open/4 and load_files/2. If such an option is present, then the Value is passed on to the hook function.

The wcx flag has a reserved value. The value wci (wide character internal encoding) signifies that the stream should use the SICStus Prolog internal encoding (UTF-8), bypassing the hook functions supplied by the user. This is appropriate, e.g., if a file with wide characters is to be produced, which has to be readable irrespective of the (possibly user supplied) encoding scheme.

Wide characters generally require several bytes to be input or output.

Therefore, for each stream, SICStus Prolog keeps track of the number of bytes input or output, in addition to the number of (wide) characters. Accordingly there is a built-in predicate byte_ count(+Stream,?N ) for accessing the number of bytes read/written on a stream.

Note that the predicate character_count/2 returns the number of characters read or written, which may be less than the number of bytes, if some of the characters are multibyte.

(On output streams the byte_count/2 can also be less than the character_count/2, if some codes, not belonging to the code-set handled, are not written out.)

Note that if a stream is opened as a binary stream: open(..., ..., ..., [type(binary)]) then no wide character handling will take place; every character output will produce a single byte on the stream, and every byte input will be considered a separate character.

10.4 Selecting the WCX mode using environment variables

When the SICStus Prolog system starts up, its WCX mode is selected according to the value of the SP_CTYPE environment variable. The supported values of the SP_CTYPE environment variable are the following: iso_8859_1 (default)

224 SICStus Prolog character code set:

0..255

character-type mapping: according to the ISO 8859/1 standard, see

[Token String], page 574

.

external encoding: each character code is mapped to a single byte on the stream with the same value (trivial encoding).

utf8 character code set:

0..2147483647 (= 2^31-1) character-type mapping: according to ISO 8859/1 for codes 0..255. All codes above 255 are considered small-letters.

external encoding:

UTF-8

This WCX mode is primarily intended to support the UNICODE character set, but it also allows the input and output of character codes above the 16-bit

UNICODE character code range.

euc character code set: a subset of 0..8388607 The exact character code set is described in

Section 10.9 [Representation of EUC Wide Characters], page 235

, together with its mapping to the standard external encoding.

character-type mapping: according to ISO 8859/1 for codes 0..127. All codes above 127 are considered small-letters.

external encoding:

EUC encoding with the lengths of the sub-code-sets dependent on the locale.

In all three cases the system encoding is implemented as truncation to 8-bits, i.e. any code output to the operating system is taken modulo 256, any byte coming from the operating system is mapped to the code with the same value.

The figure below shows an example interaction with SICStus Prolog in EUC mode. For the role of the SP_CSETLEN environment variable, see

Section 10.9 [Representation of EUC

Wide Characters], page 235 .

Chapter 10: Handling Wide Characters 225

SICStus Prolog in EUC mode

10.5 Selecting the WCX mode using hooks

Users can have complete control over the way wide characters are handled by SICStus Prolog if they supply their own definitions of appropriate hook functions. A set of such functions, implementing a specific environment for handling wide characters is called a WCX box. A sample WCX box is described below (see

Section 10.10 [A Sample WCX Box], page 236 ).

Plugging-in of the WCX hook functions can be performed by calling void SP_set_wcx_hooks ( int usage,

SP_WcxOpenHook *wcx_open,

SP_WcxCloseHook *wcx_close,

SP_WcxCharTypeHook *wcx_chartype,

SP_WcxConvHook *wcx_from_os,

226 SICStus Prolog

SP_WcxConvHook *wcx_to_os);

The effect of SP_set_wcx_hooks() is controlled by the value of usage. The remaining arguments are pointers to appropriate hook functions or NULL values, the latter implying that the hook should take some default value.

There are three independent aspects to be controlled, and usage should be supplied as a bitwise OR of chosen constant names for each aspect. The defaults have value 0, so need not be included. The aspects are the following: a. decide on the default code-set

This decides the default behavior of the wcx_open and wcx_chartype hook functions

(if both are supplied by the user, the choice of the default is irrelevant). The possible values are:

WCX_USE_LATIN1 (default)

WCX_USE_UTF8

WCX_USE_EUC

Select the behavior described above under titles iso_8859_1, utf8, and euc, respectively; see

Section 10.4 [WCX Environment Variables], page 223

.

b. decide on the default system encoding

The flags below determine what function to use for conversion from/to the operating system encoding, if such functions are not supplied by the user through the wcx_from_ os and wcx_to_os arguments (if both are supplied by the user, the choice of default is irrelevant).

WCX_OS_8BIT (default)

Select the “truncation to 8-bits” behavior.

WCX_OS_UTF8

Select the UTF-8 encoding to be used for all communication with the operating system.

c. decide on the preservation of ASCII, i.e., the codes in 0..127

This is important if some of the conversion functions (wcx_from_os, wcx_to_os, and wcx_getc, wcx_putc, see later) are user-defined. In such cases it may be beneficial for the user to inform SICStus Prolog whether the supplied encoding functions preserve

ASCII characters. (The default encodings do preserve ASCII.)

WCX_PRESERVES_ASCII (default)

Declare that the encodings preserve all ASCII characters, i.e. getting or putting an ASCII character need not go through the conversion functions, and for strings containing ASCII characters only, the system encoding conversions need not be invoked.

WCX_CHANGES_ASCII

Force the system to use the conversion functions even for ASCII characters and strings.

We now describe the role of the arguments following usage in the argument list of SP_set_ wcx_hooks().

Chapter 10: Handling Wide Characters 227

SP_WcxOpenHook *wcx_open where typedef void (SP_WcxOpenHook) (SP_stream *s, unsigned long option, int context);

This function is called by SICStus Prolog for each s stream opened, except when the encoding to be used for the stream is pre-specified (binary files, files opened using the wci option, and the C streams created with contexts SP_

STREAMHOOK_WCI and SP_STREAMHOOK_BIN).

The main task of the wcx_open hook is to associate the two WCX-processing functions with the stream, by storing them in the appropriate fields of the

SP_stream data structure:

SP_WcxGetcHook *wcx_getc;

SP_WcxPutcHook *wcx_putc;

These fields are pointers to the functions performing the external decoding and encoding as described below. They are initialized to functions that truncate to

8 bits on output and zero-extend to 31 bits on input.

SP_WcxGetcHook *wcx_getc where typedef int (SP_WcxGetcHook) (int first_byte,

SP_stream *s, long *pbyte_count);

This function is generally invoked whenever a character has to be read from a stream. Before invoking this function, however, a byte is read from the stream by SICStus Prolog itself. If the byte read is an ASCII character (its value is < 128), and WCX_PRESERVES_ASCII is in force, then the byte read is deemed to be the next character code, and wcx_getc is not invoked. Otherwise, wcx_getc is invoked with the byte and stream in question and is expected to return the next character code.

The wcx_getc function may need to read additional bytes from the stream, if first byte signifies the start of a multi-byte character.

A byte may be read from the stream s in the following way: byte = s->sgetc((long)s->user_handle);

The wcx_getc function is expected to increment its *pbyte_count argument by 1 for each such byte read.

The default wcx_open hook will install a wcx_getc function according to the usage argument. The three default external decoding functions are also available to users through the SP_wcx_getc() function (see

Section 10.8 [WCX Utility Functions], page 233 ).

SP_WcxPutcHook *wcx_putc where typedef int (SP_WcxPutcHook) (int char_code,

SP_stream *s, long *pbyte_count);

This function is generally invoked whenever a character has to be written to a stream. However, if the character code to be written is an ASCII character (its value is < 128), and WCX_PRESERVES_

ASCII is in force, then the code is written directly on the stream,

228 SICStus Prolog and wcx_putc is not invoked. Otherwise, wcx_putc is invoked with the character code and stream in question and is expected to do whatever is needed to output the character code to the stream.

This will require outputting one or more bytes to the stream. A byte byte can be written to the stream s in the following way: return_code = s->sputc(byte,(long)s->user_handle);

The wcx_putc function is expected to return the return value of the last invocation of s->sputc, or -1 as an error code, if incapable of outputting the character code. The latter may be the case, for example, if the code to be output does not belong to the character code set in force. It is also expected to increment its *pbyte_count argument by 1 for each byte written.

The default wcx_open hook function will install a wcx_putc function according to the usage argument. The three default external encoding functions are also available to users through the SP_ wcx_putc() function (see

Section 10.8 [WCX Utility Functions], page 233 ).

In making a decision regarding the selection of these WCX-processing functions, the context and option arguments of the wcx_open hook can be used. The context argument encodes the context of invocation. It is one of the following values

SP_STREAMHOOK_STDIN

SP_STREAMHOOK_STDOUT

SP_STREAMHOOK_STDERR for the three standard streams,

SP_STREAMHOOK_OPEN for streams created by open

SP_STREAMHOOK_NULL for streams created by open_null_stream

SP_STREAMHOOK_LIB for streams created from the libraries

SP_STREAMHOOK_C, SP_STREAMHOOK_C+1, ...

for streams created from C code via SP_make_stream()

The option argument comes from the user and it can carry some WCX-related information to be associated with the stream opened. For example, this can be used to implement a scheme supporting multiple encodings, supplied on a stream-by-stream basis, as shown in the sample WCX-box (see

Section 10.10

[A Sample WCX Box], page 236

).

If the stream is opened from Prolog code, the option argument for this hook function is derived from the wcx(Option) option of open/4 and load_files/2.

If this option is not present, or the stream is opened using some other built-in, then the value of the wcx prolog flag will be passed on to the open hook.

Chapter 10: Handling Wide Characters 229

If the stream is opened from C, via SP_make_stream(), then the option argument will be the value of the prolog flag wcx.

There is also a variant of SP_make_stream(), called SP_make_stream_ context() which takes two additional arguments, the option and the context, to be passed on to the wcx_open hook (see

Section 10.6 [WCX Foreign Interface], page 231

).

The wcx_open hook can associate the information derived from option with the stream in question using a new field in the SP_stream data structure: void

*wcx_info, initialized to NULL. If there is more information than can be stored in this field, or if the encoding to be implemented requires keeping track of a state, then the wcx_open hook should allocate sufficient amount of memory for storing the information and/or the state, using SP_malloc(), and deposit a pointer to that piece of memory in wcx_info.

The default wcx_open hook function ignores its option and context arguments and sets the wcx_getc and wcx_putc stream fields to functions performing the external decoding and encoding according to the usage argument of SP_set_ wcx_hooks().

SP_WcxCloseHook *wcx_close where typedef void (SP_WcxCloseHook) (SP_stream *s);

This hook function is called whenever a stream is closed, for which the wcx_open hook was invoked at its creation. The argument s points to the stream being closed. It can be used to implement the closing activities related to external encoding, e.g. freeing any memory allocated in wcx_open hook.

The default wcx_close hook function does nothing.

SP_WcxCharTypeHook *wcx_chartype where typedef int (SP_WcxCharTypeHook) (int char_code);

This function should be prepared to take any char_code >= 128 and return one of the following constants:

CHT_LAYOUT_CHAR for additional characters in the syntactic category layout-char,

CHT_SMALL_LETTER for additional characters in the syntactic category small-letter,

CHT_CAPITAL_LETTER for additional characters in the syntactic category capital-letter,

CHT_SYMBOL_CHAR for additional characters in the syntactic category symbol-char,

CHT_SOLO_CHAR for additional characters in the syntactic category solo-char.

Regarding the meaning of these syntactic categories, see

[Token String], page 574

.

The value returned by this function is not expected to change over time, therefore, for efficiency reasons, its behavior is cached. The cache is cleared by

SP_set_wcx_hooks().

230 SICStus Prolog

As a help in implementing this function, SICStus Prolog provides the function SP_latin1_chartype(), which returns the character type category for the codes 1..255 according to the ISO 8859/1 standard.

Note that if a character code >= 512 is categorized as a layout-char, and a character with this code occurs within an atom being written out in quoted form (e.g. using writeq) in native sicstus mode (as opposed to iso mode), then this code will be output as itself, rather than an octal escape sequence.

This is because in sicstus mode escape sequences consist of at most 3 octal digits.

SP_WcxConvHook *wcx_to_os where typedef char* (SP_WcxConvHook) (char *string, int context);

This function is normally called each time SICStus Prolog wishes to communicate a string of possibly wide characters to the operating system. However, if the string in question consists of ASCII characters only, and WCX_PRESERVES_

ASCII is in force, then wcx_to_os may not be called, and the original string may be passed to the operating system.

The first argument of wcx_to_os is a zero terminated string, using the internal encoding of SICStus Prolog, namely UTF-8. The function is expected to convert the string to a form required by the operating system, in the context described by the second, context argument, and to return the converted string.

The conversion should be done in a memory area controlled by this function

(preferably a static buffer, reused each time the function is called).

The second argument specifies the context of conversion. It can be one of the following integer values:

WCX_FILE the string is a file-name,

WCX_OPTION the string is a command, a command line argument or an environment variable,

WCX_WINDOW_TITLE the string is a window title,

WCX_C_CODE the string is a C identifier (used, e.g., in the glue code)

SICStus Prolog provides a utility function SP_wci_code(), see below, for obtaining a wide character code from a UTF-8 encoded string, which can be used to implement the wcx_to_os hook function.

The default of the wcx_to_os function depends on the usage argument of SP_ set_wcx_hooks(). If the value of usage includes WCX_OS_UTF8, then the function does no conversion, as the operating system uses the same encoding as

SICStus Prolog. If the value of usage includes WCX_OS_8BIT, then the function decodes the UTF-8 encoded string and converts this sequence of codes into a sequence of bytes by truncating each code to 8 bits.

Note that the default wcx_to_os functions ignore their context argument.

Chapter 10: Handling Wide Characters 231

SP_WcxConvHook *wcx_from_os where typedef char* (SP_WcxConvHook) (char *string, int context);

This function is called each time SICStus Prolog receives from the operating system a zero terminated sequence of bytes possibly encoding a wide character string. The function is expected to convert the byte sequence, if needed, to a string in the internal encoding of SICStus Prolog (UTF-8), and return the converted string. The conversion should be done in a memory area controlled by this function (preferably a static buffer, reused each time the function is called, but different from the buffer used in wcx_to_os).

The second argument specifies the context of conversion, as in the case of wcx_ to_os.

SICStus Prolog provides a utility function SP_code_wci(), see below, for converting a character code (up to 31 bits) into UTF-8 encoding, which can be used to implement the wcx_from_os hook function.

The default of the wcx_from_os function depends on the usage argument of

SP_set_wcx_hooks(). If the value of usage includes WCX_OS_UTF8, then the function does no conversion. If the value of usage includes WCX_OS_8BIT, then the function transforms the string of 8-bit codes into an UTF-8 encoded string.

Note that the default wcx_from_os functions ignore their context argument.

10.6 Summary of WCX features in the foreign interface

All strings passed to foreign code, or expected from foreign code, which correspond to atoms or lists of character codes on the Prolog side, are in the internal encoding form, UTF-8.

Note that this is of concern only if the strings contain non-ASCII characters (e.g. accented letters in the latin1 encoding).

Specifically, the C arguments corresponding to the following foreign specifications are passed and received as strings in the internal encoding:

+chars +string +string(N)

-chars -string -string(N)

[-chars] [-string] [-string(N)]

Similarly, the following functions defined in the foreign interface expect and deliver internally encoded strings in their char * and char ** arguments.

int SP_put_string(SP_term_ref t, char *name) int SP_put_list_chars(SP_term_ref t, SP_term_ref tail, char *s) int SP_put_list_n_chars(SP_term_ref t, SP_term_ref tail, long n, char *s) int SP_get_string(SP_term_ref t, char **name) int SP_get_list_chars(SP_term_ref t, char **s) int SP_get_list_n_chars(SP_term_ref t, SP_term_ref tail, long n, long *w, char *s) void SP_puts(char *string) void SP_fputs(char *string, SP_stream *s)

232 SICStus Prolog int SP_printf(char *format, ...) int SP_fprintf(SP_stream *s, char *format, ...) unsigned long SP_atom_from_string(char *s) char *SP_string_from_atom(unsigned long a)

SP_pred_ref SP_predicate(char *name_string, long arity, char *module_string) int SP_load(char *filename) int SP_restore(char *filename)

The following functions deliver or accept wide character codes (up to 31 bits), and read or write them on the appropriate stream in the external encoding form: int SP_getc(void) int SP_fgetc(SP_stream *s) void SP_putc(int c) void SP_fputc(int c, SP_stream *s)

In the following function, strings are expected in the encoding format relevant for the operating system: int SP_initialize(int argc, char **argv, char *boot_path)

Here argv is an array of strings, as received from the operating system. These strings will be transformed to internal form using the wcx_from_os(WCX_OPTION,...) hook function.

Also boot_path is expected to be in the format file names are encoded, and wcx_from_ os(WCX_FILE,...) will be used to decode it.

There are other functions in the foreign interface that take or return strings. For these, the encoding is not relevant, either because the strings are guaranteed to be ASCII (SP_error_ message(), SP_put_number_chars(), SP_get_number_chars()), or because the strings in question have no relation to Prolog code, as in SP_on_fault(), SP_raise_fault().

The SP_make_stream_context() foreign interface function is a variant of SP_make_ stream() with two additional arguments: option and context. This extended form can be used to create streams from C with specified WCX features.

The context argument the SP_make_stream_context function can be one of the following values:

SP_STREAMHOOK_WCI

SP_STREAMHOOK_BIN

SP_STREAMHOOK_C, SP_STREAMHOOK_C+1, ...

SP_STREAMHOOK_WCI means that input and output on the given stream should be performed using the SICStus internal encoding scheme, UTF-8, while SP_STREAMHOOK_BIN indicates that no encoding should be applied (binary files).

In the last two cases the wcx_open hook will not be called. In all other cases SP_make_ stream_context will call the wcx_open hook function, with the option and context sup-

Chapter 10: Handling Wide Characters 233 plied to it. The option argument of SP_make_stream_context can be the standard representation of a Prolog atom, or the constant SP_WCX_FLAG, which prescribes that the value of the prolog flag wcx should be supplied to the open hook function.

The user may add further context constants for his own use, with values greater than

SP_STREAMHOOK_C.

10.7 Summary of WCX-related features in the libraries

Some libraries are affected by the introduction of wide characters.

When using library(jasper) SICStus Prolog properly receives non-ASCII strings from

Java, and similarly, non-ASCII strings can be correctly passed to Java. This is in contrast with versions of SICStus Prolog earlier then 3.8 (i.e. without the WCX extensions), where, for example, strings containing non-ASCII characters passed from Java to Prolog resulted in an UTF-8 encoded atom or character code list on the Prolog side.

Several predicates in libraries sockets, system and tcltk create streams. These now use the SP_make_stream_context() function, with SP_WCX_FLAG as the option and the relevant

SP_STREAMHOOK_LIB constant as the context argument. For example, if the WCX mode is set using environment variables (see

Section 10.4 [WCX Environment Variables], page 223

), then this implies that the selected encoding will be used for streams created in the libraries.

E.g., if the SP_CTYPE environment variable is set to utf8, then the output of non-ASCII characters to a socket stream will be done using UTF-8 encoding. If a wcx_open hook is supplied, then the user is free to select a different encoding for the libraries, as he is informed about the stream being opened by a library through the context argument of the wcx_open function.

Some of the arguments of library predicates contain atoms which are file names, environment variable names, commands, etc. If these contain non-ASCII characters, then they will be passed to the appropriate operating system function following a conversion to the system encoding in force (wcx_to_os hook), and similarly such atoms coming from the OS functions undergo a conversion from system encoding (wcx_from_os). Note however that host names

(e.g. in system:host_name(S)) are assumed to be consisting of ASCII characters only.

10.8 WCX related utility functions

The default functions for reading in and writing out character codes using one of the three supported encodings are available through

SP_WcxGetcHook *SP_wcx_getc(int usage);

SP_WcxPutcHook *SP_wcx_putc(int usage);

These functions return the decoding/encoding functions appropriate for usage, where the latter is one of the constants WCX_USE_LATIN1, WCX_USE_UTF8, WCX_USE_EUC.

234 SICStus Prolog

The following utility functions may be useful when dealing with wide characters in internal encoding (WCI). These functions are modeled after multibyte character handling functions of Solaris.

int SP_wci_code(int *pcode, char *wci);

SP_wci_code() determines the number of bytes that comprise the internally encoded character pointed to by wci. Also, if pcode is not a null pointer, SP_ wci_code() converts the internally encoded character to a wide character code and places the result in the object pointed to by pcode. (The value of the wide character corresponding to the null character is zero.) At most WCI_MAX_BYTES bytes will be examined, starting at the byte pointed to by wci.

If wci is a null pointer, SP_wci_code() simply returns 0. If wci is not a null pointer, then, if wci points to the null character, SP_wci_code() returns 0; if the next bytes form a valid internally encoded character, SP_wci_code() returns the number of bytes that comprise the internal encoding; otherwise wci does not point to a valid internally encoded character and SP_wci_code() returns the negated length of the invalid byte sequence. This latter case can not happen, if wci points to the beginning of a Prolog atom string, or to a position within such a string reached by repeated stepping over correctly encoded wide characters.

WCI_MAX_BYTES

WCI_MAX_BYTES is a constant defined by SICStus Prolog showing the maximal length (in bytes) of the internal encoding of a single character code. (As the internal encoding is UTF-8, this constant has the value 6).

int SP_wci_len(char *wci);

SP_wci_len() determines the number of bytes comprising the multi-byte character pointed to by wci. It is equivalent to:

SP_wci_code((int *)0, wci); int SP_code_wci(char *wci, int code);

SP_code_wci() determines the number of bytes needed to represent the internal encoding of the character code, and, if wci is not a null pointer, stores the internal encoding in the array pointed to by wci. At most WCI_MAX_BYTES bytes are stored.

SP_code_wci() returns -1 if the value of code is outside the wide character code range; otherwise it returns the number of bytes that comprise the internal encoding of code.

int SP_latin1_chartype(int char_code);

SP_latin1_chartype returns the character type category of the character code char_code, according to the ISO 8859/1 code-set. The char_code value is assumed to be in the 1..255 range.

Chapter 10: Handling Wide Characters 235

10.9 Representation of EUC wide characters

As opposed to UNICODE, the definition of EUC specifies only the external representation.

The actual wide character codes assigned to the multibyte characters are not specified.

UNIX systems supporting EUC have their own C data type, wchar_t, which stores a wide character, but the mapping between this type and the external representation is not standardized.

We have decided to use a custom made mapping from the EUC encoding to the character code set, as opposed to using the UNIX type wchar_t. This decision was made so that the code set is machine independent and results in a compact representation of atoms.

EUC consists of four sub-code-sets, three of which can have multibyte external representation. Sub-code-set 0 consists of ASCII characters and is mapped one-to-one to codes 0..127.

Sub-code-set 1 has an external representation of one to three bytes in the range 128-255, the length determined by the locale. Sub-code-sets 2 and 3 are similar, but their external representation is started by a so called single shift character code, known as SS2 and SS3, respectively. The following table shows the mapping from the EUC external encoding to

SICStus Prolog character codes.

Subcode-set External encoding Character code (binary)

0 0xxxxxxx 00000000 00000000 0xxxxxxx

1

2

1xxxxxxx

1xxxxxxx 1yyyyyyy

1xxxxxxx 1yyyyyyy 1zzzzzzzz

00000000 00000000 1xxxxxxx

00000000 xxxxxxx0 1yyyyyyy

0xxxxxxx yyyyyyy0 1zzzzzzz

SS2 1xxxxxxx

SS2 1xxxxxxx 1yyyyyyy

00000000 00000001 0xxxxxxx

00000000 xxxxxxx1 0yyyyyyy

SS2 1xxxxxxx 1yyyyyyy 1zzzzzzzz 0xxxxxxx yyyyyyy1 0zzzzzzz

3 SS3 1xxxxxxx 00000000 00000001 1xxxxxxx

SS3 1xxxxxxx 1yyyyyyy 00000000 xxxxxxx1 1yyyyyyy

SS3 1xxxxxxx 1yyyyyyy 1zzzzzzzz 0xxxxxxx yyyyyyy1 1zzzzzzz

For sub-code-sets other than 0, the sub-code-set length indicated by the locale determines which of three mappings are used (but see below the SP_CSETLEN environment variable).

When converting SICStus Prolog character codes to EUC on output, we ignore bits that have no significance in the mapping selected by the locale.

The byte lengths associated with the EUC sub-code-sets are determined by using the csetlen() function.

If this function is not available in the system configuration used, then Japanese Solaris lengths are assumed, namely 2, 1, 2 for sub-code-sets 1, 2, and 3, respectively (the lengths exclude the single shift character).

To allow experimentation with sub-code-sets differing from the locale, the sub-code-set length values can be overridden by setting the SP_CSETLEN environment variable to xyz,

236 SICStus Prolog where x, y, and z are digits in the range 1..3. Such a setting will cause the sub-code-sets 1,

2, 3 to have x, y, and z associated with them as their byte lengths.

10.10 A sample Wide Character Extension (WCX) box

This example implements a WCX box supporting the use of four external encodings within the same SICStus Prolog invocation: ISO Latin1, ISO Latin2 (ISO 8859/2), UNICODE, and EUC. The code is included in the distribution as library(wcx_example).

The default encoding functions supplied in SICStus Prolog deal with a single encoding only.

However, the interface does allow the implementation of WCX boxes supporting different encodings for different streams.

A basic assumption in SICStus Prolog is that there is a single character set. If we are to support multiple encodings we have to map them into a single character set. For example, the single-byte character sets ISO Latin1 and ISO Latin2 can be easily mapped to the

Unicode character set. On the other hand there does not seem to be a simple mapping of the whole of EUC character set to UNICODE or the other way round.

Therefore, in this example, we use a composite character set, which covers both EUC and

Unicode, but does not deal with unifying the character codes of characters which appear in both character sets, except for the case of ASCII characters.

The figure below depicts the structure of the composite character set of the sample WCX box.

|

|

|

.------------------.

| EUC |

|

|

.+++++++++++++++++++++++++++.

| + ASCII * LATIN1 |

.--------+=========*==========

+ LATIN2 *

+**********

+

+

+ UNICODE +

.+++++++++++++++++++++++++++.

+

+

+

+

+

+

This character code set uses character codes up to 24 bit wide:

0 =< code =< 2^16-1

A UNICODE character with the given code, including ASCII.

code = 2^16 + euc_code

A non-ASCII EUC character with code euc_code (as described in

Section 10.9

[Representation of EUC Wide Characters], page 235 ).

Chapter 10: Handling Wide Characters 237

The four external encodings supported by the sample WCX box can be specified on a stream-by-stream basis, by supplying a wcx(ENC) option to open/4, where ENC is one of the atoms latin1, latin2, unicode or euc.

The mapping of these external encodings to the composite character code set is done in the following way: latin1 latin2 is mapped one-to-one to UNICODE codes 0x0..0xff

is mapped to UNICODE codes 0x0..0x02dd, using an appropriate conversion table for the non-ASCII part.

unicode assumes UTF-8 external encoding and maps one-to-one to the 0x0..0xffff UNI-

CODE range.

euc assumes EUC external encoding and maps sub-code-set 0 to UNICODE range

0x0..0x7f, and sub-code-sets 1-3 to internal codes above 0xffff, as shown above.

Note that in order to support this composite character code set, we had to give up the ability to read and write UTF-8-encoded files with character codes above 0xffff (which is possible using the built-in utf8 WCX-mode of SICStus Prolog, (see

Section 10.3 [Prolog

Level WCX Features], page 223

)).

The example uses a primitive character-type mapping: characters in the 0x80-0xff range are classified according to the latin1 encoding, above that range all characters are considered small-letters. However, as an example of re-classification, code 0xa1 (inverted exclamation mark) is categorized as solo-char.

The default system encoding is used (truncate to 8-bits).

The box has to be initialized by calling the C function wcx_setup(), which first reads the environment variable WCX_TYPE, and uses its value as the default encoding. It then calls

SP_set_wcx_hooks(), and initializes its own conversion tables. In a runtime system wcx_ setup() should be called before SP_initialize(), so that it effects the standard streams created there. The second phase of initialization, wcx_init_atoms(), has to be called after

SP_initialize(), to set up variables storing the atoms naming the external encodings.

In a development system the two initialization phases can be put together, this is implemented as wcx_init(), and is declared to be a foreign entry point in wcx.pl.

On any subsequent creation of a stream, the hook function my_wcx_open() is called. This sets the wide character get and put function pointers in the stream according to the atom supplied in the wcx(...) option, or according to the value of the prolog flag wcx.

Within the put function it may happen that a character code is to be output, which the given encoding cannot accommodate (a non-ASCII Unicode character on an EUC stream or vice-versa). No bytes are output in such a case and -1 is returned as an error code.

There is an additional foreign C function implemented in the sample WCX box: wcx_set_ encoding(), available from Prolog as set_encoding/2. This allows changing the encoding

238 SICStus Prolog of an already open stream. This is used primarily for standard input-output streams, while experimenting with the box.

Chapter 11: Programming Tips and Examples 239

11 Programming Tips and Examples

This chapter describes how to write clean programs that will execute efficiently. To some extent, writing efficient code in any language requires basic knowledge of its compiler, and we will mention some important properties of the SICStus Prolog compiler. A number of simple examples of Prolog programming are also given.

11.1 Programming Guidelines

A lot of clarity and efficiency is gained by sticking to a few basic rules. This list is necessarily very incomplete. The reader is referred to textbooks such as [O’Keefe 90] for a thorough exposition of the elements of Prolog programming style and techniques.

• Don’t write code in the first place if there is a library predicate that will do the job.

• Write clauses representing base case before clauses representing recursive cases.

• Input arguments before output arguments in clause heads and goals.

• Use pure data structures instead of data base changes.

• Use cuts sparingly, and only at proper places (see

Section 3.5 [Cut], page 48 ). A cut

should be placed at the exact point that it is known that the current choice is the correct one: no sooner, no later.

• Make cuts as local in their effect as possible. If a predicate is intended to be determinate, define it as such; do not rely on its callers to prevent unintended backtracking.

• Binding output arguments before a cut is a common source of programming errors, so don’t do it.

• Replace cuts by if-then-else constructs if the test is simple enough (see

Section 11.4

[If-Then-Else], page 240 ).

• Use disjunctions sparingly, always put parentheses around them, never put parentheses around the individual disjuncts, never put the ‘;’ at the end of a line.

• Write the clauses of a predicate so that they discriminate on the principal functor of the first argument (see below). For maximum efficiency, avoid “defaulty” programming

(“catch-all” clauses).

• Don’t use lists ([...]), “round lists” ((...)), or braces ({...}) to represent compound terms, or “tuples”, of some fixed arity. The name of a compound term comes for free.

11.2 Indexing

The clauses of any predicate are indexed according to the principal functor of the first argument in the head of the clause. This means that the subset of clauses which match a given goal, as far as the first step of unification is concerned, is found very quickly, in practically constant time. This can be very important where there is a large number of clauses for a predicate. Indexing also improves the Prolog system’s ability to detect determinacy—important for conserving working storage, and strongly related to last call optimization (see below).

240 SICStus Prolog

Indexing applies to interpreted clauses as well as to compiled clauses.

11.3 Last Call Optimization

The compiler incorporates last call optimization to improve the speed and space efficiency of determinate predicates.

When execution reaches the last goal in a clause belonging to some predicate, and provided there are no remaining backtrack points in the execution so far of that predicate, all of the predicate’s local working storage is reclaimed before the final call, and any terms it has created become eligible for garbage collection. This means that programs can now recurse to arbitrary depths without necessarily exceeding core limits. For example: cycle(State) :- transform(State, State1), cycle(State1).

where transform/2 is a determinate predicate, can continue executing indefinitely, provided each individual term, State, is not too large. The predicate cycle is equivalent to an iterative loop in a conventional language.

To take advantage of last call optimization one must ensure that the Prolog system can recognize that the predicate is determinate at the point where the recursive call takes place.

That is, the system must be able to detect that there are no other solutions to the current goal to be found by subsequent backtracking. In general this involves reliance on the Prolog compiler’s indexing and/or use of cut; see

Section 3.5 [Cut], page 48 .

11.4 If-Then-Else Compilation

Ordinary disjunction, (P;Q), is treated by the compiler as an anonymous predicate with two clauses, and the execution of a disjunction relies on backtracking to explore the two disjuncts.

If-then-else statements of the form:

(If -> Then; Else) are recognized by the compiler and are under certain conditions compiled to code that is much more efficient than the corresponding disjunction, essentially turning the If test to a conditional jump and often avoiding costly backtracking altogether.

For this optimization to be effective, the test must be a conjunction of a restricted set of built-in predicates (roughly, arithmetic tests, type tests and term comparisons).

This optimization is actually somewhat more general than what is described above. A sequence of guarded clauses:

Head1 :- Guard1, !, Body1.

...

Headm :- Guardm, !, Bodym.

Chapter 11: Programming Tips and Examples 241

Headn :- Bodym.

is eligible for the same optimization, provided that the arguments of the clause heads are all unique variables and that the “guards” are simple tests as described above.

11.5 Programming Examples

The rest of this chapter contains a number of simple examples of Prolog programming, illustrating some of the techniques described above.

11.5.1 Simple List Processing

The goal concatenate(L1,L2,L3) is true if list L3 consists of the elements of list L1 concatenated with the elements of list L2. The goal member(X,L) is true if X is one of the elements of list L. The goal reverse(L1,L2) is true if list L2 consists of the elements of list L1 in reverse order.

concatenate([], L, L).

concatenate([X|L1], L2, [X|L3]) :- concatenate(L1, L2, L3).

member(X, [X|_]).

member(X, [_|L]) :- member(X, L).

reverse(L, L1) :- reverse_concatenate(L, [], L1).

reverse_concatenate([], L, L).

reverse_concatenate([X|L1], L2, L3) :reverse_concatenate(L1, [X|L2], L3).

11.5.2 Family Example (descendants)

The goal descendant(X,Y ) is true if Y is a descendant of X.

descendant(X, Y) :- offspring(X, Y).

descendant(X, Z) :- offspring(X, Y), descendant(Y, Z).

offspring(abraham, ishmael).

offspring(abraham, isaac).

offspring(isaac, esau).

offspring(isaac, jacob).

If for example the query

| ?- descendant(abraham, X).

is executed, Prolog’s backtracking results in different descendants of Abraham being returned as successive instances of the variable X, i.e.

242 SICStus Prolog

X = ishmael

X = isaac

X = esau

X = jacob

11.5.3 Association List Primitives

These predicates implement “association list” primitives. They use a binary tree representation. Thus the time complexity for these predicates is O(lg N), where N is the number of keys. These predicates also illustrate the use of compare/3 (see

Section 7.3 [Term Compare], page 129 ) for case analysis.

The goal get_assoc(Key, Assoc, Value) is true when Key is identical to one of the keys in Assoc, and Value unifies with the associated value.

get_assoc(Key, t(K,V,L,R), Val) :compare(Rel, Key, K), get_assoc(Rel, Key, V, L, R, Val).

get_assoc(=, _, Val, _, _, Val).

get_assoc(<, Key, _, Tree, _, Val) :get_assoc(Key, Tree, Val).

get_assoc(>, Key, _, _, Tree, Val) :get_assoc(Key, Tree, Val).

11.5.4 Differentiation

The goal d(E1, X, E2) is true if expression E2 is a possible form for the derivative of expression E1 with respect to X.

d(X, X, D) :- atomic(X), !, D = 1.

d(C, X, D) :- atomic(C), !, D = 0.

d(U+V, X, DU+DV) :- d(U, X, DU), d(V, X, DV).

d(U-V, X, DU-DV) :- d(U, X, DU), d(V, X, DV).

d(U*V, X, DU*V+U*DV) :- d(U, X, DU), d(V, X, DV).

d(U**N, X, N*U**N1*DU) :- integer(N), N1 is N-1, d(U, X, DU).

d(-U, X, -DU) :- d(U, X, DU).

11.5.5 Use of Meta-Logical Predicates

This example illustrates the use of the meta-logical predicates var/1, arg/3, and functor/3

(see

Section 7.7 [Meta Logic], page 145 ). The procedure call variables(Term, L, [])

instantiates variable L to a list of all the variable occurrences in the term Term. e.g.

| ?- variables(d(U*V, X, DU*V+U*DV), L, []).

L = [U,V,X,DU,V,U,DV]

Chapter 11: Programming Tips and Examples 243 variables(X, [X|L0], L) :- var(X), !, L = L0.

variables(T, L0, L) :-

% nonvar(T), functor(T, _, A), variables(0, A, T, L0, L).

variables(A, A, _, L0, L) :- !, L = L0.

variables(A0, A, T, L0, L) :-

% A0<A,

A1 is A0+1, arg(A1, T, X), variables(X, L0, L1), variables(A1, A, T, L1, L).

11.5.6 Use of Term Expansion

This example illustrates the use of user:term_expansion/[2,4] to augment the built-in predicate expand_term/2 which works as a filter on the input to compile and consult. The code below will allow the declaration ‘:- wait f/3’ as an alias for ‘:- block f(-,?,?)’.

Wait declarations were used in previous versions of SICStus Prolog.

Note the multifile declaration, which prevents this user:term_expansion/[2,4] clause from erasing any other clauses for the same predicate that might have been loaded.

:- op(1150, fx, [wait]).

:- multifile user:term_expansion/2.

user:term_expansion((:- wait F/N), (:- block Head)) :functor(Head, F, N), wb_args(N, Head).

wb_args(0, _Head).

wb_args(1, Head) :- arg(1, Head, -).

wb_args(N, Head) :-

N>1, arg(N, Head, ?),

N1 is N-1, wb_args(N1, Head).

11.5.7 Prolog in Prolog

This example shows how simple it is to write a Prolog interpreter in Prolog, and illustrates the use of a variable goal. In this mini-interpreter, goals and clauses are represented as ordinary Prolog data structures (i.e. terms). Terms representing clauses are specified using the predicate my_clause/1, e.g.

my_clause( (grandparent(X, Z) :- parent(X, Y), parent(Y, Z)) ).

A unit clause will be represented by a term such as

244 SICStus Prolog my_clause( (parent(john, mary) :- true) ).

The mini-interpreter consists of three clauses: execute((P,Q)) :- !, execute(P), execute(Q).

execute(P) :- predicate_property(P, built_in), !, P.

execute(P) :- my_clause((P :- Q)), execute(Q).

The second clause enables the mini-interpreter to cope with calls to ordinary Prolog predicates, e.g. built-in predicates. The mini-interpreter needs to be extended to cope with the other control structures, i.e. !, (P;Q), (P->Q), (P->Q;R), (\+ P), and if(P,Q,R).

11.5.8 Translating English Sentences into Logic Formulae

The following example of a definite clause grammar defines in a formal way the traditional mapping of simple English sentences into formulae of classical logic. By way of illustration, if the sentence

Every man that lives loves a woman.

is parsed as a sentence by the call

| ?- phrase(sentence(P), [every,man,that,lives,loves,a,woman]).

then P will get instantiated to all(X):(man(X)&lives(X) => exists(Y):(woman(Y)&loves(X,Y))) where :, & and => are infix operators defined by

:- op(900, xfx, =>).

:- op(800, xfy, &).

:- op(550, xfy, :). /* predefined */

The grammar follows:

Chapter 11: Programming Tips and Examples 245 sentence(P) --> noun_phrase(X, P1, P), verb_phrase(X, P1).

noun_phrase(X, P1, P) --> determiner(X, P2, P1, P), noun(X, P3), rel_clause(X, P3, P2).

noun_phrase(X, P, P) --> name(X).

verb_phrase(X, P) --> trans_verb(X, Y, P1), noun_phrase(Y, P1, P).

verb_phrase(X, P) --> intrans_verb(X, P).

rel_clause(X, P1, P1&P2) --> [that], verb_phrase(X, P2).

rel_clause(_, P, P) --> [].

determiner(X, P1, P2, all(X):(P1=>P2)) --> [every].

determiner(X, P1, P2, exists(X):(P1&P2)) --> [a].

noun(X, man(X)) --> [man].

noun(X, woman(X)) --> [woman].

name(john) --> [john].

trans_verb(X, Y, loves(X,Y)) --> [loves].

intrans_verb(X, lives(X)) --> [lives].

246 SICStus Prolog

Chapter 12: The Prolog Library 247

12 The Prolog Library

The Prolog library comprises a number of packages which are thought to be useful in a number of applications. Note that the predicates in the Prolog library are not built-in predicates. One has to explicitly load each package to get access to its predicates. The following packages are provided: arrays assoc atts provides an implementation of extendible arrays with logarithmic access time.

uses AVL trees to implement “association lists”, i.e. extendible finite mappings from terms to terms.

provides a means of associating with variables arbitrary attributes, i.e. named properties that can be used as storage locations as well as hooks into Prolog’s unification.

heaps lists terms implements binary heaps, the main application of which are priority queues.

provides basic operations on lists.

provides a number of operations on terms.

ordsets defines operations on sets represented as lists with the elements ordered in

Prolog standard order.

queues random system trees defines operations on queues (FIFO stores of information).

provides a random number generator.

provides access to operating system services.

uses binary trees to represent non-extendible arrays with logarithmic access time.

The functionality is very similar to that of library(arrays), but library(trees) is slightly more efficient if the array does not need to be extendible.

ugraphs provides an implementation of directed and undirected graphs with unlabeled edges.

wgraphs provides an implementation of directed and undirected graphs where each edge has an integral weight.

sockets provides an interface to system calls for manipulating sockets.

linda/client linda/server provides an implementation of the Linda concept for process communication.

bdb provides an interface to Berkeley DB, for storage and retrieval of terms on disk files with user-defined multiple indexing.

provides constraint solving over Booleans.

clpb clpq clpr provides constraint solving over Q (Rationals) or R (Reals).

248 SICStus Prolog clpfd chr provides constraint solving over Finite (Integer) Domains provides Constraint Handling Rules objects provides the combination of the logic programming and the object-oriented programming paradigms.

tcltk

An interface to the Tcl/Tk language and toolkit.

gauge is a profiling tool for Prolog programs with a graphical interface based on tcltk.

charsio defines I/O predicates that read from, or write to, a list of character codes.

jasper

An interface to the Java language.

flinkage is a utility program for generating glue code for the Foreign Language Interface when building statically linked runtime systems or development systems.

timeout provides a way of running goals with an execution time limit.

xref provides a cross reference producer for debugging and program analysis.

wcx_example provides a sample implementation of a Wide Character Extension (WCX) box.

To load a library package Package, you will normally enter a query

| ?- use_module(library(Package)).

A library package normally consists of one or more hidden modules.

An alternative way of loading from the library is using the built-in predicate require/1

(see

Section 7.1.1 [Read In], page 100 ). The index file ‘INDEX.pl’ needed by require/1 can

be created by the make_index program. This program is loaded as:

| ?- use_module(library(mkindex)).

make_index:make_library_index(+LibraryDirectory )

Creates a file ‘INDEX.pl’ in LibraryDirectory. All ‘*.pl’ files in the directory and all its subdirectories are scanned for module/2 declarations. From these declarations, the exported predicates are entered into the index.

Chapter 13: Array Operations 249

13 Array Operations

This package provides an implementation of extendible arrays with logarithmic access time.

Beware: the atom $ is used to indicate an unset element, and the functor $ /4 is used to indicate a subtree. In general, array elements whose principal function symbol is $ will not work.

To load the package, enter the query

| ?- use_module(library(arrays)).

new_array(-Array )

Binds Array to a new empty array. Example:

| ?- new_array(A).

A = array($($,$,$,$),2) ?

yes is_array(+Array )

Is true when Array actually is an array.

aref(+Index, +Array, ?Element)

Element is the element at position Index in Array. It fails if Array[Index] is undefined.

arefa(+Index, +Array, ?Element)

Is like aref/3 except that Element is a new array if Array[Index] is undefined.

Example:

| ?- arefa(3, array($($,$,$,$),2), E).

E = array($($,$,$,$),2) ?

yes arefl(+Index, +Array, ?Element)

Is as aref/3 except that Element is [] for undefined cells. Example:

| ?- arefl(3, array($($,$,$,$),2), E).

E = [] ?

yes array_to_list(+Array, -List)

List is a list with the pairs Index-Element of all the elements of Array. Example:

| ?- array_to_list(array($(a,b,c,d),2), List).

List = [0-a,1-b,2-c,3-d] ?

250 SICStus Prolog yes aset(+Index, +Array, +Element, -NewArray )

NewArray is the result of setting Array[Index] to Element. Example:

| ?- aset(3,array($($,$,$,$),2), a, Newarr).

Newarr = array($($,$,$,a),2) ?

yes

Chapter 14: Association Lists 251

14 Association Lists

In this package, finite mappings (“association lists”) are represented by AVL trees, i.e. they are subject to the Adelson-Velskii-Landis balance criterion:

A tree is balanced iff for every node the heights of its two subtrees differ by at most 1.

The empty tree is represented as t. A tree with key K, value V, and left and right subtrees

L and R is represented as t(K,V,|R|-|L|,L,R), where |T| denotes the height of T.

The advantage of this representation is that lookup, insertion and deletion all become—in the worst case—O(log n) operations.

The algorithms are from [Wirth 76], section 4.4.6–4.4.8.

To load the package, enter the query

| ?- use_module(library(assoc)).

empty_assoc(?Assoc)

Assoc is an empty AVL tree.

assoc_to_list(+Assoc, ?List)

List is a list of Key-Value pairs in ascending order with no duplicate Keys specifying the same finite function as the association tree Assoc. Use this to convert an association tree to a list.

is_assoc(+Assoc)

Assoc is a (proper) AVL tree. It checks both that the keys are in ascending order and that Assoc is properly balanced.

min_assoc(+Assoc, ?Key, ?Val)

Key is the smallest key in Assoc and Val is its value.

max_assoc(+Assoc, ?Key, ?Val)

Key is the greatest key in Assoc and Val is its value.

gen_assoc(?Key, +Assoc, ?Value)

Key is associated with Value in the association tree Assoc. Can be used to enumerate all Values by ascending Keys.

get_assoc(+Key, +Assoc, ?Value)

Key is identical (==) to one of the keys in the association tree Assoc, and

Value unifies with the associated value.

get_assoc(+Key, +OldAssoc, ?OldValue, ?NewAssoc, ?NewValue)

OldAssoc and NewAssoc are association trees of the same shape having the same elements except that the value for Key in OldAssoc is OldValue and the value for Key in NewAssoc is NewValue.

get_next_assoc(+Key, +Assoc, ?Knext, ?Vnext)

Knext and Vnext is the next key and associated value after Key in Assoc.

252 SICStus Prolog get_prev_assoc(+Key, +Assoc, ?Kprev, ?Vprev )

Kprev and Vprev is the previous key and associated value after Key in Assoc.

list_to_assoc(+List, ?Assoc)

List is a proper list of Key-Value pairs (in any order) and Assoc is an association tree specifying the same finite function from Keys to Values.

ord_list_to_assoc(+List, ?Assoc)

List is a proper list of Key-Value pairs (keysorted) and Assoc is an association tree specifying the same finite function from Keys to Values.

map_assoc(:Pred, ?Assoc)

Assoc is an association tree, and for each Key, if Key is associated with Value in Assoc, Pred(Value) is true.

map_assoc(:Pred, ?OldAssoc, ?NewAssoc)

OldAssoc and NewAssoc are association trees of the same shape, and for each

Key, if Key is associated with Old in OldAssoc and with New in NewAssoc,

Pred(Old,New) is true.

put_assoc(+Key, +OldAssoc, +Val, ?NewAssoc)

OldAssoc and NewAssoc define the same finite function, except that NewAssoc associates Val with Key. OldAssoc need not have associated any value at all with Key.

del_assoc(+Key, +OldAssoc, ?Val, ?NewAssoc)

OldAssoc and NewAssoc define the same finite function except that OldAssoc associates Key with Val and NewAssoc doesn’t associate Key with any value.

del_min_assoc(+OldAssoc, ?Key, ?Val, ?NewAssoc)

OldAssoc and NewAssoc define the same finite function except that OldAssoc associates Key with Val and NewAssoc doesn’t associate Key with any value and Key precedes all other keys in OldAssoc.

del_max_assoc(+OldAssoc, ?Key, ?Val, -NewAssoc)

OldAssoc and NewAssoc define the same finite function except that OldAssoc associates Key with Val and NewAssoc doesn’t associate Key with any value and Key is preceded by all other keys in OldAssoc.

Chapter 15: Attributed Variables 253

15 Attributed Variables

This package implements attributed variables. It provides a means of associating with variables arbitrary attributes, i.e. named properties that can be used as storage locations as well as to extend the default unification algorithm when such variables are unified with other terms or with each other. This facility was primarily designed as a clean interface between Prolog and constraint solvers, but has a number of other uses as well. The basic idea is due to Christian Holzbaur and he was actively involved in the final design. For background material, see the dissertation [Holzbaur 90].

To load the package, enter the query

| ?- use_module(library(atts)).

The package provides a means to declare and access named attributes of variables. The attributes are compound terms whose arguments are the actual attribute values.

The attribute names are private to the module in which they are defined. They are defined with a declaration

:- attribute AttributeSpec, ..., AttributeSpec.

where each AttributeSpec has the form (Name/Arity). There must be at most one such declaration in a module Module.

Having declared some attribute names, these attributes can now be added, updated and deleted from unbound variables. For each declared attribute name, any variable can have at most one such attribute (initially it has none).

The declaration causes the following two access predicates to become defined by means of the user:goal_expansion/3 mechanism. They take a variable and an AccessSpec as arguments where an AccessSpec is either +(Attribute), -(Attribute), or a list of such. The

+ prefix may be dropped for convenience. The meaning of the +/- prefix is documented below:

Module:get_atts(-Var, ?AccessSpec)

Gets the attributes of Var according to AccessSpec. If AccessSpec is unbound, it will be bound to a list of all set attributes of Var. Non-variable terms cause a type error to be raised. The prefixes in the AccessSpec have the following meaning:

+(Attribute)

The corresponding actual attribute must be present and is unified with Attribute.

-(Attribute)

The corresponding actual attribute must be absent. The arguments of Attribute are ignored, only the name and arity are relevant.

254 SICStus Prolog

Module:put_atts(-Var, +AccessSpec)

Sets the attributes of Var according to AccessSpec. Non-variable terms cause a type error to be raised. The effects of put_atts/2 are undone on backtracking.

+(Attribute)

The corresponding actual attribute is set to Attribute. If the actual attribute was already present, it is simply replaced.

-(Attribute)

The corresponding actual attribute is removed. If the actual attribute was already absent, nothing happens.

A module that contains an attribute declaration has an opportunity to extend the default unification algorithm by defining the following predicate:

Module:verify_attributes(-Var, +Value, -Goals) [Hook]

This predicate is called whenever a variable Var that might have attributes in

Module is about to be bound to Value (it might have none). The unification resumes after the call to verify_attributes/3. Value is a non-variable term, or another attributed variable. Var might have no attributes present in Module; the unification extension mechanism is not sophisticated enough to filter out exactly the variables that are relevant for Module.

verify_attributes/3 is called before Var has actually been bound to Value.

If it fails, the unification is deemed to have failed.

It may succeed nondeterministically, in which case the unification might backtrack to give another answer. It is expected to return, in Goals, a list of goals to be called after Var has been bound to Value.

verify_attributes/3 may invoke arbitrary Prolog goals, but Var should not be bound by it. Binding Var will result in undefined behavior.

If Value is a non-variable term, verify_attributes/3 will typically inspect the attributes of Var and check that they are compatible with Value and fail otherwise. If Value is another attributed variable, verify_attributes/3 will typically copy the attributes of Var over to Value, or merge them with Value’s, in preparation for Var to be bound to Value. In either case, verify_attributes/3 may determine Var’s current attributes by calling get_atts(Var,List) with an unbound List.

An important use for attributed variables is in implementing coroutining facilities as an alternative or complement to the built-in coroutining mechanisms. In this context it might be useful to be able to interpret some of the attributes of a variable as a goal that is blocked on that variable. Certain built-in predicates (frozen/2, call_residue/2) and the Prolog top level need to access blocked goals, and so need a means of getting the goal interpretation of attributed variables by calling:

Chapter 15: Attributed Variables 255

Module:attribute_goal(-Var, -Goal) [Hook]

This predicate is called in each module that contains an attribute declaration, when an interpretation of the attributes as a goal is needed. It should unify

Goal with the interpretation, or merely fail if no such interpretation is available.

An important use for attributed variables is to provide an interface to constraint solvers. An important function for a constraint solver in the constraint logic programming paradigm is to be able to perform projection of the residual constraints onto the variables that occurred in the top-level query. A module that contains an attribute declaration has an opportunity to perform such projection of its residual constraints by defining the following predicate:

Module:project_attributes(+QueryVars, +AttrVars) [Hook]

This predicate is called by the Prolog top level and by the built-in predicate call_residue/2 in each module that contains an attribute declaration. Query-

Vars is the list of variables occurring in the query, or in terms bound to such variables, and AttrVars is a list of possibly attributed variables created during the execution of the query. The two lists of variables may or may not be disjoint.

If the attributes on AttrVars can be interpreted as constraints, this predicate will typically “project” those constraints onto the relevant QueryVars. Ideally, the residual constraints will be expressed entirely in terms of the QueryVars, treating all other variables as existentially quantified. Operationally, project_ attributes/2 must remove all attributes from AttrVars, and add transformed attributes representing the projected constraints to some of the QueryVars.

Projection has the following effect on the Prolog top level.

When the top level query has succeeded, project_attributes/2 is called first. The top level then prints the answer substition and residual constraints. While doing so, it searches for attributed variables created during the execution of the query. For each such variable, it calls attribute_goal/2 to get a printable representation of the constraint encoded by the attribute. Thus, project_attributes/2 is a mechanism for controlling how the residual constraints should be displayed at top level.

Similarly during the execution of call_residue(Goal,Residue), when Goal has succeeded, project_attributes/2 is called.

After that, all attributed variables created during the execution of Goal are located. For each such variable, attribute_goal/2 produces a term representing the constraint encoded by the attribute, and Residue is unified with the list of all such terms.

The exact definition of project_attributes/2 is constraint system dependent, but see

Section 31.5 [Projection], page 324

for details about projection in clp(Q,R).

In the following example we sketch the implementation of a finite domain “solver”. Note that an industrial strength solver would have to provide a wider range of functionality and that it quite likely would utilize a more efficient representation for the domains proper. The

256 SICStus Prolog module exports a single predicate domain(-Var,?Domain) which associates Domain (a list of terms) with Var. A variable can be queried for its domain by leaving Domain unbound.

We do not present here a definition for project_attributes/2. Projecting finite domain constraints happens to be difficult.

:- module(domain, [domain/2]).

:- use_module(library(atts)).

:- use_module(library(ordsets), [ ord_intersection/3, ord_intersect/2, list_to_ord_set/2

]).

:- attribute dom/1.

verify_attributes(Var, Other, Goals) :get_atts(Var, dom(Da)), !,

( var(Other) ->

% are we involved?

% must be attributed then

( get_atts(Other, dom(Db)) -> % has a domain?

ord_intersection(Da, Db, Dc),

Dc = [El|Els], % at least one element

( Els = [] ->

Goals = [Other=El]

; Goals = [],

% exactly one element

% implied binding put_atts(Other, dom(Dc))% rescue intersection

)

; Goals = [], put_atts(Other, dom(Da))

)

; Goals = [], ord_intersect([Other], Da)

).

verify_attributes(_, _, []).

% rescue the domain

% value in domain?

% unification triggered

% because of attributes

% in other modules

% interpretation as goal attribute_goal(Var, domain(Var,Dom)) :get_atts(Var, dom(Dom)).

domain(X, Dom) :var(Dom), !, get_atts(X, dom(Dom)).

domain(X, List) :list_to_ord_set(List, Set),

Set = [El|Els],

( Els = [] ->

X = El

% at least one element

% exactly one element

% implied binding

Chapter 15: Attributed Variables 257

; put_atts(Fresh, dom(Set)),

X = Fresh % may call

% verify_attributes/3

).

Note that the “implied binding” Other=El was deferred until after the completion of verify_attribute/3. Otherwise, there might be a danger of recursively invoke verify_ attribute/3, which might bind Var, which is not allowed inside the scope of verify_ attribute/3. Deferring unifications into the third argument of verify_attribute/3 effectively serializes th calls to verify_attribute/3.

Assuming that the code resides in the file ‘domain.pl’, we can use it via:

| ?- use_module(domain).

Let’s test it:

| ?- domain(X,[5,6,7,1]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]).

domain(X,[1,5,6,7]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]) ?

yes

| ?- domain(X,[5,6,7,1]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]),

X=Y.

Y = X, domain(X,[5,6]), domain(Z,[1,6,7,8]) ?

yes

| ?- domain(X,[5,6,7,1]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]),

X=Y, Y=Z.

X = 6,

Y = 6,

Z = 6

To demonstrate the use of the Goals argument of verify_attributes/3, we give an implementation of freeze/2. We have to name it myfreeze/2 in order to avoid a name clash with the built-in predicate of the same name.

:- module(myfreeze, [myfreeze/2]).

:- use_module(library(atts)).

:- attribute frozen/1.

verify_attributes(Var, Other, Goals) :-

258 SICStus Prolog get_atts(Var, frozen(Fa)), !,

( var(Other) ->

% are we involved?

% must be attributed then

( get_atts(Other, frozen(Fb)) % has a pending goal?

-> put_atts(Other, frozen((Fa,Fb))) % rescue conjunction

; put_atts(Other, frozen(Fa)) % rescue the pending goal

),

Goals = []

; Goals = [Fa]

).

verify_attributes(_, _, []).

attribute_goal(Var, Goal) :get_atts(Var, frozen(Goal)).

% interpretation as goal myfreeze(X, Goal) :put_atts(Fresh, frozen(Goal)),

Fresh = X.

Assuming that this code lives in file ‘myfreeze.pl’, we would use it via:

| ?- use_module(myfreeze).

| ?- myfreeze(X,print(bound(x,X))), X=2.

bound(x,2)

X = 2

The two solvers even work together:

% side effect

% bindings

| ?- myfreeze(X,print(bound(x,X))), domain(X,[1,2,3]), domain(Y,[2,10]), X=Y.

bound(x,2)

X = 2,

Y = 2

% side effect

% bindings

The two example solvers interact via bindings to shared attributed variables only. More complicated interactions are likely to be found in more sophisticated solvers. The corresponding verify_attributes/3 predicates would typically refer to the attributes from other known solvers/modules via the module prefix in Module:get_atts/2.

Chapter 16: Heap Operations 259

16 Heap Operations

A binary heap is a tree with keys and associated values that satisfies the heap condition: the key of every node is greater than or equal to the key of its parent, if it has one. The main application of binary heaps are priority queues. To load the package, enter the query

| ?- use_module(library(heaps)).

add_to_heap(+OldHeap, +Key, +Datum, ?NewHeap)

Inserts the new Key-Datum pair into the current heap OldHeap producing the new heap NewHeap. The insertion is not stable, that is, if you insert several pairs with the same Key it is not defined which of them will come out first, and it is possible for any of them to come out first depending on the history of the heap. Example:

| ?- add_to_heap(t(0,[],t),3,678,N).

N = t(1,[],t(3,678,t,t)) ?

yes get_from_heap(+OldHeap, ?Key, ?Datum, ?NewHeap)

Returns the Key-Datum pair in OldHeap with the smallest Key, and also a

NewHeap which is the OldHeap with that pair deleted. Example: get_from_heap(t(1,[],t(1,543,t,t)),K,D,N).

D = 543,

K = 1,

N = t(0,[1],t) ?

yes empty_heap(?Heap) is true when Heap is the empty heap.

heap_size(+Heap, ?Size)

Size is the number of elements in the heap Heap.

heap_to_list(+Heap, -List)

Returns the current set of Key-Datum pairs in the Heap as a keysorted List.

is_heap(+Heap) is true when Heap is a valid heap.

list_to_heap(+List, -Heap)

Takes a list List of Key-Datum pairs and forms them into a heap Heap. Example:

| ?- list_to_heap([1-34,2-345,5-678],H).

H = t(3,[],t(1,34,t(2,345,t,t),t(5,678,t,t))) ?

yes

260 SICStus Prolog min_of_heap(+Heap, ?Key, ?Datum)

Returns the Key-Datum pair at the top of the heap Heap without removing it.

Fails if the heap is empty.

min_of_heap(+Heap, ?Key1, ?Datum1, ?Key2, ?Datum2)

Returns the smallest (Key1-Datum1) and second smallest (Key2-Datum2) pairs in the Heap, without deleting them. It fails if the heap does not have at least two elements.

delete_from_heap(+OldHeap, +Key, ?Datum, ?NewHeap) deletes a single Key-Datum pair in OldHeap producing NewHeap. This is useful if you want to e.g. change the priority of Datum. Beware: this operation needs to search the whole heap in the worst case.

Chapter 17: List Operations 261

17 List Operations

This package defines operations on lists. Lists are a very basic data structure, but nevertheless certain very frequent operations are provided in this package.

To load the package, enter the query

| ?- use_module(library(lists)).

append(?Prefix, ?Suffix, ?Combined)

Combined is the combined list of the elements in Prefix followed by the elements in Suffix. It can be used to form Combined or it can be used to find Prefix and/or Suffix from a given Combined.

delete(+List, +Element, ?Residue)

Residue is the result of removing all identical occurrences of Element in List.

is_list(+List)

List is a proper list.

last(?List, ?Last)

Last is the last element in List. Example:

| ?- last([x,y,z], Z).

Z = z ?

yes max_list(+ListOfNumbers, ?Max)

Max is the largest of the elements in ListOfNumbers.

member(?Element, ?List)

Element is a member of List. It may be used to test for membership in a list, but it can also be used to enumerate all the elements in List. Example:

| ?- member(X, [a,b,c]).

X = a ? ;

X = b ? ;

X = c ?

yes memberchk(+Element, +List)

Element is a member of List, but memberchk/2 only succeeds once and can therefore not be used to enumerate the elements in List. Example:

| ?- memberchk(X, [a,b,c]).

262 SICStus Prolog

X = a ? ; no min_list(+ListOfNumbers, ?Min)

Min is the smallest of the numbers in the list ListOfNumbers.

nextto(?X, ?Y, ?List)

X and Y appears side-by-side in List. Example:

| ?- nextto(X, Y, [1,2,3]).

X = 1,

Y = 2 ? ;

X = 2,

Y = 3 ? ; no no_doubles(?List)

List contains no duplicated elements. This is true when dif(X, Y ) holds for all pairs of members X and Y of the list.

non_member(?Element, ?List)

Element does not occur in List. This is true when dif(Element, Y ) holds for all members Y of the list.

nth(?N, ?List, ?Element)

Element is the N th element of List. The first element is number 1. Example:

| ?- nth(N, [a,b,c,d,e,f,g,h,i], f).

N = 6 ?

yes nth(?N, ?List, ?Element, ?Rest)

Element is in position N in the List and Rest is all elements in List except

Element.

nth0(?N, ?List, ?Element)

Element is the N th element of List, counting the first element as 0.

nth0(?N, ?List, ?Element, ?Rest)

Element is the N th element of List, counting the first element as 0. Rest is all the other elements in List. Example:

| ?- nth0(N, [a,b,c,d,e,f,g,h,i,j], f, R).

N = 5,

R = [a,b,c,d,e,g,h,i,j] ?

yes

Chapter 17: List Operations 263 permutation(?List, ?Perm)

Perm is a permutation of List.

prefix(?Prefix, ?List)

Prefix is a prefix of List. Example:

| ?- prefix([1,2,3], [1,2,3,4,5,6]).

yes remove_duplicates(+List, ?Pruned)

Pruned is the result of removing all identical duplicate elements in List. Example:

| ?- remove_duplicates([1,2,3,2,3,1], P).

P = [1,2,3] ? ; no reverse(?List, ?Reversed)

Reversed has the same elements as List but in a reversed order.

same_length(?List1, ?List2)

List1 and List2 have the same number of elements.

same_length(?List1, ?List2, ?Length)

List1 and List2 have the same number of elements and that number is Length.

Example:

| ?- same_length([1,2,3], [9,8,7], N).

N = 3 ? ; no select(?Element, ?List, ?List2)

The result of removing an occurrence of Element in List is List2.

sublist(?Sub, ?List)

Sub contains some of the elements of List, in the same order.

substitute(+X, +Xlist, +Y, ?Ylist)

Xlist and Ylist are equal except for replacing identical occurrences of X by Y.

Example:

| ?- substitute(1, [1,2,3,4], 5, X).

X = [5,2,3,4] ?

yes sum_list(+ListOfNumbers, ?Sum)

Sum is the result of adding the ListOfNumbers together.

suffix(?Suffix, ?List)

Suffix is a suffix of List.

264 SICStus Prolog

Chapter 18: Term Utilities 265

18 Term Utilities

This package defines operations on terms for subsumption checking, “anti-unification”, unification with occurs-check, testing acyclicity, and getting the variables.

To load the package, enter the query

| ?- use_module(library(terms)).

subsumes_chk(?General, ?Specific)

Specific is an instance of General, i.e. if there is a substitution that leaves

Specific unchanged and makes General identical to Specific. It doesn’t bind any variables.

subsumes_chk(f(X), f(a)).

true

| ?- subsumes_chk(f(a), f(X)).

no

| ?- subsumes_chk(A-A, B-C).

no

| ?- subsumes_chk(A-B, C-C).

true subsumes(?General, ?Specific)

Specific is an instance of General. It will bind variables in General (but not those in Specific) so that General becomes identical to Specific.

variant(?Term, ?Variant)

Term and Variant are identical modulo renaming of variables, provided Term and Variant have no variables in common.

term_subsumer(?Term1, ?Term2, ?General)

General is the most specific term that generalizes Term1 and Term2. This process is sometimes called anti-unification, as it is the dual of unification.

| ?- term_subsumer(f(g(1,h(_))), f(g(_,h(1))), T).

T = f(g(_B,h(_A)))

| ?- term_subsumer(f(1+2,2+1), f(3+4,4+3), T).

T = f(_A+_B,_B+_A)

266 SICStus Prolog term_hash(?Term, ?Hash) term_hash(?Term, +Depth, +Range, ?Hash)

If Term is instantiated up to the given Depth, an integer hash value in the range

[0,Range) as a function of Term is unified with Hash. Otherwise, the goal just succeeds, leaving Hash uninstantiated.

If Term contains floats or integers outside the range [-33554432,33554431], the hash value will be platform dependent. Otherwise, the hash value will be identical across runs and platforms.

The depth of a term is defined as follows: the (principal functor of) the term itself has depth 1, and an argument of a term with depth i has depth i+1.

Depth should be an integer >= -1. If Depth = -1 (the default), Term must be ground, and all subterms of Term are relevant in computing Hash. Otherwise, only the subterms up to depth Depth of Term are used in the computation.

Range should be an integer >= 1. The default will give hash values in a range appropriate for all platforms.

| ?- term_hash([a,b,_], 3, 4, H).

H = 2

| ?- term_hash([a,b,_], 4, 4, H).

true

| ?- term_hash(f(a,f(b,f(_,[]))), 2, 4, H).

H = 2 term_hash/[2,4] is provided primarily as a tool for the construction of sophisticated Prolog clause access schemes. Its intended use is to generate hash values for terms that will be used with first argument clause indexing, yielding compact and efficient multi-argument or deep argument indexing.

term_variables(?Term, ?Variables)

Variables is the set of variables occurring in Term.

acyclic_term(?X )

True if X is finite (acyclic). Runs in linear time.

cyclic_term(?X )

True if X is infinite (cyclic). Runs in linear time.

Chapter 19: Ordered Set Operations 267

19 Ordered Set Operations

This package defines operations on ordered sets. Ordered sets are sets represented as lists with the elements ordered in a standard order. The ordering is defined by the @< family of term comparison predicates and it is the ordering produced by the built-in predicate sort/2

(see

Section 7.3 [Term Compare], page 129

).

To load the package, enter the query

| ?- use_module(library(ordsets)).

is_ordset(+Set)

Set is an ordered set.

list_to_ord_set(+List, ?Set)

Set is the ordered representation of the set denoted by the unordered representation List. Example:

| ?- list_to_ord_set([p,r,o,l,o,g], P).

P = [g,l,o,p,r] ?

yes ord_add_element(+Set1, +Element ?Set2)

Set2 is Set1 with Element inserted in it, preserving the order. Example:

| ?- ord_add_element([a,c,d,e,f], b, N).

N = [a,b,c,d,e,f] ?

yes ord_del_element(+Set1, +Element, ?Set2)

Set2 is like Set1 but with Element removed.

ord_disjoint(+Set1, +Set2)

The two ordered sets have no elements in common.

ord_intersect(+Set1, +Set2)

The two ordered sets have at least one element in common.

ord_intersection(+Set1, +Set2, ?Intersect)

Intersect is the ordered set representation of the intersection between Set1 and

Set2.

ord_intersection(+Set1, +Set2, ?Intersect, ?Diff )

Intersect is the intersection between Set1 and Set2, and Diff is the difference between Set2 and Set1.

ord_intersection(+Sets, ?Intersection)

Intersection is the ordered set representation of the intersection of all the sets in Sets. Example:

268 SICStus Prolog

| ?- ord_intersection([[1,2,3],[2,3,4],[3,4,5]], I).

I = [3] ?

yes ord_member(+Elt, +Set) is true when Elt is a member of Set.

ord_seteq(+Set1, +Set2)

Is true when the two arguments represent the same set. Since they are assumed to be ordered representations, they must be identical.

ord_setproduct(+Set1, +Set2, ?SetProduct)

SetProduct is the Cartesian Product of the two Sets. The product is represented as pairs: Elem1-Elem2 where Elem1 is an element from Set1 and Elem2 is an element from Set2. Example

| ?- ord_setproduct([1,2,3], [4,5,6], P).

P = [1-4,1-5,1-6,2-4,2-5,2-6,3-4,3-5,3-6] ?

yes ord_subset(+Set1, +Set2)

Every element of the ordered set Set1 appears in the ordered set Set2.

ord_subtract(+Set1, +Set2, ?Difference)

Difference contains all and only the elements of Set1 which are not also in Set2.

Example:

| ?- ord_subtract([1,2,3,4], [3,4,5,6], S).

S = [1,2] ?

yes ord_symdiff(+Set1, +Set2, ?Difference)

Difference is the symmetric difference of Set1 and Set2. Example:

| ?- ord_symdiff([1,2,3,4], [3,4,5,6], D).

D = [1,2,5,6] ?

yes ord_union(+Set1, +Set2, ?Union)

Union is the union of Set1 and Set2.

ord_union(+Set1, +Set2, ?Union, ?New )

Union is the union of Set1 and Set2, and New is the difference between Set2 and Set1. This is useful if you are accumulating members of a set and you want to process new elements as they are added to the set.

Chapter 19: Ordered Set Operations ord_union(+Sets, ?Union)

Union is the union of all the sets in Sets. Example:

| ?- ord_union([[1,2,3],[2,3,4],[3,4,5]], U).

U = [1,2,3,4,5] ?

yes

269

270 SICStus Prolog

Chapter 20: Queue Operations 271

20 Queue Operations

A queue is a first-in, first-out store of information. This implementation of queues uses difference-lists, the head of the difference-list represents the beginning of the queue and the tail represents the end of the queue. The members of the difference-list are the elements in the queue. The first argument in the queue-representation is the number of elements in the queue in unary representation.

Thus, a queue with n elements is represented as follows: q(s(...s(0)...), [X1,...,Xn,Y1,...,Ym], [Y1,...,Ym]) where n is the length of the queue and X1...Xn are the elements of the queue.

To load the package, enter the query

| ?- use_module(library(queues)).

empty_queue(?Queue)

Is true if Queue has no elements.

is_queue(+Queue) is true when Queue is a valid queue.

queue(?X, ?Queue)

Is true if Queue has one element and that is X.

queue_head(?Head, ?Queue1, ?Queue2)

Queue1 and Queue2 are the same queues except that Queue2 has Head inserted in the front. It can be used to enqueue the first element in Queue2. Example:

| ?- queue_head(Head, Nq, q(s(s(s(s(0)))),[1,2,3,4|R],R)).

Head = 1,

Nq = q(s(s(s(0))),[2,3,4|_193],_193),

R = _193 ?

yes queue_head_list(+HeadList, ?Queue1, ?Queue2)

Queue1 and Queue2 have the same elements except that Queue2 has HeadList inserted in the front.

queue_last(?Last, ?Queue1, ?Queue2)

Queue2 is like Queue1 but have Last as the last element in the queue.

queue_last_list(+LastList, ?Queue1, ?Queue2)

Queue1 and Queue2 are the same queues except that Queue2 has the list of elements LastList last in the queue. Example:

| ?- queue_last_list([5,6], q(s(s(0)))), [1,2|R], R), NQ).

272 SICStus Prolog

NQ = q(s(s(s(s(0)))))),[1,2,5,6|_360],_360),

R = [5,6|_360] ?

yes list_queue(+List, ?Queue)

Queue is the queue representation of the elements in List. Example:

| ?- list_queue([1,2,3,4], Q).

Q = q(s(s(s(s(0)))),[1,2,3,4|_138],_138) ?

yes

| ?queue_length(+Queue, ?Length)

Length is the number of elements in Queue. Example:

| ?- queue_length(q(s(s(s(s(s(0))))),[a,b,c,d,e|R],R), L).

L = 5,

R = _155 ?

yes

Chapter 21: Random Number Generator 273

21 Random Number Generator

This package provides a random number generator. To load the package, enter the query

| ?- use_module(library(random)).

random(-Number)

Binds Number to a random float in the interval [0.0, 1.0). Note that 1.0 will never be generated.

random(+Lower, +Upper, -Number)

Binds Number to a random integer in the interval [Lower,Upper) if Lower and

Upper are integers. Otherwise Number is bound to a random float between

Lower and Upper. Upper will never be generated.

randseq(+K, +N, -RandomSeq)

Generates a unordered set of K unique integers, chosen randomly in the range

1..N. RandomSeq is not returned in any particular order.

randset(+K, +N, -RandomSet)

Generates an ordered set of K unique integers, chosen randomly in the range

1..N. The set is returned in standard order.

getrand(?State)

Tries to unify State with the term rand(X,Y,Z) where X, Y, and Z are integers describing the state of the random generator.

setrand(rand(+X,+Y,+Z ))

Sets the state of the random generator. X, Y, and Z must be integers in the ranges [1,30269), [1,30307), and [1,30323), respectively.

274 SICStus Prolog

Chapter 22: Operating System Utilities 275

22 Operating System Utilities

This package contains utilities for invoking services from the operating system. To load the package, enter the query

| ?- use_module(library(system)).

Certain predicates described below take names of files or directories as arguments. These must be given as atoms, and the predicates below will not call absolute_file_name/2 on them.

Some predicates are described as invoking the default shell. Specifically this means invoking

‘/bin/sh’ on UNIX platforms. On MSDOS, Windows and OS/2, the command interpreter given by the environment variable COMSPEC is invoked.

datime(-Datime)

Datime is a timestamp of the form datime(Year,Month,Day,Hour,Min,Sec) containing the current date and time. All fields are integers.

delete_file(+FileName,+Options)

FileName is the name of an existing file or directory. Options is a list of options.

Possible options are directory, recursive or ignore. If FileName is not a directory it is deleted, otherwise if the option directory is specified but not recursive, the directory will be deleted if it is empty. If recursive is specified and FileName is a directory, the directory and all its subdirectories and files will be deleted. If the operation fails, an exception is raised unless the ignore option is specified.

delete_file(+FileName)

Equivalent to delete_file(FileName,[recursive]).

directory_files(+Directory,-FileList)

FileList is the list of entries (files, directories, etc.) in Directory.

make_directory(+DirectoryName)

Makes a new directory.

environ(?Var, ?Value)

Var is the name of an environment variable, and Value is its value. Both are atoms. Can be used to enumerate all current environment variables.

exec(+Command, [+Stdin,+Stdout,+Stderr], -Pid)

Passes Command to a new default shell process for execution. The standard

I/O streams of the new process are connected according to what is specified by the terms +Stdin, +Stdout, and +Stderr respectively. Possible values are: null

Connected to ‘/dev/null’ or equivalent.

std

The standard stream is shared with the calling process. Note that the standard stream may not be referring to a console if the calling process is windowed. To portably print the output from the subprocess on the Prolog console, pipe/1 must be used and the program

276 SICStus Prolog must explicitly read the pipe and write to the console. Similarly for the input to the subprocess.

pipe(-Stream)

A pipe is created which connects the Prolog stream Stream to the standard stream of the new process. It must be closed using close/1; it is not closed automatically when the process dies.

Pid is the process identifier of the new process.

On UNIX, the subprocess will be detached provided none of its standard streams is specified as std. This means it will not receive an interruption signal as a result of ^C being typed.

file_exists(+FileName)

FileName is the name of an existing file or directory.

file_exists(+FileName, +Permissions)

FileName is the name of an existing file or directory which can be accessed according to Permissions. Permissions is an atom, an integer (see access(2)), or a list of atoms and/or integers. The atoms must be drawn from the list

[read,write,search,exists].

file_property(+FileName, ?Property )

FileName has the property Property. The possible properties are: type(Type)

Type is one of regular, directory, fifo, symlink, socket or unknown.

size(Size)

Size is the size of FileName.

mod_time(ModTime)

ModTime is the time of the last modification of FileName.

If Property is uninstantiated, the predicate will enumerate the properties on backtracking.

host_id(-HID)

HID is the unique identifier, represented by an atom, of the host executing the current SICStus Prolog process.

host_name(-HostName)

HostName is the standard host name of the host executing the current SICStus

Prolog process.

pid(-PID)

PID is the identifier of the current SICStus Prolog process.

kill(+Pid, +Signal)

Sends the signal Signal to process Pid.

mktemp(+Template, -FileName)

Interface to the UNIX function mktemp(3). A unique file name is created and unified with FileName. Template should contain a file name with six trailing Xs.

Chapter 22: Operating System Utilities 277

The unique file name is that template with the six Xs replaced by a character string.

popen(+Command, +Mode, ?Stream)

Interface to the UNIX function popen(3). Passes Command to a new default shell process for execution. Mode may be either read or write. In the former case the output from the process is piped to Stream. In the latter case the input to the process is piped from Stream. Stream may be read/written using the ordinary StreamIO predicates. It must be closed using close/1; it is not closed automatically when the process dies.

rename_file(+OldName, +NewName)

OldName is the name of an existing file or directory, which will be renamed to

NewName. If the operation fails, an exception is raised.

shell

Starts a new interactive shell named by the environment variable SHELL. The control is returned to Prolog upon termination of the shell process.

shell(+Command)

Passes Command to a new shell named by the environment variable SHELL for execution. Succeeds if the return status value is 0.

On MSDOS, Windows or OS/2, if SHELL is defined it is expected to name a

UNIX like shell which will be invoked with the argument -c Command. If SHELL is undefined, the shell named by COMSPEC will be invoked with the argument /C

Command.

shell(+Command, -Status)

Passes Command to a new shell named by the environment variable SHELL for execution. The status value is returned in Status. See also shell/1 above.

sleep(+Seconds)

Puts the SICStus Prolog process asleep for Second seconds, where Seconds may be an integer or a float. On UNIX, the usleep function will be used if Seconds is less than one, and sleep otherwise. On MSDOS, Windows or OS/2, the

Sleep function will be used.

system

Starts a new interactive default shell process. The control is returned to Prolog upon termination of the shell process.

system(+Command)

Passes Command to a new default shell process for execution. Succeeds if the return status value is 0.

system(+Command, -Status)

Passes Command to a new default shell process for execution. The status value is returned in Status.

tmpnam(-FileName)

Interface to the ANSI C function tmpnam(3). A unique file name is created and unified with FileName.

278 SICStus Prolog wait(+Pid, -Status)

Waits for the child process Pid to terminate. The exit status is returned in

Status. The function is similar to that of the UNIX function waitpid(3).

working_directory(?OldDirectory, ?NewDirectory )

OldDirectory is the current working directory, and the working directory is set to NewDirectory. In particular, the goal working_directory(Dir,Dir) unifies

Dir with the current working directory without changing anything.

Chapter 23: Updatable Binary Trees 279

23 Updatable Binary Trees

This package uses binary trees to represent arrays of N elements where N is fixed, unlike library(arrays). To load the package, enter the query

| ?- use_module(library(trees)).

Binary trees have the following representation: t denotes the empty tree, and t(Label,Left,Right) denotes the binary tree with label Label and children Left and Right.

gen_label(?Index, +Tree, ?Label)

Label labels the Index-th element in the Tree. Can be used to enumerate all

Labels by ascending Index. Use get_label/3 instead if Index is instantiated.

get_label(+Index, +Tree, ?Label)

Label labels the Index-th element in the Tree.

list_to_tree(+List, -Tree)

Constructs a binary Tree from List where get_label(K,Tree,Lab) iff Lab is the Kth element of List.

map_tree(:Pred, ?OldTree, ?NewTree)

OldTree and NewTree are binary trees of the same shape and Pred(Old,New) is true for corresponding elements of the two trees.

put_label(+I, +OldTree, +Label, -NewTree)

Constructs NewTree which has the same shape and elements as OldTree, except that the I-th element is Label.

put_label(+I, +OldTree, +Label, -NewTree, +Label)

Constructs NewTree which has the same shape and elements as OldTree, except that the I-th element is changed from OldLabel to NewLabel.

tree_size(+Tree, ?Size)

Calculates as Size the number of elements in the Tree.

tree_to_list(+Tree, ?List)

Is the converse operation to list_to_tree/2. Any mapping or checking operation can be done by converting the tree to a list, mapping or checking the list, and converting the result, if any, back to a tree.

280 SICStus Prolog

Chapter 24: Unweighted Graph Operations 281

24 Unweighted Graph Operations

Directed and undirected graphs are fundamental data structures representing arbitrary relationships between data objects.

This package provides a Prolog implementation of directed graphs, undirected graphs being a special case of directed graphs.

An unweighted directed graph (ugraph) is represented as a list of (vertex-neighbors) pairs, where the pairs are in standard order (as produced by keysort with unique keys) and the neighbors of each vertex are also in standard order (as produced by sort), every neighbor appears as a vertex even if it has no neighbors itself, and no vertex is a neighbor to itself.

An undirected graph is represented as a directed graph where for each edge (U,V) there is a symmetric edge (V,U).

An edge (U,V) is represented as the term U-V. U and V must be distinct.

A vertex can be any term. Two vertices are distinct iff they are not identical (==).

A path from u to v is represented as a list of vertices, beginning with u and ending with v. A vertex cannot appear twice in a path. A path is maximal in a graph if it cannot be extended.

A tree is a tree-shaped directed graph (all vertices have a single predecessor, except the root node, which has none).

A strongly connected component of a graph is a maximal set of vertices where each vertex has a path in the graph to every other vertex.

Sets are represented as ordered lists (see

Chapter 19 [Ordsets], page 267 ).

To load the package, enter the query

| ?- use_module(library(ugraphs)).

The following predicates are defined for directed graphs.

vertices_edges_to_ugraph(+Vertices, +Edges, -Graph)

Is true if Vertices is a list of vertices, Edges is a list of edges, and Graph is a graph built from Vertices and Edges. Vertices and Edges may be in any order.

The vertices mentioned in Edges do not have to occur explicitly in Vertices.

Vertices may be used to specify vertices that are not connected by any edges.

vertices(+Graph, -Vertices)

Unifies Vertices with the vertices in Graph.

edges(+Graph, -Edges)

Unifies Edges with the edges in Graph.

add_vertices(+Graph1, +Vertices, -Graph2)

Graph2 is Graph1 with Vertices added to it.

282 SICStus Prolog del_vertices(+Graph1, +Vertices, -Graph2)

Graph2 is Graph1 with Vertices and all edges to and from them removed from it.

add_edges(+Graph1, +Edges, -Graph2)

Graph2 is Graph1 with Edges and their “to” and “from” vertices added to it.

del_edges(+Graph1, +Edges, -Graph2)

Graph2 is Graph1 with Edges removed from it.

transpose(+Graph, -Transpose)

Transpose is the graph computed by replacing each edge (u,v) in Graph by its symmetric edge (v,u). Takes O(N^2) time.

neighbors(+Vertex, +Graph, -Neighbors) neighbours(+Vertex, +Graph, -Neighbors)

Vertex is a vertex in Graph and Neighbors are its neighbors.

complement(+Graph, -Complement)

Complement is the complement graph of Graph, i.e. the graph that has the same vertices as Graph but only the edges that are not in Graph.

compose(+G1, +G2, -Composition)

Computes Composition as the composition of two graphs, which need not have the same set of vertices.

transitive_closure(+Graph, -Closure)

Computes Closure as the transitive closure of Graph in O(N^3) time.

symmetric_closure(+Graph, -Closure)

Computes Closure as the symmetric closure of Graph, i.e. for each edge (u,v) in Graph, add its symmetric edge (v,u). Takes O(N^2) time. This is useful for making a directed graph undirected.

top_sort(+Graph, -Sorted)

Finds a topological ordering of a Graph and returns the ordering as a list of

Sorted vertices. Fails iff no ordering exists, i.e. iff the graph contains cycles.

Takes O(N^2) time.

max_path(+V1, +V2, +Graph, -Path, -Cost)

Path is a longest path of cost Cost from V1 to V2 in Graph, there being no cyclic paths from V1 to V2. Takes O(N^2) time.

min_path(+V1, +V2, +Graph, -Path, -Cost)

Path is a shortest path of cost Cost from V1 to V2 in Graph. Takes O(N^2) time.

min_paths(+Vertex, +Graph, -Tree)

Tree is a tree of all the shortest paths from Vertex to every other vertex in

Graph. This is the single-source shortest paths problem.

path(+Vertex, +Graph, -Path)

Given a Graph and a Vertex of Graph, returns a maximal Path rooted at Vertex, enumerating more paths on backtracking.

Chapter 24: Unweighted Graph Operations 283 reduce(+Graph, -Reduced)

Reduced is the reduced graph for Graph. The vertices of the reduced graph are the strongly connected components of Graph. There is an edge in Reduced from u to v iff there is an edge in Graph from one of the vertices in u to one of the vertices in v.

reachable(+Vertex, +Graph, -Reachable)

Given a Graph and a Vertex of Graph, returns the set of vertices that are reachable from that Vertex, including Vertex itself. Takes O(N^2) time.

random_ugraph(+P, +N, -Graph)

Where P is a probability, unifies Graph with a random graph of vertices 1..N

where each possible edge is included with probability P.

The following predicates are defined for undirected graphs only.

min_tree(+Graph, -Tree, -Cost)

Tree is a spanning tree of Graph with cost Cost, if it exists.

clique(+Graph, +K, -Clique)

Clique is a maximal clique (complete subgraph) of N vertices of Graph, where

N>=K. N is not necessarily maximal.

independent_set(+Graph, +K, -Set)

Set is a maximal independent (unconnected) set of N vertices of Graph, where

N>=K. N is not necessarily maximal.

coloring(+Graph, +K, -Coloring ) colouring(+Graph, +K, -Coloring )

Coloring is a mapping from vertices to colors 1..N of Graph such that all edges have distinct end colors, where N=<K. The mapping is represented as an ordered list of Vertex-Color pairs. N is not necessarily minimal.

284 SICStus Prolog

Chapter 25: Weighted Graph Operations 285

25 Weighted Graph Operations

A weighted directed graph (wgraph) is represented as a list of (vertex-edgelist) pairs, where the pairs are in standard order (as produced by keysort with unique keys), the edgelist is a list of (neighbor-weight) pair also in standard order (as produced by keysort with unique keys), every weight is a nonnegative integer, every neighbor appears as a vertex even if it has no neighbors itself, and no vertex is a neighbor to itself.

An undirected graph is represented as a directed graph where for each edge (U,V) there is a symmetric edge (V,U).

An edge (U,V) with weight W is represented as the term U-(V-W). U and V must be distinct.

A vertex can be any term. Two vertices are distinct iff they are not identical (==).

A path from u to v is represented as a list of vertices, beginning with u and ending with v. A vertex cannot appear twice in a path. A path is maximal in a graph if it cannot be extended.

A tree is a tree-shaped directed graph (all vertices have a single predecessor, except the root node, which has none).

A strongly connected component of a graph is a maximal set of vertices where each vertex has a path in the graph to every other vertex.

Sets are represented as ordered lists (see

Chapter 19 [Ordsets], page 267 ).

To load the package, enter the query

| ?- use_module(library(wgraphs)).

The following predicates are defined for directed graphs.

wgraph_to_ugraph(+WeightedGraph, -Graph)

Graph has the same vertices and edges as WeightedGraph, except the edges of

Graph are unweighted.

ugraph_to_wgraph(+Graph, -WeightedGraph)

WeightedGraph has the same vertices and edges as Graph, except the edges of

WeightedGraph all have weight 1.

vertices_edges_to_wgraph(+Vertices, +Edges, -WeightedGraph)

Vertices is a list of vertices, Edges is a list of edges, and WeightedGraph is a graph built from Vertices and Edges. Vertices and Edges may be in any order.

The vertices mentioned in Edges do not have to occur explicitly in Vertices.

Vertices may be used to specify vertices that are not connected by any edges.

vertices(+WeightedGraph, -Vertices)

Unifies Vertices with the vertices in WeightedGraph.

286 SICStus Prolog edges(+WeightedGraph, -Edges)

Unifies Edges with the edges in WeightedGraph.

add_vertices(+WeightedGraph1, +Vertices, -WeightedGraph2)

WeightedGraph2 is WeightedGraph1 with Vertices added to it.

del_vertices(+WeightedGraph1, +Vertices, -WeightedGraph2)

WeightedGraph2 is WeightedGraph1 with Vertices and all edges to and from them removed from it.

add_edges(+WeightedGraph1, +Edges, -WeightedGraph2)

WeightedGraph2 is WeightedGraph1 with Edges and their “to” and “from” vertices added to it.

del_edges(+WeightedGraph1, +Edges, -WeightedGraph2)

WeightedGraph2 is WeightedGraph1 with Edges removed from it.

transpose(+WeightedGraph, -Transpose)

Transpose is the graph computed by replacing each edge (u,v) in Weighted-

Graph by its symmetric edge (v,u). It can only be used one way around. Takes

O(N^2) time.

neighbors(+Vertex, +WeightedGraph, -Neighbors) neighbours(+Vertex, +WeightedGraph, -Neighbors)

Vertex is a vertex in WeightedGraph and Neighbors are its weighted neighbors.

transitive_closure(+WeightedGraph, -Closure)

Computes Closure as the transitive closure of WeightedGraph in O(N^3) time.

symmetric_closure(+WeightedGraph, -Closure)

Computes Closure as the symmetric closure of WeightedGraph, i.e. for each edge (u,v) in WeightedGraph, add its symmetric edge (v,u). Takes O(N^2) time. This is useful for making a directed graph undirected.

top_sort(+WeightedGraph, -Sorted)

Finds a topological ordering of a WeightedGraph and returns the ordering as a list of Sorted vertices. Fails iff no ordering exists, i.e. iff the graph contains cycles. Takes O(N^2) time.

max_path(+V1, +V2, +WeightedGraph, -Path, -Cost)

Path is a maximum-cost path of cost Cost from V1 to V2 in WeightedGraph, there being no cyclic paths from V1 to V2. Takes O(N^2) time.

min_path(+V1, +V2, +WeightedGraph, -Path, -Cost)

Path is a minimum-cost path of cost Cost from V1 to V2 in WeightedGraph.

Takes O(N^2) time.

min_paths(+Vertex, +WeightedGraph, -Tree)

Tree is a tree of all the minimum-cost paths from Vertex to every other vertex in WeightedGraph. This is the single-source minimum-cost paths problem.

path(+Vertex, +WeightedGraph, -Path)

Given a WeightedGraph and a Vertex of WeightedGraph, returns a maximal

Path rooted at Vertex, enumerating more paths on backtracking.

Chapter 25: Weighted Graph Operations 287 reduce(+WeightedGraph, -Reduced)

Reduced is the reduced graph for WeightedGraph. The vertices of the reduced graph are the strongly connected components of WeightedGraph. There is an edge in Reduced from u to v iff there is an edge in WeightedGraph from one of the vertices in u to one of the vertices in v.

reachable(+Vertex, +WeightedGraph, -Reachable)

Given a WeightedGraph and a Vertex of WeightedGraph, returns the set of vertices that are reachable from that Vertex. Takes O(N^2) time.

random_wgraph(+P, +N, +W, -WeightedGraph)

Where P is a probability, unifies WeightedGraph with a random graph of vertices 1..N where each possible edge is included with probability P and random weight in 1..W.

The following predicate is defined for undirected graphs only.

min_tree(+WeightedGraph, -Tree, -Cost)

Tree is a minimum-cost spanning tree of WeightedGraph with cost Cost, if it exists.

288 SICStus Prolog

Chapter 26: Socket I/O 289

26 Socket I/O

This library package defines a number of predicates manipulating sockets. They are all rather straight-forward interfaces to the corresponding BSD-type socket functions with the same name (except current_host/1). The reader should therefore study the appropriate documents for a deeper description.

The Domain is either the atom ’AF_INET’ or ’AF_UNIX’. They correspond directly to the same domains in BSD-type sockets. ’AF_UNIX’ may not be available on non-UNIX platforms.

An Address is either ’AF_INET’(Host,Port) or ’AF_UNIX’(SocketName).

Host is an atomic hostname, Port is a portnumber and SocketName is an atom denoting a socket.

A reader familiar with BSD sockets will understand this immediately.

All streams below can be both read from and written on. All I/O-predicates operating on streams can be used, for example read/2, write/2, format/3, current_stream/3, etc.

Socket streams are block buffered both on read and write by default. This can be changed by calling socket_buffering/4.

To load the package, enter the query

| ?- use_module(library(sockets)).

socket(+Domain, -Socket)

A socket Socket in the domain Domain is created.

socket_close(+Socket)

Socket is closed. Sockets used in socket_connect/2 should not be closed by socket_close/1 as they will be closed when the corresponding stream is closed.

socket_bind(+Socket, ’AF_UNIX’(+SocketName)) socket_bind(+Socket, ’AF_INET’(?Host,?Port))

The socket Socket is bound to the address. If Port is uninstantiated, the operative system picks a port number to which Port is bound.

socket_connect(+Socket, ’AF_UNIX’(+SocketName), -Stream) socket_connect(+Socket, ’AF_INET’(+Host,+Port), -Stream)

The socket Socket is connected to the address. Stream is a special stream on which items can be both read and written.

socket_listen(+Socket, +Length)

The socket Socket is defined to have a maximum backlog queue of Length pending connections.

socket_accept(+Socket, -Stream) socket_accept(+Socket, -Client, -Stream)

The first connection to socket Socket is extracted. The stream Stream is opened for read and write on this connection. For the ’AF_INET’ domain, Client will unified with an atom containing the Internet host address of the connecting

290 SICStus Prolog entity in numbers-and-dots notation. For other domains, Client will not be used.

socket_buffering(+Stream, +Direction, -OldBuf, +NewBuf )

The buffering in the Direction of the socket stream Stream is changed from

OldBuf to NewBuf. Direction should be read or write. OldBuf and NewBuf should be unbuf for unbuffered I/O or fullbuf for block buffered I/O.

socket_select(+TermsSockets, -NewTermsStreams, +TimeOut, +Streams,

-ReadStreams)

The list of streams in Streams is checked for readable characters. A stream can be any stream associated with an I/O descriptor. The list ReadStreams returns the streams with readable data.

socket_select/5 also waits for connections to the sockets specified by

TermsSockets. This argument should be a list of Term-Socket pairs, where

Term, which can be any term, is used as an identifier. NewTermsStreams is a list of Term-connection(Client,Stream) pairs, where Stream is a new stream open for communicating with a process connecting to the socket identified with

Term, Client is the client host address (see socket_accept/3).

If TimeOut is instantiated to off, the predicate waits until something is available. If TimeOut is S:U the predicate waits at most S seconds and U microseconds. Both S and U must be integers >=0. If there is a timeout, ReadStreams and NewTermsStreams are [].

socket_select(+Sockets, -NewStreams, +TimeOut, +Streams, -ReadStreams) socket_select(+Socket, -NewStream, +TimeOut, +Streams, -ReadStreams) socket_select(+Sockets, -NewStreams, -NewClients, +TimeOut, +Streams,

-ReadStreams) socket_select(+Socket, -NewStream, -NewClient, +TimeOut, +Streams,

-ReadStreams)

These forms, which are provided for backward compatibility only, differs in how sockets are specified and new streams returned.

socket_select/[5,6] also wait for connections to the sockets in the list Sockets. NewStreams is the list of new streams opened for communicating with the connecting processes. NewClients is the corresponding list of client host addresses (see socket_accept/3).

The second form requires one socket (not a list) for the first argument and returns a stream, NewStream, if a connection is made.

current_host(?HostName)

HostName is unified with the fully qualified name of the machine the process is executing on. The call will also succeed if HostName is instantiated to the unqualified name of the machine.

hostname_address(+HostName, -HostAddress) hostname_address(-HostName, +HostAddress)

The Internet host is resolved given either the host name or address. HostAddress should be an atom containing the Internet host address in numbers-and-

Chapter 26: Socket I/O 291 dots notation. The predicate will fail if the host name or address cannot be resolved.

292 SICStus Prolog

Chapter 27: Linda—Process Communication 293

27 Linda—Process Communication

Linda is a concept for process communication.

For an introduction and a deeper description, see [Carreiro & Gelernter 89a] or [Carreiro &

Gelernter 89b], respectively.

One process is running as a server and one or more processes are running as clients. The processes are communicating with sockets and supports networks.

The server is in principle a blackboard on which the clients can write (out/1), read (rd/1) and remove (in/1) data. If the data is not present on the blackboard, the predicates suspend the process until they are available.

There are some more predicates besides the basic out/1, rd/1 and in/1. The in_noblock/1 and rd_noblock/1 does not suspend if the data is not available—they fail instead. A blocking fetch of a conjunction of data can be done with in/2 or rd/2.

Example: A simple producer-consumer. In client 1: producer :produce(X), out(p(X)), producer.

produce(X) :- .....

In client 2: consumer :in(p(A)), consume(A), consumer.

consume(A) :- .....

Example: Synchronization

..., in(ready), %Waits here until someone does out(ready)

...,

Example: A critical region

..., in(region_free), % wait for region to be free critical_part, out(region_free), % let next one in

...,

294 SICStus Prolog

Example: Reading global data

..., rd(data(Data)),

..., or, without blocking:

..., rd_noblock(data(Data)) -> do_something(Data)

; write(’Data not available!’),nl

),

...,

Example: Waiting for one of several events

..., in([e(1),e(2),...,e(n)], E),

% Here is E instantiated to the first tuple that became available

...,

27.1 Server

The server is the process running the "blackboard process". It is an ordinary SICStus process which can be run on a separate machine if necessary.

To load the package, enter the query

| ?- use_module(library(’linda/server’)).

and start the server with linda/0 or linda/1.

linda

Starts a Linda-server in this SICStus. The network address is written to current output stream as Host:PortNumber.

linda(+Hook)

Starts a Linda-server in this SICStus. When it is started, a goal passed in Hook is evaluated. Hook must have the form Address-Goal where Address must be unifiable with Host:Port and Goal must be instantiated to a goal. Example:

| ?- linda((Host:Port)-(my_module:mypred(Host,Port))).

will call mypred/2 in module my_module when the server is started. mypred/2 could start the client-processes, save the address for the clients etc. Note that the module must be present in Goal.

27.2 Client

The clients are one or more sicstus processes which have connection(s) to the server.

Chapter 27: Linda—Process Communication 295

To load the package, enter the query

| ?- use_module(library(’linda/client’)).

Some of the following predicates fail if they don’t receive an answer from the Linda-server in a reasonable amount of time. That time is set with the predicate linda_timeout/2.

linda_client(+Address)

Establishes a connection to a Linda-server specified by Address. The Address is of the format Host:PortNumber as given by linda/0 and linda/1.

It is not possible to be connected to two Linda-servers in the same time.

This predicate can fail due to a timeout.

close_client

Closes the connection to the Linda-server.

linda_timeout(?OldTime, ?NewTime)

This predicate controls Linda’s timeout. OldTime is unified with the old timeout and then timeout is set to NewTime. The value is either off or of the form

Seconds:Milliseconds. The former value indicates that the timeout mechanism is disabled, that is, eternal waiting. The latter form is the timeout-time.

out(+Tuple)

Places the tuple Tuple in Linda’s tuple-space.

in(?Tuple)

Removes the tuple Tuple from Linda’s tuple-space if it is there. If not, the predicate blocks until it is available (that is, someone performs an out/1).

in_noblock(?Tuple)

Removes the tuple Tuple from Linda’s tuple-space if it is there. If not, the predicate fails.

This predicate can fail due to a timeout.

in(+TupleList, ?Tuple)

As in/1 but succeeds when either of the tuples in TupleList is available. Tuple is unified with the fetched tuple.

If that unification fails, the tuple is not reinserted in the tuple-space.

rd(?Tuple)

Succeeds if Tuple is available in the tuple-space, suspends otherwise until it is available. Compare this with in/1: the tuple is not removed.

rd_noblock(?Tuple)

Succeeds if Tuple is available in the tuple-space, fails otherwise.

This predicate can fail due to a timeout.

rd(+TupleList, ?Tuple)

As in/2 but does not remove any tuples.

bagof_rd_noblock(?Template, +Tuple, ?Bag )

Bag is the list of all instances of Template such that Tuple exists in the tuplespace.

296 SICStus Prolog

The behavior of variables in Tuple and Template is as in bagof/3. The variables could be existentially quantified with ^/2 as in bagof/3.

The operation is performed as an atomic operation.

This predicate can fail due to a timeout.

Example: Assume that only one client is connected to the server and that the tuple-space initially is empty.

| ?- out(x(a,3)), out(x(a,4)), out(x(b,3)), out(x(c,3)).

yes

| ?- bagof_rd_noblock(C-N, x(C,N), L).

C = _32,

L = [a-3,a-4,b-3,c-3],

N = _52 ?

yes

| ?- bagof_rd_noblock(C, N^x(C,N), L).

C = _32,

L = [a,a,b,c],

N = _48 ?

yes

Chapter 28: DB - External Storage of Terms 297

28 DB - External Storage of Terms

This library module is obsolete and has been replaced by library(bdb). It is included in the SICStus distribution for backwards compatibility only. It will be removed in the next major release.

The sources to library(db) can be downloaded freely from ftp://ftp.sics.se/archive/sicstus3/libdb.tgz

298 SICStus Prolog

Chapter 29: External Storage of Terms (Berkeley DB) 299

29 External Storage of Terms (Berkeley DB)

This library module handles storage and retrieval of terms on files. By using indexing, the store/retrieve operations are efficient also for large data sets. The package is an interface to the Berkeley DB toolset.

The package is loaded by the query:

| ?- use_module(library(bdb)).

29.1 Basics

The idea is to get a behavior similar to assert/1, retract/1 and clause/2 but the terms are stored on files instead of in primary memory.

The differences compared with the internal database are:

• A database must be opened before any access and closed after the last access. (There are special predicates for this: db_open/[4,5] and db_close/1.)

• The functors and the indexing specifications of the terms to be stored have to be given when the database is created. (see

Section 29.7 [The DB-Spec], page 305 ).

• The indexing is specified when the database is created. It is possible to index on other parts of the term than just the functor and first argument.

• Changes affect the database immediately.

• The database will store variables with blocked goals as ordinary variables.

Some commercial databases can’t store non-ground terms or more than one instance of a term. This library module can however store terms of either kind.

29.2 Current Limitations

• The terms are not necessarily fetched in the same order as they were stored.

• If the process dies during an update operation (db_store/3, db_erase/[2,3]), the database can be inconsistent.

• Databases can only be shared between processes running on the machine where the environment is created (see

Section 29.5 [Predicates], page 301 ). The database itself

can be on a different machine.

• The number of terms ever inserted in a database cannot exceed 2^32-1.

• Duplicate keys are not handled efficiently by Berkeley DB. This limitation is supposed to get lifted in the future. Duplicate keys can result from indexing on non-key attribute sets, inserting terms with variables on indexing positions, or simply from storing the same term more than once.

300 SICStus Prolog

29.3 Berkeley DB

This library module is an interface to the Berkeley DB toolset to support persistent storage of Prolog terms. Some of the notions of Berkeley DB are directly inherited, e.g. the environment.

The interface uses the Concurrent Access Methods product of Berkeley DB. This means that multiple processes can open the same database but transactions and disaster recovery are not supported.

The environment and the database files are ordinary Berkeley DB entities which means that the standard support utilities (e.g. db_stat) will work.

29.4 The DB-Spec—Informal Description

The db-spec defines which functors are allowed and which parts of a term are used for indexing in a database. The syntax of a spec resembles to that of the mode specification.

The db-spec is a list of atoms and compound terms where the arguments are either + or

-. A term can be inserted in the database if there is a spec in the spec list with the same functor.

Multilevel indexing is not supported, terms have to be “flattened”.

Every spec with the functor of the indexed term specifies an indexing. Every argument where there is a + in the spec is indexed on.

The idea of the db-spec is illustrated with a few examples. (A section further down explains the db-spec in a more formal way).

Given a spec of [f(+,-), .(+,-), g, f(-,+)] the indexing works as follows. (The parts with indexing are underlined.)

Term g(x,y) f(A,B)

Store domain error f(A,B)

f(a,b) f(a,b)

- - - -

.(a,.(b,[]))

- g

-

Fetch domain error instantiation error f(a,b)

[a,b] g g

f(a,b)

- -

.(a,.(b,[]))

- -

The specification [f(+,-), f(-,+)] is different from [f(+,+)]. The first specifies that two indices are to be made whereas the second specifies that only one index is to be made on both arguments of the term.

Chapter 29: External Storage of Terms (Berkeley DB) 301

29.5 Predicates

29.5.1 Conventions

The following conventions are used in the predicate descriptions below.

• Mode is either update or read or enumerate. In mode read no updates can be made.

Mode enumerate is like mode read but also indexing cannot be used, i.e. you can only sequentially enumerate the items in the database. In mode enumerate only the file storing the terms along with their references is used.

• EnvRef is a reference to an open database environment. The environment is returned when it is opened. The reference becomes invalid after the environment has been closed.

• DBRef is a reference to an open database. The reference is returned when the database is opened. The reference becomes invalid after the database has been closed.

• TermRef is a reference to a term in a given database. The reference is returned when a term is stored. The reference stays valid even after the database has been closed and hence can be stored permanently as part of another term. However, if such references are stored in the database, automatic compression of the database (using db_ compress/[2,3]) is not possible, in that case the user has to write her own compressing predicate.

• SpecList is a description of the indexing scheme, see

Section 29.7 [The DB-Spec], page 305

.

• Term is any Prolog term.

• Iterator is a non-backtrackable mutable object. It can be used to iterate through a set of terms stored in a database. The iterators are unidirectional.

29.5.2 The environment

To enable sharing of databases between process, programs have to create environments and the databases should be opened in these environments. A database can be shared between processes that open it in the same environment. An environment physically consists of a directory containing the files needed to enable sharing databases between processes. The directory of the environment has to be located in a local file system.

Databases can be opened outside any environment (see db_open/4), but in that case a process writing the database must ensure exclusive access or the behavior of the predicates is undefined.

29.5.3 Memory leaks

In order to avoid memory leaks, environments, databases and iterators should always be closed. Consider using call_cleanup/2 to automate the closing/deallocation of these objects. You can always use db_current_env/1, db_current/5 and db_current_iterator/3 to enumerate the currently living objects.

302 SICStus Prolog

29.5.4 The predicates db_open_env(+EnvName, -EnvRef ) db_open_env(+EnvName, +CacheSize, -EnvRef )

Opens an environment with the name EnvName. A directory with this name is created for the environment if necessary. EnvName is not subject to absolute_ file_name/2 conversion.

By using db_open_env/3 one can specify the size of the cache: CacheSize is the (integer) size of the cache in kilobytes. The size of the cache cannot be less than 20 kilobytes. db_open_env/2 will create a cache of the system’s default size.

The size of the cache is determined when the environment is created and cannot be changed by future openings.

A process cannot open the same environment more than once.

db_close_env(+EnvRef )

Closes an environment. All databases opened in the environment will be closed as well. abort/0 does not close environments.

db_current_env(?EnvName, ?EnvRef )

Unifies the arguments with the open environments. This predicate can be used for enumerating all currently open environments through backtracking.

db_open(+DBName, +Mode, ?SpecList, -DBRef ) db_open(+DBName, +Mode, ?SpecList, +EnvRef, -DBRef )

Opens a database with the name DBName. The database physically consists of a directory with the same name, containing the files that make up the database.

If the directory does not exist, it is created. In that case Mode must be update and the db-spec SpecList must be ground. If an existing database is opened and Mode is read or update, SpecList is unified with the db-spec given when the database was created. If the unification fails an error is raised. DBRef is unified with a reference to the opened database. DBName is not subject to absolute_file_name/2 conversion.

If Mode is enumerate then the indexing specification is not read, and SpecList is left unbound.

db_open/5 opens the database in the environment referred to by EnvRef. db_ open/4 opens the database outside any environment. (db_open/4 is equivalent to calling db_open/5 with EnvRef bound to the atom none.)

To avoid inconsistency, if multiple processes open the same database with db_ open/4 then all of them should do that with Mode set to read or enumerate.

(This is not enforced by the system.) db_close(+DBRef )

Closes the database referenced by DBRef. Any iterators opened in the database will be deallocated. abort/0 does not close databases.

Chapter 29: External Storage of Terms (Berkeley DB) 303 db_current(?DBName, ?Mode, ?SpecList, ?EnvRef, ?DBRef )

Unifies the arguments with the open databases. This predicate can be used to enumerate all currently open databases through backtracking. If the database was opened with db_open/4 then EnvRef will be unified with the atom none.

db_store(+DBRef, +Term, -TermRef )

Stores Term in the database DBRef. TermRef is unified with a corresponding term reference. The functor of Term must match the functor of a spec in the db-spec associated with DBRef.

db_fetch(+DBRef, ?Term, ?TermRef )

Unifies Term with a term from the database DBRef. At the same time, TermRef is unified with a corresponding term reference. Backtracking over the predicate unifies with all terms matching Term.

If TermRef is not instantiated then both the functor and the instantiatedness of Term must match a spec in the db-spec associated with DBRef.

If TermRef is instantiated, the referenced term is read and unified with Term.

If you simply want to find all matching terms, it is more efficient to use db_ findall/5 or db_enumerate/3.

db_findall(+DBRef, +Template, +Term, :Goal, ?Bag )

Unifies Bag with the list of instances of Template in all proofs of Goal found when Term is unified with a matching term from the database DBRef. Both the functor and the instantiatedness of Term must match a spec in the dbspec associated with DBRef.

Conceptually, this predicate is equivalent to findall(Template, (db_fetch(DBRef, Term, _), Goal), Bag ).

db_erase(+DBRef, +TermRef ) db_erase(+DBRef, +TermRef, +Term)

Deletes the term from the database DBRef that is referenced by TermRef.

In the case of db_erase/2 the term associated with TermRef has to be looked up. db_erase/3 assumes that the term Term is identical with the term associated with TermRef (modulo variable renaming). If this is not the case, the behavior is undefined.

db_enumerate(+DBRef, ?Term, ?TermRef )

Unifies Term with a term from the database DBRef. At the same time, TermRef is unified with a corresponding term reference. Backtracking over the predicate unifies with all terms matching Term.

Implemented by linear search—the db-spec associated with DBRef is ignored.

It is not useful to call this predicate with TermRef instantiated.

db_compress(+DBRef, +DBName) db_compress(+DBRef, +DBName, +SpecList)

Copies the database given by DBRef to a new database named by DBName.

The new database will be a compressed version of the first one in the sense that it will not have “holes” resulting from deletion of terms. Deleted term references will also be reused, which implies that references that refer to terms in the old database will be invalid in the new one.

304 SICStus Prolog db_compress/2 looks for a database with the db-spec of the original one. db_ compress/3 stores the terms found in the original database with the indexing specification SpecList. db_compress/2 cannot be used if the database DBRef was opened in mode enumerate.

If the database DBName already exists then the terms of DBRef will be appended to it. Of course DBName must have an indexing specification which enables the terms in DBRef to be inserted into it.

In the case of db_compress/3 if the database DBName does not exist, then

SpecList must be a valid indexing specification.

db_make_iterator(+DBRef, -Iterator) db_make_iterator(+DBRef, +Term, -Iterator)

Creates a new iterator and unifies it with Iterator. Iterators created with db_ make_iterator/2 iterate through the whole database. Iterators created with db_make_iterator/3 iterate through the terms that would be found by db_ fetch(DBRef, Term, _).

Every iterator created by db_make_iterator/[2,3] must be destroyed with db_iterator_done/1.

db_iterator_next(+Iterator, ?Term, ?TermRef )

Iterator advances to the next term, Term and TermRef is unified with the term and its reference pointed to by Iterator. If there is no next term, the predicate fails.

db_iterator_done(+Iterator)

Deallocates Iterator, which must not be used anymore. abort/0 does not deallocate iterators.

db_current_iterator(?DBRef, ?Term, ?Iterator)

Unifies the the variables with the respective properties of the living iterators.

This predicate can be used to enumerate all currently alive iterators through backtracking. If Iterator was made with db_make_iterator/2 then Term will be left unbound.

29.6 An Example Session

| ?- db_open(’/tmp/db’, update, [a(+,-)], ’$db_env’(-33470544), DBRef).

DBRef = ’$db’(-33470432) ?

yes

| ?- db_store(’$db’(-33470432), a(b,1), _).

yes

| ?- db_store(’$db’(-33470432), a(c,2), _).

yes

| ?- db_fetch(’$db’(-33470432), a(b,X), _).

Chapter 29: External Storage of Terms (Berkeley DB) 305

X = 1 ? ; no

| ?- db_enumerate(’$db’(-33470432), X, _).

X = a(b,1) ? ;

X = a(c,2) ? ; no

| ?- db_current(DBName, Mode, Spec, EnvRef, DBRef).

Mode = update,

Spec = [a(+,-)],

DBRef = ’$db’(-33470432),

DBName = ’/tmp/db’,

EnvRef = ’$db_env’(-33470544) ? ; no

| ?- db_close_env(’$db_env’(-33470544)).

yes

29.7 The DB-Spec

A db-spec has the form of a speclist: speclist spec argspec

= [spec1, ..., specM ]

= functor(argspec1, ..., argspecN )

= + | where functor is a Prolog atom. The case N = 0 is allowed.

A spec F(argspec1, ..., argspecN ) is applicable to any nonvar term with principal functor

F/N.

When storing a term T we generate a hash code for every applicable spec in the db-spec, and a reference to T is stored with each of them. (More precisely with each element of the set of generated hash codes). If T contains nonvar elements on each + position in the spec, then the hash code depends on each of these elements. If T does contain some variables on

+ position, then the hash code depends only on the functor of T.

When fetching a term Q we look for an applicable spec for which there are no variables in

Q on positions maked +. If no applicable spec can be found a domain error is raised. If no spec can be found where on each + position a nonvar term occurs in Q an instantiation error is raised. Otherwise, we choose the the spec with the most + postitions in it breaking ties by choosing the leftmost one.

306 SICStus Prolog

The terms that contain nonvar terms on every + postition will be looked up using indexing based on the principal functor of the term and the principal functor of terms on + postitions.

The other (more general) terms will be looked up using an indexing based on the principal functor of the term only.

As can be seen, storing and fetching terms with variables on + positions are not vigorously supported operations.

Chapter 30: Boolean Constraint Solver 307

30 Boolean Constraint Solver

The clp(B) system provided by this library module is an instance of the general Constraint

Logic Programming scheme introduced in [Jaffar & Michaylov 87]. It is a solver for constraints over the Boolean domain, i.e. the values 0 and 1. This domain is particularly useful for modeling digital circuits, and the constraint solver can be used for verification, design, optimization etc. of such circuits.

To load the solver, enter the query:

| ?- use_module(library(clpb)).

The solver contains predicates for checking the consistency and entailment of a constraint wrt. previous constraints, and for computing particular solutions to the set of previous constraints.

The underlying representation of Boolean functions is based on Boolean Decision Diagrams

[Bryant 86]. This representation is very efficient, and allows many combinatorial problems to be solved with good performance.

Boolean expressions are composed from the following operands: the constants 0 and 1

(FALSE and TRUE), logical variables, and symbolic constants, and from the following connectives. P and Q are Boolean expressions, X is a logical variable, Is is a list of integers or integer ranges, and Es is a list of Boolean expressions:

~ P

P * Q

True if P is false.

True if P and Q are both true.

P + Q

P # Q

True if at least one of P and Q is true.

True if exactly one of P and Q is true.

X ^ P True if there exists an X such that P is true. Same as P[X /0] + P[X /1].

P =:= Q Same as ~P # Q.

P =\= Q Same as P # Q.

P =< Q Same as ~P + Q.

P >= Q

P < Q

P > Q

Same as P + ~Q.

Same as ~P * Q.

Same as P * ~Q.

card(Is, Es)

True if the number of true expressions in Es is a member of the set denoted by

Is.

Symbolic constants (Prolog atoms) denote parametric values and can be viewed as allquantified variables whose quantifiers are placed outside the entire expression. They are useful for forcing certain variables of an equation to be treated as input parameters.

308 SICStus Prolog

30.1 Solver Interface

The following predicates are defined: sat(+Expression)

Expression is a Boolean expression. This checks the consistency of the expression wrt. the accumulated constraints, and, if the check succeeds, tells the constraint that the expression be true.

If a variable X, occurring in the expression, is subsequently unified with some term T, this is treated as a shorthand for the constraint

?- sat(X=:=T).

taut(+Expression, ?Truth)

Expression is a Boolean expression. This asks whether the expression is now entailed by the accumulated constraints (Truth=1), or whether its negation is entailed by the accumulated constraints (Truth=0). Otherwise, it fails.

labeling(+Variables)

Variables is a list of variables. The variables are instantiated to a list of 0s and 1s, in a way that satisfies any accumulated constraints. Enumerates all solutions by backtracking, but creates choicepoints only if necessary.

30.2 Examples

30.2.1 Example 1

| ?- sat(X + Y).

sat(X=\=_A*Y#Y) ?

illustrates three facts. First, any accumulated constraints affecting the top-level variables are displayed as floundered goals, since the query is not true for all X and Y. Secondly, accumulated constraints are displayed as sat(V =:=Expr) or sat(V =\=Expr) where V is a variable and Expr is a “polynomial”, i.e. an exclusive or of conjunctions of variables and constants. Thirdly, _A had to be introduced as an artificial variable, since Y cannot be expressed as a function of X. That is, X + Y is true iff there exists an _A such that

X=\=_A*Y#Y. Let’s check it!

| ?- taut(_A ^ (X=\=_A*Y#Y) =:= X + Y, T).

T = 1 ?

verifies the above answer. Notice that the formula in this query is a tautology, and so it is entailed by an empty set of constraints.

30.2.2 Example 2

| ?- taut(A =< C, T).

Chapter 30: Boolean Constraint Solver 309 no

| ?- sat(A =< B), sat(B =< C), taut(A =< C, T).

T = 1, sat(A=:=_A*_B*C), sat(B=:=_B*C) ?

| ?- taut(a, T).

T = 0 ?

yes

| ?- taut(~a, T).

T = 0 ?

illustrates the entailment predicate. In the first query, the expression “A implies C” is neither known to be true nor false, so the query fails. In the second query, the system is told that “A implies B” and “B implies C”, so “A implies C” is entailed. The expressions in the third and fourth queries are to be read “for each a, a is true” and “for each a, a is false”, respectively, and so T = 0 in both cases since both are unsatisfiable. This illustrates the fact that the implicit universal quantifiers introduced by symbolic constants are placed in front of the entire expression.

30.2.3 Example 3

| ?- [user].

| adder(X, Y, Sum, Cin, Cout) :sat(Sum =:= card([1,3],[X,Y,Cin])), sat(Cout =:= card([2-3],[X,Y,Cin])).

| {user consulted, 40 msec 576 bytes} yes

| ?- adder(x, y, Sum, cin, Cout).

sat(Sum=:=cin#x#y), sat(Cout=:=x*cin#x*y#y*cin) ?

yes

| ?- adder(x, y, Sum, 0, Cout).

sat(Sum=:=x#y), sat(Cout=:=x*y) ?

yes

| ?- adder(X, Y, 0, Cin, 1), labeling([X,Y,Cin]).

Cin = 0,

310 SICStus Prolog

X = 1,

Y = 1 ? ;

Cin = 1,

X = 0,

Y = 1 ? ;

Cin = 1,

X = 1,

Y = 0 ? ; illustrates the use of cardinality constraints and models a one-bit adder circuit. The first query illustrates how representing the input signals by symbolic constants forces the output signals to be displayed as functions of the inputs and not vice versa. The second query computes the simplified functions obtained by setting carry-in to 0. The third query asks for particular input values satisfying sum and carry-out being 0 and 1, respectively.

30.2.4 Example 4

The predicate fault/3 below describes a 1-bit adder consisting of five gates, with at most one faulty gate. If one of the variables Fi is equal to 1, the corresponding gate is faulty, and its output signal is undefined (i.e., the constraint representing the gate is relaxed).

Assuming that we have found some incorrect output from a circuit, we are interesting in finding the faulty gate. Two instances of incorrect output are listed in fault_ex/2: fault([F1,F2,F3,F4,F5], [X,Y,Cin], [Sum,Cout]) :sat( card([0-1],[F1,F2,F3,F4,F5]) *

(F1 + (U1 =:= X * Cin)) *

(F2 + (U2 =:= Y * U3)) *

(F3 + (Cout =:= U1 + U2)) *

(F4 + (U3 =:= X # Cin)) *

(F5 + (Sum =:= Y # U3))

).

fault_ex(1, Faults) :- fault(Faults, [1,1,0], [1,0]).

fault_ex(2, Faults) :- fault(Faults, [1,0,1], [0,0]).

To find the faulty gates, we run the query

| ?- fault_ex(I,L), labeling(L).

I = 1,

L = [0,0,0,1,0] ? ;

I = 2,

L = [1,0,0,0,0] ? ;

Chapter 30: Boolean Constraint Solver 311

I = 2,

L = [0,0,1,0,0] ? ; no

Thus for input data [1,1,0], gate 4 must be faulty. For input data [1,0,1], either gate 1 or gate 3 must be faulty.

To get a symbolic representation of the outputs interms of the input, we run the query

| ?- fault([0,0,0,0,0], [x,y,cin], [Sum,Cout]).

sat(Cout=:=x*cin#x*y#y*cin), sat(Sum=:=cin#x#y) which shows that the sum and carry out signals indeed compute the intended functions if no gate is faulty.

312 SICStus Prolog

Chapter 31: Constraint Logic Programming over Rationals or Reals

31 Constraint Logic Programming over Rationals or Reals

313

31.1 Introduction

The clp(Q,R) system described in this document is an instance of the general Constraint

Logic Programming scheme introduced by [Jaffar & Michaylov 87].

The implementation is at least as complete as other existing clp(R) implementations: It solves linear equations over rational or real valued variables, covers the lazy treatment of nonlinear equations, features a decision algorithm for linear inequalities that detects implied equations, removes redundancies, performs projections (quantifier elimination), allows for linear dis-equations, and provides for linear optimization.

The full clp(Q,R) distribution, including a stand-alone manual and an examples directory that is possibly more up to date than the version in the SICStus Prolog distribution, is available from: http://www.ai.univie.ac.at/clpqr/

.

Referencing this Software

When referring to this implementation of clp(Q,R) in publications, you should use the following reference:

Holzbaur C.: OFAI clp(q,r) Manual, Edition 1.3.3, Austrian Research Institute for Artificial Intelligence, Vienna, TR-95-09, 1995.

Acknowledgments

The development of this software was supported by the Austrian Fonds zur Foerderung der

Wissenschaftlichen Forschung under grant P9426-PHY. Financial support for the Austrian

Research Institute for Artificial Intelligence is provided by the Austrian Federal Ministry for Science and Research.

We include a collection of examples that has been distributed with the Monash University version of clp(R) [Heintze et al. 87], and its inclusion into this distribution was kindly permitted by Roland Yap.

31.2 Solver Interface

Until rational numbers become first class citizens in SICStus Prolog, rational arithmetics has to be emulated. Because of the emulation it is too expensive to support arithmetics with automatic coercion between all sorts of numbers, like you find it in CommonLisp, for example.

You must choose whether you want to operate in the field of Q (Rationals) or R (Reals):

| ?- use_module(library(clpq)).

314 SICStus Prolog or

| ?- use_module(library(clpr)).

You can also load both modules, but the exported predicates listed below will nameclash (see

Section 4.4 [Importation], page 57

).

You can avoid the interactive resolution dialog if the importation is skipped, e.g. via: use_module(library(clpq),[]),use_ module(library(clpr),[]).

Notational Conventions

Throughout this chapter, the prompts clp(q) ?- and clp(r) ?- are used to differentiate between clp(Q) and clp(R) in exemplary interactions.

In general there are many ways to express the same linear relationship. This degree of freedom is manifest in the fact that the printed manual and an actual interaction with the current version of clp(Q,R) may show syntactically different answer constraints, despite the fact the same semantic relationship is being expressed. There are means to control the presentation; see

Section 31.5.1 [Variable Ordering], page 325 . The approximative nature

of floating point numbers may also produce numerical differences between the text in this manual and the actual results of clp(R), for a given edition of the software.

31.2.1 Solver Predicates

The solver interface for both Q and R consists of the following predicates which are exported from module(linear).

{+Constraint}

Constraint is a term accepted by the the grammar below. The corresponding constraint is added to the current constraint store and checked for satisfiability.

Use the module prefix to distinguish the solvers if both clp(Q) and clp(R) were loaded

| ?- clpr:{Ar+Br=10}, Ar=Br, clpq:{Aq+Bq=10}, Aq=Bq.

Aq = 5,

Ar = 5.0,

Bq = 5,

Br = 5.0

Although clp(Q) and clp(R) are independent modules, you are asking for trouble if you (accidently) share variables between them:

| ?- clpr:{A+B=10}, clpq:{A=B}.

{TYPE ERROR: _118=5.0 - arg 2: expected ’a rational number’, found 5.0}

This is because both solvers eventually compute values for the variables and

Reals are incompatible with Rationals.

Here is the constraint grammar:

Chapter 31: Constraint Logic Programming over Rationals or Reals 315

Constraint

-->

C -->

C

| C , C

Expr =:= Expr

| Expr =

Expr

| Expr <

Expr

| Expr >

Expr

| Expr =<

Expr

| Expr >=

Expr

| Expr =\= Expr conjunction equation equation strict inequation strict inequation nonstrict inequation nonstrict inequation disequation

Expr --> variable

| number

| + Expr

| - Expr

| Expr + Expr

| Expr - Expr

| Expr * Expr

| Expr / Expr

| abs(Expr)

| sin(Expr)

| cos(Expr)

| tan(Expr)

| pow(Expr,Expr)

| exp(Expr,Expr)

| min(Expr,Expr)

| max(Expr,Expr)

| #(Const)

Prolog variable floating point or integer unary plus unary minus addition subtraction multiplication division absolute value trigonometric sine trigonometric cosine trigonometric tangent raise to the power raise to the power minimum of the two arguments maximum of the two arguments symbolic numerical constants

Conjunctive constraints {C,C} have been made part of the syntax to control the granularity of constraint submission, which will be exploited by future versions of this software. Symbolic numerical constants are provided for compatibility only; see

Section 31.7.1 [Monash Examples], page 332

.

entailed(+Constraint)

Succeeds iff the linear Constraint is entailed by the current constraint store.

This predicate does not change the state of the constraint store.

clp(q) ?- {A =< 4}, entailed(A=\=5).

{A=<4} yes clp(q) ?- {A =< 4}, entailed(A=\=3).

no inf(+Expr, -Inf ) inf(+Expr, -Inf, +Vector, -Vertex)

Computes the infimum of the linear expression Expr and unifies it with Inf. If given, Vector should be a list of variables relevant to Expr, and Vertex will be

316 SICStus Prolog unified a list of the same length as Vector containing the values for Vector, such that the infimum is produced when assigned. Failure indicates unboundedness.

sup(+Expr, -Sup) sup(+Expr, -Sup, +Vector, -Vertex)

Computes the supremum of the linear expression Expr and unifies it with Sup.

If given, Vector should be a list of variables relevant to Expr, and Vertex will be unified a list of the same length as Vector containing the values for Vector, such that the supremum is produced when assigned. Failure indicates unboundedness.

clp(q) ?- { 2*X+Y =< 16, X+2*Y =< 11,

X+3*Y =< 15, Z = 30*X+50*Y

}, sup(Z, Sup, [X,Y], Vertex).

Sup = 310,

Vertex = [7,2],

{Z=30*X+50*Y},

{X+1/2*Y=<8},

{X+3*Y=<15},

{X+2*Y=<11} minimize(+Expr)

Computes the infimum of the linear expression Expr and equates it with the expression, i.e. as if defined as: minimize(Expr) :- inf(Expr, Expr).

maximize(+Expr)

Computes the supremum of the linear expression Expr and equates it with the expression.

clp(q) ?- { 2*X+Y =< 16, X+2*Y =< 11,

X+3*Y =< 15, Z = 30*X+50*Y

}, maximize(Z).

X = 7,

Y = 2,

Z = 310 bb_inf(+Ints, +Expr, -Inf )

Computes the infimum of the linear expression Expr under the additional constraint that all of variables in the list Ints assume integral values at the infimum.

This allows for the solution of mixed integer linear optimization problems; see

Section 31.8 [MIP], page 333

.

clp(q) ?- {X >= Y+Z, Y > 1, Z > 1}, bb_inf([Y,Z],X,Inf).

Inf = 4,

{Y>1},

{Z>1},

{X-Y-Z>=0}

Chapter 31: Constraint Logic Programming over Rationals or Reals 317 bb_inf(+Ints, +Expr, -Inf, -Vertex, +Eps)

Computes the infimum of the linear expression Expr under the additional constraint that all of variables in the list Ints assume integral values at the infimum.

Eps is a positive number between 0 and 0.5 that specifies how close a number X must be to the next integer to be considered integral: abs(round(X)-X) < Eps.

The predicate bb_inf/3 uses Eps = 0.001. With clp(Q), Eps = 0 makes sense.

Vertex is a list of the same length as Ints and contains the (integral) values for Ints, such that the infimum is produced when assigned. Note that this will only generate one particular solution, which is different from the situation with minimize/1, where the general solution is exhibited.

ordering(+Spec)

Provides a means to control one aspect of the presentation of the answer constraints; see

Section 31.5.1 [Variable Ordering], page 325 .

dump(+Target, -NewVars, -CodedAnswer)

Reflects the constraints on the target variables into a term, where Target and

NewVars are lists of variables of equal length and CodedAnswer is the term representation of the projection of constraints onto the target variables where the target variables are replaced by the corresponding variables from NewVars

(see

Section 31.5.2 [Turning Answers into Terms], page 326

).

clp(q) ?- {A+B =< 10, A>=4}, dump([A,B],Vs,Cs), dump([B],Bp,Cb).

Cb = [_A=<6],

Bp = [_A],

Cs = [_B>=4,_C+_B=<10],

Vs = [_C,_B],

{A>=4},

{A+B=<10}

The current version of dump/3 is incomplete with respect to nonlinear constraints. It only reports nonlinear constraints that are connected to the target variables. The following example has no solution. From the top level’s report we have a chance to deduce this fact, but dump/3 currently has no means to collect global constraints ...

q(X) :-

{X>=10},

{sin(Z)>3}.

clp(r) ?- q(X), dump([X],V,C).

C = [_A>=10.0],

V = [_A], clpr:{3.0-sin(_B)<0.0},

{X>=10.0}

318 SICStus Prolog

31.2.2 Unification

Equality constraints are added to the store implicitly each time variables that have been mentioned in explicit constraints are bound - either to another such variable or to a number.

clp(r) ?- {2*A+3*B=C/2}, C=10.0, A=B.

A = 1.0,

B = 1.0,

C = 10.0

Is equivalent modulo rounding errors to clp(r) ?- {2*A+3*B=C/2, C=10, A=B}.

A = 1.0,

B = 0.9999999999999999,

C = 10.0

The shortcut bypassing the use of {}/1 is allowed and makes sense because the interpretation of this equality in Prolog and clp(R) coincides. In general, equations involving interpreted functors, +/2 in this case, must be fed to the solver explicitly: clp(r) ?- X=3.0+1.0, X=4.0.

no

Further, variables known by clp(R) may be bound directly to floats only. Likewise, variables known by clp(Q) may be bound directly to rational numbers only; see

Section 31.9.1.1

[Rationals], page 335

. Failing to do so is rewarded with an exception: clp(q) ?- {2*A+3*B=C/2}, C=10.0, A=B.

{TYPE ERROR: _165=10.0 - arg 2: expected ’a rational number’, found 10.0}

This is because 10.0 is not a rational constant. To make clp(Q) happy you have to say: clp(q) ?- {2*A+3*B=C/2}, C=rat(10,1), A=B.

A = 1,

B = 1,

C = 10

If you use {}/1, you don’t have to worry about such details. Alternatively, you may use the automatic expansion facility, check

Section 31.7 [Syntactic Sugar], page 331

.

31.2.3 Feedback and Bindings

What was covered so far was how the user populates the constraint store.

The other direction of the information flow consists of the success and failure of the above predicates and the binding of variables to numerical values. Example:

Chapter 31: Constraint Logic Programming over Rationals or Reals 319 clp(r) ?- {A-B+C=10, C=5+5}.

{A = B},

C = 10.0

The linear constraints imply C=10.0 and the solver consequently exports this binding to the

Prolog world. The fact that A=B is deduced and represented by the solver but not exported as a binding. More about answer presentation in

Section 31.5 [Projection], page 324 .

31.3 Linearity and Nonlinear Residues

The clp(Q,R) system is restricted to deal with linear constraints because the decision algorithms for general nonlinear constraints are prohibitively expensive to run. If you need this functionality badly, you should look into symbolic algebra packages. Although the clp(Q,R) system cannot solve nonlinear constraints, it will collect them faithfully in the hope that through the addition of further (linear) constraints they might get simple enough to solve eventually. If an answer contains nonlinear constraints, you have to be aware of the fact that success is qualified modulo the existence of a solution to the system of residual (nonlinear) constraints: clp(r) ?- {sin(X) = cos(X)}.

clpr:{sin(X)-cos(X)=0.0}

There are indeed infinitely many solutions to this constraint (X = 0.785398 + n*Pi), but clp(Q,R) has no direct means to find and represent them.

The systems goes through some lengths to recognize linear expressions as such. The method is based on a normal form for multivariate polynomials. In addition, some simple isolation axioms, that can be used in equality constraints, have been added. The current major limitation of the method is that full polynomial division has not been implemented. Examples:

This is an example where the isolation axioms are sufficient to determine the value of X.

clp(r) ?- {sin(cos(X)) = 1/2}.

X = 1.0197267436954502

If we change the equation into an inequation, clp(Q,R) gives up: clp(r) ?- {sin(cos(X)) < 1/2}.

clpr:{sin(cos(X))-0.5<0.0}

The following is easy again: clp(r) ?- {sin(X+2+2)/sin(4+X) = Y}.

Y = 1.0

320 SICStus Prolog

And so is this: clp(r) ?- {(X+Y)*(Y+X)/X = Y*Y/X+99}.

{Y=49.5-0.5*X}

An ancient symbol manipulation benchmark consists in rising the expression X+Y+Z+1 to the 15th power: clp(q) ?{exp(X+Y+Z+1,15)=0}.

clpq:{Z^15+Z^14*15+Z^13*105+Z^12*455+Z^11*1365+Z^10*3003+...

... polynomial continues for a few pages ...

=0}

Computing its roots is another story.

31.3.1 How Nonlinear Residues are made to disappear

Binding variables that appear in nonlinear residues will reduce the complexity of the nonlinear expressions and eventually results in linear expressions: clp(q) ?- {exp(X+Y+1,2) = 3*X*X+Y*Y}.

clpq:{Y*2-X^2*2+Y*X*2+X*2+1=0}

Equating X and Y collapses the expression completely and even determines the values of the two variables: clp(q) ?- {exp(X+Y+1,2) = 3*X*X+Y*Y}, X=Y.

X = -1/4,

Y = -1/4

31.3.2 Isolation Axioms

These axioms are used to rewrite equations such that the variable to be solved for is moved to the left hand side and the result of the evaluation of the right hand side can be assigned to the variable. This allows, for example, to use the exponentiation operator for the computation of roots and logarithms, see below.

A = B * C

Residuates unless B or C is ground or A and B or C are ground.

A = B / C

Residuates unless C is ground or A and B are ground.

X = min(Y,Z)

Residuates unless Y and Z are ground.

X = max(Y,Z)

Residuates unless Y and Z are ground.

Chapter 31: Constraint Logic Programming over Rationals or Reals 321

X = abs(Y)

Residuates unless Y is ground.

X = pow(Y,Z), X = exp(Y,Z)

Residuates unless any pair of two of the three variables is ground. Example: clp(r) ?- { 12=pow(2,X) }.

X = 3.5849625007211565

clp(r) ?- { 12=pow(X,3.585) }.

X = 1.9999854993443926

clp(r) ?- { X=pow(2,3.585) }.

X = 12.000311914286545

X = sin(Y)

Residuates unless X or Y is ground. Example: clp(r) ?- { 1/2 = sin(X) }.

X = 0.5235987755982989

X = cos(Y)

Residuates unless X or Y is ground.

X = tan(Y)

Residuates unless X or Y is ground.

31.4 Numerical Precision and Rationals

The fact that you can switch between clp(R) and clp(Q) should solve most of your numerical problems regarding precision. Within clp(Q), floating point constants will be coerced into rational numbers automatically. Transcendental functions will be approximated with rationals. The precision of the approximation is limited by the floating point precision.

These two provisions allow you to switch between clp(R) and clp(Q) without having to change your programs.

What is to be kept in mind however is the fact that it may take quite big rationals to accommodate the required precision. High levels of precision are for example required if your linear program is ill-conditioned, i.e., in a full rank system the determinant of the coefficient matrix is close to zero. Another situation that may call for elevated levels of precision is when a linear optimization problem requires exceedingly many pivot steps before the optimum is reached.

If your application approximates irrational numbers, you may be out of space particularly soon. The following program implements N steps of Newton’s approximation for the square root function at point 2.

322 SICStus Prolog

%

% from file: library(’clpqr/examples/root’)

% root(N, R) :root(N, 1, R).

root(0, S, R) :- !, S=R.

root(N, S, R) :-

N1 is N-1,

{ S1 = S/2 + 1/S }, root(N1, S1, R).

It is known that this approximation converges quadratically, which means that the number of correct digits in the decimal expansion roughly doubles with each iteration. Therefore the numerator and denominator of the rational approximation have to grow likewise: clp(q) ?- use_module(library(’clpqr/examples/root’)).

clp(q) ?- root(3,R),print_decimal(R,70).

1.4142156862 7450980392 1568627450 9803921568 6274509803 9215686274

5098039215

R = 577/408 clp(q) ?- root(4,R),print_decimal(R,70).

1.4142135623 7468991062 6295578890 1349101165 5962211574 4044584905

0192000543

R = 665857/470832 clp(q) ?- root(5,R),print_decimal(R,70).

1.4142135623 7309504880 1689623502 5302436149 8192577619 7428498289

4986231958

R = 886731088897/627013566048 clp(q) ?- root(6,R),print_decimal(R,70).

1.4142135623 7309504880 1688724209 6980785696 7187537723 4001561013

1331132652

R = 1572584048032918633353217/1111984844349868137938112 clp(q) ?- root(7,R),print_decimal(R,70).

1.4142135623 7309504880 1688724209 6980785696 7187537694 8073176679

7379907324

R = 4946041176255201878775086487573351061418968498177 /

3497379255757941172020851852070562919437964212608

Chapter 31: Constraint Logic Programming over Rationals or Reals 323

Iterating for 8 steps produces no further change in the first 70 decimal digits of sqrt(2).

After 15 steps the approximating rational number has a numerator and a denominator with

12543 digits each, and the next step runs out of memory.

Another irrational number that is easily computed is e. The following program implements an alternating series for 1/e, where the absolute value of last term is an upper bound on the error.

%

% from file: library(’clpqr/examples/root’)

% e(N, E) :-

{ Err =:= exp(10,-(N+2)), Half =:= 1/2 }, inv_e_series(Half, Half, 3, Err, Inv_E),

{ E =:= 1/Inv_E }.

inv_e_series(Term, S0, _, Err, Sum) :-

{ abs(Term) =< Err }, !,

S0 = Sum.

inv_e_series(Term, S0, N, Err, Sum) :-

N1 is N+1,

{ Term1 =:= -Term/N, S1 =:= Term1+S0 }, inv_e_series(Term1, S1, N1, Err, Sum).

The computation of the rational number E that approximates e up to at least 1000 digits in its decimal expansion requires the evaluation of 450 terms of the series, i.e. 450 calls of inv_e_series/5.

clp(q) ?- e(1000,E).

E = 7149056228932760213666809592072842334290744221392610955845565494

3708750229467761730471738895197792271346693089326102132000338192

0131874187833985420922688804220167840319199699494193852403223700

5853832741544191628747052136402176941963825543565900589161585723

4023097417605004829991929283045372355639145644588174733401360176

9953973706537274133283614740902771561159913069917833820285608440

3104966899999651928637634656418969027076699082888742481392304807

9484725489080844360397606199771786024695620205344042765860581379

3538290451208322129898069978107971226873160872046731879753034549

3130492167474809196348846916421782850086985668680640425192038155

4902863298351349469211627292865440876581064873866786120098602898

8799130098877372097360065934827751120659213470528793143805903554

7928682131082164366007016698761961066948371407368962539467994627

1374858249110795976398595034606994740186040425117101588480000000

0000000000000000000000000000000000000000000000000000000000000000

00000000000000000000000000000000000000

/

2629990810403002651095959155503002285441272170673105334466808931

6863103901346024240326549035084528682487048064823380723787110941

324 SICStus Prolog

6809235187356318780972302796570251102928552003708556939314795678

1978390674393498540663747334079841518303636625888963910391440709

0887345797303470959207883316838346973393937778363411195624313553

8835644822353659840936818391050630360633734935381528275392050975

7271468992840907541350345459011192466892177866882264242860412188

0652112744642450404625763019639086944558899249788084559753723892

1643188991444945360726899532023542969572584363761073528841147012

2634218045463494055807073778490814692996517359952229262198396182

1838930043528583109973872348193806830382584040536394640895148751

0766256738740729894909630785260101721285704616818889741995949666

6303289703199393801976334974240815397920213059799071915067856758

6716458821062645562512745336709063396510021681900076680696945309

3660590933279867736747926648678738515702777431353845466199680991

73361873421152165477774911660108200059

The decimal expansion itself looks like this: clp(q) ?- e(1000, E), print_decimal(E, 1000).

2.

7182818284 5904523536 0287471352 6624977572 4709369995 9574966967

6277240766 3035354759 4571382178 5251664274 2746639193 2003059921

8174135966 2904357290 0334295260 5956307381 3232862794 3490763233

8298807531 9525101901 1573834187 9307021540 8914993488 4167509244

7614606680 8226480016 8477411853 7423454424 3710753907 7744992069

5517027618 3860626133 1384583000 7520449338 2656029760 6737113200

7093287091 2744374704 7230696977 2093101416 9283681902 5515108657

4637721112 5238978442 5056953696 7707854499 6996794686 4454905987

9316368892 3009879312 7736178215 4249992295 7635148220 8269895193

6680331825 2886939849 6465105820 9392398294 8879332036 2509443117

3012381970 6841614039 7019837679 3206832823 7646480429 5311802328

7825098194 5581530175 6717361332 0698112509 9618188159 3041690351

5988885193 4580727386 6738589422 8792284998 9208680582 5749279610

4841984443 6346324496 8487560233 6248270419 7862320900 2160990235

3043699418 4914631409 3431738143 6405462531 5209618369 0888707016

7683964243 7814059271 4563549061 3031072085 1038375051 0115747704

1718986106 8739696552 1267154688 9570350354

31.5 Projection and Redundancy Elimination

Once a derivation succeeds, the Prolog system presents the bindings for the variables in the query. In a CLP system, the set of answer constraints is presented in analogy. A complication in the CLP context are variables and associated constraints that were not mentioned in the query. A motivating example is the familiar mortgage relation:

%

% from file: library(’clpqr/examples/mg’)

% mg(P,T,I,B,MP):-

{

Chapter 31: Constraint Logic Programming over Rationals or Reals 325

}.

mg(P,T,I,B,MP):-

{

T = 1,

B + MP = P * (1 + I)

T > 1,

P1 = P * (1 + I) - MP,

T1 = T - 1

}, mg(P1, T1, I, B, MP).

A sample query yields: clp(r) ?- use_module(library(’clpqr/examples/mg’)).

clp(r) ?- mg(P,12,0.01,B,Mp).

{B=1.1268250301319698*P-12.682503013196973*Mp}

Without projection of the answer constraints onto the query variables we would observe the following interaction: clp(r) ?- mg(P,12,0.01,B,Mp).

{B=12.682503013196973*_A-11.682503013196971*P},

{Mp= -(_A)+1.01*P},

{_B=2.01*_A-1.01*P},

{_C=3.0301*_A-2.0301*P},

{_D=4.060401000000001*_A-3.0604009999999997*P},

{_E=5.101005010000001*_A-4.10100501*P},

{_F=6.152015060100001*_A-5.152015060099999*P},

{_G=7.213535210701001*_A-6.213535210700999*P},

{_H=8.285670562808011*_A-7.285670562808009*P},

{_I=9.368527268436091*_A-8.36852726843609*P},

{_J=10.462212541120453*_A-9.46221254112045*P},

{_K=11.566834666531657*_A-10.566834666531655*P}

The variables A . . . K are not part of the query, they originate from the mortgage program proper. Although the latter answer is equivalent to the former in terms of linear algebra, most users would prefer the former.

31.5.1 Variable Ordering

In general, there are many ways to express the same linear relationship between variables.

clp(Q,R) does not care to distinguish between them, but the user might. The predicate ordering(+Spec) gives you some control over the variable ordering. Suppose that instead of B, you want Mp to be the defined variable: clp(r) ?- mg(P,12,0.01,B,Mp).

326 SICStus Prolog

{B=1.1268250301319698*P-12.682503013196973*Mp}

This is achieved with: clp(r) ?mg(P,12,0.01,B,Mp), ordering([Mp]).

{Mp= -0.0788487886783417*B+0.08884878867834171*P}

One could go one step further and require P to appear before (to the left of) B in a addition: clp(r) ?- mg(P,12,0.01,B,Mp), ordering([Mp,P]).

{Mp=0.08884878867834171*P-0.0788487886783417*B}

Spec in ordering(+Spec) is either a list of variables with the intended ordering, or of the form A<B.

The latter form means that A goes to the left of B. In fact, ordering([A,B,C,D]) is shorthand for: ordering(A < B), ordering(A < C), ordering(A < D), ordering(B < C), ordering(B < D), ordering(C < D)

The ordering specification only affects the final presentation of the constraints. For all other operations of clp(Q,R), the ordering is immaterial. Note that ordering/1 acts like a constraint: you can put it anywhere in the computation, and you can submit multiple specifications.

clp(r) ?- ordering(B < Mp), mg(P,12,0.01,B,Mp).

{B= -12.682503013196973*Mp+1.1268250301319698*P} yes clp(r) ?- ordering(B < Mp), mg(P,12,0.01,B,Mp), ordering(P < Mp).

{P=0.8874492252651537*B+11.255077473484631*Mp}

31.5.2 Turning Answers into Terms

In meta-programming applications one needs to get a grip on the results computed by the clp(Q,R) solver. You can use the predicates dump/3 and/or call_residue/2 for that purpose: clp(r) ?- {2*A+B+C=10,C-D=E,A<10}, dump([A,B,C,D,E],[a,b,c,d,e],Constraints).

Constraints = [e<10.0,a=10.0-c-d-2.0*e,b=c+d],

{C=10.0-2.0*A-B},

{E=10.0-2.0*A-B-D},

{A<10.0} clp(r) ?- call_residue({2*A+B+C=10,C-D=E,A<10}, Constraints).

Chapter 31: Constraint Logic Programming over Rationals or Reals 327

Constraints = [

[A]-{A<10.0},

[B]-{B=10.0-2.0*A-C},

]

[D]-{D=C-E}

31.5.3 Projecting Inequalities

As soon as linear inequations are involved, projection gets more demanding complexity wise.

The current clp(Q,R) version uses a Fourier-Motzkin algorithm for the projection of linear inequalities. The choice of a suitable algorithm is somewhat dependent on the number of variables to be eliminated, the total number of variables, and other factors. It is quite easy to produce problems of moderate size where the elimination step takes some time. For example, when the dimension of the projection is 1, you might be better off computing the supremum and the infimum of the remaining variable instead of eliminating n-1 variables via implicit projection.

In order to make answers as concise as possible, redundant constraints are removed by the system as well. In the following set of inequalities, half of them are redundant.

%

% from file: library(’clpqr/examples/eliminat’)

% example(2, [X0,X1,X2,X3,X4]) :-

{

+87*X0 +52*X1 +27*X2 -54*X3 +56*X4 =< -93,

+33*X0 -10*X1 +61*X2 -28*X3 -29*X4 =< 63,

-68*X0 +8*X1 +35*X2 +68*X3 +35*X4 =< -85,

+90*X0 +60*X1 -76*X2 -53*X3 +24*X4 =< -68,

-95*X0 -10*X1 +64*X2 +76*X3 -24*X4 =< 33,

+43*X0 -22*X1 +67*X2 -68*X3 -92*X4 =< -97,

+39*X0 +7*X1 +62*X2 +54*X3 -26*X4 =< -27,

+48*X0 -13*X1 +7*X2 -61*X3 -59*X4 =< -2,

+49*X0 -23*X1 -31*X2 -76*X3 +27*X4 =<

-50*X0 +58*X1 -1*X2 +57*X3 +20*X4 =<

3,

6,

-13*X0 -63*X1 +81*X2 -3*X3 +70*X4 =< 64,

+20*X0 +67*X1 -23*X2 -41*X3 -66*X4 =< 52,

-81*X0 -44*X1 +19*X2 -22*X3 -73*X4 =< -17,

-43*X0 -9*X1 +14*X2 +27*X3 +40*X4 =< 39,

+16*X0 +83*X1 +89*X2 +25*X3 +55*X4 =< 36,

+2*X0 +40*X1 +65*X2 +59*X3 -32*X4 =< 13,

-65*X0 -11*X1 +10*X2 -13*X3 +91*X4 =< 49,

+93*X0 -73*X1 +91*X2 -1*X3 +23*X4 =< -87

}.

Consequently, the answer consists of the system of nine non-redundant inequalities only: clp(q) ?- use_module(library(’clpqr/examples/elimination’)).

328 SICStus Prolog clp(q) ?- example(2, [X0,X1,X2,X3,X4]).

{X0-2/17*X1-35/68*X2-X3-35/68*X4>=5/4},

{X0-73/93*X1+91/93*X2-1/93*X3+23/93*X4=<-29/31},

{X0-29/25*X1+1/50*X2-57/50*X3-2/5*X4>=-3/25},

{X0+7/39*X1+62/39*X2+18/13*X3-2/3*X4=<-9/13},

{X0+2/19*X1-64/95*X2-4/5*X3+24/95*X4>=-33/95},

{X0+2/3*X1-38/45*X2-53/90*X3+4/15*X4=<-34/45},

{X0-23/49*X1-31/49*X2-76/49*X3+27/49*X4=<3/49},

{X0+44/81*X1-19/81*X2+22/81*X3+73/81*X4>=17/81},

{X0+9/43*X1-14/43*X2-27/43*X3-40/43*X4>=-39/43}

The projection (the shadow) of this polyhedral set into the X0,X1 space can be computed via the implicit elimination of non-query variables: clp(q) ?- example(2, [X0,X1|_]).

{X0+2619277/17854273*X1>=-851123/17854273},

{X0+6429953/16575801*X1=<-12749681/16575801},

{X0+19130/1213083*X1>=795400/404361},

{X0-1251619/3956679*X1>=21101146/3956679},

{X0+601502/4257189*X1>=220850/473021}

Projection is quite a powerful concept that leads to surprisingly terse executable specifications of nontrivial problems like the computation of the convex hull from a set of points in an n-dimensional space: Given the program

%

% from file: library(’clpqr/examples/elimination’)

% conv_hull(Points, Xs) :lin_comb(Points, Lambdas, Zero, Xs), zero(Zero), polytope(Lambdas).

polytope(Xs) :positive_sum(Xs, 1).

positive_sum([], Z) :- {Z=0}.

positive_sum([X|Xs], SumX) :-

{ X >= 0, SumX = X+Sum }, positive_sum(Xs, Sum).

zero([]).

zero([Z|Zs]) :- {Z=0}, zero(Zs).

lin_comb([], [], S1, S1).

lin_comb([Ps|Rest], [K|Ks], S1, S3) :lin_comb_r(Ps, K, S1, S2), lin_comb(Rest, Ks, S2, S3).

Chapter 31: Constraint Logic Programming over Rationals or Reals 329 lin_comb_r([], _, [], []).

lin_comb_r([P|Ps], K, [S|Ss], [Kps|Ss1]) :-

{ Kps = K*P+S }, lin_comb_r(Ps, K, Ss, Ss1).

we can post the following query: clp(q) ?- conv_hull([ [1,1], [2,0], [3,0], [1,2], [2,2] ], [X,Y]).

{Y=<2},

{X+1/2*Y=<3},

{X>=1},

{Y>=0},

{X+Y>=2}

This answer is easily verified graphically:

|

2 -

|

|

1 -

* *

*

|

|

0 -----|----*----*----

1 2 3

The convex hull program directly corresponds to the mathematical definition of the convex hull. What does the trick in operational terms is the implicit elimination of the Lambdas from the program formulation. Please note that this program does not limit the number of points or the dimension of the space they are from. Please note further that quantifier elimination is a computationally expensive operation and therefore this program is only useful as a benchmark for the projector and not so for the intended purpose.

31.6 Why Disequations

A beautiful example of disequations at work is due to [Colmerauer 90]. It addresses the task of tiling a rectangle with squares of all-different, a priori unknown sizes. Here is a translation of the original Prolog-III program to clp(Q,R):

%

% from file: library(’clpqr/examples/squares’)

% filled_rectangle(A, C) :-

{ A >= 1 }, distinct_squares(C), filled_zone([-1,A,1], _, C, []).

330 SICStus Prolog distinct_squares([]).

distinct_squares([B|C]) :-

{ B > 0 }, outof(C, B), distinct_squares(C).

outof([], _).

outof([B1|C], B) :-

{ B =\= B1 }, outof(C, B).

% *** note disequation *** filled_zone([V|L], [W|L], C0, C0) :-

{ V=W,V >= 0 }.

filled_zone([V|L], L3, [B|C], C2) :-

{ V < 0 }, placed_square(B, L, L1), filled_zone(L1, L2, C, C1),

{ Vb=V+B }, filled_zone([Vb,B|L2], L3, C1, C2).

placed_square(B, [H,H0,H1|L], L1) :-

{ B > H, H0=0, H2=H+H1 }, placed_square(B, [H2|L], L1).

placed_square(B, [B,V|L], [X|L]) :-

{ X=V-B }.

placed_square(B, [H|L], [X,Y|L]) :-

{ B < H, X= -B, Y=H-B }.

There are no tilings with less than nine squares except the trivial one where the rectangle equals the only square. There are eight solutions for nine squares. Six further solutions are rotations of the first two.

clp(q) ?- use_module(library(’clpqr/examples/squares’)).

clp(q) ?- filled_rectangle(A, Squares).

A = 1,

Squares = [1] ? ;

A = 33/32,

Squares = [15/32,9/16,1/4,7/32,1/8,7/16,1/32,5/16,9/32] ? ;

A = 69/61,

Squares = [33/61,36/61,28/61,5/61,2/61,9/61,25/61,7/61,16/61]

Depending on your hardware, the above query may take a few minutes. Supplying the knowledge about the minimal number of squares beforehand cuts the computation time by a factor of roughly four: clp(q) ?- length(Squares, 9), filled_rectangle(A, Squares).

Chapter 31: Constraint Logic Programming over Rationals or Reals 331

A = 33/32,

Squares = [15/32,9/16,1/4,7/32,1/8,7/16,1/32,5/16,9/32] ? ;

A = 69/61,

Squares = [33/61,36/61,28/61,5/61,2/61,9/61,25/61,7/61,16/61]

31.7 Syntactic Sugar

There is a package that transforms programs and queries from a eval-quote variant of clp(Q,R) into corresponding programs and queries in a quote-eval variant. Before you use it, you need to know that in an eval-quote language, all symbols are interpreted unless explicitly quoted. This means that interpreted terms cannot be manipulated syntactically directly. Meta-programming in a CLP context by definition manipulates interpreted terms, therefore you need quote/1 (just as in LISP) and some means to put syntactical terms back to their interpreted life: {}/1.

In a quote-eval language, meta-programming is (pragmatically) simpler because everything is implicitly quoted until explicitly evaluated. On the other hand, now object programming suffers from the dual inconvenience.

We chose to make our version of clp(Q,R) of the quote-eval type because this matches the intended use of the already existing boolean solver of SICStus. In order to keep the users of the eval-quote variant happy, we provide a source transformation package. It is activated via:

| ?- use_module(library(’clpqr/expand’)).

Loading the package puts you in a mode where the arithmetic functors like +/2, */2 and all numbers (functors of arity 0) are interpreted semantically.

clp(r) ?- 2+2=X.

X = 4.0

The package works by purifying programs and queries in the sense that all references to interpreted terms are made explicit. The above query is expanded prior to evaluation into:

{2.0+2.0=X}

The same mechanism applies when interpreted terms are nested deeper: some_predicate(10, f(A+B/2), 2*cos(A))

Expands into:

{Xc=2.0*cos(A)},

{Xb=A+B/2},

{Xa=10.0}, some_predicate(Xa, f(Xb), Xc)

332 SICStus Prolog

This process also applies when files are consulted or compiled. In fact, this is the only situation where expansion can be applied with relative safety. To see this, consider what happens when the top level evaluates the expansion, namely some calls to the clp(Q,R) solver, followed by the call of the purified query. As we learned in

Section 31.2.3 [Feedback], page 318

, the solver may bind variables, which produces a goal with interpreted functors in it (numbers), which leads to another stage of expansion, and so on.

We recommend that you only turn on expansion temporarily while consulting or compiling files needing expansion with expand/0 and noexpand/0.

31.7.1 Monash Examples

This collection of examples has been distributed with the Monash University Version of clp(R) [Heintze et al. 87], and its inclusion into this distribution was kindly permitted by

Roland Yap.

In order to execute the examples, a small compatibility package has to be loaded first: clp(r) ?- use_module(library(’clpqr/monash’)).

Then, assuming you are using clp(R): clp(r) ?- expand, [library(’clpqr/examples/monash/rkf45’)], noexpand.

clp(r) ?- go.

Point 0.00000 :

Point

Point

Point

0.50000 :

1.00000 :

0.75000

0.61969

0.29417

1.50000 : -0.10556

Point

Point

Point

2.00000 : -0.49076

2.50000 : -0.81440

3.00000 : -1.05440

0.00000

0.47793

0.81233

0.95809

0.93977

0.79929

0.57522

Iteration finished

------------------

439 derivative evaluations

31.7.1.1 Compatibility Notes

The Monash examples have been written for clp(R). Nevertheless, all but rkf45 complete nicely in clp(Q). With rkf45, clp(Q) runs out of memory. This is an instance of the problem discussed in

Section 31.4 [Numerical Precision], page 321

.

The Monash University clp(R) interpreter features a dump/n predicate. It is used to print the target variables according to the given ordering. Within this version of clp(Q,R), the corresponding functionality is provided via ordering/1. The difference is that ordering/1 does only specify the ordering of the variables and no printing is performed. We think

Chapter 31: Constraint Logic Programming over Rationals or Reals 333

Prolog has enough predicates to perform output already. You can still run the examples referring to dump/n from the Prolog top level: clp(r) ?- expand, [library(’clpqr/examples/monash/mortgage’)], noexpand.

% go2

% clp(r) ?- mg(P,120,0.01,0,MP), dump([P,MP]).

{P=69.7005220313972*MP}

% go3

% clp(r) ?- mg(P,120,0.01,B,MP), dump([P,B,MP]).

{P=0.30299477968602706*B+69.7005220313972*MP}

% go4

% clp(r) ?- mg(999, 3, Int, 0, 400), dump.

clpr:{_B-_B*Int+_A+400.0=0.0}, clpr:{_A-_A*Int+400.0=0.0},

{_B=599.0+999.0*Int}

31.8 A Mixed Integer Linear Optimization Example

The predicates bb_inf/3, bb_inf/5 implement a simple Branch and Bound search algorithm for Mixed Integer Linear (MIP) Optimization examples. Serious MIP is not trivial.

The implementation library(’clpqr/bb.pl’) is to be understood as a starting point for more ambitious users who need control over branching, or who want to add cutting planes, for example.

Anyway, here is a small problem from miplib, a collection of MIP models, housed at Rice

University:

NAME:

ROWS:

COLUMNS:

INTEGER:

NONZERO: flugpl

18

18

11

46

BEST SOLN:

LP SOLN:

SOURCE:

1201500 (opt)

1167185.73

Harvey M. Wagner

John W. Gregory (Cray Research)

E. Andrew Boyd (Rice University)

APPLICATION: airline model

COMMENTS: no integer variables are binary

334 SICStus Prolog

%

% from file: library(’clpqr/examples/mip’)

% example(flugpl, Obj, Vs, Ints, []) :-

Vs = [ Anm1,Anm2,Anm3,Anm4,Anm5,Anm6,

Stm1,Stm2,Stm3,Stm4,Stm5,Stm6,

UE1,UE2,UE3,UE4,UE5,UE6],

Ints = [Stm6, Stm5, Stm4, Stm3, Stm2,

Anm6, Anm5, Anm4, Anm3, Anm2, Anm1],

Obj = 2700*Stm1 + 1500*Anm1 + 30*UE1

+ 2700*Stm2 + 1500*Anm2 + 30*UE2

+ 2700*Stm3 + 1500*Anm3 + 30*UE3

+ 2700*Stm4 + 1500*Anm4 + 30*UE4

+ 2700*Stm5 + 1500*Anm5 + 30*UE5

+ 2700*Stm6 + 1500*Anm6 + 30*UE6, allpos(Vs),

{ Stm1 = 60, 0.9*Stm1 +1*Anm1 -1*Stm2 = 0,

0.9*Stm2 +1*Anm2 -1*Stm3 = 0, 0.9*Stm3 +1*Anm3 -1*Stm4 = 0,

0.9*Stm4 +1*Anm4 -1*Stm5 = 0, 0.9*Stm5 +1*Anm5 -1*Stm6 = 0,

150*Stm1 -100*Anm1 +1*UE1 >= 8000,

150*Stm2 -100*Anm2 +1*UE2 >= 9000,

150*Stm3 -100*Anm3 +1*UE3 >= 8000,

150*Stm4 -100*Anm4 +1*UE4 >= 10000,

150*Stm5 -100*Anm5 +1*UE5 >= 9000,

150*Stm6 -100*Anm6 +1*UE6 >= 12000,

-20*Stm1 +1*UE1 =< 0, -20*Stm2 +1*UE2 =< 0, -20*Stm3 +1*UE3 =< 0,

-20*Stm4 +1*UE4 =< 0, -20*Stm5 +1*UE5 =< 0, -20*Stm6 +1*UE6 =< 0,

Anm1 =< 18, 57 =< Stm2, Stm2 =< 75, Anm2 =< 18,

57 =< Stm3, Stm3 =< 75, Anm3 =< 18, 57 =< Stm4,

Stm4 =< 75, Anm4 =< 18, 57 =< Stm5, Stm5 =< 75,

}.

Anm5 =< 18, 57 =< Stm6, Stm6 =< 75, Anm6 =< 18 allpos([]).

allpos([X|Xs]) :- {X >= 0}, allpos(Xs).

We can first check whether the relaxed problem has indeed the quoted infimum: clp(r) ?- example(flugpl, Obj, _, _, _), inf(Obj, Inf).

Inf = 1167185.7255923203

Computing the infimum under the additional constraints that Stm6, Stm5, Stm4, Stm3, Stm2,

Anm6, Anm5, Anm4, Anm3, Anm2, Anm1 assume integer values at the infimum is computationally harder, but the query does not change much: clp(r) ?- example(flugpl, Obj, _, Ints, _), bb_inf(Ints, Obj, Inf, Vertex, 0.001).

Chapter 31: Constraint Logic Programming over Rationals or Reals 335

Inf = 1201500.0000000005,

Vertex = [75.0,70.0,70.0,60.0,60.0,0.0,12.0,7.0,16.0,6.0,6.0]

31.9 Implementation Architecture

The system consists roughly of the following components:

• A polynomial normal form expression simplification mechanism.

• A solver for linear equations [Holzbaur 92a].

• A simplex algorithm to decide linear inequalities [Holzbaur 94].

31.9.1 Fragments and Bits

31.9.1.1 Rationals

The internal data structure for rational numbers is rat(Num,Den). Den is always positive, i.e. the sign of the rational number is the sign of Num. Further, Num and Den are relative prime. Note that integer N looks like rat(N,1) in this representation. You can control printing of terms with portray/1.

31.9.1.2 Partial Evaluation, Compilation

Once one has a working solver, it is obvious and attractive to run the constraints in a clause definition at read time or compile time and proceed with the answer constraints in place of the original constraints. This gets you constant folding and in fact the full algebraic power of the solver applied to the avoidance of computations at runtime. The mechanism to realize this idea is to use dump/3, call_residue/2 for the expansion of {}/1, via hook predicate user:goal_expansion/3).

31.9.1.3 Asserting with Constraints

If you use the dynamic data base, the clauses you assert might have constraints on the variables occurring in the clause. This works as expected: clp(r) ?- {A < 10}, assert(p(A)).

{A<10.0} yes clp(r) ?- p(X).

{X<10.0}

31.9.2 Bugs

336 SICStus Prolog

• The fuzzy comparison of floats is the source for all sorts of weirdness. If a result in R surprises you, try to run the program in Q before you send me a bug report.

• The projector for floundered nonlinear relations keeps too many variables. Its output is rather unreadable.

• Disequations are not projected properly.

• This list is probably incomplete.

Please send bug reports to <[email protected]>.

Chapter 32: Constraint Logic Programming over Finite Domains

32 Constraint Logic Programming over Finite

Domains

337

32.1 Introduction

The clp(FD) solver described in this chapter is an instance of the general Constraint Logic

Programming scheme introduced in [Jaffar & Michaylov 87]. This constraint domain is particularly useful for modeling discrete optimization and verification problems such as scheduling, planning, packing, timetabling etc. The treatise [Van Hentenryck 89] is an excellent exposition of the theoretical and practical framework behind constraint solving in finite domains, and summarizes the work up to 1989.

This solver has the following highlights:

• Two classes of constraints are handled internally: primitive constraints and global constraints.

• The constraints described in this chapter are automatically translated to conjunctions of primitive and global library constraints.

• The truth value of a primitive constraint can be reflected into a 0/1-variable (reification).

• New primitive constraints can be added by writing so-called indexicals.

• New global constraints can be written in Prolog, by means of a programming interface.

The rest of this chapter is organized as follows: How to load the solver and how to write simple programs is explained in

Section 32.2 [CLPFD Interface], page 338

. A description of all constraints that the solver provides is contained in

Section 32.3 [Available Constraints], page 341

. The predicates for searching for solution are documented in

Section 32.4 [Enumeration Predicates], page 347

. The predicates for getting execution statistics are documented in

Section 32.5 [Statistics Predicates], page 350 . A few example programs are given in

Section 32.10 [Example Programs], page 365

. Finally,

Section 32.11 [Syntax Summary], page 368

contains syntax rules for all expressions.

The following sections discuss advanced features and are probably only relevant to experienced users: How to control the amount of information presented in answers to queries is explained in

Section 32.6 [Answer Constraints], page 351 . The solver’s execution mechanism

and primitives are described in

Section 32.7 [The Constraint System], page 351

. How to add new global constraints via a programming interface is described in

Section 32.8 [Defining

Global Constraints], page 352 . How to define new primitive constraints with indexicals is

described in

Section 32.9 [Defining Primitive Constraints], page 358 .

Referencing this Software

When referring to this implementation of clp(FD) in publications, please use the following reference:

338 SICStus Prolog

Carlsson M., Ottosson G., Carlson B. “An Open-Ended Finite Domain Constraint Solver” Proc. Programming Languages: Implementations, Logics, and

Programs, 1997.

Acknowledgments

The first version of this solver was written as part of Key Hyckenberg’s MSc thesis in 1995, with contributions from Greger Ottosson at the Computing Science Department, Uppsala

University. The code was later rewritten by Mats Carlsson. Peter Szeredi contributed material for this manual chapter.

The development of this software was supported by the Swedish National Board for Technical and Industrial Development (NUTEK) under the auspices of Advanced Software Technology (ASTEC) Center of Competence at Uppsala University.

We include a collection of examples, some of which have been distributed with the INRIA implementation of clp(FD) [Diaz & Codognet 93].

32.2 Solver Interface

The solver is available as a library module and can be loaded with a query

:- use_module(library(clpfd)).

The solver contains predicates for checking the consistency and entailment of finite domain constraints, as well as solving for solution values for your problem variables.

In the context of this constraint solver, a finite domain is a subset of the integers, and a finite domain constraint denotes a relation over one or more finite domains. All domain variables, i.e. variables that occur as arguments to finite domain constraints get associated with a finite domain, either explicitly declared by the program, or implicitly imposed by the constraint solver. Temporarily, the domain of a variable may actually be infinite, if it does not have a finite lower or upper bound.

The domain of all variables gets narrower and narrower as more constraints are added. If a domain becomes empty, the accumulated constraints are unsatisfiable, and the current computation branch fails. At the end of a successful computation, all domains have usually become singletons, i.e. the domain variables have become assigned.

The domains do not become singletons automatically. Usually, it takes some amount of search to find an assignment that satisfies all constraints. It is the programmer’s responsibility to do so. If some domain variables are left unassigned in a computation, the garbage collector will preserve all constraint data that is attached to them. Furthermore, the behavior of the predicates assert/1, findall/3, copy_term/2 and friends is undefined on non-ground terms containing domain variables.

The heart of the constraint solver is a scheduler for indexicals [Van Hentenryck et al. 92] and global constraints. Both entities act as coroutines performing incremental constraint

Chapter 32: Constraint Logic Programming over Finite Domains 339 solving or entailment checking. They wake up by changes in the domains of its arguments.

All constraints provided by this package are implemented as indexicals or global constraints.

New constraints can be defined by the user.

Indexicals are reactive functional rules that take part in the solver’s basic constraint solving algorithm, whereas each global constraint is associated with its particular constraint solving algorithm. The solver maintains two scheduling queues, giving priority to the queue of indexicals.

The feasibility of integrating the indexical approach with a Prolog based on the WAM was clearly demonstrated by Diaz’s clp(FD) implementation [Diaz & Codognet 93], one of the fastest finite domains solvers around.

32.2.1 Posting Constraints

A constraint is called as any other Prolog predicate. When called, the constraint is posted to the store. For example:

| ?- X in 1..5, Y in 2..8, X+Y #= T.

X in 1..5,

Y in 2..8,

T in 3..13 ?

yes

| ?- X in 1..5, T in 3..13, X+Y #= T.

X in 1..5,

T in 3..13,

Y in -2..12 ?

yes

Note that the answer constraint shows the domains of nonground query variables, but not any constraints that may be attached to them.

32.2.2 A Constraint Satisfaction Problem

Constraint satisfaction problems (CSPs) are a major class of problems for which this solver is ideally suited. In a CSP, the goal is to pick values from pre-defined domains for certain variables so that the given constraints on the variables are all satisfied.

As a simple CSP example, let us consider the Send More Money puzzle. In this problem, the variables are the letters S, E, N, D, M, O, R, and Y. Each letter represents a digit between 0 and 9. The problem is to assign a value to each digit, such that SEND + MORE equals MONEY.

340 SICStus Prolog

A program which solves the puzzle is given below. The program contains the typical three steps of a clp(FD) program:

1. declare the domains of the variables

2. post the problem constraints

3. look for a feasible solution via backtrack search, or look for an optimal solution via branch-and-bound search

Sometimes, an extra step precedes the search for a solution: the posting of surrogate constraints to break symmetries or to otherwise help prune the search space. No surrogate constraints are used in this example.

The domains of this puzzle are stated via the domain/3 goal and by requiring that S and M be greater than zero. The two problem constraint of this puzzle are the equation (sum/8) and the constraint that all letters take distinct values (all_different/1). Finally, the backtrack search is performed by labeling/2. Different search strategies can be encoded in the Type parameter. In the example query, the default search strategy is used (select the leftmost variable, try values in ascending order).

:- use_module(library(clpfd)).

mm([S,E,N,D,M,O,R,Y], Type) :domain([S,E,N,D,M,O,R,Y], 0, 9),

S#>0, M#>0, all_different([S,E,N,D,M,O,R,Y]), sum(S,E,N,D,M,O,R,Y), labeling(Type, [S,E,N,D,M,O,R,Y]).

% step 1

% step 2

% step 3 sum(S, E, N, D, M, O, R, Y) :-

+

1000*S + 100*E + 10*N + D

1000*M + 100*O + 10*R + E

#= 10000*M + 1000*O + 100*N + 10*E + Y.

| ?- mm([S,E,N,D,M,O,R,Y], []).

D = 7,

E = 5,

M = 1,

N = 6,

O = 0,

R = 8,

S = 9,

Y = 2 ?

32.2.3 Reified Constraints

Instead of merely posting constraints it is often useful to reflect its truth value into a 0/1variable B, so that:

Chapter 32: Constraint Logic Programming over Finite Domains 341

• the constraint is posted if B is set to 1

• the negation of the constraint is posted if B is set to 0

• B is set to 1 if the constraint becomes entailed

• B is set to 0 if the constraint becomes disentailed

This mechanism is known as reification. Several frequently used operations can be defined in terms of reified constraints, such as blocking implication [Saraswat 90] and the cardinality operator [Van Hentenryck & Deville 91], to name a few. A reified constraint is written:

| ?- Constraint #<=> B.

where Constraint is reifiable. As an example of a constraint that uses reification, consider exactly(X,L,N) which is true if X occurs exactly N times in the list L. It can be defined thus: exactly(_, [], 0).

exactly(X, [Y|L], N) :-

X #= Y #<=> B,

N #= M+B, exactly(X, L, M).

32.3 Available Constraints

This section describes the classes of constraints that can be used with this solver.

32.3.1 Arithmetic Constraints

?Expr RelOp ?Expr

defines an arithmetic constraint. The syntax for Expr and RelOp is defined by a grammar (see

[Syntax of Arithmetic Expressions], page 371 ). Note that

the expressions are not restricted to being linear. Constraints over non-linear expressions, however, will usually yield less constraint propagation than constraints over linear expressions. Any variables occurring in the expression must be unbound or bound to integers at run time.

Arithmetic constraints can be reified as e.g.

| ?- X in 1..2, Y in 3..5, X#=<Y #<=> B.

B = 1,

X in 1..2,

Y in 3..5 ?

Linear arithmetic constraints maintain (at least) interval-consistency and their reified versions detect (at least) interval-entailment and -disentailment; see

Section 32.7 [The Constraint System], page 351

.

The following constraints are among the library constraints that general arithmetic constraints compile to. They express a relation between a sum or a scalar product and a

342 SICStus Prolog value, using a dedicated algorithm that avoids creating any temporary variables holding intermediate values. If you are computing a sum or a scalar product, it can be much more efficient to compute lists of coefficients and variables and post a single sum or scalar product constraint than to post a sequence of elementary constraints.

sum(+Xs, +RelOp, ?Value) where Xs is a list of integers or domain variables, RelOp is a relational symbol as above, and Value is an integer or a domain variable. True if Xs RelOp Value.

Cannot be reified.

scalar_product(+Coeffs, +Xs, +RelOp, ?Value) where Coeffs is a list of length n of integers, Xs is a list of length n of integers or domain variables, RelOp is a relational symbol as above, and Value is an integer or a domain variable. True if Coeffs*Xs RelOp Value. Cannot be reified.

32.3.2 Membership Constraints domain(+Variables, +Min, +Max) where Variables is a list of domain variables or integers, Min is an integer or the atom inf (minus infinity), and Max is an integer or the atom sup (plus infinity). True if the variables all are elements of the range Min..Max. Cannot be reified.

?X in +Range defines a membership constraint. X is an integer or a domain variable and

Range is a ConstantRange (see

[Syntax of Indexicals], page 369 ). True if X is

an element of the range.

?X in_set +FDSet defines a membership constraint. X is an integer or a domain variable and

FDSet is an FD set term (see

Section 32.8.3 [FD Set Operations], page 355

).

True if X is an element of the FD set.

in/2 and in_set/2 constraints can be reified.

They maintain domain-consistency and their reified versions detect domain-entailment and -disentailment; see

Section 32.7 [The

Constraint System], page 351

.

32.3.3 Propositional Constraints

Propositional combinators can be used to combine reifiable constraints into propositional formulae over such constraints. Such formulae are goal expanded by the system into sequences of reified constraints and arithmetic constraints. For example,

X #= 4 #\/ Y #= 6 expresses the disjunction of two equality constraints.

The leaves of propositional formulae can be reifiable constraints, the constants 0 and 1, or 0/1-variables. New primitive, reifiable constraints can be defined with indexicals as

Chapter 32: Constraint Logic Programming over Finite Domains 343 described in

Section 32.9 [Defining Primitive Constraints], page 358 . The following propo-

sitional combinators are available:

#\ :Q

True if the constraint Q is false.

:P #/\ :Q

True if the constraints P and Q are both true.

:P #\ :Q

True if exactly one of the constraints P and Q is true.

:P #\/ :Q

True if at least one of the constraints P and Q is true.

:P #=> :Q

:Q #<= :P

True if the constraint Q is true or the constraint P is false.

:P #<=> :Q

True if the constraints P and Q are both true or both false.

Note that the reification scheme introduced in

Section 32.2.3 [Reified Constraints], page 340

is a special case of a propositional constraint.

32.3.4 Combinatorial Constraints

The constraints listed here are sometimes called symbolic constraints. They are currently not reifiable.

count(+Val,+List,+RelOp,?Count) where Val is an integer, List is a list of integers or domain variables, Count an integer or a domain variable, and RelOp is a relational symbol as in

Section 32.3.1 [Arithmetic Constraints], page 341

. True if N is the number of elements of List that are equal to Val and N RelOp Count. Thus, count/4 is a generalization of exactly/3 (not an exported constraint) that was used in an example earlier.

count/4 maintains domain-consistency; see

Section 32.7 [The Constraint System], page 351 .

element(?X,+List,?Y ) where X and Y are integers or domain variables and List is a list of integers or domain variables. True if the X :th element of List is Y. Operationally, the domains of X and Y are constrained so that for every element in the domain of X, there is a compatible element in the domain of Y, and vice versa.

This constraint uses an optimized algorithm for the special case where List is ground.

element/3 maintains domain-consistency in X and interval-consistency in List and Y ; see

Section 32.7 [The Constraint System], page 351

.

344 SICStus Prolog relation(?X,+MapList,?Y ) where X and Y are integers or domain variables and MapList is a list of integer-

ConstantRange pairs, where the integer keys occur uniquely (see

[Syntax of

Indexicals], page 369 ). True if MapList contains a pair X -R and Y is in the

range denoted by R.

Operationally, the domains of X and Y are constrained so that for every element in the domain of X, there is a compatible element in the domain of Y, and vice versa.

If MapList is not ground, the constraint must be wrapped in call/1 to postpone goal expansion until runtime.

An arbitrary binary constraint can be defined with relation/3. relation/3 maintains domain-consistency; see

Section 32.7 [The Constraint System], page 351

.

all_different(+Variables) all_different(+Variables, +Options) all_distinct(+Variables) all_distinct(+Variables, +Options) where Variables is a list of domain variables or integers. Each variable is constrained to take a value that is unique among the variables. Declaratively, this is equivalent to an inequality constraint for each pair of variables.

Options is a list of zero or more of the following, where Boolean must be true or false (false is the default): on(On)

How eagerly to wake up the constraint. One of value (the default for all_ different/[1,2]), to wake up when a variable becomes ground; range, to wake up when a bound of a variable is changed; or domain (the default for all_distinct/[1,2]), to wake up when the domain of a variable is changed.

complete(Boolean)

Which algorithm to use.

If true (the default for all_distinct/[1,2]), a complete algorithm [Regin 94] is used, i.e. it maintains domain-consistency.

Otherwise (the default for all_different/[1,2]), an incomplete one is used, achieving exactly the same pruning as a set of pairwise inequality constraints.

The following is a constraint over two lists of length n of variables. Each variable is constrained to take a value in 1,...,n that is unique for its list. Furthermore, the lists are dual in a sense described below.

assignment(+Xs, +Ys) assignment(+Xs, +Ys, +Options) where Xs and Ys are lists of domain variables or integers, both of length n.

Options is a list of the same form as in all_different/2 with the default value [on(domain),complete(true)]. True if all Xi, Yi in 1,...,n and Xi=j iff

Yj=i.

Chapter 32: Constraint Logic Programming over Finite Domains 345

The following constraint can be thought of as constraining n nodes in a graph to form a

Hamiltonian circuit. The nodes are numbered from 1 to n. The circuit starts in node 1, visits each node, and returns to the origin.

circuit(+Succ) circuit(+Succ, +Pred) where Succ is a list of length n of domain variables or integers. The i:th element of Succ (Pred) is the successor (predecessor) of i in the graph. True if the values form a Hamiltonian circuit.

The following four constraints can be thought of as constraining n tasks, each with a start time Sj and a duration Dj, so that no tasks ever overlap. The tasks can be seen as competing for some exclusive resource.

serialized(+Starts,+Durations) serialized(+Starts,+Durations,+Options) where Starts = [S1,...,Sn] and Durations = [D1,...,Dn] are lists of domain variables with finite bounds or integers. Durations must be non-negative. True if

Starts and Durations denote a set of non-overlapping tasks, i.e.:

Si+Di =< Sj OR Sj+Dj =< Si, for all 1 =< i<j =< n

The serialized/[2,3] constraint is merely a special case of cumulative/[4,5]

(see below).

Options is a list of zero or more of the following, where Boolean must be true or false (false is the default): precedences(Ps)

Ps encodes a set of precedence constraint to apply to the tasks. Ps should be a list of terms of the form d(i,j,d), where i and j should be task numbers, and d should be a positive integer or sup. Each term adds the constraint:

Si+d =< Sj OR Sj =< Si, if d is an integer

Sj =< Si, if d is sup resource(R)

R is unified with a term which can be passed to order_resource/2

(see

Section 32.4 [Enumeration Predicates], page 347 ) in order to

find a consistent ordering of the tasks.

path_consistency(Boolean) if true, a redundant path consistency algorithm will be used inside the constraint in an attempt to improve the pruning.

static_sets(Boolean) if true, a redundant algorithm will be used which reasons about the set of tasks that must precede (be preceded by) a given task, in an attempt to tighten the lower (upper) bound of a given start variable.

346 SICStus Prolog edge_finder(Boolean) if true, a redundant algorithm will be used which attempts to identify tasks that necessarily precede or are preceded by some set of tasks.

decomposition(Boolean) if true, an attempt is made to decompose the constraint each time it is resumed.

Whether it’s worthwhile to switch on any of the latter four options is highly problem dependent.

serialized/3 can model a set of tasks to be serialized with sequence-dependent setup times. For example, the following constraint models three tasks, all with duration 5, where task 1 must precede task 2 and task 3 must either complete before task 2 or start at least 10 time units after task 2 started:

?- domain([S1,S2,S3], 0, 20), serialized([S1,S2,S3], [5,5,5], [precedences([d(2,1,sup),d(2,3,10)])]).

S1 in 0..15,

S2 in 5..20,

S3 in 0..20 ?

The bounds of S1 and S2 changed because of the precedence constraint. Setting

S2 to 5 will propagate S1=0 and S3 in 15..20.

The following constraint can be thought of as constraining n tasks, each with a start time

Sj, a duration Dj, and a resource amount Rj, so that the total resource consumption does not exceed Limit at any time: cumulative(+Starts,+Durations,+Resources,?Limit) cumulative(+Starts,+Durations,+Resources,?Limit,+Options) where Starts = [S1,...,Sn], Durations = [D1,...,Dn], Resource = [R1,...,Rn] are lists of domain variables with finite bounds or integers, and Limit is a domain variable with finite bounds or an integer. Durations, Resources and Limit must be non-negative. Let: a = min(S1,...,Sn), b = max(S1+D1,...,Sn+Dn)

Rij = Rj, if Sj =< i < Sj+Dj

Rij = 0 otherwise

The constraint holds if:

Ri1+...+Rin =< Limit, for all a =< i < b

If given, Options should be of the same form as in serialized/3, except the resource(R) option is not useful in cumulative/5.

The cumulative/4 constraint is due to Aggoun and Beldiceanu [Aggoun &

Beldiceanu 93]. The algorithm is incomplete.

Chapter 32: Constraint Logic Programming over Finite Domains 347

32.3.5 User-Defined Constraints

New, primitive constraints can be added defined by the user on two different levels. On a higher level, constraints can be defined using the global constraint programming interface; see

Section 32.8 [Defining Global Constraints], page 352 . Such constraints can embody

specialized algorithms and use the full power of Prolog. They cannot be reified.

On a lower level, new primitive constraints can be defined with indexicals. In this case, they take part in the basic constraint solving algorithm and express custom designed rules for special cases of the overall local propagation scheme. Such constraints are called FD predicates; see

Section 32.9 [Defining Primitive Constraints], page 358 . They can optionally

be reified.

32.4 Enumeration Predicates

As is usually the case with finite domain constraint solvers, this solver is not complete.

That is, it does not ensure that the set of posted constraints is satisfiable. One must resort to search (enumeration) to check satisfiability and get particular solutions.

The following predicates provide several variants of search: indomain(?X ) where X is a domain variable with a bounded domain or an integer. Assigns, in increasing order via backtracking, a feasible value to X.

labeling(:Options, +Variables) where Variables is a list of domain variables or integers and Options is a list of search options. The domain variables must all have bounded domains. True if an assignment of the variables can be found which satisfies the posted constraints.

apply_bound(BB)

Provides an auxiliary service for the value(Enum) option (see below).

minimize(:Goal,?X ) maximize(:Goal,?X )

Uses a branch-and-bound algorithm with restart to find an assignment that minimizes (maximizes) the domain variable X. Goal should be a Prolog goal that constrains X to become assigned, and could be a labeling/2 goal. The algorithm calls Goal repeatedly with a progressively tighter upper (lower) bound on X until a proof of optimality is obtained, at which time Goal and X are unified with values corresponding to the optimal solution.

The Options argument of labeling/2 controls the order in which variables are selected for assignment (variable choice heuristic), the way in which choices are made for the selected variable (value choice heuristic), and whether all solutions or a single, optimal solution should be found. The options are divided into four groups. One option may be selected per group. Finally, the number of assumptions (choices) made during the search can be collected.

348 SICStus Prolog

The following options control the order in which the next variable is selected for assignment.

leftmost

The leftmost variable is selected. This is the default.

min max ff

The leftmost variable with the smallest lower bound is selected.

The leftmost variable with the greatest upper bound is selected.

ffc

The first-fail principle is used: the leftmost variable with the smallest domain is selected.

The most constrained heuristic is used: a variable with the smallest domain is selected, breaking ties by (a) selecting the variable that has the most constraints suspended on it and (b) selecting the leftmost one.

variable(Sel)

Sel is a predicate to select the next variable.

Given Vars, the variables that remain to label, it will be called as

Sel(Vars,Selected,Rest).

Sel is expected to succeed deterministically, unifying Selected and

Rest with the selected variable and the remaining list, respectively.

Sel should be a callable term, optionally with a module prefix, and the arguments Vars,Selected,Rest will be appended to it.

For example, if Sel is mod:sel(Param), it will be called as mod:sel(Param,Vars,Selected,Rest).

The following options control the way in which choices are made for the selected variable X : step enum bisect

Makes a binary choice between X #= B and X #\= B, where B is the lower or upper bound of X. This is the default.

Makes a multiple choice for X corresponding to the values in its domain.

Makes a binary choice between X #=< M and X #> M , where M is the midpoint of the domain of X. This strategy is also known as domain splitting.

value(Enum)

Enum is a predicate which should narrow the domain of X, possibly but not necessarily to a singleton.

It will be called as

Enum(X,Rest,BB) where Rest is the list of variables that need labeling except X, and BB is a parameter described below.

Enum is expected to succeed non-deterministically, narrowing the domain of X, and to backtrack one or more times, providing alternative narrowings. In all solutions except the first one, it must call the auxiliary predicate apply_bound(BB) to ensure that branchand-bound search works correctly.

Chapter 32: Constraint Logic Programming over Finite Domains 349

Enum should be a callable term, optionally with a module prefix, and the arguments X,Rest,BB will be appended to it.

For example, if Enum is mod:enum(Param), it will be called as mod:enum(Param,X,Rest,BB).

The following options control the order in which the choices are made for the selected variable X. Not useful with the value(Enum) option: up down

The domain is explored in ascending order. This is the default.

The domain is explored in descending order.

The following options control whether all solutions should be enumerated by backtracking or whether a single solution that minimizes (maximizes) X is returned, if one exists.

all

All solutions are enumerated. This is the default.

minimize(X ) maximize(X )

Uses a branch-and-bound algorithm to find an assignment that minimizes (maximizes) the domain variable X. The labeling should constrain X to become assigned for all assignments of Variables.

Finally, the following option counts the number of assumptions (choices) made during the search: assumptions(K )

When a solution is found, K is unified with the number of choices made.

For example, to enumerate solutions using a static variable ordering, use:

| ?- constraints(Variables), labeling([], Variables).

%same as [leftmost,step,up,all]

To minimize a cost function using branch-and-bound search, a dynamic variable ordering using the first-fail principle, and domain splitting exploring the upper part of domains first, use:

| ?- constraints(Variables, Cost), labeling([ff,bisect,down,minimize(Cost)], Variables).

The file library(’clpfd/examples/tsp.pl’) contains an example of user-defined variable and value choice heuristics.

As opposed to the predicates above which search for consistent assignments to domain variables, the following predicate searches for a consistent ordering among tasks competing for an exclusive resource, without necessarily fixing their start times:

350 SICStus Prolog order_resource(+Options, +Resource) where Options is a list of search options and Resource represents a resource as returned by serialized/3 (see

Section 32.3.4 [Combinatorial Constraints], page 343

) on which tasks must be serialized. True if a total ordering can be imposed on the tasks, enumerating all such orderings via backtracking.

The search options control the construction of the total ordering. It may contain at most one of the following atoms, selecting a strategy: first last

The ordering is built by repetitively selecting some task to be placed before all others.

The ordering is built by repetitively selecting some task to be placed after all others.

and at most one of the following atoms, controlling which task to select at each step. If first is chosen (the default), the task with the smallest value is selected, otherwise the task with the greatest value is selected.

est lst ect lct

The tasks are ordered by earliest start time.

The tasks are ordered by latest start time.

The tasks are ordered by earliest completion time.

The tasks are ordered by latest completion time.

[first,est] (the default) and [last,lct] can be good heuristics.

32.5 Statistics Predicates

The following predicates can be used to get execution statistics.

fd_statistics(?Key, ?Value)

This allows a program to access execution statistics specific to this solver. General statistics about CPU time and memory consumption etc. is available from the built-in predicate statistics/2.

For each of the possible keys Key, Value is unified with the current value of a counter which is simultaneously zeroed. The following counters are maintained.

See

Section 32.7 [The Constraint System], page 351

, for details of what they all mean: resumptions

The number of times a constraint was resumed.

entailments

The number of times a (dis)entailment was detected by a constraint.

prunings

The number of times a domain was pruned.

backtracks

The number of times a contradiction was found by a domain being wiped out, or by a global constraint signalling failure. Other causes of backtracking, such as failed Prolog tests, are not covered by this counter.

Chapter 32: Constraint Logic Programming over Finite Domains 351 constraints

The number of constraints created.

fd_statistics

Displays on the standard error stream a summary of the above statistics. All counters are zeroed.

32.6 Answer Constraints

By default, the answer constraint only shows the projection of the store onto the variables that occur in the query, but not any constraints that may be attached to these variables, nor any domains or constraints attached to other variables. This is a conscious decision, as no efficient algorithm for projecting answer constraints onto the query variables is known for this constraint system.

It is possible, however, to get a complete answer constraint including all variables that took part in the computation and their domains and attached constraints. This is done by asserting a clause for the following predicate: clpfd:full_answer

[Hook]

If false (the default), the answer constraint only contains the domains of the query variables. If true, the answer constraints contains the domains and any attached constraints of all variables. Initially defined as a dynamic predicate with no clauses.

32.7 The Constraint System

32.7.1 Definitions

The constraint system is based on domain constraints and indexicals. A domain constraint is an expression X ::I , where X is a domain variable and I is a nonempty set of integers.

A set S of domain constraints is called a store. D(X,S), the domain of X in S, is defined as the intersection of all I such that X ::I belongs to S. The store is contradictory if the domain of some variable is empty; otherwise, it is consistent. A consistent store S’ is an extension of a store S iff, for all variables X, D(X,S’) is contained in D(X,S).

The following definitions, adapted from [Van Hentenryck et al. 95], define important notions of consistency and entailment of constraints wrt. stores.

A ground constraint is true if it holds and false otherwise.

A constraint C is domain-consistent wrt. S iff, for each variable Xi and value Vi in D(Xi,S), there exist values Vj in D(Xj,S), 1 =< j =< n, i\=j, such that C(V1,...,Vn) is true.

A constraint C is domain-entailed by S iff, for all values Vj in D(Xj,S), 1 =< j =< n,

C(V1,...,Vn) is true.

352 SICStus Prolog

Let D’(X,S) denote the interval min(D(X,S))..max(D(X,S)).

A constraint C is interval-consistent wrt. S iff, for each variable Xi there exist values Vj in D’(Xj,S), 1 =< j =< n, i\=j, such that C(V1,...,min(D(Xi,S)),...,Vn) and

C(V1,...,max(D(Xi,S)),...,Vn) are both true.

A constraint C is interval-entailed by S iff, for all values Vj in D’(Xj,S), 1 =< j =< n,

C(V1,...,Vn) is true.

Finally, a constraint is domain-disentailed (interval-disentailed) by S iff its negation is domain-entailed (interval-entailed) by S.

32.7.2 Pitfalls of Interval Reasoning

In most circumstances, arithmetic constraints only maintain interval-consistency and only detect interval-entailment and -disentailment. Note that there are cases where an intervalconsistency maintaining constraint may detect a contradiction when the constraint is not yet interval-disentailed, as the following example illustrates. Note that X #\= Y maintains domain consistency if both arguments are constants or variables:

| ?- X+Y #= Z, X=1, Z=6, Y in 1..10, Y #\= 5.

no

| ?- X+Y #= Z #<=> B, X=1, Z=6, Y in 1..10, Y #\= 5.

X = 1,

Z = 6,

Y in(1..4)\/(6..10),

B in 0..1

Since 1+5#=6 holds, X+Y #= Z is not interval-disentailed, although any attempt to make it interval-consistent wrt. the store results in a contradictory store.

32.8 Defining Global Constraints

32.8.1 The Global Constraint Programming Interface

This section describes a programming interface by means of which new constraints can be written. The interface consists of a set of predicates provided by this library module.

Constraints defined in this way can take arbitrary arguments and may use any constraint solving algorithm, provided it makes sense. Reification cannot be expressed in this interface; instead, reification may be achieved by explicitly passing a 0/1-variable to the constraint in question.

Global constraints have state which may be updated each time the constraint is resumed.

The state information may be used e.g. in incremental constraint solving.

Chapter 32: Constraint Logic Programming over Finite Domains 353

The following two predicates are the principal entrypoints for defining and posting new global constraints: clpfd:dispatch_global(+Constraint, +State0, -State, -Actions)

[Hook]

Tells the solver how to solve constraints of the form Constraint. Defined as a dynamic, multifile predicate.

When defining a new constraint, a clause of this predicate must be added. Its body defines a constraint solving method and should always succeed deterministically. When a global constraint is called or resumed, the solver will call this predicate to deal with the constraint.

State0 and State are the old and new state respectively.

The constraint solving method must not invoke the constraint solver recursively e.g. by binding variables or posting new constraints; instead, Actions should be unified with a list of requests to the solver. Each request should be of the following form: exit

The constraint has become entailed, and ceases to exist.

fail

The constraint has become disentailed, causing the solver to backtrack.

X = V

X in R

The solver binds X to V.

The solver constrains X to be a member of the ConstantRange R

(see

[Syntax of Indexicals], page 369

).

X in_set S

The solver constrains X to be a member of the FD set S (see

Section 32.8.3 [FD Set Operations], page 355

).

call(Goal)

The solver calls the goal or constraint Goal, which should be module prefixed unless it is a built-in predicate or an exported predicate of the clpfd module.

The constraint solving method is assumed to be idempotent. That is, if the

Actions were performed by the solver and the constraint resumed, it should not produce any further Actions. Thus the solver will not check for the resumption conditions for the given constraint, while performing its Actions.

fd_global(+Constraint, +State, +Susp) where Constraint is a constraint goal, State is its initial state, and Susp is a term encoding how the constraint should wake up in response to domain changes.

This posts the constraint.

Susp is a list of F(Var) terms where Var is a variable to suspend on and F is a functor encoding when to wake up: dom(X ) wake up when the domain of X has changed min(X ) wake up when the lower bound of X has changed max(X ) wake up when the upper bound of X has changed

354 SICStus Prolog minmax(X ) wake up when the lower or upper of X has changed val(X ) wake up when X has become ground

For an example of usage, see

Section 32.8.4 [A Global Constraint Example], page 356

.

32.8.2 Reflection Predicates

The constraint solving method needs access to information about the current domains of variables.

This is provided by the following predicates, which are all constant time operations.

fd_min(?X, ?Min) where X is a domain variable (or an integer). Min is unified with the smallest value in the current domain of X, i.e. an integer or the atom inf denoting minus infinity.

fd_max(?X, ?Max) where X is a domain variable (or an integer). Max is unified with the upper bound of the current domain of X, i.e. an integer or the atom sup denoting infinity.

fd_size(?X, ?Size) where X is a domain variable (or an integer). Size is unified with the size of the current domain of X, if the domain is bounded, or the atom sup otherwise.

fd_set(?X, ?Set) where X is a domain variable (or an integer). Set is unified with an FD set term denoting the internal representation of the current domain of X ; see below.

fd_dom(?X, ?Range) where X is a domain variable (or an integer). Range is unified with a ConstantRange (see

[Syntax of Indexicals], page 369 ) denoting the the current do-

main of X.

fd_degree(?X, ?Degree) where X is a domain variable (or an integer). Degree is unified with the number of constraints that are attached to X. NOTE: this number may include some constraints that have been detected as entailed.

The following predicate can be used for computing the set of variables that are transitively connected via constraints to some given variables.

fd_closure(+Vars, -Closure)

Given a list Vars of domain variables, Closure is the set of variables (including

Vars) that can be transitively reached via constraints posted so far.

Chapter 32: Constraint Logic Programming over Finite Domains 355

32.8.3 FD Set Operations

The domains of variables are internally represented compactly as FD set terms. The details of this representation are subject to change and should not be relied on. Therefore, a number of operations on FD sets are provided, as such terms play an important role in the interface. The following operations are the primitive ones: is_fdset(+Set)

Set is a valid FD set.

empty_fdset(?Set)

Set is the empty FD set.

fdset_parts(?Set, ?Min, ?Max, ?Rest)

Set is an FD set which is a union of the non-empty interval Min..Max and the

FD set Rest, and all elements of Rest are greater than Max+1. Min and Max are both integers or the atoms inf and sup, denoting minus and plus infinity, respectively. Either Set or all the other arguments must be ground.

The following operations can all be defined in terms of the primitive ones, but in most cases, a more efficient implementation is used: empty_interval(+Min, +Max)

Min..Max is an empty interval.

fdset_interval(?Set, ?Min, ?Max)

Set is an fdset which is the non-empty interval Min..Max.

fdset_singleton(?Set, ?Elt)

Set is an FD set containing Elt only. At least one of the arguments must be ground.

fdset_min(+Set, -Min)

Min is the lower bound of Set.

fdset_max(+Set, -Min)

Max is the upper bound of Set. This operation is linear in the number of intervals of Set.

fdset_size(+Set, -Size)

Size is the cardinality of Set, represented as sup if Set is infinite. This operation is linear in the number of intervals of Set.

list_to_fdset(+List, -Set)

Set is the FD set containing the elements of List. Slightly more efficient if List is ordered.

fdset_to_list(+Set, -List)

List is an ordered list of the elements of Set, which must be finite.

range_to_fdset(+Range, -Set)

Set is the FD set containing the elements of the ConstantRange (see

[Syntax of Indexicals], page 369

) Range.

356 SICStus Prolog fdset_to_range(+Set, -Range)

Range is a constant interval, a singleton constant set, or a union of such, denoting the same set as Set.

fdset_add_element(+Set1, +Elt -Set2)

Set2 is Set1 with Elt inserted in it.

fdset_del_element(+Set1, +Elt, -Set2)

Set2 is like Set1 but with Elt removed.

fdset_disjoint(+Set1, +Set2)

The two FD sets have no elements in common.

fdset_intersect(+Set1, +Set2)

The two FD sets have at least one element in common.

fdset_intersection(+Set1, +Set2, -Intersection)

Intersection is the intersection between Set1 and Set2.

fdset_intersection(+Sets, -Intersection)

Intersection is the intersection of all the sets in Sets.

fdset_member(?Elt, +Set) is true when Elt is a member of Set. If Elt is unbound, Set must be finite.

fdset_eq(+Set1, +Set2)

Is true when the two arguments represent the same set i.e. they are identical.

fdset_subset(+Set1, +Set2)

Every element of Set1 appears in Set2.

fdset_subtract(+Set1, +Set2, -Difference)

Difference contains all and only the elements of Set1 which are not also in Set2.

fdset_union(+Set1, +Set2, -Union)

Union is the union of Set1 and Set2.

fdset_union(+Sets, -Union)

Union is the union of all the sets in Sets.

fdset_complement(+Set, -Complement)

Complement is the complement of Set wrt. inf..sup.

32.8.4 A Global Constraint Example

The following example defines a new global constraint exactly(X,L,N ) which is true if X occurs exactly N times in the list L of integers and domain variables. N must be an integer when the constraint is posted. A version without this restriction and defined in terms of reified equalities was presented earlier; see

Section 32.2.3 [Reified Constraints], page 340 .

This example illustrates the use of state information. The state has two components: the list of variables that could still be X, and the number of variables still required to be X.

The constraint is defined to wake up on any domain change.

Chapter 32: Constraint Logic Programming over Finite Domains 357

/*

An implementation of exactly(I, X[1]...X[m], N):

Necessary condition: 0 =< N =< m.

Rewrite rules:

[1] |= X[i]=I ==> exactly(I, X[1]...X[i-1],X[i+1]...X[m], N-1):

[2] |= X[i]\=I ==> exactly(I, X[1]...X[i-1],X[i+1]...X[m], N):

[3] |= N=0 ==> X[1]\=I ... X[m]\=I

[4] |= N=m ==> X[1]=I ... X[m]=I

*/

:- use_module(library(clpfd)).

% the entrypoint exactly(I, Xs, N) :dom_suspensions(Xs, Susp), fd_global(exactly(I,Xs,N), state(Xs,N), Susp).

dom_suspensions([], []).

dom_suspensions([X|Xs], [dom(X)|Susp]) :dom_suspensions(Xs, Susp).

% the solver method

:- multifile clpfd:dispatch_global/4.

clpfd:dispatch_global(exactly(I,_,_), state(Xs0,N0), state(Xs,N), Actions) :exactly_solver(I, Xs0, Xs, N0, N, Actions).

exactly_solver(I, Xs0, Xs, N0, N, Actions) :ex_filter(Xs0, Xs, N0, N, I), length(Xs, M),

( N=:=0 -> Actions = [exit|Ps], ex_neq(Xs, I, Ps)

; N=:=M -> Actions = [exit|Ps], ex_eq(Xs, I, Ps)

; N>0, N<M -> Actions = []

; Actions = [fail]

).

% rules [1,2]: filter the X’s, decrementing N ex_filter([], [], N, N, _).

ex_filter([X|Xs], Ys, L, N, I) :- X==I, !,

M is L-1, ex_filter(Xs, Ys, M, N, I).

ex_filter([X|Xs], Ys0, L, N, I) :fd_set(X, Set), fdset_member(I, Set), !,

Ys0 = [X|Ys], ex_filter(Xs, Ys, L, N, I).

ex_filter([_|Xs], Ys, L, N, I) :ex_filter(Xs, Ys, L, N, I).

358 SICStus Prolog

% rule [3]: all must be neq I ex_neq(Xs, I, Ps) :fdset_singleton(Set0, I), fdset_complement(Set0, Set), eq_all(Xs, Set, Ps).

% rule [4]: all must be eq I ex_eq(Xs, I, Ps) :fdset_singleton(Set, I), eq_all(Xs, Set, Ps).

eq_all([], _, []).

eq_all([X|Xs], Set, [X in_set Set|Ps]) :eq_all(Xs, Set, Ps).

end_of_file.

% sample queries:

| ?- exactly(5,[A,B,C],1), A=5.

A = 5,

B in(inf..4)\/(6..sup),

C in(inf..4)\/(6..sup)

| ?- exactly(5,[A,B,C],1), A in 1..2, B in 3..4.

C = 5,

A in 1..2,

B in 3..4

32.9 Defining Primitive Constraints

Indexicals are the principal means of defining constraints, but it is usually not necessary to resort to this level of programming—most commonly used constraints are available in a library and/or via macro-expansion. The key feature about indexicals is that they give the programmer precise control over aspects of the operational semantics of the constraints.

Trade-offs can be made between the computational cost of the constraints and their pruning power. The indexical language provides many degrees of freedom for the user to select the level of consistency to be maintained depending on application-specific needs.

32.9.1 Indexicals

An indexical is a reactive functional rule of the form X in R, where R is a set valued range expression (see below). See

[Syntax of Indexicals], page 369

, for a grammar defining indexicals and range expressions.

Indexicals can play one of two roles: propagating indexicals are used for constraint solving, and checking indexicals are used for entailment checking. When a propagating indexical

Chapter 32: Constraint Logic Programming over Finite Domains 359 fires, R is evaluated in the current store S, which is extended by adding the new domain constraint X ::S(R) to the store, where S(R) denotes the value of R in S. When a checking indexical fires, it checks if D(X,S) is contained in S(R), and if so, the constraint corresponding to the indexical is detected as entailed.

32.9.2 Range Expressions

A range expression has one of the following forms, where Ri denote range expressions, Ti denote integer valued term expressions, S(Ti) denotes the integer value of Ti in S, X denotes a variable, I denotes an integer, and S denotes the current store.

dom(X ) evaluates to D(X,S)

{T1,...,Tn} evaluates to {S(T1),...,S(Tn)}. Any term expression containing a subexpression which is a variable that is not “quantified” by unionof/3 will only be evaluated when this variable has been assigned.

T1..T2

evaluates to the interval between S(T1) and S(T2).

R1/\R2 evaluates to the intersection of S(R1) and S(R2)

R1\/R2 evaluates to the union of S(R1) and S(R2)

\R2

R1+R2

R1+T2 evaluates to the complement of S(R2)

-R2

R1-R2

R1-T2

T1-R2

R1 mod R2

R1 mod T2 evaluates to S(R2) or S(T2) added pointwise to S(R1) evaluates to S(R2) negated pointwise evaluates to S(R2) or S(T2) subtracted pointwise from S(R1) or S(T1) evaluates to S(R1) pointwise modulo S(R2) or S(T2)

R1 ? R2 evaluates to S(R2) if S(R1) is a non-empty set; otherwise, evaluates to the empty set. This expression is commonly used in the context (R1 ? (inf..sup)

\/ R3), which evaluates to S(R3) if S(R1) is an empty set; otherwise, evaluates to inf..sup. As an optimization, R3 is not evaluated while the value of R1 is a non-empty set.

unionof(X,R1,R2) evaluates to the union of S(Expr 1)...S(Expr N), where each Expr I has been formed by substituting K for X in R2, where K is the I :th element of S(R1).

See

Section 32.10.2 [N Queens], page 366 , for an example of usage. N.B. If

S(R1) is infinite, the evaluation of the indexical will be abandoned, and the indexical will simply suspend.

switch(T1,MapList) evaluates to S(Expr) if S(T1) equals Key and MapList contains a pair Key-

Expr. Otherwise, evaluates to the empty set.

360 SICStus Prolog

When used in the body of an FD predicate (see

Section 32.9.8 [Goal Expanded Constraints], page 365 ), a relation/3 expression expands to two indexicals, each consisting of a switch/2

expression nested inside a unionof/3 expression. Thus, the following constraints are equivalent: p(X, Y) +: relation(X, [1-{1},2-{1,2},3-{1,2,3}], Y).

q(X, Y) +:

X in unionof(B,dom(Y),switch(B,[1-{1,2,3},2-{2,3},3-{3}])),

Y in unionof(B,dom(X),switch(B,[1-{1},2-{1,2},3-{1,2,3}])).

32.9.3 Term Expressions

A term expression has one of the following forms, where T1 and T2 denote term expressions,

X denotes a variable, I denotes an integer, and S denotes the current store.

min(X ) evaluates to the minimum of D(X,S) max(X ) evaluates to the maximum of D(X,S) card(X ) evaluates to the size of D(X,S)

X evaluates to the integer value of X. A subexpression of this form, not “quantified” by unionof/3, will cause the evaluation to suspend until the variable is assigned.

I an integer inf sup minus infinity plus infinity

-T1

T1+T2

T1-T2 evaluates to S(T1) negated evaluates to the sum of S(T1) and S(T2) evaluates to the difference of S(T1) and S(T2)

T1*T2 evaluates to the product of S(T1) and S(T2), where S(T2) must not be negative

T1/>T2 evaluates to the quotient of S(T1) and S(T2), rounded up, where S(T2) must be positive

T1/<T2 evaluates to the quotient of S(T1) and S(T2), rounded down, where S(T2) must be positive

T1 mod T2 evaluates to the modulo of S(T1) and S(T2)

32.9.4 Monotonicity of Indexicals

A range R is monotone in S iff the value of R in S’ is contained in the value of R in S, for every extension S’ of S. A range R is anti-monotone in S iff the value of R in S is contained in the value of R in S’, for every extension S’ of S. By abuse of notation, we will say that

X in R is (anti-)monotone iff R is (anti-)monotone.

Chapter 32: Constraint Logic Programming over Finite Domains 361

The consistency or entailment of a constraint C expressed as indexicals X in R in a store S is checked by considering the relationship between D(X,S) and S(R), together with the (anti-

)monotonicity of R in S. The details are given in

Section 32.9.6 [Execution of Propagating

Indexicals], page 363

and

Section 32.9.7 [Execution of Checking Indexicals], page 364

.

The solver checks (anti-)monotonicity by requiring that certain variables occurring in the indexical be ground. This sufficient condition can sometimes be false for an (anti-)monotone indexical, but such situations are rare in practice.

32.9.5 FD predicates

The following example defines the constraint X+Y=T as an FD predicate in terms of three indexicals. Each indexical is a rule responsible for removing values detected as incompatible from one particular constraint argument. Indexicals are not Prolog goals; thus, the example does not express a conjunction. However, an indexical may make the store contradictory, in which case backtracking is triggered: plus(X,Y,T) +:

X in min(T) - max(Y) .. max(T) - min(Y),

Y in min(T) - max(X) .. max(T) - min(X),

T in min(X) + min(Y) .. max(X) + max(Y).

The above definition contains a single clause used for constraint solving. The first indexical wakes up whenever the bounds of S(T) or S(Y) are updated, and removes from D(X,S) any values that are not compatible with the new bounds of T and Y. Note that in the event of

“holes” in the domains of T or Y, D(X,S) may contain some values that are incompatible with X+Y=T but go undetected.

Like most built-in arithmetic constraints, the above definition maintains interval-consistency, which is significantly cheaper to maintain than domain-consistency and suffices in most cases. The constraint could for example be used as follows:

| ?- X in 1..5, Y in 2..8, plus(X,Y,T).

X in 1..5,

Y in 2..8,

T in 3..13 ?

yes

Thus, when an FD predicate is called, the ‘+:’ clause is activated.

The definition of a user constraint has to specify what domain constraints should be added to the constraint store when the constraint is posted. Therefore the FD predicate contains a set of indexicals, each representing a domain constraint to be added to the constraint store.

The actual domain constraint depends on the constraint store itself. For example, the third indexical in the above FD predicate prescribes the domain constraint ‘T :: 3..13’ if the store contains ‘X :: 1..5, Y :: 2..8’. As the domain of some variables gets narrower, the indexical may enforce a new, stricter constraint on some other variables. Therefore such an

362 SICStus Prolog indexical (called a propagating indexical) can be viewed as an agent reacting to the changes in the store by enforcing further changes in the store.

In general there are three stages in the lifetime of a propagating indexical. When it is posted it may not be evaluated immediately (e.g. has to wait until some variables are ground before being able to modify the store). Until the preconditions for the evaluation are satisfied, the agent does not enforce any constraints. When the indexical becomes evaluable the resulting domain constraint is added to the store. The agent then waits and reacts to changes in the domains of variables occurring in the indexical by re-evaluating it and adding the new, stricter constraint to the store. Eventually the computation reaches a phase when no further refinement of the store can result in a more precise constraint (the indexical is entailed by the store), and then the agent can cease to exist.

A necessary condition for the FD predicate to be correctly defined is the following: for any store mapping each variable to a singleton domain the execution of the indexicals should succeed without contradiction exactly when the predicate is intended to be true.

There can be several alternative definitions for the same user constraint with different strengths in propagation. For example, the definition of plusd below encodes the same

X+Y=T constraint as the plus predicate above, but maintaining domain consistency: plusd(X,Y,T) +:

X in dom(T) - dom(Y),

Y in dom(T) - dom(X),

T in dom(X) + dom(Y).

| ?- X in {1}\/{3}, Y in {10}\/{20}, plusd(X, Y, T).

X in{1}\/{3},

Y in{10}\/{20},

T in{11}\/{13}\/{21}\/{23} ?

yes

This costs more in terms of execution time, but gives more precise results. For singleton domains plus and plusd behave in the same way.

In our design, general indexicals can only appear in the context of FD predicate definitions.

The rationale for this restriction is the need for general indexicals to be able to suspend and resume, and this ability is only provided by the FD predicate mechanism.

If the program merely posts a constraint, it suffices for the definition to contain a single clause for solving the constraint.

If a constraint is reified or occurs in a propositional formula, the definition must contain four clauses for solving and checking entailment of the constraint and its negation. The role of each clause is reflected in the “neck” operator.

The following table summarizes the different forms of indexical clauses corresponding to a constraint C. In all cases, Head should be a compound term with all arguments being distinct variables:

Chapter 32: Constraint Logic Programming over Finite Domains 363

Head +: Indexicals.

The clause consists of propagating indexicals for solving C.

Head -: Indexicals.

The clause consists of propagating indexicals for solving the negation of C.

Head +? Indexical.

The clause consists of a single checking indexical for testing entailment of C.

Head -? Indexical.

The clause consists of a single checking indexical for testing entailment of the negation of C.

When a constraint is reified, the solver spawns two reactive agents corresponding to detecting entailment and disentailment. Eventually, one of them will succeed in this and consequently will bind B to 0 or 1. A third agent is spawned, waiting for B to become assigned, at which time the constraint (or its negation) is posted. In the mean time, the constraint may have been detected as (dis)entailed, in which case the third agent is dismissed. The waiting is implemented by means of the coroutining facilities of SICStus Prolog.

As an example of a constraint with all methods defined, consider the following library constraint defining a disequation between two domain variables:

’x\\=y’(X,Y) +:

X in \{Y},

Y in \{X}.

’x\\=y’(X,Y) -:

X in dom(Y),

Y in dom(X).

’x\\=y’(X,Y) +?

X in \dom(Y).

’x\\=y’(X,Y) -?

X in {Y}.

The following sections provide more precise coding rules and operational details for indexicals. X in R denotes an indexical corresponding to a constraint C. S denotes the current store.

32.9.6 Execution of Propagating Indexicals

Consider the definition of a constraint C containing a propagating indexical X in R. Let

TV(X,C,S) denote the set of values for X that can make C true in some ground extension of the store S. Then the indexical should obey the following coding rules:

• all arguments of C except X should occur in R

• if R is ground in S, S(R) = TV(X,C,S)

If the coding rules are observed, S(R) can be proven to contain TV(X,C,S) for all stores in which R is monotone. Hence it is natural for the implementation to wait until R becomes

364 SICStus Prolog monotone before admitting the propagating indexical for execution. The execution of X in

R thus involves the following:

• If D(X,S) is disjoint from S(R), a contradiction is detected.

• If D(X,S) is contained in S(R), D(X,S) does not contain any values known to be incompatible with C, and the indexical suspends, unless R is ground in S, in which case

C is detected as entailed.

• Otherwise, D(X,S) contains some values that are known to be incompatible with C.

Hence, X ::S(R) is added to the store (X is pruned), and the indexical suspends, unless

R is ground in S, in which case C is detected as entailed.

A propagating indexical is scheduled for execution as follows:

• it is evaluated initially as soon as it has become monotone

• it is re-evaluated when one of the following conditions occurs:

1. the domain of a variable Y that occurs as dom(Y ) or card(Y ) in R has been updated

2. the lower bound of a variable Y that occurs as min(Y ) in R has been updated

3. the upper bound of a variable Y that occurs as max(Y ) in R has been updated

32.9.7 Execution of Checking Indexicals

Consider the definition of a constraint C containing a checking indexical X in R. Let

FV(X,C,S) denote the set of values for X that can make C false in some ground extension of the store S. Then the indexical should obey the following coding rules:

• all arguments of C except X should occur in R

• if R is ground in S, S(R) = TV(X,C,S)

If the coding rules are observed, S(R) can be proven to exclude FV(X,C,S) for all stores in which R is anti-monotone. Hence it is natural for the implementation to wait until R becomes anti-monotone before admitting the checking indexical for execution. The execution of X in R thus involves the following:

• If D(X,S) is contained in S(R), none of the possible values for X can make C false, and so C is detected as entailed.

• Otherwise, if D(X,S) is disjoint from S(R) and R is ground in S, all possible values for

X will make C false, and so C is detected as disentailed.

• Otherwise, D(X,S) contains some values that could make C true and some that could make C false, and the indexical suspends.

A checking indexical is scheduled for execution as follows:

• it is evaluated initially as soon as it has become anti-monotone

• it is re-evaluated when one of the following conditions occurs:

1. the domain of X has been pruned, or X has been assigned

Chapter 32: Constraint Logic Programming over Finite Domains 365

2. the domain of a variable Y that occurs as dom(Y ) or card(Y ) in R has been pruned

3. the lower bound of a variable Y that occurs as min(Y ) in R has been increased

4. the upper bound of a variable Y that occurs as max(Y ) in R has been decreased

32.9.8 Goal Expanded Constraints

The arithmetic, membership, and propositional constraints described earlier are transformed at compile time to conjunctions of goals of library constraints.

Sometimes it is necessary to postpone the expansion of a constraint until runtime, e.g. if the arguments are not instantiated enough. This can be achieved by wrapping call/1 around the constraint.

Although space economic (linear in the size of the source code), the expansion of a constraint to library goals can have an overhead compared to expressing the constraint in terms of indexicals. Temporary variables holding intermediate values may have to be introduced, and the grain size of the constraint solver invocations can be rather small. The translation of constraints to library goals has been greatly improved in the current version, so these problems have virtually disappeared. However, for backward compatibility, an implementation by compilation to indexicals of the same constraints is also provided. An FD predicate may be defined by a single clause:

Head +: Constraint.

where Constraint is an arithmetic constraint or an element/3 or a relation/3 constraint.

This translation is only available for ‘+:’ clauses; thus, Head cannot be reified.

In the case of arithmetic constraints, the constraint must be over linear terms (see

[Syntax of Indexicals], page 369

). The memory consumption of the FD predicate will be quadratic in the size of the source code. The alternative version of sum/8 in

Section 32.10.1 [Send

More Money], page 366

illustrates this technique.

In the case of element(X,L,Y) or relation(X,L,Y), the memory consumption of the FD predicate will be linear in the size of the source code. The execution time of the initial evaluation of the FD predicate will be linear in the size of the initial domains for X and Y ; if these domains are infinite, no propagation will take place.

32.10 Example Programs

This section contains a few example programs. The first two programs are included in a benchmark suite that comes with the distribution. The benchmark suite is run by typing:

| ?- compile(library(’clpfd/examples/bench’)).

| ?- bench.

366 SICStus Prolog

32.10.1 Send More Money

Let us return briefly to the Send More Money problem (see

Section 32.2.2 [A Constraint

Satisfaction Problem], page 339

). Its sum/8 predicate will expand to a space-efficient conjunction of library constraints. A faster but more memory consuming version is defined simply by changing the neck symbol of sum/8 from ‘:-’ to ‘+:’, thus turning it into an FD predicate: sum(S, E, N, D, M, O, R, Y) +:

+

1000*S + 100*E + 10*N + D

1000*M + 100*O + 10*R + E

#= 10000*M + 1000*O + 100*N + 10*E + Y.

32.10.2 N Queens

The problem is to place N queens on an NxN chess board so that no queen is threatened by another queen.

The variables of this problem are the N queens. Each queen has a designated row. The problem is to select a column for it.

The main constraint of this problem is that no queen threaten another. This is encoded by the no_threat/3 constraint and holds between all pairs (X,Y) of queens. It could be defined as no_threat(X, Y, I) :-

X #\= Y,

X+I #\= Y,

X-I #\= Y.

However, this formulation introduces new temporary domain variables and creates twelve fine-grained indexicals. Worse, the arithmetic constraints are only guaranteed to maintain interval-consistency and so may miss some opportunities for pruning elements in the middle of domains.

A better idea is to formulate no_threat/3 as an FD predicate with two indexicals, as shown in the program below. This constraint will not fire until one of the queens has been assigned

(the corresponding indexical does not become monotone until then). Hence, the constraint is still not as strong as it could be.

For example, if the domain of one queen is (2..3), then it will threaten any queen placed in column 2 or 3 on an adjacent row, no matter which of the two open positions is chosen for the first queen. The commented out formulation of the constraint captures this reasoning, and illustrates the use of the unionof/3 operator. This stronger version of the constraint indeed gives less backtracking, but is computationally more expensive and does not pay off in terms of execution time, except possibly for very large chess boards.

It is clear that no_threat/3 cannot detect any incompatible values for a queen with domain of size greater than three. This observation is exploited in the third version of the constraint.

Chapter 32: Constraint Logic Programming over Finite Domains

The first-fail principle is appropriate in the enumeration part of this problem.

:- use_module(library(clpfd)).

queens(N, L, LabelingType) :length(L, N), domain(L, 1, N), constrain_all(L), labeling(LabelingType, L).

constrain_all([]).

constrain_all([X|Xs]) :constrain_between(X, Xs, 1), constrain_all(Xs).

constrain_between(_X, [], _N).

constrain_between(X, [Y|Ys], N) :no_threat(X, Y, N),

N1 is N+1, constrain_between(X, Ys, N1).

% version 1: weak but efficient no_threat(X, Y, I) +:

X in \({Y} \/ {Y+I} \/ {Y-I}),

Y in \({X} \/ {X+I} \/ {X-I}).

/*

% version 2: strong but very inefficient version no_threat(X, Y, I) +:

X in unionof(B,dom(Y),\({B} \/ {B+I} \/ {B-I})),

Y in unionof(B,dom(X),\({B} \/ {B+I} \/ {B-I})).

% version 3: strong but somewhat inefficient version no_threat(X, Y, I) +:

X in (4..card(Y)) ? (inf..sup) \/ unionof(B,dom(Y),\({B} \/ {B+I} \/ {B-I})),

Y in (4..card(X)) ? (inf..sup) \/ unionof(B,dom(X),\({B} \/ {B+I} \/ {B-I})).

*/

| ?- queens(8, L, [ff]).

L = [1,5,8,6,3,7,2,4] ?

367

368 SICStus Prolog

32.10.3 Cumulative Scheduling

This example is a very small scheduling problem. We consider seven tasks where each task has a fixed duration and a fixed amount of used resource: t3 t4 t5 t6 t7

TASK

==== t1 t2

DURATION

========

16

6

13

7

5

18

4

RESOURCE

========

2

9

3

7

10

1

11

The goal is to find a schedule that minimizes the completion time for the schedule while not exceeding the capacity 13 of the resource. The resource constraint is succinctly captured by a cumulative/4 constraint. Branch-and-bound search is used to find the minimal completion time.

This example was adapted from [Beldiceanu & Contejean 94].

:- use_module(library(clpfd)).

:- use_module(library(lists), [append/3]).

schedule(Ss, End) :length(Ss, 7),

Ds = [16, 6,13, 7, 5,18, 4],

Rs = [ 2, 9, 3, 7,10, 1,11], domain(Ss, 1, 30), domain([End], 1, 50), after(Ss, Ds, End), cumulative(Ss, Ds, Rs, 13), append(Ss, [End], Vars), labeling([minimize(End)], Vars). % label End last after([], [], _).

after([S|Ss], [D|Ds], E) :- E #>= S+D, after(Ss, Ds, E).

%% End of file

| ?- schedule(Ss, End).

Ss = [1,17,10,10,5,5,1],

End = 23 ?

32.11 Syntax Summary

Chapter 32: Constraint Logic Programming over Finite Domains

Syntax of Indexicals

369

370 SICStus Prolog

X --> variable

{ domain variable }

Constant --> integer

| inf

| sup

Term --> Constant

|

X

| min(X )

| max(X )

| card(X )

| - Term

|

Term + Term

|

Term - Term

|

Term * Term

|

Term /> Term

|

Term /< Term

|

Term mod Term

TermSet --> {Term,...,Term}

{ minus infinity }

{ plus infinity }

{ suspend until assigned }

{ min. of domain of X }

{ max. of domain of X }

{ size of domain of X }

{ division rounded up }

{ division rounded down }

Range --> TermSet

| dom(X )

|

Term..Term

|

Range/\Range

|

Range\/Range

| \Range

| - Range

|

Range + Range

|

Range - Range

|

Range mod Range

|

Range + Term

|

Range - Term

|

Term - Range

|

Range mod Term

|

Range ? Range

| unionof(X,Range,Range)

| switch(Term,MapList)

ConstantSet --> {integer,...,integer}

ConstantRange --> ConstantSet

|

Constant..Constant

|

ConstantRange/\ConstantRange

|

ConstantRange\/ConstantRange

| \ConstantRange

MapList --> []

| [integer-ConstantRange|MapList]

CList --> []

| [integer|CList]

{ domain of X }

{ interval }

{ intersection }

{ union }

{ complement }

{ pointwise negation }

{ pointwise addition }

{ pointwise subtraction }

{ pointwise modulo }

{ pointwise addition }

{ pointwise subtraction }

{ pointwise subtraction }

{ pointwise modulo }

Chapter 32: Constraint Logic Programming over Finite Domains

Indexical --> X in Range

Indexicals --> Indexical

|

Indexical, Indexicals

ConstraintBody --> Indexicals

|

LinExpr RelOp LinExpr

| element(X,CList,X )

| relation(X,MapList,X )

Head --> term

{ a compound term with unique variable args }

TellPos --> Head +: ConstraintBody.

TellNeg --> Head -: ConstraintBody.

AskPos --> Head +? Indexical.

AskNeg --> Head -? Indexical.

ConstraintDef -->

TellPos. [TellNeg.] [AskPos.] [AskNeg.]

Syntax of Arithmetic Expressions

X --> variable

{ domain variable }

N --> integer

LinExpr --> N

|

X

|

N * X

|

N * N

|

LinExpr + LinExpr

|

LinExpr - LinExpr

{ linear expression }

Expr --> LinExpr

|

Expr + Expr

|

Expr - Expr

|

Expr * Expr

|

Expr / Expr

|

Expr mod Expr

| min(Expr,Expr)

| max(Expr,Expr)

| abs(Expr)

{ integer division }

RelOp --> #= | #\= | #< | #=< | #> | #>=

Operator Declarations

371

372

:- op(1200, xfx, [+:,-:,+?,-?]).

:- op(760, yfx, #<=>).

:- op(750, xfy, #=>).

:- op(750, yfx, #<=).

:- op(740, yfx, #\/).

:- op(730, yfx, #\).

:- op(720, yfx, #/\).

:- op(710, fy, #\).

:- op(700, xfx, [in,in_set]).

:- op(700, xfx, [#=,#\=,#<,#=<,#>,#>=]).

:- op(550, xfx, ..).

:- op(500, fy, \).

:- op(490, yfx, ?).

:- op(400, yfx, [/>,/<]).

SICStus Prolog

Chapter 33: Constraint Handling Rules

33 Constraint Handling Rules

373

Copyright

LMU (Ludwig-Maximilians-University)

Munich, Germany

Permission is granted to make and distribute verbatim copies of this chapter provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this chapter under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this chapter into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by LMU.

33.1 Introduction

Experience from real-life applications using constraint-based programming has shown that typically, one is confronted with a heterogeneous mix of different types of constraints.

To be able to express constraints as they appear in the application and to write and combine constraint systems, a special purpose language for writing constraint systems called constraint handling rules (CHR) was developed.

CHR have been used to encode a wide range of constraint handlers (solvers), including new domains such as terminological and temporal reasoning.

Several CHR libraries exist in declarative languages such as Prolog and LISP, worldwide more than 20 projects use CHR. You can find more information about CHR in [Fruehwirth 98] or at URL: http://www.pst.informatik.uni-muenchen.de/personen/fruehwir/chr-intro.html

The high-level CHR are an excellent tool for rapid prototyping and implementation of constraint handlers. The usual abstract formalism to describe a constraint system, i.e.

inference rules, rewrite rules, sequents, formulas expressing axioms and theorems, can be written as CHR in a straightforward way. Starting from this executable specification, the rules can be refined and adapted to the specifics of the application.

The CHR library includes a compiler, which translates CHR programs into Prolog programs on the fly, and a runtime system, which includes a stepper for debugging. Many constraint handlers are provided in the example directory of the library.

CHR are essentially a committed-choice language consisting of guarded rules that rewrite constraints into simpler ones until they are solved. CHR define both simplification of and propagation over constraints.

Simplification replaces constraints by simpler constraints

374 SICStus Prolog while preserving logical equivalence (e.g. X>Y,Y>X <=> fail). Propagation adds new constraints which are logically redundant but may cause further simplification (e.g. X>Y,Y>Z

==> X>Z). Repeatedly applying CHR incrementally simplifies and finally solves constraints

(e.g. A>B,B>C,C>A leads to fail.

With multiple heads and propagation rules, CHR provide two features which are essential for non-trivial constraint handling. The declarative reading of CHR as formulas of first order logic allows one to reason about their correctness. On the other hand, regarding CHR as a rewrite system on logical formulas allows one to reason about their termination and confluence.

In case the implementation of CHR disagrees with your expectations based on this chapter, drop a line to the current maintainer: [email protected] (Christian Holzbaur).

33.2 Introductory Examples

We define a CHR constraint for less-than-or-equal, leq, that can handle variable arguments.

This handler can be found in the library as the file leq.pl. (The code works regardless of options switched on or off.)

:- use_module(library(chr)).

handler leq.

constraints leq/2.

:- op(500, xfx, leq).

reflexivity @ X leq Y <=> X=Y | true.

antisymmetry @ X leq Y , Y leq X <=> X=Y.

idempotence @ X leq Y \ X leq Y <=> true.

transitivity @ X leq Y , Y leq Z ==> X leq Z.

The CHR specify how leq simplifies and propagates as a constraint. They implement reflexivity, idempotence, antisymmetry and transitivity in a straightforward way. CHR reflexivity states that X leq Y simplifies to true, provided it is the case that X=Y. This test forms the (optional) guard of a rule, a precondition on the applicability of the rule.

Hence, whenever we see a constraint of the form A leq A we can simplify it to true.

The rule antisymmetry means that if we find X leq Y as well as Y leq X in the constraint store, we can replace it by the logically equivalent X=Y. Note the different use of X=Y in the two rules: In the reflexivity rule the equality is a precondition (test) on the rule, while in the antisymmetry rule it is enforced when the rule fires. (The reflexivity rule could also have been written as reflexivity X leq X <=> true.)

The rules reflexivity and antisymmetry are simplification CHR. In such rules, the constraints found are removed when the rule applies and fires. The rule idempotence is a simpagation CHR, only the constraints right of ’\’ will be removed. The rule says that if we find X leq Y and another X leq Y in the constraint store, we can remove one.

Chapter 33: Constraint Handling Rules 375

Finally, the rule transitivity states that the conjunction X leq Y, Y leq Z implies X leq

Z. Operationally, we add X leq Z as (redundant) constraint, without removing the constraints X leq Y, Y leq Z. This kind of CHR is called propagation CHR.

Propagation CHR are useful, as the query A leq B,C leq A,B leq C illustrates: The first two constraints cause CHR transitivity to fire and add C leq B to the query. This new constraint together with B leq C matches the head of CHR antisymmetry, X leq Y, Y leq

X. So the two constraints are replaced by B=C. Since B=C makes B and C equivalent, CHR antisymmetry applies to the constraints A leq B, C leq A, resulting in A=B. The query contains no more CHR constraints, the simplification stops. The constraint handler we built has solved A leq B, C leq A, B leq C and produced the answer A=B, B=C:

A leq B,C leq A,B leq C.

% C leq A, A leq B propagates C leq B by transitivity.

% C leq B, B leq C simplifies to B=C by antisymmetry.

% A leq B, C leq A simplifies to A=B by antisymmetry since B=C.

A=B,B=C.

Note that multiple heads of rules are essential in solving these constraints.

Also note that this handler implements a (partial) order constraint over any constraint domain, this generality is only possible with CHR.

As another example, we can implement the sieve of Eratosthenes to compute primes simply as (for variations see the handler ‘primes.pl’):

:- use_module(library(chr)).

handler eratosthenes.

constraints primes/1,prime/1.

primes(1) <=> true.

primes(N) <=> N>1 | M is N-1,prime(N),primes(M). % generate candidates absorb(J) @ prime(I) \ prime(J) <=> J mod I =:= 0 | true.

The constraint primes(N) generates candidates for prime numbers, prime(M), where M is between 1 and N. The candidates react with each other such that each number absorbs multiples of itself. In the end, only prime numbers remain.

Looking at the two rules defining primes/1, note that head matching is used in CHR, so the first rule will only apply to primes(1). The test N>1 is a guard (precondition) on the second rule. A call with a free variable, like primes(X), will delay (suspend). The third, multi-headed rule absorb(J) reads as follows: If there is a constraint prime(I) and some other constraint prime(J) such that J mod I =:= 0 holds, i.e. J is a multiple of I, then keep prime(I) but remove prime(J) and execute the body of the rule, true.

376 SICStus Prolog

33.3 CHR Library

CHR extend the Prolog syntax by a few constructs introduced in the next sections. Technically, the extension is achieved through the user:term_expansion/2 mechanism. A file that contains a constraint handler may also contain arbitrary Prolog code. Constraint handling rules can be scattered across a file. Declarations and options should precede rules.

There can only be at most one constraint handler per module.

33.3.1 Loading the Library

Before you can load or compile any file containing a constraint handler (solver) written in

CHR, the chr library module has to be imported:

| ?- use_module(library(chr)).

It is recommended to include the corresponding directive at the start of your files containing handlers:

:- use_module(library(chr)).

33.3.2 Declarations

Declarations in files containing CHR affect the compilation and thus the behavior of the rules at runtime.

The mandatory handler declaration precedes any other CHR specific code. Example: handler minmax.

A handler name must be a valid Prolog atom. Per module, only one constraint handler can be defined.

The constraints must be declared before they are used by rules. With this mandatory declaration one lists the constraints the rules will later talk about. The declaration can be used more than once per handler. Example: constraints leq/2, minimum/3, maximum/3.

The following optional declaration allows for conditional rule compilation. Only the rules mentioned get compiled. Rules are referred to by their names (see

Section 33.3.3 [CHR

Syntax], page 377 ). The latest occurrence takes precedence if used more than once per

handler. Although it can be put anywhere in the handler file, it makes sense, as with other declarations, to use it early. Example: rules antisymmetry, transitivity.

To simplify the handling of operator declarations, in particular during fcompile/1, operator/3 declarations with the same denotation as op/3, but taking effect during compilation and loading, are helpful. Example:

Chapter 33: Constraint Handling Rules 377 operator(700, xfx, ::).

operator(600, xfx, :).

33.3.3 Constraint Handling Rules, Syntax

A constraint handling rule has one or more heads, an optional guard, a body and an optional name. A Head is a Constraint. A constraint is a callable Prolog term, whose functor is a declared constraint. The Guard is a Prolog goal. The Body of a rule is a Prolog goal

(including constraints). A rule can be named with a Name which can be any Prolog term

(including variables from the rule).

There are three kinds of constraint handling rules:

Rule --> [Name @]

(Simplification | Propagation | Simpagation)

[pragma Pragma].

Simplification --> Heads

Propagation

Simpagation

--> Heads

<=> [Guard ’|’] Body

==> [Guard ’|’] Body

--> Heads \ Heads <=> [Guard ’|’] Body

Heads

Head

Constraint

Id

Guard

Ask

Tell

Goal

Body

--> Head | Head, Heads

--> Constraint | Constraint # Id

--> a callable term declared as constraint

--> a unique variable

--> Ask | Ask & Tell

--> Goal

--> Goal

--> a callable term, including conjunction and disjunction etc.

--> Goal

Pragma --> a conjunction of terms usually referring to one or more heads identified via #/2

The symbol ‘|’ separates the guard (if present) from the body of a rule. Since ‘|’ is read as ‘;’ (disjunction) by the reader, care has to be taken when using disjunction in the guard or body of the rule. The top level disjunction will always be interpreted as guard-body separator ‘|’, so proper bracketing has to be used, e.g. a <=> (b;c) | (d;e) instead of a

<=> b;c | d;e and a <=> true | (d;e) instead of a <=> (d;e).

In simpagation rules, ‘\’ separates the heads of the rule into two parts.

Individual head constraints may be tagged with variables via ‘#’, which may be used as identifiers in pragma declarations, for example.

Constraint identifiers must be distinct variables, not occurring elsewhere in the heads.

378 SICStus Prolog

Guards test the applicability of a rule. Guards come in two parts, tell and ask, separated by ‘&’. If the ‘&’ operator is not present, the whole guard is assumed to be of the ask type.

Declaratively, a rule relates heads and body provided the guard is true. A simplification rule means that the heads are true if and only if the body is true. A propagation rule means that the body is true if the heads are true. A simpagation rule combines a simplification and a propagation rule. The rule Heads1 \ Heads2 <=> Body is equivalent to the simplification rule Heads1, Heads2 <=> Heads1, Body. However, the simpagation rule is more compact to write, more efficient to execute and has better termination behavior than the corresponding simplification rule, since the constraints comprising Heads1 will not be removed and inserted again.

33.3.4 How CHR work

Each CHR constraint is associated with all rules in whose heads it occurs by the CHR compiler. Every time a CHR constraint is executed (called) or woken and reconsidered, it checks itself the applicability of its associated CHR by trying each CHR. By default, the rules are tried in textual order, i.e. in the order they occur in the defining file. To try a CHR, one of its heads is matched against the constraint. Matching succeeds if the constraint is an instance of the head. If a CHR has more than one head, the constraint store is searched for partner constraints that match the other heads. Heads are tried from left to right, except that in simpagation rules, the heads to be removed are tried before the head constraints to be kept (this is done for efficiency reasons). If the matching succeeds, the guard is executed. Otherwise the next rule is tried.

The guard either succeeds or fails. A guard succeeds if the execution of its Ask and Tell parts succeeds and in the ask part no variable that occurs also in the heads was touched or the cause of an instantiation error. The ask guard will fail otherwise. A variable is touched if it is unified with a term (including other variables from other constraints) different from itself. Tell guards, on the contrary, are trusted and not checked for that property. If the guard succeeds, the rule applies. Otherwise the next rule is tried.

If the firing CHR is a simplification rule, the matched constraints are removed from the store and the body of the CHR is executed. Similarly for a firing simpagation rule, except that the constraints that matched the heads preceding ‘\’ are kept. If the firing CHR is a propagation rule the body of the CHR is executed without removing any constraints.

It is remembered that the propagation rule fired, so it will not fire again with the same constraints if the constraint is woken and reconsidered. If the currently active constraint has not been removed, the next rule is tried.

If the current constraint has not been removed and all rules have been tried, it delays until a variable occurring in the constraint is touched. Delaying means that the constraint is inserted into the constraint store. When a constraint is woken, all its rules are tried again.

(This process can be watched and inspected with the CHR debugger, see below.)

Chapter 33: Constraint Handling Rules 379

33.3.5 Pragmas

Pragmas are annotations to rules and constraints that enable the compiler to generate more specific, more optimized code. A pragma can be a conjunction of the following terms: already_in_heads

The intention of simplification and simpagation rules is often to combine the heads into a stronger version of one of them. Depending on the strength of the guard, the new constraint may be identical to one of the heads to removed by the rule. This removal followed by addition is inefficient and may even cause termination problems. If the pragma is used, this situation is detected and the corresponding problems are avoided. The pragma applies to all constraints removed by the rule.

already_in_head(Id)

Shares the intention of the previous pragma, but affects only the constraint indicated via Id. Note that one can use more than one pragma per rule.

passive(Id)

No code will be generated for the specified constraint in the particular head position. This means that the constraint will not see the rule, it is passive in that rule. This changes the behavior of the CHR system, because normally, a rule can be entered starting from each head constraint. Usually this pragma will improve the efficiency of the constraint handler, but care has to be taken in order not to lose completeness.

For example, in the handler leq, any pair of constraints, say A leq B, B leq A, that matches the head X leq Y , Y leq X of the antisymmetry rule, will also match it when the constraints are exchanged, B leq A, A leq B. Therefore it is enough if a currently active constraint enters this rule in the first head only, the second head can be declared to be passive. Similarly for the idempotence rule. For this rule, it is more efficient to declare the first head passive, so that the currently active constraint will be removed when the rule fires (instead of removing the older constraint and redoing all the propagation with the currently active constraint). Note that the compiler itself detects the symmetry of the two head constraints in the simplification rule antisymmetry, thus it is automatically declared passive and the compiler outputs CHR eliminated code for head 2 in antisymmetry.

antisymmetry X leq Y , Y leq X # Id <=> X=Y pragma passive(Id).

idempotence X leq Y # Id \ X leq Y <=> true pragma passive(Id).

transitivity X leq Y # Id , Y leq Z ==> X leq Z pragma passive(Id).

Declaring the first head of rule transitivity passive changes the behavior of the handler. It will propagate less depending on the order in which the constraints arrive:

?- X leq Y, Y leq Z.

X leq Y,

Y leq Z,

X leq Z ?

380 SICStus Prolog

?- Y leq Z, X leq Y.

Y leq Z,

X leq Y ?

?- Y leq Z, X leq Y, Z leq X.

Y = X,

Z = X ?

The last query shows that the handler is still complete in the sense that all circular chains of leq-relations are collapsed into equalities.

33.3.6 Options

Options parametrise the rule compilation process. Thus they should precede the rule definitions. Example: option(check_guard_bindings, off).

The format below lists the names of the recognized options together with the acceptable values. The first entry in the lists is the default value.

option(debug_compile, [off,on]).

Instruments the generated code such that the execution of the rules may be traced (see

Section 33.4 [CHR Debugging], page 384 ).

option(check_guard_bindings, [on,off]).

Per default, for guards of type ask the CHR runtime system makes sure that no variables are touched or the cause of an instantiation error. These checks may be turned off with this option, i.e. all guards are treated as if they were of the tell variety. The option was kept for backward compatibility. Tell and ask guards offer better granularity.

option(already_in_store, [off,on]).

If this option is on, the CHR runtime system checks for the presence of an identical constraint upon the insertion into the store. If present, the attempted insertion has no effect. Since checking for duplicates for all constraints costs, duplicate removal specific to individual constraints, using a few simpagation rules of the following form instead, may be a better solution.

Constraint \ Constraint <=> true.

option(already_in_heads, [off,on]).

The intention of simplification and simpagation rules is often to combine the heads into a stronger version of one of them. Depending on the strength of the guard, the new constraint may be identical to one of the heads removed by the rule. This removal followed by addition is inefficient and may even cause termination problems. If the option is enabled, this situation is detected and the corresponding problems are avoided. This option applies to all constraints and is provided mainly for backward compatibility. Better grained control can be achieved with corresponding pragmas. (see

Section 33.3.5 [CHR Pragmas], page 379

).

Chapter 33: Constraint Handling Rules 381

The remaining options are meant for CHR implementors only: option(flatten, [on,off]).

option(rule_ordering, [canonical,heuristic]).

option(simpagation_scheme, [single,multi]).

option(revive_scheme, [new,old]).

option(dead_code_elimination, [on,off]).

33.3.7 Built-In Predicates

This table lists the predicates made available by the CHR library. They are meant for advanced users, who want to tailor the CHR system towards their specific needs.

current_handler(?Handler, ?Module)

Non-deterministically enumerates the defined handlers with the module they are defined in.

current_constraint(?Handler, ?Constraint)

Non-deterministically enumerates the defined constraints in the form Functor/Arity and the handlers they are defined in.

insert_constraint(+Constraint, -Id)

Inserts Constraint into the constraint store without executing any rules. The constraint will be woken and reconsidered when one of the variables in Constraint is touched. Id is unified with an internal object representing the constraint. This predicate only gets defined when a handler and constraints are declared (see

Section 33.3.2 [CHR Declarations], page 376 ).

insert_constraint(+Constraint, -Id, ?Term)

Inserts Constraint into the constraint store without executing any rules. The constraint will be woken and reconsidered when one of the variables in Term is touched. Id is unified with an internal object representing the constraint. This predicate only gets defined when a handler and constraints are declared (see

Section 33.3.2 [CHR Declarations], page 376

).

find_constraint(?Pattern, -Id)

Non-deterministically enumerates constraints from the constraint store that match Pattern, i.e. which are instances of Pattern. Id is unified with an internal object representing the constraint.

find_constraint(-Var, ?Pattern, -Id)

Non-deterministically enumerates constraints from the constraint store that delay on Var and match Pattern, i.e. which are instances of Pattern.

The identifier Id can be used to refer to the constraint later, e.g. for removal.

findall_constraints(?Pattern, ?List)

Unifies List with a list of Constraint # Id pairs from the constraint store that match Pattern.

382 SICStus Prolog findall_constraints(-Var, ?Pattern, ?List)

Unifies List with a list of Constraint # Id pairs from the constraint store that delay on Var and match Pattern.

remove_constraint(+Id)

Removes the constraint Id, obtained with one of the previous predicates, from the constraint store.

unconstrained(?Var)

Succeeds if no CHR constraint delays on Var. Defined as: unconstrained(X) :find_constraint(X, _, _), !, fail.

unconstrained(_).

notify_constrained(?Var)

Leads to the reconsideration of the constraints associated with Var. This mechanism allows solvers to communicate reductions on the set of possible values of variables prior to making bindings.

33.3.8 Consulting and Compiling Constraint Handlers

The CHR compilation process has been made as transparent as possible. The user deals with files containing CHR just as with files containing ordinary Prolog predicates. Thus

CHR may be consulted, compiled with various compilation modes, and compiled to file (see

Chapter 5 [Load Intro], page 59 ).

33.3.9 Compiler-generated Predicates

Besides predicates for the defined constraints, the CHR compiler generates some support predicates in the module containing the handler. To avoid naming conflicts, the following predicates must not be defined or referred to by user code in the same module: verify_attributes/3 attribute_goal/2 attach_increment/2

’attach_F/A’/2 for every defined constraint F/A.

’F/A_N_M_...’/Arity for every defined constraint F/A. N,M is are integers, Arity > A.

For the prime number example that is: attach_increment/2 attach_prime/1/2 attach_primes/1/2 attribute_goal/2 goal_expansion/3

Chapter 33: Constraint Handling Rules 383 prime/1 prime/1_1/2 prime/1_1_0/3 prime/1_2/2 primes/1 primes/1_1/2 verify_attributes/3

If an author of a handler wants to avoid naming conflicts with the code that uses the handler, it is easy to encapsulate the handler. The module declaration below puts the handler into module primes, which exports only selected predicates - the constraints in our example.

:- module(primes, [primes/1,prime/1]).

:- use_module(library(chr)).

handler eratosthenes.

constraints primes/1,prime/1.

...

33.3.10 Operator Declarations

This table lists the operators as used by the CHR library:

:- op(1200, xfx, @).

:- op(1190, xfx, pragma).

:- op(1180, xfx, [==>,<=>]).

:- op(1180, fy, chr_spy).

:- op(1180, fy, chr_nospy).

:- op(1150, fx, handler).

:- op(1150, fx, constraints).

:- op(1150, fx, rules).

:- op(1100, xfx, ’|’).

:- op(1100, xfx, \ ).

:- op(1050, xfx, &).

:- op( 500, yfx, #).

33.3.11 Exceptions

The CHR runtime system reports instantiation and type errors for the predicates: find_constraint/2 findall_constraints/3 insert_constraint/2 remove_constraint/1 notify_constrained/1

The only other CHR specific runtime error is:

384 SICStus Prolog

{CHR ERROR: registering <New>, module <Module> already hosts <Old>}

An attempt to load a second handler New into module <Module> already hosting handler <Old> was made.

The following exceptional conditions are detected by the CHR compiler:

{CHR Compiler ERROR: syntax rule <N>: <Term>}

If the N-th <Term> in the file being loaded violates the CHR syntax (see

Section 33.3.3 [CHR Syntax], page 377

).

{CHR Compiler ERROR: too many general heads in <Name>}

Unspecific heads in definitions like C \ C <=> true must not be combined with other heads in rule <Name>.

{CHR Compiler ERROR: bad pragma <Pragma> in <Name>}

The pragma <Pragma> used in rule <Name> does not qualify. Currently this only happens if <Pragma> is unbound.

{CHR Compiler ERROR: found head <F/A> in <Name>, expected one of: <F/A list>}

Rule <Name> has a head of given F/A which is not among the defined constraints.

{CHR Compiler ERROR: head identifiers in <Name> are not unique variables}

The identifiers to refer to individual constraints (heads) via ‘#’ in rule <Name> do not meet the indicated requirements.

{CHR Compiler ERROR: no handler defined}

CHR specific language elements, declarations or rules for example, are used before a handler was defined. This error is usually reported a couple of times, i.e. as often as there are CHR forms in the file expecting the missing definition.

{CHR Compiler ERROR: compilation failed}

Not your fault. Send us a bug report.

33.4 Debugging CHR Programs

Use option(debug_compile,on) preceding any rules in the file containing the handler to enable CHR debugging. The CHR debugging mechanism works by instrumenting the code generated by the CHR compiler. Basically, the CHR debugger works like the Prolog debugger. The main differences are: there are extra ports specific to CHR, and the CHR debugger provides no means for the user to change the flow of control, i.e. there are currently no retry and fail options available.

33.4.1 Control Flow Model

The entities reflected by the CHR debugger are constraints and rules. Constraints are treated like ordinary Prolog goals with the usual ports: [call,exit,redo,fail].

In addition, constraints may get inserted into or removed from the constraint store (ports: insert,remove), and stored constraints containing variables will be woken and reconsidered (port: wake) when variables are touched.

Chapter 33: Constraint Handling Rules 385

The execution of a constraint consists of trying to apply the rules mentioning the constraint in their heads. Two ports for rules reflect this process: At a try port the active constraint matches one of the heads of the rule, and matching constraints for the remaining heads of the rule, if any, have been found as well. The transition from a try port to an apply port takes place when the guard has been successfully evaluated, i.e. when the rule commits. At the apply port, the body of the rule is just about to be executed. The body is a Prolog goal transparent to the CHR debugger. If the rule body contains CHR constraints, the CHR debugger will track them again. If the rules were consulted, the Prolog debugger can be used to study the evaluations of the other predicates in the body.

33.4.2 CHR Debugging Predicates

The following predicates control the operation of the CHR debugger: chr_trace

Switches the CHR debugger on and ensures that the next time control enters a CHR port, a message will be produced and you will be asked to interact.

At this point you have a number of options. See

Section 33.4.5 [CHR Debugging

Options], page 388

. In particular, you can just type h cr i

(Return) to creep (or single-step) into your program. You will notice that the CHR debugger stops at many ports. If this is not what you want, the predicate chr_leash gives full control over the ports at which you are prompted.

chr_debug

Switches the CHR debugger on and ensures that the next time control enters a CHR port with a spypoint set, a message will be produced and you will be asked to interact.

chr_nodebug

Switches the CHR debugger off. If there are any spypoints set then they will be kept.

chr_notrace

Equivalent to chr_nodebug.

chr_debugging

Prints onto the standard error stream information about the current CHR debugging state. This will show:

1. Whether the CHR debugger is switched on.

2. What spypoints have been set (see below).

3. What mode of leashing is in force (see below).

chr_leash(+Mode)

The leashing mode is set to Mode. It determines the CHR ports at which you are to be prompted when you creep through your program. At unleashed ports a tracing message is still output, but program execution does not stop to allow user interaction. Note that the ports of spypoints are always leashed

(and cannot be unleashed). Mode is a list containing none, one or more of the following port names:

386 SICStus Prolog call exit redo fail wake try

Prompt when a constraint is executed for the first time.

Prompt when the constraint is successfully processed, i.e. the applicable rules have applied.

Prompt at subsequent exits generated by non-determinate rule bodies.

Prompt when a constraint fails.

Prompt when a constraint from the constraint store is woken and reconsidered because one of its variables has been touched.

Prompt just before the guard evaluation of a rule, after constraints matching the heads have been found.

apply insert remove

Prompt upon the application of a rule, after the successful guard evaluation, when the rule commits and fires, just before evaluating the body.

Prompt when a constraint gets inserted into the constraint store, i.e. after all rules have been tried.

Prompt when a constraint gets removed from the constraint store, e.g. when a simplification rule applies.

The initial value of the CHR leashing mode is [call,exit,fail,wake,apply].

Predefined shortcuts are: chr_leash(none), chr_leash(off)

To turn leashing off.

chr_leash(all)

To prompt at every port.

chr_leash(default)

Same as chr_leash([call,exit,fail,wake,apply]).

chr_leash(call)

No need to use a list if only a singular port is to be leashed.

33.4.3 CHR spypoints

For CHR programs of any size, it is clearly impractical to creep through the entire program.

Spypoints make it possible to stop the program upon an event of interest. Once there, one can set further spypoints in order to catch the control flow a bit further on, or one can start creeping.

Setting a spypoint on a constraint or a rule indicates that you wish to see all control flow through the various ports involved, except during skips. When control passes through any port with a spypoint set on it, a message is output and the user is asked to interact. Note that the current mode of leashing does not affect spypoints: user interaction is requested on every port.

Spypoints are set and removed by the following predicates, which are declared as prefix operators:

Chapter 33: Constraint Handling Rules 387 chr_spy Spec

Sets spypoints on constraints and rules given by Spec, which is is of the form:

(variable) denoting all constraints and rules, or: constraints Cs where Cs is one of

(variable) denoting all constraints

C,...,C denoting a list of constraints C

Name denoting all constraints with this functor, regardless of arity

Name/Arity denoting the constraint of that name and arity rules Rs where Rs is one of:

(variable) denoting all rules

R,...,R

Name denoting a list of rules R where Name is the name of a rule in any handler.

already in store

The name of a rule implicitly defined by the system when the option already_in_store is in effect.

already in heads

The name of a rule implicitly defined by the system when the option already_in_heads or the corresponding pragmas are in effect.

Handler:Name where Handler is the name of a constraint handler and

Name is the name of a rule in that handler

Examples:

| ?- chr_spy rules rule(3), transitivity, already_in_store.

| ?- chr_spy constraints prime/1.

If you set spypoints, the CHR debugger will be switched on.

chr_nospy Spec

Removes spypoints on constraints and rules given by Spec, where Spec is of the form as described for chr_spy Spec. There is no chr_nospyall/0. To remove all CHR spypoints use chr_nospy _.

The options available when you arrive at a spypoint are described later. See

Section 33.4.5

[CHR Debugging Options], page 388

.

388 SICStus Prolog

33.4.4 CHR Debugging Messages

All trace messages are output to the standard error stream.

This allows you to trace programs while they are performing file I/O. The basic format is as follows:

S 3

1 try eratosthenes:absorb(10) @ prime(9)#<c4>, prime(10)#<c2> ?

S is a spypoint indicator. It is printed as ‘ ’ if there is no spypoint, as ‘r’, indicating that there is a spypoint on this rule, or as ‘c’ if one of the involved constraints has a spypoint.

The first number indicates the current depth of the execution; i.e. the number of direct ancestors the currently active constraint has.

The second number indicates the head position of the currently active constraint at rule ports.

The next item tells you which port is currently traced.

A constraint or a matching rule are printed next. Constraints print as Term#Id, where Id is a unique identifier pointing into the constraint store. Rules are printed as Handler:Name

@, followed by the constraints matching the heads.

The final ‘?’ is the prompt indicating that you should type in one of the debug options (see

Section 33.4.5 [CHR Debugging Options], page 388 ).

33.4.5 CHR Debugging Options

This section describes the options available when the system prompts you after printing out a debugging message. Most of them you know from the standard Prolog debugger. All the options are one letter mnemonics, some of which can be optionally followed by a decimal integer. They are read from the standard input stream up to the end of the line (Return, h

< cr

> i

). Blanks will be ignored.

The only option which you really have to remember is ‘h’. This provides help in the form of the following list of available options.

CHR debugging options:

<cr> creep l leap s g

& n

+

-

< a

?

skip ancestors constraints c s <i> creep skip (ancestor i) nodebug spy this nospy this

& <i> constraints (details)

= debugging

.

show rule reset printdepth < <n> set printdepth abort b break help h help

Chapter 33: Constraint Handling Rules 389 c h

< cr

> i l s s i g creep causes the debugger to single-step to the very next port and print a message. Then if the port is leashed, the user is prompted for further interaction.

Otherwise, it continues creeping. If leashing is off, creep is the same as leap (see below) except that a complete trace is printed on the standard error stream.

leap causes the debugger to resume running your program, only stopping when a spypoint is reached (or when the program terminates). Leaping can thus be used to follow the execution at a higher level than exhaustive tracing.

skip over the entire execution of the constraint.

That is, you will not see anything until control comes back to this constraint (at either the exit port or the fail port). This includes ports with spypoints set; they will be masked out during the skip. The command can be used with a numeric argument to skip the execution up to and including the ancestor indicated by the argument.

Example:

...

4 - exit

Ancestors: prime(8)#<c6> ? g

1 1 apply eratosthenes:rule(2) @ primes(10)#<c1>

2 1 apply eratosthenes:rule(2) @ primes(9)#<c3>

3 1 apply eratosthenes:rule(2) @ primes(8)#<c5>

4 - call prime(8)#<c6>

4 - exit

2 - exit prime(8)#<c6> ? s 2 primes(9)#<c3> ?

print ancestors provides you with a list of ancestors to the currently active constraint, i.e. all constraints not yet exited that led to the current constraint in the derivation sequence. The format is the same as with trace messages.

Constraints start with call entries in the stack. The subsequent application of a rule replaces the call entry in the stack with an apply entry. Later the constraint shows again as redo or fail entry. Example:

0 - call

1 1 try primes(10)#<c1> ?

eratosthenes:rule(2) @ primes(10)#<c1> ? g

Ancestors:

1 - call primes(10)#<c1>

1 1 try eratosthenes:rule(2) @ primes(10)#<c1> ?

1 1 apply eratosthenes:rule(2) @ primes(10)#<c1> ?

1 - call prime(10)#<c2> ?

2 - insert prime(10)#<c2>

2 - exit prime(10)#<c2> ? g

Ancestors:

1 1 apply eratosthenes:rule(2) @ primes(10)#<c1>

2 - call prime(10)#<c2>

390 SICStus Prolog a b

& n

=

+

-

.

<

< n print constraints prints a list of the constraints in the constraint store. With a numeric argument, details relevant primarily to CHR implementors are shown.

nodebug switches the CHR debugger off.

debugging outputs information concerning the status of the CHR debugger as via chr_debugging/0 spy this sets a spypoint on the current constraint or rule.

nospy this removes the spypoint from the current constraint or rule, if it exists.

show rule prints the current rule instantiated by the matched constraints. Example:

8 1 apply era:absorb(8) @ prime(4)#<c14> \ prime(8)#<c6> ? .

absorb(8) @ prime(4)#<c14> \ prime(8)#<c6> <=>

8 mod 4=:=0

| true.

While in the debugger, a printdepth is in effect for limiting the subterm nesting level when printing rules and constraints. The limit is initially 10. This command, without arguments, resets the limit to 10. With an argument of n, the limit is set to n, treating 0 as infinity.

abort calls the built-in predicate abort/0.

break calls the built-in predicate break/0, thus putting you at a recursive toplevel. When you end the break (entering ^D) you will be re-prompted at the port at which you broke. The CHR debugger is temporarily switched off as you call the break and will be switched on again when you finish the break and go back to the old execution. Any changes to the CHR leashing or to spypoints during the break will remain in effect.

help displays the table of options given above.

?

h

33.5 Programming Hints

This section gives you some programming hints for CHR. For maximum efficiency of your constraint handler, see also the previous subsections on declarations and options.

Constraint handling rules for a given constraint system can often be derived from its definition in formalisms such as inference rules, rewrite rules, sequents, formulas expressing axioms and theorems. CHR can also be found by first considering special cases of each constraint and then looking at interactions of pairs of constraints sharing a variable. Cases that do not occur in the application can be ignored.

Chapter 33: Constraint Handling Rules 391

It is important to find the right granularity of the constraints. Assume one wants to express that n variables are different from each other. It is more efficient to have a single constraint all_different(List_of_n_Vars) than n*n inequality constraints between each pair of different variables. However, the extreme case of having a single constraint modeling the whole constraint store will usually be inefficient.

Starting from an executable specification, the rules can then be refined and adapted to the specifics of the application. Efficiency can be improved by weakening the guards to perform simplification as early as needed and by strengthening the guards to do the just right amount of propagation. Propagation rules can be expensive, because no constraints are removed.

The more heads a rule has, the more expensive it is. Rules with several heads are more efficient, if the heads of the rule share a variable (which is usually the case). Then the search for a partner constraint has to consider less candidates. In the current implementation, constraints are indexed by their functors, so that the search is only performed among the constraints containing the shared variable. Moreover, two rules with identical (or sufficiently similar) heads can be merged into one rule so that the search for a partner constraint is only performed once instead of twice.

As guards are tried frequently, they should be simple tests not involving side-effects. Head matching is more efficient than explicitly checking equalities in the ask-part of the guard.

In the tell part of a guard, it should be made sure that variables from the head are never touched (e.g. by using nonvar or ground if necessary). For efficiency and clarity reasons, one should also avoid using constraints in guards. Besides conjunctions, disjunctions are allowed in the guard, but they should be used with care. The use of other control built-in predicates in the guard is discouraged. Negation and if-then-else in the ask part of a guard can give wrong results, since e.g. failure of the negated goal may be due to touching its variables.

Several handlers can be used simultaneously if they do not share constraints with the same name. The implementation will not work correctly if the same constraint is defined in rules of different handlers that have been compiled separately. In such a case, the handlers must be merged by hand. This means that the source code has to be edited so that the rules for the shared constraint are together (in one module). Changes may be necessary (like strengthening guards) to avoid divergence or loops in the computation.

33.6 Constraint Handlers

The CHR library comes with plenty of constraint handlers written in CHR. The most recent versions of these are maintained at: http://www.pst.informatik.uni-muenchen.de/~fruehwir/chr-solver.html

‘arc.pl’ classical arc-consistency over finite domains

‘bool.pl’ simple Boolean constraints

‘cft.pl’ feature term constraints according to the CFT theory

392 SICStus Prolog

‘domain.pl’ finite domains over arbitrary ground terms and interval domains over integers and reals, but without arithmetic functions

‘gcd.pl’ elegant two-liner for the greatest common divisor

‘interval.pl’ straightforward interval domains over integers and reals, with arithmetic functions

‘kl-one.pl’ terminological reasoning similar to KL-ONE or feature trees

‘leq.pl’ standard introductory CHR example handler for less-than-or-equal

‘list.pl’ equality constraints over concatenations of lists (or strings)

‘listdom.pl’ a straightforward finite enumeration list domains over integers, similar to

‘interval.pl’

‘math-elim.pl’ solves linear polynomial equations and inequations using variable elimination, several variations possible

‘math-fougau.pl’ solves linear polynomial equations and inequations by combining variable elimination for equations with Fourier’s algorithm for inequations, several variations possible

‘math-fourier.pl’ a straightforward Fouriers algorithm to solve polynomial inequations over the real or rational numbers

‘math-gauss.pl’ a straightforward, elegant implementation of variable elimination for equations in one rule

‘minmax.pl’ simple less-than and less-than-or-equal ordering constraints together with minimum and maximum constraints

‘modelgenerator.pl’ example of how to use CHR for model generation in theorem proving

‘monkey.pl’ classical monkey and banana problem, illustrates how CHR can be used as a fairly efficient production rule system

‘osf.pl’ constraints over order sorted feature terms according to the OSF theory

‘oztype.pl’ rational trees with disequality and OZ type constraint with intersection

‘pathc.pl’ the most simple example of a handler for path consistency - two rules

Chapter 33: Constraint Handling Rules 393

‘primes.pl’ elegant implementations of the sieve of Eratosthenes reminiscent of the chemical abstract machine model, also illustrates use of CHR as a general purpose concurrent constraint language

‘scheduling.pl’ simple classical constraint logic programming scheduling example on building a house

‘tarski.pl’ most of Tarski’s axiomatization of geometry as constraint system

‘term.pl’ Prolog term manipulation built-in predicates functor/3, arg/3, =../2 as constraints

‘time-pc.pl’ grand generic handler for path-consistency over arbitrary constraints, load via

‘time.pl’ to get a powerful solver for temporal constraints based on Meiri’s unifying framework. ‘time-rnd.pl’ contains a generator for random test problems.

‘time-point.pl’ quantitative temporal constraints over time points using path-consistency

‘tree.pl’ equality and disequality over finite and infinite trees (terms)

‘type.pl’ equalities and type constraints over finite and infinite trees (terms)

You can consult or compile a constraint handler from the CHR library using e.g.:

?- [library(’chr/examples/gcd’)].

?- compile(library(’chr/examples/gcd’)).

If you want to learn more about the handlers, look at their documented source code.

In addition, there are files with example queries for some handlers, their file name starts with ‘examples-’ and the file extension indicates the handler, e.g. ‘.bool’: examples-adder.bool

examples-benchmark.math

examples-deussen.bool

examples-diaz.bool

examples-fourier.math

examples-holzbaur.math

examples-lim1.math

examples-lim2.math

examples-lim3.math

examples-puzzle.bool

examples-queens.bool

examples-queens.domain

examples-stuckey.math

examples-thom.math

394 SICStus Prolog

33.7 Backward Compatibility

In this section, we discuss backward compatibility with the CHR library of Eclipse Prolog.

1. The restriction on at most two heads in a rule has been abandoned. A rule can have as many heads as you like. Note however, that searching for partner constraints can be expensive.

2. By default, rules are compiled in textual order. This gives the programmer more control over the constraint handling process. In the Eclipse library of CHR, the compiler was optimizing the order of rules. Therefore, when porting a handler, rules may have to be reordered. A good heuristic is to prefer simplification to simpagation and propagation and to prefer rules with single heads to rules with several heads. Instead of manually rearranging an old handler one may also use the following combination of options to get the corresponding effect: option(rule_ordering,heuristic).

option(revive_scheme,old).

3. For backward compatibility, the already_in_store, already_in_head and guard_ bindings options are still around, but there are CHR syntax extensions:

Section 33.3.3

[CHR Syntax], page 377

and pragmas

Section 33.3.5 [CHR Pragmas], page 379

offering better grained control.

4. The Eclipse library of CHR provided automatic built-in labeling through the label_ with declaration. Since it was not widely used and can be easily simulated, builtin labeling was dropped. The same effect can be achieved by replacing the declaration label_with Constraint if Guard by the simplification rule chr_labeling,

Constraint <=> Guard | Constraint’, chr_labeling and by renaming the head in each clause Constraint :- Body into Constraint’ :- Body where Constraint’ is a new predicate.

Efficiency can be improved by declaring Constraint to be passive: chr_labeling, Constraint#Id <=> Guard | Constraint’, chr_labeling pragma passive(Id).

This translation will not work if option(already_in_ heads,on).

In that case use e.g. chr_labeling(_), Constraint <=> Guard |

Constraint’, chr_labeling(_) to make the new call to chr_labeling differ from the head occurrence.

5. The set of built-in predicates for advanced CHR users is now larger and better designed.

Also the debugger has been improved.

The Opium debugging environment is not available in SICStus Prolog.

Chapter 34: SICStus Objects 395

34 SICStus Objects

SICStus Objects is an extension to SICStus Prolog for flexible structuring, sharing and reuse of knowledge in large logic programming applications. It enhances Prolog with an expressive and efficient object-oriented programming component.

SICStus Objects is based on the notion of prototypes. In object-oriented programming a prototype is an object that represents a typical behavior of a certain concept. A prototype can be used as is or as a model to construct other objects that share some of the characteristics of the prototypical object. These specialized objects can themselves become prototypes used to construct other objects and so forth. The basic mechanism for sharing is by inheritance and delegation. Inheritance is known for most readers. By using the delegation mechanism an object can forward a message to another object to invoke a method defined by the recipient but interpreted in the context of the sender.

In SICStus Objects, an object is a named collection of predicate definitions. In this sense an object is similar to a Prolog module. The object system can be seen as an extension of SICStus Prolog’s module system. In addition an object may have attributes that are modifiable. Predicate definitions belonging to an object are called methods. So, an object is conceptually a named collection of methods and attributes. Some of the methods defined for an object need not be stored explicitly within the object, but are rather shared with other objects by the inheritance mechanism.

The Object system allows objects to be defined in a file, or dynamically created during the execution of a program. Objects defined in a file are integrated into SICStus Prolog in a way similar to definite clause grammars. That is to say, objects have a specific syntax as Prolog terms, and can be loaded and expanded into Prolog code. When an object is created, during load-time, or run-time, it inherits the methods and attributes of its prototypical object(s).

Objects defined in a file can be either static or dynamic. Also, methods can be either dynamic or static. these properties are inherited by sub-objects. Objects created during execution are dynamic.

The inheritance mechanism is implemented using the importation mechanism of the module system. The default inheritance is an inheritance by overriding mechanism, which means that if a method is defined locally, and the same method is defined in a super-object, then the clauses of the super-method are not part of the definition of the local one. As usual in Prolog, methods can be non-determinately defined, and alternative answers can be retrieved through backtracking. Using the delegation mechanism, other methods for knowledge sharing can be implemented by the user. In SICStus Objects, there is an initial prototypical proto-object called object, from which other objects may be constructed, directly or indirectly.

34.1 Getting Started

To load the SICStus Objects library, enter the query:

| ?- use_module(library(objects)).

396 SICStus Prolog

SICStus Objects defines some new infix and prefix operators, and redefines some of the built-in ones. The following operators become installed:

:- op(1200, xfy, [ & ]).

:- op(1198, xfx, [ :- ]).

:- op(1198, fx, [ :- ]).

:- op(550, xfx, [ ::, <: ]).

:- op(550, fx, [ ::, <: ]).

34.2 Declared Objects

Declared objects are created when the files defining them are loaded into the system.

34.2.1 Object Declaration

An object object-identifier is declared by writing it in the following form: object-identifier :: { sentence-1 & sentence-2 &

: sentence-n

}.

where object-identifier is a Prolog term that is either an atom or a compound term of the form functor(V1,...,Vn), where V1,...,Vn are distinct variables. The object body consists of a number of sentences, possibly none, surrounded by braces, where each sentence is either a method-directive, to be executed when the object is created, or a method-clause. A method is a number of method-clauses with the same principal functor. A method-clause has a clausal syntax similar to that of Prolog, but instead of usual predicate calls in the body of a clause there are method-calls. Ordinary Prolog goals are also allowed in a prefixed form, using ‘:’ as a prefix. A method-directive is a directive which contains method-calls.

All sentences are subject to term expansion (see

Section 7.1.2 [Definite], page 103

, built-in expand_term/2) before further processing, so in particular definite clause grammar syntax can be used in method-clauses. In addition, before expand_term/2, sentences are expanded by the predicate user:method_expansion/3.

method_expansion(+Term1,+ObjectIdentifier,?Term2) user:method_expansion(+Term1,+ObjectIdentifier,?Term2)

[Hook]

Defines transformations on methods similarly as user:term_expansion/[2,4].

At the end of an object definition, user:method_expansion/3 is called with end_of_object.

Chapter 34: SICStus Objects 397

34.2.2 Method Declarations

Method-clauses are declared similarly to Prolog clauses. Thus a method-clause can be either a unit-clause or a rule. We also allow a default catch-all method-clause as the last clause in an object body. The catch-all clause has as its head a Prolog variable, in order to match messages that are not previously defined or inherited in the object. It can be used to implement alternative inheritance mechanisms.

Goals in the body of a rule have the normal control structures of Prolog:

:P, :Q

:P; :Q

!

\+ :P

Conjunction

Disjunction

Cut

Negation

:P -> :Q

:P -> :Q; :R if(:P, :Q, :R)

If-then[-else]

?A = ?B

Unification

Atomic goals in the body of a method-clause may be one of the following:

:goal m:goal goal

::goal to call the Prolog predicate goal in the source module.

to call the Prolog predicate goal in module m.

to send the message goal to the object Self.

<:goal to send the message goal to a method that may be defined locally or inherited by the object.

to delegate the message goal to a method that may be defined locally or inherited by the object.

object::goal to send the message goal to object object.

object<:goal to delegate the message goal to object object.

Message sending and delegation will be explained later (see

Section 34.3 [Obj Self], page 399 ).

The following is a definition for the object list_object. It is constructed from three methods: append/3, member/2, and length/2. Note that the calls to append/3 and length/2 are to the local definition, whereas the member/2 call is to the predicate imported from the

Prolog library module lists.

list_object :: {

398 SICStus Prolog

:- :use_module(library(lists), [append/3,member/2]) & append([], L, L) & append([X|L1], L2, [X|L3]) :-

:: append(L1, L2, L3) & member(X, L) :-

:member(X,L) & length([], 0) & length([_|L], N) :-

:: length(L, N1),

:(N is N1+1)

}.

The following object apt_1 could be part of a larger database about free apartments in a real-estate agency: apt_1 :: { super(apartment) & street_name(’York’) & street_number(100) & wall_color(white) & floor_surface(wood)

}.

Another way to define apt_1 is by using attributes. These can be retrieved and modified efficiently by the methods get/1 and set/1 respectively.

apt_1 :: { super(apartment) & attributes([ street_name(’York’), street_number(100), wall_color(white), floor_surface(wood)])

}.

34.2.3 Generic Objects for Easy Reuse

Defining objects for easy reuse is a very important property for reducing the cost of large projects.

One important technique is to define prototypes in a parameterized way, so that various instantiations of a prototype correspond to different uses. Parameterized or generic objects have been used for this purpose in other object-oriented systems. An objectidentifier can be a compound term. The arguments of the term are parameters that are visible in the object-body. Here we show one example. Other examples and techniques that use this facility has been investigated extensively in [McCabe 92].

Chapter 34: SICStus Objects 399

The following is an object sort that sorts lists of different types. sort has a parameter that defines the type of the elements of the list. Notice that Type is visible to all methods in the body of sort, and is used in the method partition/4. In the query, we use sort(rat) to sort a list of terms denoting rational numbers. We must therefore define a rat object and its < method also: rat :: {

(P/Q < R/S) :- :(P*S < Q*R)

}.

sort(Type) :: {

:- :use_module(library(lists), [append/3]) & qsort([], []) & qsort([P|L], S) :partition(L, P, Small, Large), qsort(Small, S0), qsort(Large, S1),

:append(S0, [P|S1], S) & partition([], _P, [], []) & partition([X|L1], P, Small, Large) :-

( Type :: (X < P) ->

Small = [X|Small1], Large = Large1

; Small = Small1, Large = [X|Large1]

), partition(L1, P, Small1, Large1)

}.

| ?- sort(rat) :: qsort([23/3, 34/11, 45/17], L).

L = [45/17,34/11,23/3]

Parameterized objects are interesting in their own right in Prolog even if one is not interested in the object-oriented paradigm. They provide global context variables in a Prolog program without having to add such variables as additional context arguments to each clause that potentially uses the context.

34.3 Self, Message Sending, and Message Delegation

In SICStus Objects, each method is executed in the context of an object. This object may not be the static object where the method is declared. The current contextual object is used to determine dynamically which attributes are accessed, and which methods are called.

This leads to a mechanism known as dynamic binding. This object can be retrieved using the universal method self(S), where S will be bound to the current contextual object.

400 SICStus Prolog

When a message is sent to an object, the corresponding method will be executed in the context of the target object. A message delegated to an object will invoke a method that is executed in the context of the message-delegation operation.

object :: message

:: message

Message sending. Sends message to object, setting Self of the recipient to the recipient, i.e. object. If object is omitted, the recipient is the object in which the goal textually appears.

object <: message

<: message

Message delegation. Sends message to object, setting Self of the recipient to

Self of the sender. If object is omitted, the recipient is the object in which the goal textually appears. Delegation preserves Self .

The following objects physical_object, a, and b are written using the default notations for sending and delegation, hiding the contextual variable Self : physical_object :: { volume(50) & density(100) & weight(X) :volume(V), density(D),

:(X is V*D)

}.

a :: { volume(5) & density(10) &

Method :physical_object <: Method

}.

b :: { volume(5) & density(10) &

Method :physical_object :: Method

}.

Notice that the difference between the objects a and b is that a delegates any message except volume(_) and density(_) to physical_object while b sends the message to physical_ object. We may now ask

| ?- a :: weight(X), b :: weight(Y).

Chapter 34: SICStus Objects 401

X = 50

Y = 5000

To get hold of the current contextual object, the universal method self(S) is provided.

Another way to send a message to Self is to use the constant self. So the following two alternative definition of physical_object are equivalent to the previous one: physical_object :: { volume(50) & density(100) & weight(X) :self(S),

S::volume(V),

S::density(D),

:(X is V*D)

}.

physical_object :: { volume(50) & density(100) & weight(X) :self::volume(V), self::density(D),

:(X is V*D)

}.

34.4 Object Hierarchies, Inheritance, and Modules

The SICStus Objects system implements a default inheritance mechanism. By declaring within an object which objects are super-objects, the hierarchy of objects are maintained.

The system also maintains for each object its immediate sub-objects (i.e. immediate children).

Each object may also call Prolog predicates.

At the top of the hierarchy, the proto-object object provides various services for other objects. If object is not used at the top of the hierarchy many services will not be available for other objects (check what methods are available in object by sending the message method/1 to object).

34.4.1 Inheritance

Immediate super-objects are declared by defining the method super/2 within the object.

(Any definition super(Super) is transformed to super(Super,[])). The objects declared by super/2 are the immediate objects from which a method is inherited if not defined within the object. This implies that the inheritance mechanism is an overriding one. One could possibly have a union inheritance, whereby all clauses defining a method are collected from the super hierarchy and executed in a Prolog fashion. This can easily be programmed in SICStus Objects, using delegation to super objects.

The following example shows some objects used for animal classification.

402 SICStus Prolog animal :: {}.

bird :: { super(animal) & skin(feather) & habitat(tree) & motions(fly)

}.

penguin :: { super(bird) & habitat(land) & motions(walk) & motions(swim) & size(medium)

}.

| ?- penguin :: motions(M).

M = walk ;

M = swim ; no

| ?- penguin :: skin(S).

S = feather ; no

The following is an example of multiple inheritance: an object john is both a sportsman and a professor: john :: { super(sportsman) & super(professor) &

:

}.

Inheritance will give priority to the super-objects by the order defined in the super/2 method. Therefore in the above example John’s characteristics of being a sportsman will dominate those of being professor. Other kinds of hierarchy traversal can be programmed explicitly using the delegation mechanism.

34.4.2 Differential Inheritance

It is possible to be selective about what is inherited by using the method super/2. Its first argument is the super object, and its second is a list of the methods that will not be inherited from the super object.

Chapter 34: SICStus Objects 403

34.4.3 Use of Modules

In SICStus Objects, the visible predicates of the source module (context) for the object definition may be called in the body of a method. (The : prefix is used to distinguish such calls from method calls.) Any (: prefixed) directives occurring among the methodclauses are also executed in the same source module. For example, to import into the source module and call the public predicates of a module, the built-in predicate use_module/2 and its variants may be used: some_object :: {

:- :use_module(library(lists), [append/3]) & double_list(X, XX) :- :append(X,X,XX)

}.

34.4.4 Super and Sub

Two methods provided by the initial object object are super/1 and sub/1. (Note that any definition of super/1, except the one in object, is transformed to super/2). super/1 if sent to an object will return the immediate parents of the object. sub/1 will return the immediate children of the object if any. It is important to note that this service is provided only for objects that have object as their initial ancestor.

| ?- john :: super(S), S :: sub(john).

S = sportsman ;

S = professor ; no

The sub/1 property allows programs to traverse object hierarchies from a root object object down to the leaves.

34.4.5 The Keyword Super

To be able to send or delegate messages to the super-objects in a convenient way while following the inheritance protocol, the keyword super is provided. The calls: super :: method, or super <: method mean: send or delegate (respectively) method to the super-objects according to the inheritance protocol. A simple example illustrates this concept: assume that john in the above example has three id-cards, one stored in his sportsman prototype identifying the club he is member of, one stored in his professor prototype identifying the university he works in, and finally one stored locally identifying his social-security number. Given the following methods in the object john: m1(X) :super <: id_card(X) & m2(X) :super(S), S <: id_card(X) &

404 SICStus Prolog one may ask the following:

| ?- john :: m1(X).

% will follow the default inheritance and returns:

X = johns_club ;

| ?- john :: m2(X).

% will backtrack through the possible supers returning:

X = johns_club ;

X = johns_university ;

34.4.6 Semantic Links to Other Objects

Some object-oriented languages have syntactic constructs for redirecting the inheritance chain for certain methods to completely other objects which are not defined in the object’s inheritance hierarchy. This is not needed in SICStus Objects due to delegation. Assume that the method m/n is linked to object some_object, we just add a method for this: m(X1, ..., Xn) :- some_object <: m(X1, ..., Xn) &

34.4.7 Dynamically Declared Objects

When an object is declared and compiled into SICStus Objects, its methods cannot be changed during execution. Such an object is said to be static. To be able to update any method in an object, the object has to be declared dynamic. There is one exception, the inheritance hierarchy declared by super/[1,2] cannot be changed. By including the fact dynamic as part of the object body, the object becomes dynamic: dynamic_object :: { dynamic &

:

}.

34.4.8 Dynamic Methods

To be able to change a method with functor F and arity N in a static object, the method has to be declared dynamic by storing the following fact in the object: some_object :: { dynamic F/N &

:

}.

Each book in a library can be represented as an object, in which the name of the book is stored, the authors, and a borrowing history indicating when a book is borrowed and when it is returned. A history item may have the form history_item(Person,Status,Date) where Status is either borrowed or returned, and Date has the form YY-MM-DD, for YY year, MM month, DD day.

Chapter 34: SICStus Objects 405

A typical book book_12 could have the following status. Note that history_item/3 is dynamic: book_12 :: { super(book) & title(’The Art of Prolog’) & authors([’Leon Sterling’, ’Ehud Shapiro’]) & dynamic history_item/3 & history_item(’Dan Sahlin’, returned, 92-01-10) & history_item(’Dan Sahlin’, borrowed, 91-06-10) &

:

}.

Dynamic methods that are stored in an object can be updated, as in usual Prolog programs, by sending assert and retract messages directly to the object.

For example, to borrow a book the following method could be defined in the object book.

We assume that the top most history_item fact is the latest transaction, and there is an object date from which we can get the current date.

borrow(Person) :history_item(_Person0, Status, _Date0), !,

( Status = returned -> date::current(Date), asserta(history_item(Person, borrowed, Date))

; :display(’book not available’), :ttynl

) &

34.4.9 Inheritance of Dynamic Behavior

When an object is created, it will inherit from its parents their dynamic behavior. Methods that are declared dynamic in a parent, will be copied into the object, and its dynamic behavior preserved.

a:: { super(object) & dynamic p/1 & p(1) & p(2)

} b :: { super(a)

}

| ?- b::p(X).

406 SICStus Prolog

X = 1 ? ;

X = 2 ? ; no

| ?- b::asserta(p(3)).

yes

| ?- b::p(X).

X = 3 ? ;

X = 1 ? ;

X = 2 ? ; no

Notice that by redeclaring a method to be dynamic in a sub-object, amounts to redefining the method, and overriding of the parent definition will take effect.

c :: { super(a) & dynamic p/1

}

| ?- c::p(X).

no

34.5 Creating Objects Dynamically

As with dynamically declared objects, the full flexibility of SICStus Objects is achieved when objects are created at runtime. Anything, except the inheritance hierarchy, can be changed: methods can be added or deleted. The services for object creation, destruction, and method modification are defined in the proto-object object.

34.5.1 Object Creation

+SomeObject :: new(?NewObject)

NewObject is created with SomeObject as super. NewObject could be an atom, variable, or compound term whose arguments are distinct variables.

+SomeObject :: new(?NewObject,+Supers)

NewObject is created with Supers specifying the super objects (prototypes).

Supers is a list containing super specifications. A super specification is either an object identifier or a pair Object-NotInheritList where NotInheritList specifies methods not to inherit from Object. NewObject could be an atom, variable, or compound term whose arguments are distinct variables.

The object vehicle is created having the proto-object object as super, followed by creating moving_van with vehicle as super, followed by creating truck.

Chapter 34: SICStus Objects 407

| ?- object :: new(vehicle), vehicle :: new(moving_van), moving_van :: new(truck).

yes

| ?- truck :: super(X), vehicle :: sub(X).

X = moving_van ; no

34.5.2 Method Additions

+SomeObject :: asserta(+SomeMethod)

+SomeObject :: assertz(+SomeMethod)

+SomeObject :: assert(+SomeMethod)

Asserts SomeMethod in SomeObject with normal Prolog semantics.

Add some facts to vehicle and truck with initial value equal to [].

| ?- vehicle :: assert(fuel_level([])), vehicle :: assert(oil_level([])), vehicle :: assert(location([])), truck :: assert(capacity([])), truck :: assert(total_weight([])).

yes

34.5.3 Parameter Passing to New Objects

When new objects are created, it is possible to pass parameters. The following example shows:

• How general methods are asserted

In the previous examples one could pass parameters to an object as follows, using the method augment/1.

| ?- vehicle :: augment({ new_attrs(Instance, Attribute_list) :self :: new(Instance),

:: assign_list(Attribute_list, Instance) & assign_list([], Instance) & assign_list([Att|List], Instance) :-

:: assign(Att, Instance),

:: assign_list(List, Instance) & assign(P, Instance) :-

Instance :: assert(P)

408 SICStus Prolog

}).

yes

% create a new ’truck’

| ?- vehicle :: new_attrs(truck, [capacity([]),total_weight([])]).

yes

34.6 Access Driven Programming—Daemons

Access based programming is a paradigm where certain actions are performed, or some constraints are checked, when “access operations” are invoked. Access operations for updates (i.e. assert, retract) can be redefined in an object by redefining these operations and delegating the same operation to super. Notice that without a delegation mechanism this would not be possible, since the Self would have changed. So assume that we want to print on the screen “p is augmented” whenever the fact p(X) is asserted in an object foo, we just redefine assert/1: foo :: { super(object) & dynamic p/1 & p(0) & p(1) & assert(p(X)) :- !, /* assert/1 is redefined for p(X) */ super <: assert(p(X)),

:display(’p is augmented’), :ttynl & assert(M) :/* delegating assert(_) messages */ super <: assert(M) &

:

}.

34.7 Instances

Objects are relatively heavy weight. To be able to create efficiently light weight objects, we introduce the notion of instances. An instance is an object with restricted capability. It is created from an object that is considered its class. It gets a copy of the attributes of its class. These can be modified by get/1 and set/1. An instance cannot be a class for other instances. Instances are in general very efficient, both in space and access/modification time. The attribute ’$class’/1 will store the identity of the class of the instance including parameters.

34.8 Built-In Objects and Methods

Chapter 34: SICStus Objects 409

34.8.1 Universal Methods

The following methods are “universal”, i.e. they are defined locally, if appropriate, for every object: super(?Object,?NotInheritList)

Object is a parent (a super-object) of Self. NotInheritList specifies methods of Object explicitly not inherited by Self. The definition super(Object) is translated to super(Object,[]).

attributes(+Attributes)

Attributes is a list of compound terms specifying the local attributes of Self and the initial values.

34.8.2 Inlined Methods

The following methods are compiled inline i.e. calls are replaced by definitions. This implies

(in the current implementation) that they have a fixed semantics an can not be redefined.

There are also definitions for these methods in object covering the cases of unexpanded calls.

self(?Self )

Unifies Self with "self".

get(+Attribute)

Gets the attribute value(s) of the attribute specified by the head functor of

Attribute. The value(s) are unified with the argument(s) of Attribute.

set(+Attribute)

Sets the attribute value(s) of the attribute specified by the head functor of

Attribute. The value(s) are taken from the argument(s) of Attribute.

34.8.3 The Proto-Object "object"

The proto-object object provides basic methods that are available to all other objects by delegation: super(?Object)

Object is a parent (a super-object) of Self. Note that any other definition of super(Object) are translated to the universal method super/2.

sub(?Object)

Object is a child (a sub-object) of Self.

self(?Self )

Unifies Self with "self". NOTE: this method is inlined when possible.

410 SICStus Prolog object(?Object)

One of the defined objects in the system is Object.

dynamic

Self is a dynamic object.

static

Self is a static object.

dynamic ?Name/?Arity

Name/Arity is a dynamic method of Self.

static ?Name/?Arity

Name/Arity is a static method of Self.

new(?Object)

Creates a new dynamic Object. Self will be the prototype of Object. Object can be a compound term, an atom, or a variable. In the last case the method generates a unique name for Object.

+SomeObject :: new(?NewObject,+Supers)

NewObject is created with Supers specifying the super objects (prototypes).

Supers is a list containing super specifications. A super specification is either an object identifier or a pair Object-NotInheritList where NotInheritList specifies methods not to inherit from Object. NewObject could be an atom, variable, or compound term whose arguments are distinct variables.

instance(?Instance)

Creates a new instance Instance. Self will be the class of Instance. Instance can be a compound term, an atom, or a variable. In the last case the method generates a unique name for Instance.

has_instance(?Instance)

Self has the instance Instance.

has_attribute(?AttributeSpec)

Self has the attribute AttributeSpec, locally defined or inherited. AttributeSpec is on the format Name/Arity.

get(+Attribute)

Gets the attribute value(s) of the attribute specified by the head functor of

Attribute. The value(s) are unified with the argument(s) of Attribute. NOTE: this method is inlined when possible.

set(+Attribute)

Sets the attribute value(s) of the attribute specified by the head functor of

Attribute. The value(s) are taken from the argument(s) of Attribute. NOTE: this method is inlined when possible.

Chapter 34: SICStus Objects 411 assert(+Fact) assert(+Fact, -Ref ) asserta(+Fact) asserta(+Fact, -Ref ) assertz(+Fact) assertz(+Fact, -Ref )

Asserts a new Fact in Self. If Self is static, the name and arity of Fact must be declared as a dynamic method. asserta places Fact before any old facts.

The other forms place it after any old facts. A pointer to the asserted fact is returned in the optional argument Ref, and can be used by the Prolog built-in predicates erase/1 and instance/2.

retract(+Fact)

Retracts a Fact from Self. If Self is static, the name and arity of Fact must be declared as a dynamic method.

update(+Fact)

Replaces the first fact with the same name and arity as Fact in Self by Fact.

If Self is static, the name and arity of Fact must be declared as a dynamic method.

retractall(?Head)

Removes facts from Self that unify with Head. If Self is static, the name and arity of Fact must be declared as a dynamic method.

abolish

Abolishes Self if dynamic.

augment(?ObjectBody ) augmenta(?ObjectBody ) augmentz(?ObjectBody )

ObjectBody, having the form { sentence-1 & ... & sentence-n }, is added to

Self. augmenta places the new clauses before any old clauses. The other forms place it after any old clauses.

34.8.4 The built-in object "utility"

The base object utility provides methods that could be used in user programs. utility has object as its super-object.

subs(?Objects)

Gives a list of all the children of Self.

supers(?Objects)

Gives a list of all parents of Self.

objects(?Objects)

Gives a list of all objects.

dynamic_objects(?Objects)

Gives a list of all dynamic objects.

412 SICStus Prolog static_objects(?Objects)

Gives a list of all static objects.

methods(?Methods)

Gives a list of all the methods of Self.

dynamic_methods(?Methods)

Gives a list of all dynamic methods of Self.

static_methods(?Methods)

Gives a list of all static methods of Self.

descendant(?Object)

One of the descendants of Self is Object.

descendant(?Object, ?Level)

Object a descendant at depth Level of Self. A child of Self is at level 1.

descendants(?Objects)

The list of all descendants of Self is Objects.

descendants(?Objects, ?Level)

Objects is the list of descendants at depth Level of Self.

ancestor(?Object)

One of the ancestors of Self is Object.

ancestor(?Object, ?Level)

Object is an ancestor of Self at height Level. A super-object of Self has level

1.

ancestors(?Object)

The list of all ancestors of Self is Objects.

ancestors(?Object, ?Level)

Objects is the list of ancestors at height Level of Self.

restart

Removes all dynamic objects. Note that dynamic methods added to static objects are not removed.

and_cast(+Objects, ?Message)

Sends the same message Message to all objects in the list Objects.

or_cast(+Objects, ?Message)

Sends the same message Message to one of the objects in the list Objects, backtracking through the alternative objects.

34.9 Expansion to Prolog Code

As already mentioned, object definitions are expanded to Prolog clauses much as definite clause grammars. This expansion is usually transparent to the user. While debugging a SICStus Objects program, however, the expanded representation may become exposed.

This section will explain in detail the source expansion, so as to give the user the possibility

Chapter 34: SICStus Objects 413 to relate back to the source code during a debugging session. The inheritance mechanism, based on module importation, is also described.

First of all, every statically defined object will translate to several Prolog clauses belonging to a unique object module with the same identity as the object-identifier. Object modules are significantly cheaper to create than ordinary modules, as they do not import the built-in

Prolog predicates.

The module will contain predicates implementing an object declaration, the method code, imported methods and parameter transfer predicates. These predicates will be described in detail below, using the notational convention that variable names in italics are syntactic variables that will be replaced by something else in the translation process.

34.9.1 The Inheritance Mechanism

The inheritance mechanism is based on the importation mechanism of the Prolog module system. When an object is created, whether loaded from file or at runtime by new/[1,2], the method predicates (i.e. predicates implementing the methods) visible in the immediate supers are collected. After subtracting from this set the method predicates which are locally defined, and those that are specified in the don’t-inherit-list , the resulting set is made visible in the module of the inheriting object by means of importation. This implies that inherited methods are shared, expect dynamic methods.

Dynamic methods are inherited in a similar way with the big difference that they are not imported but copied. Even dynamic declarations (methods without clauses) are inherited.

Inheritance from dynamic objects differs in one aspect: Static predicates visible in a dynamic object are not imported directly from the dynamic object but from the static object from where it was imported to the dynamic object. This makes an inheriting object independent of any dynamic ancestor object after its creation.

34.9.2 Object Attributes

Attributes are based on an efficient term storage associated to modules. The attributes for an object is collected from its ancestors and itself at compile time and used for initialization at load time. The methods for accessing attributes, get/1 and set/1, are inlined to primitive calls whenever possible. They should hence not be redefined.

34.9.3 Object Instances

Instances are different from other objects in that they do not inherit. Instead they share the predicate name space with its class object. They do however have their own attributes.

At creation, an instance gets a copy of its class objects attributes. The reserved attribute

’$class’/1, which is present in any object, is used for an instance to hold its class object identifier. The purpose of this is mainly to store the parameters of the class object when the instance is created.

414 SICStus Prolog

34.9.4 The Object Declaration

The object declaration is only used by certain meta-programming operations. It consists of a fact

’$so_type’(Object, Type).

where Object is the object-identifier, and Type is either static or dynamic. If the type is static, the other generated predicates will be static, otherwise they will be dynamic.

34.9.5 The Method Code

Each method clause translates to a Prolog clause with two extra arguments: Self (a variable) and Myself. The latter argument is needed to cater for passing object parameters to the method body which is desribed further in next section.

The method body is translated to a Prolog-clause body as follows. The code is traversed, and the goals are transformed according to the following transformation patterns and rules.

In the transformation rules, the notation Msg(X,Y ) denotes the term produced by augmenting Msg by the two arguments X and Y :

Goal

:: Msg

<: Msg where Goal is a variable, is translated to objects:call_from_body(Goal,Self,Myself,Src) where Src is the source module. objects:call_from_body/4 will meta-interpret Goal at runtime.

is translated to Myself :Msg(Myself,Myself ) if Msg is a non variable. Otherwise, it is translated to objects:call_object(Myself, Msg, Myself ).

is translated to Myself :Msg(Self,Myself ) if Msg is a non variable. Otherwise, it is translated to objects:call_object(Myself, Msg, Self ).

super :: Msg is translated to objects:call_super_exp(Myself,Msg (Super,Myself ),Super) if Msg is a non variable. call_super_exp/3 searches the supers of Myself. Super is bound to the super object where the method is found. If Msg is a variable, the goal is translated to objects:call_super(Myself,Msg,Super,Super) which expands

Msg and performs otherwise the same actions as call_super_exp/3.

super <: Msg is translated to objects:call_super_exp(Myself,Msg(Self,Myself ),Super) if Msg is a non variable. call_super_exp/3 searches the supers of Myself. Super is bound to the super object where the method is found. If Msg is a variable, the goal is translated to objects:call_super(Myself,Msg,Self,Super) which expands Msg and performs otherwise the same actions as call_super_exp/3.

Obj :: Msg

* If Msg is non-variable, this is translated to Obj:Msg(Obj,Obj).

* Otherwise, it is translated to objects:call_object(Obj,Msg,Obj).

Obj <: Msg

* If Msg is non-variable, this is translated to Obj:Msg(Self,Obj).

Chapter 34: SICStus Objects 415

* Otherwise, if Msg is a non-variable, it is translated to functor(Obj,O, ),

O:Msg(Self,Obj).

* Otherwise, it is translated to objects:call_object(Obj,Msg,Self ).

self <: Msg self :: Msg

Msg are all translated like Self :: Msg.

Module:Goal is translated to Module:Goal.

:Goal is translated to Src:Goal where Src is the source module.

To illustrate the expansion, consider the object history_point directives, all executed in the history_point module:

:-objects:create_object(history_point,

[point-[]],

[attributes/3,display/3,move/4,new/4,print_history/3,super/4],

[],

[y(0),x(0),history([])], tree(history_point,[tree(point,[tree(object,[])])])).

history_point:super(point, [], _, history_point).

history_point:attributes([history([])], _, _).

history_point:display(A, B, _) :objects:call_super_exp(history_point, display(A,B,C), C), history_point:print_history(A, B, history_point).

history_point:’$so_type’(history_point, static).

history_point:move(A, B, C, _) :objects:call_super_exp(history_point, move(A,B,C,E), E), prolog:’$get_module_data’(C, history, D), prolog:’$set_module_data’(C, history, [(A,B)|D]).

history_point:print_history(A, B, _) :prolog:’$get_module_data’(B, history, C),

A:format(’with location history ~w~n’, [C], A, A).

history_point:new(A, xy(D,E), B, _) :objects:call_super_exp(history_point, new(A,xy(D,E),B,C), C), prolog:’$set_module_data’(A, history, [(D,E)]).

The directive create_object/6 creates the object, performs the inheritance by importation, and initializes attributes. The last argument is a tree representing the ancestor hierarchy during compilation. It is used to check that the load time and compile time environments are consistent.

416 SICStus Prolog

34.9.6 Parameter Transfer

As can be seen in the expanded methods above, the second additional argument is simply ignored if the object has no parameter. In contrast regard the following objects: ellipse(RX,RY,Color) :: { color(Color) & area(A) :-

:(A is RX*RY*3.14159265)

}.

circle(R,Color) :: { super(ellipse(R,R,Color))

}.

red_circle(R) :: { super(circle(R,red))

}.

... and their expansions: ellipse(_, _, _):’$so_type’(ellipse(_,_,_), static).

ellipse(_, _, _):area(A, _, B) :-

B:’$fix_param’(ellipse(C,D,_), B), user:(A is C*D*3.14159265).

ellipse(_, _, _):color(A, _, B) :-

B:’$fix_param’(ellipse(_,_,A), B).

ellipse(_, _, _):’$fix_param’(ellipse(B,C,D), A) :objects:object_class(ellipse(B,C,D), A).

circle(_, _):’$so_type’(circle(_,_), static).

circle(_, _):super(ellipse(A,A,B), [], _, circle(A,B)).

circle(_, _):’$fix_param’(circle(B,C), A) :objects:object_class(circle(B,C), A).

circle(_, _):’$fix_param’(ellipse(B,B,C), A) :objects:object_class(circle(B,C), A).

red_circle(_):’$so_type’(red_circle(_), static).

red_circle(_):super(circle(A,red), [], _, red_circle(A)).

red_circle(_):’$fix_param’(red_circle(B), A) :objects:object_class(red_circle(B), A).

red_circle(_):’$fix_param’(circle(B,red), A) :-

Chapter 34: SICStus Objects 417 objects:object_class(red_circle(B), A).

red_circle(_):’$fix_param’(ellipse(B,B,red), A) :objects:object_class(red_circle(B), A).

The second additional argument contains the receiver of a method call. If the method makes use of any parameter of the object where it is defined, it places a call to the reserved predicate $fix_param/2 in the module of the receiver. The purpose of this call is to bind the parameters used in the method to appropriate values given by the receiver. The receiver may be the object where the method is defined or any of its subs. In order to service these calls, a clause of $fix_param/2 is generated for each ancestor having parameters. Such a clause may be regarded as the collapsed chain of super/[1,2] definitions leading up to the ancestor.

The call objects:object_class(Class,Object) serves to pick up the ’$class’/1 attribute if Object is an instance, otherwise Class is unified with Object.

The following trace illustrates how parameters are transfered:

| ?- red_circle(2.5)::area(A).

1 1 Call: red_circle(2.5)::area(_A) ?

2 2 Call: ellipse(_,_,_):area(_A,red_circle(2.5),red_circle(2.5)) ?

3 3 Call: red_circle(_):$fix_param(ellipse(_B,_,_),red_circle(2.5)) ?

4 4 Call: objects:object_class(red_circle(_B),red_circle(2.5)) ?

4 4 Exit: objects:object_class(red_circle(2.5),red_circle(2.5)) ?

3 3 Exit: red_circle(_):$fix_param(ellipse(2.5,2.5,red),red_circle(2.5)) ?

5 3 Call: _A is 2.5*2.5*3.14159265 ?

5 3 Exit: 19.6349540625 is 2.5*2.5*3.14159265 ?

2 2 Exit: ellipse(_,_,_):area(19.6349540625,red_circle(2.5),red_circle(2.5)) ?

1 1 Exit: red_circle(2.5)::area(19.6349540625) ?

A = 19.6349540625 ?

34.10 Examples

34.10.1 Classification of Birds

This example illustrates how Prolog object can be used in classification of certain concepts.

This style is common in expert system application for describing its domain.

animal :: { super(object) & relative_size(S) :size(Obj_size), super(Obj_prototype),

Obj_prototype :: size(Prototype_size),

:(S is Obj_size/Prototype_size * 100)

}.

418 SICStus Prolog bird :: { super(animal) & moving_method(fly) & active_at(daylight)

}.

albatross :: { super(bird) & color(black_and_white) & size(115)

}.

kiwi :: { super(bird) & moving_method(walk) & active_at(night) & size(40) & color(brown)

}.

albert :: { super(albatross) & size(120)

}.

ross :: { super(albatross) & size(40)

}.

| ?- ross :: relative_size(R).

R = 34.78

34.10.2 Inheritance and Delegation

The following example illustrates a number of concepts. Firstly, how to use SICStus Objects for defining traditional classes a la Smalltalk, or other traditional object oriented languages.

Secondly, how to create instances of these classes. Finally, how to access instance variables.

The concept of instance variables is readily available as the variables belonging to the instances created dynamically and not to the class of the instances. For example, each instance of the class point will have two instance variables, x and y, represented by the attributes x/1 and y/1. The traditional class variables are easily available by accessing the same attributes in the associated class.

Another issue is the pattern used to create new instances. For example, to create an instance of the class history_point, the following code is used:

Chapter 34: SICStus Objects 419 new(Instance, xy(IX,IY)) :super <: new(Instance, xy(IX,IY)),

Instance :: set(history([(IX,IY)])) &

Note that the delegation of new/2 to super is necessary in order to create an object whose super is history_point and not point.

The example shows how delegation can be effective as a tool for flexible sharing of concepts in multiple inheritance. Four prototypes are defined: point, history_point, bounded_ point, and bh_point. The latter is a bounded history point.

An instance of the point class is a point that moves in 2-D space and that can be displayed.

An instance of the history_point class is similar to an instance of the point class but also keeps a history of all the moves made so far. An instance of bounded_point is similar to an instance of point but moves only in a region of the 2-D space. Finally an instance of bh_point inherits most of the features of a bounded_point and a history_point.

The default inheritance does not work for the methods display/1 and move/2 in bh_point.

Inheritance by delegating messages to both supers of bh_point results in redundant actions,

(moving and displaying the point twice). Selective delegation solves the problem. Taken from [Elshiewy 90].

point :: { super(object) & attributes([x(0),y(0)]) & xy(X, Y) :- get(x(X)), get(y(Y)) & new(Instance, xy(IX,IY)) :super <: instance(Instance),

Instance :: set(x(IX)),

Instance :: set(y(IY)) & location((X,Y)) :- <: xy(X,Y) & move_horizontal(X) :set(x(X)) & move_vertical(Y) :set(y(Y)) & move(X, Y) :-

<: move_horizontal(X),

<: move_vertical(Y) & display(Terminal) :-

<: xy(X, Y),

Terminal :: format(’point at (~d,~d)~n’,[X,Y])

420 SICStus Prolog

}.

history_point :: { super(point) & attributes([history([])]) & new(Instance, xy(IX,IY)) :super <: new(Instance, xy(IX,IY)),

Instance :: set(history([(IX,IY)])) & move(X, Y) :super <: move(X, Y), get(history(History)), set(history([(X,Y)|History])) & display(Terminal) :super <: display(Terminal),

<: print_history(Terminal) & print_history(Terminal) :get(history(History)),

Terminal :: format(’with location history ~w~n’,

[History])

}.

bounded_point :: { super(point) & attributes([bounds(0,0,0,0)]) & new(Instance, Coords, Bounds) :super <: new(Instance, Coords),

Instance :: set_bounds(Bounds) & set_bounds(Bounds) :set(Bounds) & move(X, Y) :-

<: bound_constraint(X, Y), !, super <: move(X, Y) & move(_, _) & bound_constraint(X, Y) :get(bounds(X0, X1, Y0, Y1)),

:(X >= X0),

:(X =< X1),

:(Y >= Y0),

:(Y =< Y1) &

Chapter 34: SICStus Objects 421 display(Terminal) :super <: display(Terminal),

<: print_bounds(Terminal) & print_bounds(Terminal) :get(bounds(X0, X1, Y0, Y1)),

Terminal :: format(’xbounds=(~d,~d), \c ybounds=(~d,~d)~n’,

[X0,X1,Y0,Y1])

}.

bh_point :: { super(history_point) & super(bounded_point) & new(Instance, Coords, Bounds) :history_point <: new(Instance, Coords),

Instance :: set_bounds(Bounds) & move(X, Y) :bounded_point <: bound_constraint(X, Y), !, history_point <: move(X, Y) & move(_, _) & display(Terminal) :bounded_point <: display(Terminal), history_point <: print_history(Terminal)

}.

tty :: { format(X, Y) :- :format(X, Y)

}.

point at (8,12) xbounds=(5,15), ybounds=(5,15) with location history [(8,12),(9,11)]

34.10.3 Prolog++ programs

Prolog++ is a product by LPA Associates for object-oriented programming extensions of LPA

Prolog. Most Prolog++ programs can be easily converted into SICStus Objects programs.

The following is a translation of a program for fault diagnosis in LPA’s Prolog++ manual, page 83. The program illustrates a top-down diagnosis method starting from general objects to more specific objects. The problem is fault diagnosis for car maintenance. The objects have the following structure:

- faults

- electrical

422 SICStus Prolog

|

|

|

|

|

|

- lights

- starting

- fuel_system

- mechanical

- starter_motor

- sparking

- plugs

- distributer

The general diagnosis method is defined in the object faults, whereas the cause-effect relationships are defined in the specific objects e.g. the object distributor.

This program heavily uses the sub/1 method. We have tried to be as close as possible to the original formulation.

faults :: { super(utility) & dynamic(told/2) &

/* no fault is the default */ fault(_, _) :- :fail & findall :-

<: restart,

:: sub(Sub),

Sub :: find(Where, Fault),

<: print(Where, Fault),

:fail & findall & print(Where, Fault) :-

:writeseqnl(’Location : ’, [Where]),

:writeseqnl(’Possible Fault : ’, [Fault]),

:nl & find(Where, Fault) :self(Where), fault(FaultNum, Fault),

\+ (effect(FaultNum, S), contrary(S, S1), exhibited(S1)

),

\+ (effect(FaultNum, SymptomNum),

\+ exhibited(SymptomNum)) & find(Where, Fault) :sub(Sub),

Chapter 34: SICStus Objects

Sub :: find(Where, Fault) & exhibited(S) :-

:: told(S, R), !,

R = yes & exhibited(S) :symptom(S,Text),

( :yesno([Text]) -> R = yes

; R = no

),

:: asserta(told(S,R)),

R = yes & restart :-

:: retractall(told(_,_))

}.

electrical :: { super(faults)

}.

fuel_system :: { super(faults)

}.

mechanical :: { super(faults)

}.

lights :: { super(electrical)

}.

sparking :: { super(electrical)

}.

starting :: { super(electrical)

}.

starter_motor :: { super(electrical)

}.

plugs :: { super(sparking)

}.

423

424 SICStus Prolog engine :: { super(mechanical)

}.

cylinders :: { super(engine)

}.

distributor :: { super(sparking) &

/* faults */ fault(’F1001’, ’Condensation in distributor cap’) & fault(’F1002’, ’Faulty distributor arm’) & fault(’F1003’, ’Worn distributor brushes’) &

/* symptoms */ symptom(’S1001’, ’Starter turns, but engine does not fire’) & symptom(’S1002’, ’Engine has difficulty starting’) & symptom(’S1003’, ’Engine cuts out shortly after starting’) & symptom(’S1004’, ’Engine cuts out at speed’) &

/* symptoms contrary to each other */ contrary(’S1002’, ’S1001’) & contrary(’S1003’, ’S1001’) &

/* causal-effect relationship */ effect(’F1001’, ’S1001’) & effect(’F1002’, ’S1001’) & effect(’F1002’, ’S1004’) & effect(’F1003’, ’S1002’) & effect(’F1003’, ’S1003’)

}.

yesno(Value) :- write(Value), nl, read(yes).

writeseqnl(Prompt, L) :- write(Prompt), write_seq(L).

write_seq([]).

write_seq([X|L]) :- write(X), write(’ ’), write_seq(L), nl.

faults :- faults :: findall.

| ?- faults.

[Starter turns, but engine does not fire]

Chapter 34: SICStus Objects

|: yes.

Location : distributor

Possible Fault : Condensation in distributor cap

[Engine cuts out at speed]

|: yes.

Location : distributor

Possible Fault : Faulty distributor arm yes

| ?- faults.

[Starter turns, but engine does not fire]

|: no.

[Engine has difficulty starting]

|: yes.

[Engine cuts out shortly after starting]

|: yes.

Location : distributor

Possible Fault : Worn distributor brushes

425

426 SICStus Prolog

Chapter 35: How to use the Tcl/Tk library

35 How to use the Tcl/Tk library

427

35.1 Introduction

This is a basic tutorial for those SICStus Prolog users who would like to add Tcl/Tk user interfaces to their Prolog applications. The tutorial assumes no prior knowledge of Tcl/Tk but, of course, does assume the reader is proficient in Prolog.

Aware that the reader may not have heard of Tcl/Tk, we will start by answering three questions: what is Tcl/Tk? what is it good for? what relationship does it have to Prolog?

35.1.1 What is Tcl/Tk?

Tcl/Tk, as its title suggests, is actually two software packages: Tcl and Tk. Tcl, pronounced tickle, stands for tool command language and is a scripting language that provides a programming environment and programming facilities such as variables, loops, and procedures.

It is designed to be easily extensible.

Tk, pronounced tee-kay, is just such an extension to Tcl which is a toolkit for windowing systems.

In other words, Tk adds facilities to Tcl for creating and manipulating user interfaces based on windows and widgets within those windows.

35.1.2 What is Tcl/Tk good for?

In combination the Tcl and Tk packages (we will call the combination simply Tcl/Tk) are useful for creating graphical user interfaces (GUIs) to applications. The GUI is described in terms of instances of Tk widgets, created through calls in Tcl, and Tcl scripts that form the glue that bind together the GUI and the application. (If you are a little lost at this point, all will be clear in a moment with a simple example.)

There are lots of systems out there for adding GUIs to applications so why choose Tcl/Tk?

Tcl/Tk has several advantages that make it attractive for this kind of work. Firstly, it is good for rapid prototyping of GUIs.

Tcl is an interpreted scripting language.

The scripts can be modified and executed quickly, with no compilation phase, so speeding up the development loop.

Secondly, it is easier to use a system based on a scripting language, such as Tcl/Tk, than many of the conventional packages available. For example, getting to grips with the X windows suite of C libraries is not an easy task. Tcl/Tk can produce the same thing using simple scripting with much less to learn. The penalty for this is that programs written in an interpreted scripting language will execute more slowly than those written using compiled

C library calls, but for many interfaces that do not need great speed Tcl/Tk is fast enough and its ease of use more than outweighs the loss of speed. In any case, Tcl/Tk can easily handle hundreds of events per mouse movement without the user noticing.

428 SICStus Prolog

Thirdly, Tcl/Tk is good for making cross-platform GUIs. The Tk toolkit has been ported to native look-and-feel widgets on Mac, PC (Windows), and UNIX (X windows) platforms.

You can write your scripts once and they will execute on any of these platforms.

Lastly, the software is distributed under a free software license and so is available in both binary and source formats free of charge.

35.1.3 What is Tcl/Tks relationship to SICStus Prolog?

SICStus Prolog comes with a Prolog library for interfacing to Tcl/Tk. The purpose of the library is to enable Prolog application developers to add GUIs to their applications rapidly and easily.

35.1.4 A quick example of Tcl/Tk in action

As a taster, we will show you two simple examples programs that use SICStus Prolog with the Tcl/Tk extensions: the ubiquitous “hello world” example; and a very simple telephone book look up example.

You are not expected to understand how these examples work at this stage. They are something for you to quickly type in to see how easy it is to add GUIs to Prolog programs through Tcl/Tk. After reading through the rest of this tutorial you will fully understand these examples and be able to write your own GUIs.

35.1.4.1 hello world

Here is the program; also in library(’tcltk/examples/ex1.pl’):

:- use_module(library(tcltk)).

go :tk_new([name(’Example 1’)], Interp), tcl_eval(Interp, ’button .fred -text "hello world" -command { puts "hello world"}’, _), tcl_eval(Interp, ’pack .fred’, _), tk_main_loop.

SICStus+Tcl/Tk hello world program.

To run it just start up SICStus, load the program, and evaluate the Prolog goal go. The first line of the go clause calls tk_new/2 which creates a Tcl/Tk interpreter and returns a handle Interp through which Prolog will interact with the interpreter. Next a call to tcl_eval/3 is made which creates a button displaying the ‘hello world’ text. Next a

Chapter 35: How to use the Tcl/Tk library 429 call is made to tcl_eval/3 that causes the button to be displayed in the main application window. Finally, a call is make to tk_main_loop/0 that passes control to Tcl/Tk, making sure that window events are serviced.

See how simple it is with just a three line Prolog program to create an application window and display a button in it. Click on the button and see what it does.

35.1.4.2 telephone book

The previous example showed us how to create a button and display some text in it. It was basically pure Tcl/Tk generated from within Prolog but did not have any interaction with Prolog. The following example demonstrates a simple callback mechanism. A name is typed into an text entry box, a button is pressed which looks up the telephone number corresponding to the name in a Prolog clause database, and the telephone number is then displayed.

Here is the code; also in library(’tcltk/examples/ex2.pl’):

:- use_module(library(tcltk)).

telephone(fred, ’123-456’).

telephone(wilbert, ’222-2222’).

telephone(taxi, ’200-0000’).

telephone(mary, ’00-36-1-666-6666’).

go :tk_new([name(’Example 2’)], T), tcl_eval(T, ’entry .name -textvariable name’,_), tcl_eval(T, ’button .search -text search -command { prolog telephone($name,X); set result $prolog_variables(X) }’, _), tcl_eval(T, ’label .result -relief raised -textvariable result’, _), tcl_eval(T, ’pack .name .search .result -side top -fill x’, _), tk_main_loop.

SICStus+Tcl/Tk telephone number lookup

Again, to run the example, start up SICStus Prolog, load the code, and run the goal go.

430 SICStus Prolog

You will notice that three widgets will appear in a window: one is for entering the name of the person or thing that you want to find the telephone number for, the button is for initiating the search, and the text box at the bottom is for displaying the result.

Type ‘fred’ into the entry box, hit the search button and you should see the phone number displayed. You can then try the same thing but with ‘wilbert’, ‘taxi’ or ‘mary’ typed into the text entry box.

What is happening is that when the button is pressed, the value in the entry box is retrieved, then the telephone/2 predicate is called in Prolog with the entry box value as first argument, then the second argument of telephone is retrieved (by this time bound to the number) and is displayed below the button.

This is a very crude example of what can be done with the Tcl/Tk module in Prolog.

For example, this program does not handle cases where there is no corresponding phone number or where there is more than one corresponding phone number. The example is just supposed to whet your appetite but all these problems can be handled by Prolog +

Tcl/Tk, although with a more sophisticated program. You will learn how to do this in the subsequent chapters.

35.1.5 Outline of this tutorial

Now we have motivated using Tcl/Tk as a means of creating GUIs for Prolog programs, this document goes into the details of using Tcl/Tk as a means of building GUIs for SICStus

Prolog applications.

Firstly, Tcl is introduced and its syntax and core commands described.

Then the Tk extensions to Tcl are introduced. We show how with Tcl and Tk together the user can build sophisticated GUIs easily and quickly. At the end of this Tcl/Tk part of the tutorial an example of a pure Tcl/Tk program will be presented together with some tips on how to design and code Tcl/Tk GUIs.

The second phase of this document describes the SICStus Prolog tcltk library. It provides extensions to Prolog that allow Prolog applications to interact with Tcl/Tk: Prolog can make calls to Tcl/Tk code and vice versa.

Having reached this point in the tutorial the user will know how to write a Tcl/Tk GUI interface and how to get a Prolog program to interact with it, but arranging which process

(the Prolog process or the Tcl/Tk process) is the dominant partner is non-trivial and so is described in a separate chapter on event handling. This will help the user choose the most appropriate method of cooperation between Tcl/Tk and Prolog to suit their particular application.

This section, the Tcl/Tk+Prolog section, will be rounded off with the presentation of some example applications that make use of Tcl/Tk and Prolog.

Chapter 35: How to use the Tcl/Tk library 431

Then there is a short discussion section on how to use other Tcl extension packages with

Tcl/Tk and Prolog. Many such extension packages have been written and when added to

Prolog enhanced with Tcl/Tk can offer further functionality to a Prolog application.

The appendices provide a full listing with description of the predicates available in the tcltk

SICStus Prolog library, and the extensions made to Tcl/Tk for interacting with Prolog.

Lastly, a section on resources gives pointers to where the reader can find more information on Tcl/Tk.

35.2 Tcl

Tcl is an interpreted scripting language. In this chapter, first the syntax of Tcl is described and then the core commands are described. It is not intended to give a comprehensive description of the Tcl language here but an overview of the core commands, enough to get the user motivated to start writing their own scripts.

For pointers to more information on Tcl; see

Section 35.7 [Resources], page 518 .

35.2.1 Syntax

A Tcl script consists of a series of strings separated from each other by a newline character.

Each string contains a command or series of semi-colon separated commands. A command is a series of words separated by spaces. The first word in a command is the name of the command and subsequent words are its arguments.

An example is: set a 1 set b 2 which is a Tcl script of two commands: the first command sets the value of variable a to 1, and the second command sets the value of variable b to 2.

An example of two commands on the same line separated by a semi-colon is: set a 1; set b 2 which is equivalent to the previous example but written entirely on one line.

A command is executed in two phases. In the first phase, the command is broken down into its constituent words and various textual substitutions are performed on those words.

In the second phase, the procedure to call is identified from the first word in the command, and the procedure is called with the remaining words as arguments.

There are special syntactic characters that control how the first phase, the substitution phase, is carried out. The three major substitution types are variable substitution, command substitution, and backslash substitution.

432 SICStus Prolog

35.2.1.1 Variable substitution

Variable substitution happens when a ‘$’ prefixed word is found in a command. There are three types of variable substitution:

− $name

− where name is a scalar variable. name is simply substituted in the word for its value. name can contain only letters, digits, or underscores.

− $name(index)

− where name is the name of an array variable and index is the index into it. This is substituted by the value of the array element. name must contain only letters, digits, or underscores. index has variable, command, and backslash substitution performed on it too.

− ${name}

− where name can have any characters in it except closing curly bracket. This is more or less the same as $name substitution except it is used to get around the restrictions in the characters that can form name.

An example of variable substitution is: set a 1 set b $a which sets the value of variable a to 1, and then sets the value of variable b to the value of variable a.

35.2.1.2 Command substitution

Command substitution happens when a word contains an open square bracket, [. The string between the open bracket and matching closing bracket are treated as a Tcl script. The script is evaluated and its result is substituted in place of the original command substitution word.

A simple example of command substitution is: set a 1 set b [set a] which does the same as the previous example but using command substitution. The result of a set a command is to return the value of a which is then passed as an argument to set b and so variable b acquires the value of variable a.

35.2.1.3 Backslash substitution

Backslash substitution is performed whenever the interpreter comes across a backslash.

The backslash is an escape character and when it is encountered is causes the interpreter to handle the next characters specially. Commonly escaped characters are ‘\a’ for audible

Chapter 35: How to use the Tcl/Tk library 433 bell, ‘\b’ for backspace, ‘\f’ for form feed, ‘\n’ for newline, ‘\r’ for carriage return, ‘\t’ for horizontal tab, and ‘\v’ for vertical tab. Double-backslash, ‘\\’, is substituted with a single backslash. Other special backslash substitutions have the following forms:

• \ooo

− the digits ooo give the octal value of the escaped character

• \xHH

− the x denotes that the following hexadecimal digits are the value of the escaped character

Any other character that is backslash escaped is simply substituted by the character itself.

For example, \W is replaced by W.

35.2.1.4 Delaying substitution

A further syntactic construction is used to delay substitution. When the beginning of a word starts with a curly bracket, {, it does not do any of the above substitutions between the opening curly bracket and its matching closing curly bracket. The word ends with the matching closing curly bracket. This construct is used to make the bodies of procedures in which substitutions happen when the procedure is called, not when it is constructed. Or it is used anywhere when the programmer does not want the normal substitutions to happen.

For example: puts {I have $20} will print the string ‘I have $20’ and will not try variable substitution on the ‘$20’ part.

A word delineated by curly brackets is replaced with the characters within the brackets without performing the usual substitutions.

35.2.1.5 Double-quotes

A word can begin with a double-quote and ending with the matching closing double-quote.

Substitutions as detailed above are done on the characters between the quotes, and the result is then substituted for the original word. Typically double-quotes are used to group sequences of characters that contain spaces into a single command word.

For example: set name "Fred the Great" puts "Hello my name is $name" outputs ‘Hello my name is Fred the Great’. The first command sets the value of variable name to the following double-quoted string "Fred the Great".

The the next command prints its argument, a single argument because it is a word delineated by double-quotes, that has had variable substitution performed on it.

Here is the same example but using curly brackets instead of double-quotes:

434 SICStus Prolog set name {Fred the Great} puts {Hello my name is $name} gives the output ‘Hello my name is $name’ because substitutions are suppressed by the curly bracket notation.

And again the same example but without either curly brackets or double-quotes: set name Fred the Great puts Hello my name is $name simply fails because both set and puts expect a single argument but without the word grouping effects of double-quotes or curly brackets they find that they have more than one argument and throw an exception.

Being a simple scripting language, Tcl does not have any real idea of data types. The interpreter simply manipulates strings. The Tcl interpreter is not concerned with whether those strings contain representations of numbers or names or lists. It is up to the commands themselves to interpret the strings that are passed to them as arguments in any manner those choose.

35.2.2 Variables

This has been dealt with implicitly above. A variable has a name and a value. A name can be any string whatsoever, as can its value.

For example, set "Old King Cole" "merry soul" sets the value of the variable named Old King Cole to the value merry soul. Variable names can also be numbers: set 123 "one two three" sets the variable with name 123 to the value one two three. In general, it is better to use the usual conventions — start with a letter then follow with a combination of letters, digits, and underscores — when giving variables names to avoid confusion.

Array variables are also available in Tcl. These are denoted by an array name followed by an array index enclosed in round brackets. As an example: set fred(one) 1 set fred(two) 2 will set the variable fred(one) to the value 1 and fred(two) to the value 2.

Tcl arrays are associative arrays in that both the array name and the array index can be arbitrary strings. This also makes multidimensional arrays possible if the index contains a comma:

Chapter 35: How to use the Tcl/Tk library 435 set fred(one,two) 12

It is cheating in that the array is not stored as a multidimensional array with a pair of indices, but as a linear array with a single index that happens to contain a comma.

35.2.3 Commands

Now that the Tcl syntax and variables have been been dealt with, we will now look at some of the commands that are available.

Each command when executed returns a value. The return value will be described along with the command.

35.2.3.1 Notation

A quick word about the notation used to describe Tcl commands. In general, a description of a command is the name of the command followed by its arguments separated by spaces.

An example is: set varName ?value?

which is a description of the Tcl set command, which takes a variable name varName and an optional argument, a value.

Optional arguments are enclosed in question mark, ?, pairs, as in the example.

A series of three dots ... represents repeated arguments. An example is a description of the unset command: unset varName ? varName varName ...?

which shows that the unset command has at least one compulsory argument varName but has any number of subsequent optional arguments.

35.2.3.2 Commands to do with variables

The most used command over variables is the set command. It has the form set varName ?value?

The value of value is determined, the variable varName is set to it, and the value is returned.

If there is no value argument then simply the value of the variable is returned. It is thus used to set and/or get the value of a variable.

The unset command is used to remove variables completely from the system: unset varName ? varName varName ...?

which given a series of variable names deletes them. The empty string is always returned.

436 SICStus Prolog

There is a special command for incrementing the value of a variable: incr varName ? increment?

which, given the name of a variable thats value is an integer string, increments it by the amount increment. If the increment part is left out then it defaults to 1. The return value is the new value of the variable.

35.2.3.3 Expressions

Expressions are constructed from operands and operators and can then be evaluated. The most general expression evaluator in Tcl is the expr command: expr arg ? arg arg ... arg ?

which evaluates its arguments as an expression and returns the value of the evaluation.

A simple example expression is expr 2 * 2 which when executed returns the value 4.

There are different classes of operators: arithmetic, relational, logical, bitwise, and choice.

Here are some example expressions involving various operators: arithmetic

$x * 2 relational

$x > 2 logical

($x == $y) || ($x == $z) bitwise

8 & 2 choice

($a == 1) ? $x : $y

Basically the operators follow the syntax and meaning of their ANSI C counterparts.

Expressions to the expr command can be contained in curly brackets in which case the usual substitutions are not done before the expr command is evaluated, but the command does its own round of substitutions. So evaluating a script such as: set a 1 expr { ($a==1) : "yes" ? "no" } will evaluate to yes.

Tcl also has a whole host of math functions that can be used in expressions. Their evaluation is again the same as that for their ANSI C counterparts. For example: expr { 2*log($x) } will return 2 times the natural log of the value of variable x.

Chapter 35: How to use the Tcl/Tk library 437

35.2.3.4 Lists

Tcl has a notion of lists, but as with everything it is implemented through strings. A list is a string that contains words.

A simple list is just a space separated series of strings: set a {one two three four five} will set the variable a to the list containing the five strings shown. The empty list is denoted by an open and close curly bracket pair with nothing in between: {}.

For the Prolog programmer, there is much confusion between a Prolog implementation of lists and the Tcl implementation of lists. In Prolog we have a definite notion of the printed representation of a list: a list is a sequence of terms enclosed in square brackets (we ignore dot notation for now); a nested list is just another term.

In Tcl, however, a list is really just a string that conforms to a certain syntax: a string of space separated words. But in Tcl there is more than one way of generating such a string.

For example, set fred {a b c d} sets fred to

"a b c d" as does set fred "a b c d" because {a b c d} evaluates to the string a b c d which has the correct syntax for a list. But what about nested lists? Those are represented in the final list-string as being contained in curly brackets. For example: set fred {a b c {1 2 3} e f} results in fred having the value

"a b c {1 2 3} e f"

The outer curly brackets from the set command have disappeared which causes confusion.

The curly brackets within a list denote a nested list, but there are no curly brackets at the top-level of the list. (We can’t help thinking that life would have been easier if the creators of Tcl would have chosen a consistent representation for lists, as Prolog and LISP do.)

So remember: a list is really a string with a certain syntax, space separated items or words; a nested list is surrounded by curly brackets.

There are a dozen commands that operate on lists.

438 SICStus Prolog concat ? list list ...?

This makes a list out of a series of lists by concatenating its argument lists together. The return result is the list resulting from the concatenation.

lindex list index returns the index-th element of the list. The first element of a list has an index of 0.

linsert list index value ? value ...?

returns a new list in which the value arguments have been inserted in turn before the index-th element of list.

list ? value value ...?

returns a list where each element is one of the value arguments.

llength list returns the number of elements in list list.

lrange list first last returns a slice of a list consisting of the elements of the list list from index first until index last.

lreplace list first last ? value ... value?

returns a copy of list list but with the elements between indices first and last replaced with a list formed from the value arguments.

lsearch ?-exact? ?-glob? ?-regexp? list pattern returns the index of the first element in the list that matches the given pattern. The type of matching done depends on which of the switch is present -exact, -glob, -regexp, is present. Default is -glob.

lsort ?-ascii? ?-integer? ?-real? ?-command command? ?-increasing? ?-decreasing{? list returns a list which is the original list list sorted by the chosen technique. If none of the switches supplies the intended sorting technique then the user can provide one through the

-command command switch.

There are also two useful commands for converting between lists and strings: join list ? joinString?

which concatenates the elements of the list together, with the separator joinString between them, and returns the resulting string. This can be used to construct filenames; for example:

Chapter 35: How to use the Tcl/Tk library 439 set a {{} usr local bin} set filename [join $a /] results in the variable filename having the value /usr/local/bin.

The reverse of the join command is the split command: split string ? splitChars?

which takes the string string and splits it into string on splitChars boundaries and returns a list with the strings as elements. An example is splitting a filename into its constituent parts: set a [split /usr/local/src /] gives a the value {{} usr local src}, a list.

35.2.3.5 Control flow

Tcl has the four usual classes of control flow found in most other programming languages: if...elseif...else, while, for, foreach, switch, and eval.

We go through each in turn.

The general form of an if command is the following: if test1 body1 ? elseif test2 body2 elseif ...? ? else bodyn?

which when evaluated, evaluates expression test1 which if true causes body1 to be evaluated, but if false, causes test2 to be evaluated, and so on. If there is a final else clause then its bodyn part is evaluated if all of the preceding tests failed. The return result of an if statement is the result of the last body command evaluated, or the empty list if none of the bodies are evaluated.

Conditional looping is done through the while command: while test body which evaluates expression test which if true then evaluates body. It continues to do that until test evaluates to 0, and returns the empty string.

A simple example is: set a 10 while {$a > 0} { puts $a; incr a -1 } which initializes variable a with value ten and then loops printing out the value of a and decrementing it until its value is 0, when the loop terminates.

The for loop has the following form:

440 SICStus Prolog for init test reinit body which initializes the loop by executing init, then each time around the loop the expression test is evaluated which if true causes body to be executed and then executes reinit. The loop spins around until test evaluates to 0. The return result of a for loop is the empty string.

An example of a for loop: for {set a 10} ($a>0) {incr a -1} {puts $a} which initializes the variable a with value 10, then goes around the loop printing the value of a and decrementing it as long as its value is greater than 0. Once it reaches 0 the loop terminates.

The foreach command has the following form: foreach varName list body where varName is the name of a variable, list is an instance of a list, and body is a series of commands to evaluate. A foreach then iterates over the elements of a list, setting the variable varName to the current element, and executes body. The result of a foreach loop is always the empty string.

An example of a foreach loop: foreach friend {joe mary john wilbert} {puts "I like $friend"} will produce the output:

I like joe

I like mary

I like john

I like wilbert

There are also a couple of commands for controlling the flow of loops: continue and break.

continue stops the current evaluation of the body of a loop and goes on to the next one.

break terminates the loop altogether.

Tcl has a general switch statement which has two forms: switch ? options? string pattern body ? pattern body ... ?

switch ? options? string { pattern body ? pattern body ...? }

When executed, the switch command matches its string argument against each of the pattern arguments, and the body of the first matching pattern is evaluated. The matching algorithm depends on the options chosen which can be one of

-exact use exact matching

Chapter 35: How to use the Tcl/Tk library 441

-glob use glob-style matching

-regexp use regular expression matchinig

An example is: set a rob switch -glob $a { a*z { puts "A to Z"} r*b { puts "rob or rab"}

} which will produce the output: rob or rab

There are two forms of the switch command. The second form has the command arguments surrounded in curly brackets. This is primarily so that multi-line switch commands can be formed, but it also means that the arguments in brackets are not evaluated (curly brackets suppress evaluation), whereas in the first type of switch statement the arguments are first evaluated before the switch is evaluated. These effects should be borne in mind when choosing which kind of switch statement to use.

The final form of control statement is eval: eval arg ? arg ...?

which takes one or more arguments, concatenates them into a string, and executes the string as a command. The return result is the normal return result of the execution of the string as a command.

An example is set a b set b 0 eval set $a 10 which results in the variable b being set to 10. In this case, the return result of the eval is

10, the result of executing the string "set b 10" as a command.

35.2.3.6 Commands over strings

Tcl has several commands over strings. There are commands for searching for patterns in strings, formatting and parsing strings (much the same as printf and scanf in the C language), and general string manipulation commands.

Firstly we will deal with formatting and parsing of strings. The commands for this are format and scan respectively.

format formatString ? value value ...?

442 SICStus Prolog which works in a similar to C’s printf; given a format string with placeholders for values and a series of values, return the appropriate string.

Here is an example of printing out a table for base 10 logarithms for the numbers 1 to 10: for {set n 1} {$n <= 10} {incr n} { puts [format "log10(%d) = %.4f" $n [expr log10($n)]]

} which produces the output ln(1) = 0.0000

ln(2) = 0.3010

ln(3) = 0.4771

ln(4) = 0.6021

ln(5) = 0.6990

ln(6) = 0.7782

ln(7) = 0.8451

ln(8) = 0.9031

ln(9) = 0.9542

ln(10) = 1.0000

The reverse function of format is scan: scan string formatString varName ? varName ...?

which parses the string according to the format string and assigns the appropriate values to the variables. it returns the number of fields successfully parsed.

An example, scan "qty 10, unit cost 1.5, total 15.0" \

"qty %d, unit cost %f, total %f" \ quantity cost_per_unit total would assign the value 10 to the variable quantity, 1.5 to the variable cost_per_unit and the value 15.0 to the variable total.

There are commands for performing two kinds of pattern matching on strings: one for matching using regular expressions, and one for matching using UNIX-style wildcard pattern matching (globbing).

The command for regular expressions matching is as follows: regexp ?-indices? ?-nocase? exp string ? matchVar? ?subVar subVar ...?

where exp is the regular expression and string is the string on which the matching is performed. The regexp command returns 1 if the expression matches the string, 0 otherwise.

The optional -nocase switch does matching without regard to the case of letters in the string. The optional matchVar and subVar variables, if present, are set to the values of

Chapter 35: How to use the Tcl/Tk library 443 string matches. In the regular expression, a match that is to be saved into a variable is enclosed in round braces. An example is regexp {([0-9]+)} "I have 3 oranges" a will assign the value 3 to the variable a.

If the optional switch -indices is present then instead of storing the matching substrings in the variables, the indices of the substrings are stored; that is a list with a pair of numbers denoting the start and end position of the substring in the string. Using the same example: regexp -indices {([0-9]+)} "I have 3 oranges" a will assign the value "7 7", because the matched numeral 3 is in the eighth position in the string, and indices count from 0.

String matching using the UNIX-style wildcard pattern matching technique is done through the string match command: string match pattern string where pattern is a wildcard pattern and string is the string to match. If the match succeeds, the command returns 1, otherwise it returns 0. An example is string match {[a-z]*[0-9]} {a_$%^_3} which matches because the command says match any string that starts with a lower case letter and ends with a number, regardless of anything in between.

There is a command for performing string substitutions using regular expressions: regsub ?-all? ?-nocase? exp string subSpec varName where exp is the regular expression and string is the input string on which the substitution is made, subSpec is the string that is substituted for the part of the string matched by the regular expression, and varName is the variable on which the resulting string is copied into.

With the -nocase switch, then the matching is done without regard to the case of letters in the input string. The -all switch causes repeated matching and substitution to happen on the input string. The result of a regsub command is the number of substitutions made.

An example of string substitution is: regsub {#name#} {My name is #name#} Rob result which sets the variable result to the value "My name is Rob". An example of using the

-all switch: regsub -all {#name#} {#name#’s name is #name#} Rob result sets the variable result to the value "Rob’s name is Rob" and it returns the value 2 because two substitutions were made.

444 SICStus Prolog

The are a host of other ways to manipulate strings through variants of the string command.

Here we will go through them.

To select a character from a string given the character position, use the string index command. An example is: string index "Hello world" 6 which returns w, the 7th character of the string. (Strings are indexed from 0).

To select a substring of a string, given a range of indices use the string range command.

An example is: string range "Hello world" 3 7 which returns the string "lo wo". There is a special index marker named end which is used to denote the the end of a string, so the code string range "Hello world" 6 end will return the string "world".

There are two ways to do simple search for a substring on a string, using the string first and string last commands. An example of string first is: string first "dog" "My dog is a big dog" find the first position in string "My dog is a big dog" that matches "dog". It will return the position in the string in which the substring was found, in this case 3. If the substring cannot be found then the value -1 is returned.

Similarly, string last "dog" "My dog is a big dog" will return the value 16 because it returns the index of the last place in the string that the substring matches. Again, if there is no match, -1 is returned.

To find the length of a string use string length which given a string simply returns its length.

string length "123456" returns the value 6.

To convert a string completely to upper case use string toupper: string toupper "this is in upper case" returns the string "THIS IS IN UPPER CASE".

Similarly,

Chapter 35: How to use the Tcl/Tk library 445 string tolower "THIS IS IN LOWER CASE" returns the string "this is in lower case".

There are commands for removing characters from strings: string trim, string trimright, and string trimleft.

string trim string ? chars?

which removes the characters in the string chars from the string string and returns the trimmed string.

If chars is not present, then whitespace characters are removed.

An example is: string string "The dog ate the exercise book" "doe" which would return the string "Th g at th xrcis bk".

string trimleft is the same as string trim except only leading characters are removed.

Similarly string trimright removes only trailing characters. For example: string trimright $my_input would return a copy of the string contained in $my_input but with all the trailing whitespace characters removed.

35.2.3.7 File I/O

There is a comprehensive set of commands for file manipulation. We will cover only the some of the more important ones here.

To open a file the open command is used: open name ? access?

where name is a string containing the filename, and the option access parameter contains a string of access flags, in the UNIX style. The return result is a handle to the open file.

If access is not present then the access permissions default to "r" which means open for reading only. The command returns a file handle that can be used with other commands.

An example of the use of the open command is set fid [open "myfile" "r+"] which means open the file myfile for both reading and writing and set the variable fid to the file handle returned.

To close a file simply use close fileId

For example,

446 SICStus Prolog close $fid will close the file that has the file handle stored in the variable fid.

To read from a file, the read command is used: read fileId numBytes which reads numBytes bytes from the file attached to file handle fileId, and returns the bytes actually read.

To read a single line from a file use gets: gets fileId ? varName?

which reads a line from the file attached to file handle fileId but chops off the trailing newline. If variable varName is specified then the string read in is stored there and the number of bytes is returned by the command. If the variable is not specified, then the command returns the string only.

To write to a file, use puts: puts ?-nonewline? ? fileId? string which outputs the string string. If the file handle fileId is present then the string is output to that file, otherwise it is printed on stdout. If the switch -nonewline is present then a trailing newline is not output.

To check if the end of a file has been reached, use eof: eof fileId which, given a file handle fileId returns 1 if the end has been reached, and 0 otherwise.

The are a host of other commands over files and processes which we will not go into here.

(For extra information on file I/O commands, refer to the Tcl manual pages.)

35.2.3.8 User defined procedures

Tcl provides a way of creating new commands, called procedures, that can be executed in scripts. The arguments of a procedure can be call-by-value or call-by-reference, and there is also a facility for creating new user defined control structures using procedures.

A procedure is declared using the proc command: proc name argList body where the name of the procedure is name, the arguments are contained in argList and the body of the procedure is the script body. An example of a procedure is:

Chapter 35: How to use the Tcl/Tk library 447 proc namePrint { first family } { puts "My first name is $first" puts "My family name is $family"

} which can be called with namePrint Tony Blair to produce the output:

My first name is Tony

My family name is Blair

A procedure with no arguments is specified with an empty argument list. An example is a procedure that just prints out a string: proc stringThing {} { puts "I just print this string"

}

Arguments can be given defaults by pairing them with a value in a list. An example here is a counter procedure: proc counter { value { inc 1 } } { eval $value + $inc

} which can be called with two arguments like this set v 10 set v [counter $v 5] which will set variable v to the value 15; or it can be called with one argument: set v 10 set v [counter $v] in which case v will have the value 11, because the default of the argument inc inside the procedure is the value 1.

There is a special argument for handling procedures with variable number of arguments, the args argument. An example is a procedure that sums a list of numbers: proc sum { args } { set result 0; foreach n $args { set result [expr $result + $n ]

}

448 SICStus Prolog return $result;

} which can be called like this: sum 1 2 3 4 5 which returns the value 15.

The restriction on using defaulted arguments is that all the arguments that come after the defaulted ones must also be defaulted. If using args then it must be the last argument in the argument list.

A procedure can return a value through the return command: return ? options? ? value?

which terminates the procedure returning value value, if specified, or just causes the procedure to return, if no value specified. (The ? options? part has to do with raising exceptions which we will will not cover here.)

The return result of a user defined procedure is the return result of the last command executed by it.

So far we have seen the arguments of a procedure are passed using the call-by-value mechanism. They can be passed call by reference using the upvar command: upvar ? level? otherVar1 myVar1 ? otherVar2 myVar2 ...?

which makes accessible variables somewhere in a calling context with the current context.

The optional argument level describes how many calling levels up to look for the variable.

This is best shown with an example: set a 10 set b 20 proc add { first second } { upvar $first f $second s expr $f+$s

} which when called with add a b will produce the result 30. If you use call-by-value instead: add $a $b then the program will fail because when executing the procedure add it will take the first argument 10 as the level argument, a bad level. (Also variable 20 doesn’t exist at any level.)

Chapter 35: How to use the Tcl/Tk library 449

New control structures can be generated using the uplevel command: uplevel ? level? arg ? arg arg ...?

which is like eval but it evaluates its arguments in a context higher up the calling stack.

How far up the stack to go is given by the optional level argument.

proc do { loop condition } { set nostop 1 while { $nostop } { uplevel $loop if {[uplevel "expr $condition"] == 0} { set nostop 0

}

}

} which when called with this set x 5 do { puts $x; incr x -1 } { $x > 0 } will print

5

4

3

2

1

(NOTE: this doesn’t quite work for all kinds of calls because of break, continue, and return. It is possible to get around these problems but that is outside the scope of this tutorial.)

35.2.3.9 Global variables

A word about the scope of variables. Variables used within procedures are normally created only for the duration of that procedure and have local scope.

It is possible to declare a variable as having global scope, through the global command: global name1 ? name2 ...?

where name1, name2, ..., are the names of global variables. Any references to those names will be taken to denote global variables for the duration of the procedure call.

Global variables are those variables declared at the topmost calling context. It is possible to run a global command at anytime in a procedure call. After such a command, the variable name will refer to a global variable until the procedure exits.

450 SICStus Prolog

An example: set x 10 proc fred { } { set y 20 global x puts [expr $x + $y]

} fred will print the result 30 where 20 comes from the local variable y and 10 comes from the global variable x.

Without the global x line, the call to fred will fail with an error because there is no variable x defined locally in the procedure for the expr to evaluate over.

35.2.3.10 source

In common with other scripting languages, there is a command for evaluating the contents of a file in the Tcl interpreter: source fileName where fileName is the filename of the file containing the Tcl source to be evaluated. Control returns to the Tcl interpreter once the file has been evaluated.

35.2.4 What we have left out (Tcl)

We have left out a number of Tcl commands as they are outside of the scope of this tutorial.

We list some of them here to show some of what Tcl can do. Please refer to the Tcl manual for more information.

http implements the http protocol for retrieving web pages namespaces a modules systems for Tcl trace commands can be attached to variables that are triggered when the variable changes value (amongst other things) processes start, stop, and manage processes sockets UNIX and Internet style socket management exception handling

3rd party extension packages load extension packages into Tcl and use their facilities as native Tcl commands

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35.3 Tk

Tk is an extension to Tcl. It provides Tcl with commands for easily creating and managing graphical objects, or widgets, so providing a way to add graphical user interfaces (GUIs) to

Tcl applications.

In this section we will describe the main Tk widgets, the Tcl commands used to manipulate them, how to give them behaviors, and generally how to arrange them into groups to create a GUI.

35.3.1 Widgets

A widget is a “window object”. It is something that is displayed that has at least two parts: a state and a behavior. An example of a widget is a button. Its state is things like what color is it, what text is written it in, and how big it is. Its behavior is things like what it does when you click on it, or what happens when the cursor is moved over or away from it.

In Tcl/Tk there are three parts to creating a useful widget. The first is creating an instance of the widget with its initial state. The second is giving it a behavior by defining how the widget behaves when certain events happen — event handling. The third is actually displaying the widget possibly in a group of widgets or inside another widget — geometry management. In fact, after creating all the widgets for a GUI, they are not displayed until handled by a geometry manager, which has rules about how to calculate the size of the widgets and how they will appear in relation to each other.

35.3.2 Types of widget

In Tcl/Tk there are currently 15 types of widget. In alphabetical order they are (see also see also library(’tcltk/examples/widgets.tcl’)): button canvas a simple press button is a container for displaying “drawn” objects such as lines, circles, and polygons.

checkbutton a button that hold a state of either on or off entry frame label a text entry field a widget that is a container for other widgets a simple label listbox a box containing a list of options menu a widget for creating menu bars menubutton a button which when pressed offers a selection of choices message a multi-line text display widget

452 SICStus Prolog radiobutton a button used to form groups of mutually interacting buttons (When one button is pressed down, the others pop up.) scale is like a slider on a music console. It consists of a trough scale and a slider.

Moving the slider to a position on the scale sets the overall value of the widget to that value.

scollbar used to add scrollbars to windows or canvases. The scrollbar has a slider which when moved changes the value of the slider widget.

text a sophisticated multi-line text widget that can also display other widgets such as buttons toplevel for creating new standalone toplevel windows. (These windows are containers for other widgets. They are not terminal windows.)

Chapter 35: How to use the Tcl/Tk library 453

Meet The Main Tk Widgets

454 SICStus Prolog

35.3.3 Widgets hierarchies

Before going further it is necessary to understand how instances of widgets are named.

Widgets are arranged in a hierarchy. The names of widget instances are formed from dot separated words. The root window is simply . on its own. So for, example, a button widget that is displayed in the root window might have the name .b1. A button that is displayed inside a frame that is displayed inside the root window may have the name .frame1.b1.

The frame would have the name .frame1.

Following this notation, it is clear that widgets are both formed in hierarchies, with the dot notation giving the path to a widget, and in groups, all widgets with the same leading path are notionaly in the same group.

(It is a similar to the way file systems are organized. A file has a path which shows where to find it in the hierarchical file system. But also files with the same leading path are in the same directory/folder and so are notionaly grouped together.)

An instance of a widget is created through the a Tcl command for that widget. The widget command my have optional arguments set for specifying various attributes of the widget that it will have when it is created. The result of a successful widget command is the name of the new widget.

For example, a command to create a button widget named .mybutton that displays the text “I am a button” would look like this: button .mybutton -text "I am a button" and this will return the name .mybutton.

A widget will only be created if all the windows/widgets in the leading path of the new widget also exist, and also that the name of the new widget does not already exist.

For example, the following button .mybutton -text "I am a button" button .mybutton -text "and so am I" will fail at the second command because there is also a widget named .mybutton from the first command.

The following will also fail button .frame.mybutton -text "I am a button" if there is no existing widget with the name .frame to be the parent of .mybutton.

All this begs the question: why are widgets named and arranged in a hierarchy? Isn’t a

GUI just a bunch of widgets displayed in a window?

This is not generally how GUIs are arranged. For example, they often have a menubar over the top of each window. The menubar contains pulldown menus. The pulldown menus may

Chapter 35: How to use the Tcl/Tk library 455 have cascading menu items that may cascade down several levels. Under the menu bar is the main part of the window that may also be split into several “frames”. A left hand frame my have a set of buttons in it, for example. And so on. From this you can see that the widgets in GUIs are naturally arranged in a hierarchy. To achieve this in Tcl/Tk instances of widgets are placed in a hierarchy which is reflected in their names.

Now we will go through each of the widget commands in turn. Each widget command has many options most of which will not be described here. Just enough will be touched on for the reader to understand the basic operation of each widget. For a complete description of each widget and its many options refer to the Tk manual.

35.3.4 Widget creation

As has already been said, a widget is a window object that has state and behavior. In terms of Tcl/Tk a widget is created by calling a widget creation command. There is a specific widget creation for each type of widget.

The widget creation command is supplied with arguments. The first argument is always the name you want to give to the resulting widget; the other arguments set the initial state of the widget.

The immediate result of calling a widget creation command is that it returns the name of the new widget. A side effect is that the instance of the widget is created and its name is defined as in the Tcl interpreter as a procedure through which the widget state can be accessed and manipulated.

This needs an example. We will use the widget creator command button to make a button widget: button .fred -text ’Fred’ -background red which creates an instance of a button widget named .fred that will display the text Fred on the button and will have a red background color. Evaluating this command returns the string .fred, the name of the newly created widget.

As a side effect, a Tcl procedure named .fred is created. A call to a widget instance has the following form: widgetName method methodArgs where widgetName is the name of the widget to be manipulated, method is the action to be performed on the widget, and methodArgs are the arguments passed to the method that is performed on the widget.

The two standard methods for widgets are configure and cget. configure - is used to change the state of a widget; for example:

.fred configure -background green -text ’Sid’

456 SICStus Prolog will change the background color of the widget .fred to green and the text displayed to

Sid.

cget is used to get part of the state of a widget; for example:

.fred cget -text will return Sid if the text on the button .fred is Sid.

In addition to these general methods, there are special methods for each widget type. For example, with button widgets you have the flash and invoke methods.

For example,

.fred invoke can be called somewhere in the Tcl code to invoke button .fred as though it had been clicked on.

.fred flash can be called somewhere in the Tcl code to cause the button to flash.

We will come across some of these special method when we discuss the widgets in detail. For a comprehensive list of widget methods, refer to entry for the appropriate widget creation command in the Tcl/Tk manual.

We now discuss the widget creation command for each widget type.

35.3.4.1 label

A label is a simple widget for displaying a single line of text. An example of creating an instance of a label is label .l -text "Hello world!" which simply creates the label named .l with the text ‘Hello world!’ displayed in it. Most widgets that display text can have a variable associated with them through the option textvariable. When the value of the variable is changed the text changes in the associated label. For example, label .l -text "Hello world!" -textvariable mytext creates a text label called .l displaying the initial text ‘Hello world!’ and associated text variable mytext; mytext will start with the value ‘Hello world!’. However, if the following script is executed: set mytext "Goodbye moon!" then magically the text in the label will change to ‘Goodbye moon!’.

Chapter 35: How to use the Tcl/Tk library 457

35.3.4.2 message

A message widget is similar to a label widget but for multi-line text. As its name suggests it is mostly used for creating popup message information boxes.

An example of a message widget is message .msg -text "Your data is incorrect.\n\n \

Please correct it and try again." \

-justify center which will create a message widget displaying the text shown, center justified. The width of the message box can be given through the -width switch. Any lines that exceed the width of the box are wrapped at word boundaries.

35.3.4.3 button

Calling the button command creates an instance of a button widget. An example is: button .mybutton -text "hello" -command {puts "howdie!"} which creates a button with name .mybutton that will display the text "hello" and will execute the Tcl script puts "howdie!" (that is print howdie! to the terminal) when clicked on.

35.3.4.4 checkbutton

Checkbuttons are buttons that have a fixed state that is either on or off. Clicking on the button toggles the state. To store the state, a checkbutton is associated with a variable.

When the state of the checkbutton changes, so does that of the variable. An example is: checkbutton .on_or_off -text "I like ice cream" -variable ice which will create a checkbutton with name .on_or_off displaying the text ‘I like ice cream’ and associated with the variable ice. If the checkbutton is checked then ice will have the value 1; if not checked then it will have the value 0. The state of the checkbutton can also be changed by changing the state of the variable. For example, executing set ice 0 will set the state of .on_or_off to not checked.

35.3.4.5 radiobutton

Radiobuttons are buttons that are grouped together to select one value among many. Each button has a value, but only one in the button group is active at any one time. In Tcl/Tk this is achieved by creating a series of radiobutton that share an associated variable. Each button has a value. When a radiobutton is clicked on, the variable has that value and all

458 SICStus Prolog the other buttons in the group are put into the off state. Similarly, setting the value of the variable is reflected in the state of the button group. An example is: radiobutton .first -value one -text one -variable count radiobutton .second -value two -text two -variable count radiobutton .third -value three -text three -variable count which creates three radiobuttons that are linked through the variable count. If button

.second is active, for example, then the other two buttons are in the inactive state and count has the value two. The following code sets the button group to make the button

.third active and the rest inactive regardless of the current state: set count three

If the value of count does not match any of the values of the radiobuttons then they will all be off. For example executing the script set count four will turn all the radiobuttons off.

35.3.4.6 entry

An entry widget allows input of a one line string. An example of a an entry widget: label .l -text "Enter your name" entry .e -width 40 -textvariable your_name would display a label widget named .l showing the string ‘Enter your name’ and an entry widget named .e of width 40 characters. The value of variable your_name will reflect the string in the entry widget: as the entry widget string is updated, so is the value of the variable. Similarly, changing the value of your_name in a Tcl script will change the string displayed in the entry field.

35.3.4.7 scale

A scale widget is for displaying an adjustable slider. As the slider is moved its value, which is displayed next to the slider, changes. To specify a scale, it must have -from and -to attributes which is the range of the scale. It can have a -command option which is set to a script to evaluate when the value of the slider changes.

An example of a scale widget is: scale .s -from 0 -to 100 which creates a scale widget with name .s that will slide over a range of integers from 0 to

100.

Chapter 35: How to use the Tcl/Tk library 459

There are several other options that scales can have. For example it is possible to display tick marks along the length of the scale through the -tickinterval attribute, and it is possible to specify both vertically and horizontally displayed scales through the -orient attribute.

35.3.4.8 listbox

A listbox is a widget that displays a list of single line strings. One or more of the strings may be selected through using the mouse. Initializing and manipulating the contents of a listbox is done through invoking methods on the instance of the listbox. As examples, the insert method is used to insert a string into a listbox, delete to delete one, and get to retrieve a particular entry. Also the currently selected list items can be retrieved through the selection command.

Here is an example of a listbox that is filled with entries of the form entry N: listbox .l

for { set i 0 } { $i<10 } { incr i } {

.l insert end "entry $i"

}

A listbox may be given a height and/or width attribute, in which case it is likely that not all of the strings in the list are visible at the same time. There are a number of methods for affecting the display of such a listbox.

The see method causes the listbox display to change so that a particular list element is in view. For example,

.l see 5 will make sure that the sixth list item is visible. (List elements are counted from element

0.)

35.3.4.9 scrollbar

A scrollbar widget is intended to be used with any widget that is likely to be able to display only part of its contents at one time. Examples are listboxes, canvases, text widgets, and frames, amongst others.

A scrollbar widget is displayed as a movable slider between two arrows. Clicking on either arrow moves the slider in the direction of the arrow. The slider can be moved by dragging it with the cursor.

The scollbar and the widget it scrolls are connected through Tcl script calls. A scrollable widgets will have a scrollcommand attribute that is set to a Tcl script to call when the widget changes its view. When the view changes the command is called, and the command is usually set to change the state of its associated scrollbar.

460 SICStus Prolog

Similarly, the scrollbar will have a command attribute that is another script that is called when an action is performed on the scrollbar, like moving the slider or clicking on one of its arrows. That action will be to update the display of the associated scrollable widget

(which redraws itself and then invokes its scrollcommand which causes the scrollbar to be redrawn).

How this is all done is best shown through an example: listbox .l -yscrollcommand ".s set" -height 10 scrollbar .s -command ".l yview" for { set i 0 } { $i < 50 } { incr i } {

.l insert end "entry $i"

} creates a listbox named .l and a scrollbar named .s. Fifty strings of the form entry N are inserted into the listbox. The clever part is the way the scrollbar and listbox are linked.

The listbox has its -yscrollcommand attribute set to the script ".s set". What happens is that if the view of .l is changed, then this script is called with 4 arguments attached: the number of entries in the listbox, the size of the listbox window, the index of the first entry currently visible, and the index of the last entry currently visible. This is exactly enough information for the scrollbar to work out how to redisplay itself. For example, changing the display of the above listbox could result in the following -yscrollcommand script being called:

.s set 50 10 5 15 which says that the listbox contains 50 elements, it can display 10 at one time, the first element displayed has index 5 and the last one on display has index 15. This call invokes the set method of the scrollbar widget .s, which causes it to redraw itself appropriately.

If, instead, the user interacts with the scrollbar, then the scrollbar will invoke its -command script, which in this example is ".l yview". Before invoking the script, the scrollbar widget calculates which element should the first displayed in its associated widget and appends its index to the call. For example, if element with index 20 should be the first to be displayed, then the following call will be made:

.l yview 20 which invokes the yview method of the listbox .l. This causes .l to be updated (which then causes its -yscrollcommand to be called which updates the scrollbar).

35.3.4.10 frame

A frame widget does not do anything by itself except reserve an area of the display. Although this does not seem to have much purpose, it is a very important widget. It is a container widget; that is, it is used to group together collections of other widgets into logical groups.

For example, a row of buttons may be grouped into a frame, then as the frame is manipulated so will the widgets displayed inside it. A frame widget can also be used to create large areas

Chapter 35: How to use the Tcl/Tk library 461 of color inside a another container widget (such as another frame widget or a toplevel widget).

An example of the use of a frame widget as a container: canvas .c -background red frame .f

button .b1 -text button1 button .b2 -text button2 button .b3 -text button3 button .b4 -text button4 button .b5 -text button5 pack .b1 .b2 .b3 .b4 .b5 -in .f -side left pack .c -side top -fill both -expand 1 pack .f -side bottom which specifies that there are two main widgets a canvas named .c and a frame named .f.

There are also 5 buttons, .b1 through .b5. The buttons are displayed inside the frame.

Then then the canvas is displayed at the top of the main window and the frame is displayed at the bottom. As the frame is displayed at the bottom, then so will the buttons because they are displayed inside the frame.

(The pack command causes the widgets to be handled for display by the packer geometry manager. The -fill and -expand 1 options to pack for .c tell the display manager that if the window is resized then the canvas is to expand to fill most of the window. You will learn about geometry managers later in the Geometry Managers section.)

35.3.4.11 toplevel

A toplevel widget is a new toplevel window. It is a container widget inside which other widgets are displayed. The root toplevel widget has path . — i.e. dot on its own. Subsequent toplevel widgets must have a name which is lower down the path tree just like any other widget.

An example of creating a toplevel widget is: toplevel .t

All the widgets displayed inside .t must also have .t as the root of their path. For example, to create a button widget for display inside the .t toplevel the following would work: button .t.b -text "Inside ’t’"

(Attributes, such as size and title, of toplevel widgets can be changed through the wm command, which we will not cover in this tutorial. The reader is referred to the Tk manual.)

462 SICStus Prolog

35.3.4.12 menu

Yet another kind of container is a menu widget. It contains a list of widgets to display inside itself, as a pulldown menu. A simple entry in a menu widget is a command widget, displayed as an option in the menu widget, which if chosen executes a Tcl command. Other types of widgets allowed inside a menu widget are radiobuttons and checkboxes. A special kind of menu item is a separator that is used to group together menu items within a menu.

(It should be noted that the widgets inside a menu widget are special to that menu widget and do not have an independent existence, and so do not have their own Tk name.)

A menu widget is built by first creating an instance of a menu widget (the container) and then invoking the add method to make entries into the menu. An example of a menu widget is as follows: menu .m

.m add command -label "Open file" -command "open_file"

.m add command -label "Open directory" -command "open_directory"

.m add command -label "Save buffer" -command "save_buffer"

.m add command -label "Save buffer as..." -command "save_buffer_as"

.m add separator

.m add command -label "Make new frame" -command "new_frame"

.m add command -label "Open new display" -command "new_display"

.m add command -label "Delete frame" -command "delete_frame" which creates a menu widget called .m which contains eight menu items, the first four of which are commands, then comes a separator widget, then the final three command entries.

(Some of you will notice that this menu is a small part of the Files menu from the menubar of the Emacs text editor.)

An example of a checkbox and some radiobutton widget entries:

.m add checkbox -label "Inverse video" -variable inv_vid

.m add radiobutton -label "black" -variable color

.m add radiobutton -label "blue" -variable color

.m add radiobutton -label "red" -variable color which gives a checkbox displaying ‘Inverse video’, keeping its state in the variable inv_ vid, and three radiobuttons linked through the variable color.

Another menu item variant is the cascade variant which is used to make cascadable menus, i.e. menus that have submenus. An example of a cascade entry is the following:

.m add cascade -label "I cascade" -menu .m.c

which adds a cascade entry to the menu .m that displays the text ‘I cascade’. If the ‘I cascade’ option is chosen from the .m menu then the menu .m.c will be displayed.

The cascade option is also used to make menubars at the top of an application window. A menu bar is simply a menu each element of which is a cascade entry, (for example). The menubar menu is attached to the application window through a special configuration option

Chapter 35: How to use the Tcl/Tk library 463 for toplevel widgets, the -menu option. Then a menu is defined for each of the cascade entry in the menubar menu.

There are a large number of other variants to menu widgets: menu items can display bitmaps instead of text; menus can be specified as tear-off menus; accelerator keys can be defined for menu items; and so on.

35.3.4.13 menubutton

A menubutton widget displays like a button but when activated a menu pops up. The menu of the menubutton is defined through the menu command and is attached to the menubutton. An example of a menu button: menubutton .mb -menu .mb.m -text "mymenu" menu .mb.m

.mb.m add command -label hello

.mb.m add command -label goodbye which crates a menubutton widget named .mb with attached menu .mb.m and displays the text ‘mymenu’. Menu .mb.m is defined as two command options, one labelled hello and the other labelled goodbye. When the menubutton .mb is clicked on then the menu .mb.m will popup and its options can be chosen.

35.3.4.14 canvas

A canvas widget is a container widget that is used to manage the drawing of complex shapes; for example, squares, circles, ovals, and polygons. (It can also handle bitmaps, text and most of the Tk widgets too.) The shapes may have borders, filled in, be clicked on, moved around, and manipulated.

We will not cover the working of the canvas widget here. It is enough to know that there is a powerful widget in the Tk toolkit that can handle all manner of graphical objects. The interested reader is referred to the Tk manual.

35.3.4.15 text

A text widget is another powerful container widget that handles multi-line texts. The textwidget can display texts with varying font styles, sizes, and colors in the same text, and can also handle other Tk widgets embedded in the text.

The text widget is a rich and complicated widget and will not be covered here. The interested reader is referred to the Tk manual.

35.3.5 Geometry managers

So far we have described each of the Tk widgets but have not mentioned how they are arranged to be displayed. Tk separates the creating of widgets from the way they are

464 SICStus Prolog arranged for display. The “geometry” of the display is handled by a “geometry manager”.

A geometry manager is handed the set of widgets to display with instructions on their layout. The layout instructions are particular to each geometry manager.

Tk comes with three distinct geometry managers: grid, place, and pack. As might be expected the grid geometry manager is useful for creating tables of widgets, for example, a table of buttons.

The place geometry manager simply gives each widget an X and Y coordinate and places them at that coordinate in their particular parent window.

The pack geometry manager places widgets according to constraints, like “these three button widgets should be packed together from the left in their parent widget, and should resize with the parent”.

(In practice the grid and pack geometry managers are the most useful because they can easily handle events such as resizing of the toplevel window, automatically adjusting the display in a sensible manner. place is not so useful for this.)

Each container widget (the master) has a geometry manager associated with it which tells the container how to display its sub-widgets (slaves) inside it. A single master has one and only one kind of geometry manager associated with it, but each master can have a different kind. For example, a frame widget can use the packer to pack other frames inside it. One of the slave frames could use the grid manager to display buttons inside it itself, while another slave frame could use the packer to pack labels inside it itself.

35.3.5.1 pack

The problem is how to display widgets. For example, there is an empty frame widget inside which a bunch of other widgets will be displayed. The pack geometry manager’s solution to this problem is to successively pack widgets into the empty space left in the container widget. The container widget is the master widget, and the widgets packed into it are its slaves. The slaves are packed in a sequence: the packing order.

What the packer does is to take the next slave to be packed. It allocates an area for the slave to be packed into from the remaining space in the master. Which part of the space is allocated depends on instructions to the packer. When the size of the space has been determined, this is sliced off the free space, and allocated to the widget which is displayed in it. Then the remaining space is available to subsequent slaves.

At any one time the space left for packing is a rectangle. If the widget is too small to use up a whole slice from the length or breadth of the free rectangle, still a whole slice is allocated so that the free space is always rectangular.

It can be tricky to get the packing instructions right to get the desired finished effect, but a large number of arrangements of widgets is possible using the packer.

Chapter 35: How to use the Tcl/Tk library 465

Let us take a simple example: three buttons packed into the root window. First we create the buttons; see also library(’tcltk/examples/ex3.tcl’): button .b1 -text b1 button .b2 -text b2 button .b3 -text b3 then we can pack them thus: pack .b1 .b2 .b3

which produces a display of the three buttons, one on top of the other, button .b1 on the top, and button .b3 on the bottom.

Three Plain Buttons

If we change the size of the text in button .b2 through the command:

.b2 config -text "hello world" then we see that the window grows to fit the middle button, but the other two buttons stay their original size.

Middle Button Widens

The packer defaults to packing widgets in from the top of the master. Other directions can be specified. For example, the command: pack .b1 .b2 .b3 -side left

466 SICStus Prolog will pack starting at the left hand side of the window. The result of this is that the buttons are formed in a horizontal row with the wider button, .b2, in the middle.

Packing From The Left pad-ding

It is possible to leave space between widgets through the padding options to the packer:

-padx and -pady. What these do is to allocate space to the slave that is padded with the padding distances. An example would be: pack .b1 .b2 .b3 -side left -padx 10

External Padding which adds 10 pixels of space to either side of the button widgets. This has the effect of leaving 10 pixels at the left side of button .b1, 20 pixels between buttons .b1 and .b2, 20 pixels between buttons .b2 and .b3, and finally 10 pixels on the right side of button .b3.

That was external padding for spacing widgets. There is also internal padding for increasing the size of widgets in the X and Y directions by a certain amount, through -ipadx and

-ipady options; i.e. internal padding. For example: pack .b1 .b2 .b3 -side left -ipadx 10 -ipady 10

Internal Padding instead of spacing out the widgets, will increase their dimensions by 10 pixels in each direction.

Chapter 35: How to use the Tcl/Tk library 467 fill-ing

Remember that space is allocated to a widget from the currently available space left in the master widget by cutting off a complete slice from that space. It is often the case that the slice is bigger that the widget to be displayed in it.

There are further options for allowing a widget to fill the whole slice allocated to it. This is done through the -fill option, which can have one of four values: none for no filling

(default), x to fill horizontally only, y to fill vertically only, and both to fill both horizontally and vertically at the same time.

Filling is useful, for example, for creating buttons that are the same size even though they display texts of differing lengths. To take our button example again, the following code produces three buttons, one on top of each other, but of the same size: button .b1 -text b1 button .b2 -text "hello world" button .b3 -text b3 pack .b1 .b2 .b3 -fill x

Using fill For Evenly Sized Widgets

How does this work? The width of the toplevel windows is dictated by button .b2 because it has the widest text. Because the three buttons are packed from top to bottom, then the slices of space allocated to them are cut progressively straight along the top of the remaining space. i.e. each widget gets a horizontal slice of space the same width cut from the top-level widget. Only the wide button .b2 would normally fit the whole width of its slice. But by allowing the other two widgets to fill horizontally, then they will also take up the whole width of their slices. The result: 3 buttons stacked on top of each other, each with the same width, although the texts they display are not the same length.

A further common example is adding a scrollbar to a listbox. The trick is to get the scrollbar to size itself to the listbox; see also library(’tcltk/examples/ex9a.tcl’): listbox .l

scrollbar .s

pack .l .s -side left

468 SICStus Prolog

Scrollbar With Listbox, First Try

So far we have a listbox on the left and a tiny scrollbar on the right. To get the scrollbar to fill up the vertical space around it add the following command: pack .s -fill y

Now the display looks like a normal listbox with a scrollbar.

Scrollbar With Listbox, Second Try

Why does this work? They are packed from the left, so first a large vertical slice of the master is given to the listbox, then a thin vertical slice is given to the scrollbar. The scrollbar has a small default width and height and so it does not fill the vertical space of its slice. But filling in the vertical direction (through the pack .s -fill y command) allows it to fill its space, and so it adjusts to the height of the listbox.

Chapter 35: How to use the Tcl/Tk library 469 expand-ing

The fill packing option specifies whether the widget should fill space left over in its slice of space. A further option to take into account is what happens when the space allocated to the master widget is much greater than the that used by its slaves. This is not usually a problem initially because the master container widget is sized to shrink-wrap around the space used by its slaves. If the container is subsequently resized, however, to a much larger size there is a question as to what should happen to the slave widgets. A common example of resizing a container widget is the resizing of a top-level window widget.

The default behavior of the packer is not to change the size or arrangement of the slave widgets. There is an option though through the expand option to cause the slices of space allocated to slaves to expand to fill the newly available space in the master. expand can have one of two values: 0 for no expansion, and 1 for expansion.

Take the listbox-scrollbar example; see also library(’tcltk/examples/ex10.tcl’): listbox .l

scrollbar .s

pack .l -side left pack .s -side left -fill y

Initially this looks good but now resize the window to a much bigger size. You will find that the listbox stays the same size and that empty space appears at the top and bottom of it, and that the scrollbar resizes in the vertical. It is now not so nice.

470 SICStus Prolog

Scrollbar And Listbox, Problems With Resizing

We can fix part of the problem by having the listbox expand to fill the extra space generated by resizing the window.

pack .l -side left -expand 1

Chapter 35: How to use the Tcl/Tk library 471

Scrollbar And Listbox, Almost There

The problem now is that expand just expands the space allocated to the listbox, it doesn’t stretch the listbox itself. To achieve that we need to apply the fill option to the listbox too.

pack .l -side left -expand 1 -fill both

472 SICStus Prolog

Scrollbar And Listbox, Problem Solved Using fill

Now whichever way the top-level window is resized, the listbox-scrollbar combination should look good.

If more than one widget has the expansion bit set, then the space is allocated equally to those widgets.

This can be used, for example, to make a row of buttons of equal size that resize to fill the widget of their container.

Try the following code; see also library(’tcltk/examples/ex11.tcl’): button .b1 -text "one" button .b2 -text "two" button .b3 -text "three" pack .b1 .b2 .b3 -side left -fill x -expand 1

Chapter 35: How to use the Tcl/Tk library 473

Resizing Evenly Sized Widgets

Now resize the window. You will see that the buttons resize to fill the width of the window, each taking an equal third of the width.

(NOTE: the best way to get the hang of the packer is to play with it. Often the results are not what you expect, especially when it comes to fill and expand options. When you have created a display that looks pleasing, always try resizing the window to see if it still looks pleasing, or whether some of your fill and expand options need revising.) anchors and packing order

There is an option to change how a slave is displayed if its allocated space is larger than itself. Normally it will be displayed centered. That can be changed by anchoring it with the -anchor option. The option takes a compass direction as its argument: n, s, e, w, nw, ne, sw, se, or c (for center).

For example, the previous example with the resizing buttons displays the buttons in the center of the window, the default anchoring point.

If we wanted the buttons to be displayed at the top of the window then we would anchor them there thus; see also library(’tcltk/examples/ex12.tcl’): button .b1 -text "one" button .b2 -text "two" button .b3 -text "three" pack .b1 .b2 .b3 -side left -fill x -expand 1 -anchor n

Anchoring Widgets

474 SICStus Prolog

Each button is anchored at the top of its slice and so in this case is displayed at the top of the window.

The packing order of widget can also be changed. For example, pack .b3 -before .b2

will change the positions of .b2 and .b3 in our examples.

Changing The Packing Order Of Widgets

35.3.5.2 grid

The grid geometry manager is useful for arranging widgets in grids or tables. A grid has a number of rows and columns and a widget can occupy one of more adjacent rows and columns.

A simple example of arranging library(’tcltk/examples/ex14.tcl’): three buttons; see also button .b1 -text b1 button .b2 -text b2 button .b3 -text b3 grid .b1 -row 0 -column 0 grid .b2 -row 1 -column 0 grid .b3 -row 0 -column 1 -rowspan 2 this will display button .b1 above button .b2. Button .b3 will be displayed in the next column and it will take up two rows.

Chapter 35: How to use the Tcl/Tk library 475

Using the grid Geometry Manager

However, .b3 will be displayed in the center of the space allocated to it. It is possible to get it to expand to fill the two rows it has using the -sticky option. The -sticky option says to which edges of its cells a widget “sticks” to, i.e. expands to reach. (This is like the fill and expand options in the pack manager.) So to get .b3 to expand to fill its space we could use the following: grid .b3 -sticky ns which says stick in the north and south directions (top and bottom). This results in .b3

taking up two rows and filling them.

grid Geometry Manager, Cells With Sticky Edges

There are plenty of other options to the grid geometry manager. For example, it is possible to give some rows/columns more “weight” than others which gives them more space in the master. For example, if in the above example you wanted to allocate 1/3 of the width of the master to column 0 and 2/3 of the width to column 1, then the following commands would achieve that: grid columnconfigure . 0 -weight 1 grid columnconfigure . 1 -weight 2 which says that the weight of column 0 for master . (the root window) is 1 and the weight of column 1 is 2. Since column 1 has more weight than column 0 it gets proportionately more space in the master.

It may not be apparent that this works until you resize the window. You can see even more easily how much space is allocated to each button by making expanding them to fill their space through the sticky option.

The whole example looks like this; see also library(’tcltk/examples/ex16.tcl’): button .b1 -text b1 button .b2 -text b2 button .b3 -text b3 grid .b1 -row 0 -column 0 -sticky nsew

476 SICStus Prolog grid .b2 -row 1 -column 0 -sticky nsew grid .b3 -row 0 -column 1 -rowspan 2 -sticky nsew grid columnconfigure . 0 -weight 1 grid columnconfigure . 1 -weight 2

Now resize the window to various sizes and we will see that button .b3 has twice the width of buttons .b1 and .b2.

Changing Row/Column Ratios

The same kind of thing can be specified for each row too via the grid rowconfigure command.

For other options and a full explanation of the grid manager see the manual.

35.3.5.3 place

Place simply places the slave widgets in the master at the given x and y coordinates. It displays the widgets with the given width and height. For example (see also library(’tcltk/examples/ex17.tcl’)): button .b1 -text b1 button .b2 -text b2 button .b3 -text b3 place .b1 -x 0 -y 0 place .b2 -x 100 -y 100

Chapter 35: How to use the Tcl/Tk library place .b3 -x 200 -y 200

477

Using The place Geometry Manager will place the buttons .b1, .b2, and .b3 along a diagonal 100 pixels apart in both the x and y directions. Heights and widths can be given in absolute sizes, or relative to the size of the master in which case they are specified as a floating point proportion of the master; 0.0

being no size and 1.0 being the size of the master. x and y coordinates can also be specified in a relative way, also as a floating point number. For example, a relative y coordinate of

0.0 refers to the top edge of the master, while 1.0 refers to the bottom edge. If both relative and absolute x and y values are specified then they are summed.

Through this system the placer allows widgets to be placed on a kind of rubber sheet. If all the coordinates are specified in relative terms, then as the master is resized then so will the slaves move to their new relative positions.

35.3.6 Event Handling

So far we have covered the widgets types, how instances of them are created, how their attributes can be set and queried, and how they can be managed for display using geometry managers. What we have not touched on is how to give each widget a behavior.

This is done through event handlers. Each widget instance can be given a window event handler for each kind of window event. A window event is something like the cursor moving into or out of the widget, a key press happening while the widget is active (in focus), or the widget being destroyed.

Event handlers are specified through the bind command: bind widgetName eventSequence command

478 SICStus Prolog where widgetName is the name or class of the widget to which the event handler should be attached, eventSqueuence is a description of the event that this event handler will handle, and command is a script that is invoked when the event happens (i.e. it is the event handler).

Common event types are

Key

KeyPress when a key was pressed

KeyRelease when a key was released

Button

ButtonPress when a mouse button was pressed

ButtonRelease when a mouse button was released

Enter when the cursor moves into a widget

Leave

Motion when the cursor moved our of a widget when the cursor moves within a widget

There are other event types. Please refer to the Tk documentation for a complete list.

The eventSequence part of a bind command is a list of one or more of these events, each event surrounded by angled brackets. (Mostly, an event sequence consists of handling a single event. Later we will show more complicated event sequences.)

An example is the following: button .b -text "click me" pack .b

bind .b <Enter> { puts "entering .b" } makes a button .b displaying text ‘click me’ and displays it in the root window using the packing geometry manager. The bind command specifies that when the cursor enters (i.e.

goes onto) the widget, then the text entering .b is printed at the terminal.

We can make the button change color as the cursor enters or leaves it like this: button .b -text "click me" -background red pack .b

bind .b <Enter> { .b config -background blue } bind .b <Leave> { .b config -background red } which causes the background color of the button to change to blue when the cursor enters it and to change back to red when the cursor leaves.

Chapter 35: How to use the Tcl/Tk library 479

An action can be appended to an event handler by prefixing the action with a + sign. An example is: bind .b <Enter> {+puts "entering .b"} which, when added to the example above, would not only change the color of the button to red when the cursor enters it, but would also print entering .b to the terminal.

A binding can be revoked simply by binding the empty command to it: bind .b <Enter> {}

A list of events that are bound can be found by querying the widget thus: bind .b

which will return a list of bound events.

To get the current command(s) bound to an event on a widget, invoke bind with the widget name and the event. An example is: bind .b <Enter> which will return a list of the commands bound to the event <Enter> on widget .b.

Binding can be generalized to sequences of events. For example, we can create an entry widget that prints spells rob each time the key sequence ESC r o b happens: entry .e

pack .e

bind .e <Escape>rob {puts "spells rob"}

(A letter on its own in an event sequence stands for that key being pressed when the corresponding widget is in focus.)

Events can also be bound for entire classes of widgets. For example, if we wanted to perform the same trick for ALL entry widgets we could use the following command: bind entry <Escape>rob {puts "spells rob"}

In fact, we can bind events over all widgets using all as the widget class specifier.

The event script can have substitutions specified in it. Certain textual substitutions are then made at the time the event is processed. For example, %x in a script gets the x coordinate of the mouse substituted for it. Similarly, %y becomes the y coordinate, %W the dot path of the window on which the event happened, %K the keysym of the button that was pressed, and so on. For a complete list, see the manual.

In this way it is possible to execute the event script in the context of the event.

A clever example of using the all widget specifier and text substitutions is given in John

Ousterhout’s book on Tcl/Tk (see

Section 35.7 [Resources], page 518

):

480 SICStus Prolog bind all <Enter> {puts "Entering %W at (%x, %y)"} bind all <Leave> {puts "Leaving %W at (%x, %y)"} bind all <Motion> {puts "Pointer at (%x, %y)"} which implements a mouse tracker for all the widgets in a Tcl/Tk application. The widget’s name and x and y coordinates are printed at the terminal when the mouse enters or leaves any widget, and also the x and y coordinates are printed when the mouse moves within a widget.

35.3.7 Miscellaneous

There are a couple of other Tk commands that we ought to mention: destroy and update.

The destroy command is used to destroy a widget, i.e. remove it from the Tk interpreter entirely and so from the display. Any children that the widget may have are also destroyed. Anything connected to the destroyed widget, such as bindings, are also cleaned up automatically.

For example, to create a window containing a button that is destroyed when the button is pressed: button .b -text "Die!" -command { destroy . } pack .b

creates a button .b displaying the text ‘Die!’ which runs the command destroy . when it is pressed. Because the widget . is the main toplevel widget or window, running that command will kill the entire application associated with that button.

The command update is used to process any pending Tk events. An event is not just such things as moving the mouse but also updating the display for newly created and displayed widgets. This may be necessary in that usually Tk draws widgets only when it is idle.

Using the update command forces Tk to stop and handle any outstanding events including updating the display to its actually current state, i.e. flushing out the pending display of any widgets. (This is analogous to the fflush command in C that flushes writes on a stream to disk. In Tk displaying of widgets is “buffered”; calling the update command flushes the buffer.)

35.3.8 What we have left out (Tk)

There are a number of Tk features that we have not described but we list some of them here in case the reader is interested. Refer to the Tk manual for more explanation.

photo wm creating full color images through the command setting and getting window attributes selection and focus commands modal interaction

(not recommended)

Chapter 35: How to use the Tcl/Tk library 481 send sending messages between Tk applications

35.3.9 Example pure Tcl/Tk program

To show some of what can be done with Tcl/Tk, we will show an example of part of a GUI for an 8-queens program. Most people will be familiar with the 8-queens problem: how to place 8 queens on a chess board such that they do not attack each other according to the normal rules of chess.

Our example will not be a program to solve the 8-queens problem (that will come later in the tutorial) but just the Tcl/Tk part for displaying a solution. The code can be found in library(’tcltk/examples/ex18.tcl’).

The way an 8-queens solution is normally presented is as a list of numbers. The position of a number in the list indicates the column the queens is placed at and the number itself indicates the row. For example, the Prolog list [8, 7, 6, 5, 4, 3, 2, 1] would indicate 8 queens along the diagonal starting a column 1, row 8 and finishing at column 8 row 1.

The problem then becomes, given this list of numbers as a solution, how to display the solution using Tcl/Tk. This can be divided into two parts: how to display the initial empty chess board, and how to display a queen in one of the squares.

Here is our code for setting up the chess board:

#! /usr/bin/wish proc setup_board { } {

# create container for the board frame .queens

# loop of rows and columns for {set row 1} {$row <= 8} {incr row} { for {set column 1} {$column <= 8} {incr column} {

# create label with a queen displayed in it label .queens.$column-$row -bitmap @bitmaps/q64s.bm -relief flat

# choose a background color depending on the position of the

# square; make the queen invisible by setting the foreground

# to the same color as the background if { [expr ($column + $row) % 2] } {

.queens.$column-$row config -background #ffff99

.queens.$column-$row config -foreground #ffff99

} else {

.queens.$column-$row config -background #66ff99

.queens.$column-$row config -foreground #66ff99

}

# place the square in a chess board grid

482 SICStus Prolog grid .queens.$column-$row -row $row -column $column -padx 1 -pady 1

}

} pack .queens

} setup_board

The first thing that happens is that a frame widget is created to contain the board. Then there are two nested loops that loop over the rows and columns of the chess board. Inside the loop, the first thing that happens is that a label widget is created. It is named using the row and column variables so that it can be easily referenced later. The label will not be used to display text but to display an image, a bitmap of a queen. The label creation command therefore has the special argument -bitmap @q64s.bm which says that the label will display the bitmap loaded from the file ‘q64s.bm’.

The label with the queen displayed in it has now been created. The next thing that happens is that the background color of the label (square) is chosen. Depending on the position of the square it becomes either a “black” or a “white” square. At the same time, the foreground color is set to the background color. This is so that the queen (displayed in the foreground color) will be invisible, at least when the board is first displayed.

The final action in the loop is to place the label (square) in relation to all the other squares for display. A chess board is a simple grid of squares, and so this is most easily done through the grid geometry manager.

After the board has been setup square-by-square it still needs to be displayed which is done by pack-ing the outermost frame widget.

To create and display a chess board widget, all that is needed is to call the procedure setup_board which creates the chess board widget.

Once the chess board has been displayed, we need to be able to take a solution, a list of rows ordered by column, and place queens in the positions indicated.

Taking a topdown approach, our procedure for taking a solution and displaying is as follows: proc show_solution { solution } { clear_board set column 1 foreach row $solution { place_queen $column $row incr column

}

}

Chapter 35: How to use the Tcl/Tk library 483

This takes a solution in solution, clears the board of all queens, and then places each queen from the solution on the board.

Next we will handle clearing the board: proc clear_board { } { for { set column 1 } {$column <= 8} {incr column} { reset_column $column

}

} proc reset_column { column } { for {set row 1 } { $row <= 8 } {incr row} { set_queens $column $row off

}

} proc set_queens { column row state } { if { $state == "on" } {

.queens.$column-$row config -foreground black

} else {

.queens.$column-$row config -foreground [.queens.$column-$row cget -background]

}

}

The procedure clear_board clears the board of queens by calling the procedure reset_ column for each of the 8 columns on a board. reset_column goes through each square of a column and sets the square to off through set_queens. In turn, set_queens sets the foreground color of a square to black if the square is turned on, thus revealing the queen bitmap, or sets the foreground color of a square to its background color, thus making the queens invisible, if it is called with something other than on.

That handles clearing the board, clearing a column or turning a queen on or off on a particular square.

The final part is place_queen: proc place_queen { column row } { reset_column $column set_queens $column $row on

}

This resets a column so that all queens on it are invisible and then sets the square with coordinates given in row and column to on.

A typical call would be: show_solution "1 2 3 4 5 6 7 6 8"

484 SICStus Prolog

8-Queens Display In Tcl/Tk which would display queens along a diagonal. (This is of course not a solution to the 8queens problem. This Tcl/Tk code only displays possible queens solutions; it doesn’t check if the solution is valid. Later we will combine this Tcl/Tk display code with Prolog code for generating solutions to the 8-queens problem.)

35.4 The Prolog library

Now we have covered the wonders of Tcl/Tk, we come to the real meat of the tutorial: how to couple the power of Tcl/Tk with the power of SICStus Prolog.

Tcl/Tk is included in SICStus Prolog by loading a special library. The library provides a bidirectional interface between Tcl/Tk and Prolog.

Chapter 35: How to use the Tcl/Tk library 485

35.4.1 How it works - an overview

Before describing the details of the Tcl/Tk library we will give an overview of how it works with the Prolog system.

The Tcl/Tk library provides a loosely coupled integration of Prolog and Tcl/Tk. By this we mean that the two systems, Prolog and Tcl/Tk, although joined through the library, are mostly separate; Prolog variables have nothing to do with Tcl variables, Prolog and Tcl program states are separate, and so on.

The Tcl/Tk library extends Prolog so that Prolog can create a number of independent Tcl interpreters with which it can interact. Basically, there is a predicate which when executed creates a Tcl interpreter and returns a handle with which Prolog can interact with the interpreter.

Prolog and a Tcl interpreter interact, and so communicate and cooperate, through two ways:

1. One system evaluates a code fragment in the other system and retrieves the result.

For example, Prolog evaluates a Tcl code fragment in an attached Tcl interpreter and gets the result of the evaluation in a Prolog variable. Similarly, a Tcl interpreter can evaluate a Prolog goal and get the result back through a Tcl variable.

This is synchronous communication in that the caller waits until the callee has finished their evaluation and reads the result.

2. One system passing a “message” to the other on an “event” queue.

This is asynchronous communication in that the receiver of the message can read the message whenever it likes, and the sender can send the message without having to wait for a reply.

The Tk part of Tcl/Tk comes in because an attached Tcl interpreter may be extended with the Tk widget set and so be a Tcl/Tk interpreter. This makes it possible to add GUIs to a Prolog application: the application loads the Tcl/Tk Prolog library, creates a Tcl/Tk interpreter, and sends commands to the interpreter to create a Tk GUI. The user interacts with the GUI and therefore with the underlying Prolog system.

There are two main ways to partition the Tcl/Tk library functions: by function, i.e. the task they perform; or by package, i.e. whether they are Tcl, Tk, or Prolog functions. We will describe the library in terms of the former because it fits in with the tutorial style better, but at the end is a summary section that summarizes the library functions both ways.

Taking the functional approach, the library can be split into six function groups:

• basic functions

− loading the library

− creating and destroying Tcl and Tcl/Tk interpreters

• evaluation functions

486 SICStus Prolog

− evaluating Tcl expressions from Prolog

− evaluating Prolog expressions from Tcl

• Prolog event functions

− handling the Prolog/Tcl event queue

• Tk event handling

• passing control to Tk

• housekeeping functions

We go through each group in turn.

35.4.2 Basic functions

35.4.2.1 Loading the library

First we need to know how to load the Tcl/Tk library into Prolog. This is done through the use_module/1 predicate thus:

| ?- use_module(library(tcltk)).

35.4.2.2 Creating a Tcl interpreter

The heart of the system is the ability to create an embedded Tcl interpreter with which the Prolog system can interact. A Tcl interpreter is created within Prolog through a call to tcl_new/1: tcl_new(-TclInterpreter) which creates a new interpreter, initializes it, and returns a reference to it in the variable

TclInterpreter. The reference can then be used in subsequent calls to manipulate the interpreter. More than one Tcl interpreter object can be active in the Prolog system at any one time.

35.4.2.3 Creating a Tcl interpreter extended with Tk

To start a Tcl interpreter extended with Tk, the tk_new/2 predicate is called from Prolog.

It has the following form: tk_new(+Options, -TclInterpreter) which returns through the variable TclInterpreter a handle to the underlying Tcl interpreter.

The usual Tcl/Tk window pops up after this call is made and it is with reference to that window that subsequent widgets are created. As with the tcl_new/1 predicate, many

Tcl/Tk interpreters may be created from Prolog at the same time through calls to tk_ new/2.

The Options part of the call is a list of some (or none) of the following elements:

Chapter 35: How to use the Tcl/Tk library 487 top_level_events

This allows Tk events to be handled while Prolog is waiting for terminal input; for example, while the Prolog system is waiting for input at the Prolog prompt.

Without this option, Tk events are not serviced while the Prolog system is waiting for terminal input. (For information on Tk events; see

Section 35.3.6

[Event Handling], page 477

).

NOTE: This option is not currently supported under Microsoft Windows.

name(+ApplicationName)

This gives the main window a title ApplicationName. This name is also used for communicating between Tcl/Tk applications via the Tcl send command. (send is not covered in this document. Please refer to the Tcl/Tk documentation.) display(+Display )

(This is X windows specific.) Gives the name of the screen on which to create the main window. If this is not given, the default display is determined by the

DISPLAY environment variable.

An example of using tk_new/2:

| ?- tk_new([top_level_events, name(’My SICStus/Tk App’)], Tcl).

which creates a Tcl/Tk interpreter, returns a handle to it in the variable Tcl and Tk events are serviced while Prolog is waiting at the Prolog prompt. The window that pops up will have the title My SICStus/Tk App.

The reference to a Tcl interpreter returned by a call to tk_new/2 is used in the same way and in the same places as a reference returned by a call to tcl_new/1. They are both references to Tcl interpreters.

35.4.2.4 Removing a Tcl interpreter

To remove a Tcl interpreter from the system, use the tcl_delete/1 predicate: tcl_delete(+TclInterpreter) which given a reference to a Tcl interpreter, closes down the interpreter and removes it.

The reference can be for a plain Tcl interpreter or for a Tk enhanced one; tcl_delete/1 removes both kinds.

35.4.3 Evaluation functions

There are two functions in this category: Prolog extended to be able to evaluate Tcl expressions in a Tcl interpreter; Tcl extended to be able to evaluate a Prolog expression in the Prolog system.

488 SICStus Prolog

35.4.3.1 Command format

There is a mechanism for describing Tcl commands in Prolog as Prolog terms. This is used in two ways: firstly, to be able to represent Tcl commands in Prolog so that they can be subsequently passed to Tcl for evaluation; and secondly for passing terms back from Tcl to

Prolog by doing the reserve transformation.

Why not represent a Tcl command as a simple atom or string? This can indeed be done, but commands are often not static and each time they are called require slightly different parameters. This means constructing different atoms or strings for each command in Prolog, which are expensive operations. A better solution is to represent a Tcl command as a

Prolog term, something that can be quickly and efficiently constructed and stored by a

Prolog system. Variable parts to a Tcl command (for example command arguments) can be passed in through Prolog variables.

In the special command format, a Tcl command is specified as follows.

Command

--> Name

| chars(PrologString)

| write(Term)

| format(Fmt,Args)

| dq(Command)

| br(Command)

| sqb(Command)

| min(Command)

| dot(ListOfNames)

| ListOfCommands

{ a list of character codes }

Name

--> Atom

| Number

{ other than [] }

ListOfCommands --> []

| [ Command | ListOfCommands ] where

ListOfNames --> []

| [ Name | ListOfNames ]

Atom

Number denote their printed representations chars(PrologString) denotes the string represented by PrologString (a list of character codes) write(Term) denotes the string that is printed by the corresponding built-in predicate format(Fmt, Args) denotes the string that is printed by the corresponding built-in predicate

Chapter 35: How to use the Tcl/Tk library 489 dq(Command) denotes the string specified by Command, enclosed in double quotes br(Command) denotes the string specified by Command, enclosed in curly brackets sqb(Command) denotes the string specified by Command, enclosed in square brackets min(Command) denotes the string specified by Command, immediately preceded by a hyphen dot(ListOfName) denotes the widget path specified by ListOfName, preceded by and separated by dots

ListOfCommands denotes the string denoted by each element, separated by spaces

Examples of command specifications and the resulting Tcl code: set x 32

==> set x 32

[set, x, br([a, b, c])]

==> set x {a b c}

[dot([panel,value_info,name]), configure, min(text), br(write(’$display’/1))]

==> {.panel.value_info.name configure -text {$display/1}}

35.4.3.2 Evaluating Tcl expressions from Prolog

Prolog calls Tcl through the predicate tcl_eval/3 which has the following form: tcl_eval(+TclInterpreter, +Command, -Result) which causes the interpreter TclInterpreter to evaluate the Tcl command Command and return the result Result. The result is a string (a list of character codes) that is the usual return string from evaluating a Tcl command. Command is not just a simple Tcl command string (although that is a possibility) but a Tcl command represented as a Prolog term in the special Command Format (see

Section 35.4.3.1 [Command Format], page 488

).

Through tcl_eval/3, Prolog has a method of synchronous communication with an embedded Tcl interpreter and a way of manipulating the state of the interpreter.

An example:

?- tcl_new(Interp), tcl_eval(Interp, ’set x 1’, _), tcl_eval(Interp, ’incr x’, R) which creates a Tcl interpreter the handle of which is stored in the variable Interp. Then variable x is set to the value "1" and then variable x is incremented and the result returned

490 SICStus Prolog in R as a string. The result will be "2". By evaluating the Tcl commands in separate tcl_eval/3 calls, we show that we are manipulating the state of the Tcl interpreter and that it remembers its state between manipulations.

It is worth mentioning here also that because of the possibility of the Tcl command causing an error to occur in the Tcl interpreter, two new exceptions are added by the tcltk library: tcl_error(Goal, Message) tk_error(Goal, Message) where Message is a list of character codes detailing the reason for the exception. Also two new user:portray_message/2 rules are provided so that any such uncaught exceptions are displayed at the Prolog top-level as

[TCL ERROR: Goal - Message]

[TK ERROR: Goal - Message] respectively.

These exception conditions can be raised/caught/displayed in the usual way through the user module builtin predicates raise_exception/3, on_exception/1, and portray_ message/2.

As an example, the following Prolog code will raise such an exception:

| ?- tcl_new(X), tcl_eval(X, ’wilbert’, R).

which causes a tcl_error/2 exception and prints the following:

{TCL ERROR: tcl_eval/3 - invalid command name "wilbert"} assuming that there is no command or procedure defined in Tcl called wilbert.

35.4.3.3 Evaluating Prolog expressions from Tcl

The Tcl interpreters created through the SICStus Prolog Tcl/Tk library have been extended to allow calls to the underlying Prolog system.

To evaluate a Prolog expression in the Prolog system from a Tcl interpreter, the new prolog

Tcl command is invoked. It has the following form: prolog PrologGoal where PrologGoal is the printed form of a Prolog goal. This causes the goal to be executed in Prolog. It will be executed in the user module unless it is prefixed by a module name.

Execution is always deterministic.

The return value of the command either of the following:

"1", if execution succeeded,

Chapter 35: How to use the Tcl/Tk library 491

"0", if execution failed,

If succeeded (and "1" was returned) then any variable in PrologGoal that has become bound to a Prolog term will be returned to Tcl in the Tcl array named prolog_variables with the variable name as index. The term is converted to Tcl using the same conversion as used

for Tcl commands ( Section 35.4.3.1 [Command Format], page 488 ).

An example: test_callback(Result) :tcl_new(Interp), tcl_eval(Interp,

’if {[prolog "foo(X,Y,Z)"] == 1} \\

{list $prolog_variables(X) \\

$prolog_variables(Y) \\

$prolog_variables(Z)}’,

Result), tcl_delete(Interp).

foo(1, bar, [a, b, c]).

When called with the query:

| ?- test_callback(Result).

will succeed, binding the variable Result to:

"1 bar {a b c}"

This is because execution of the tcl_eval/3 predicate causes the execution of the prolog command in Tcl, which executes foo(X, Y, Z) in Prolog making the following bindings:

X = 1, Y = bar, Z = [a, b, c]. The bindings are returned to Tcl in the associative array prolog_variables where prolog_variables(X) is "1", prolog_variables(Y) is "bar", and prolog_variables(Z) is "a b c". Then Tcl goes on to execute the list command as list "1" "bar" "a b c" which returns the result

"1 bar {a b c}"

(remember: nested lists magically get represented with curly brackets) which is the string returned in the Result part of the Tcl call, and is ultimately the returned in the Result variable of the top-level call to test_callback(Result).

If an error occurs during execution of the prolog Tcl command, a tcl_error/2 exception will be raised. The message part of the exception will be formed from the string ‘Exception during Prolog execution: ’ appended to the Prolog exception message. An example is the following:

?- tcl_new(T), tcl_eval(T, ’prolog wilbert’, R).

492 SICStus Prolog which will print

{TCL ERROR: tcl_eval/3 - Exception during Prolog execution: wilbert existence_error(wilbert,0,procedure,user:wilbert/0,0)} at the Prolog top-level, assuming that the predicate wilbert/0 is not defined on the Prolog side of the system. (This is a tcl_error exception containing information about the underlying exception, an existence_error exception, which was caused by trying to execute the non-existent predicate wilbert.)

35.4.4 Event functions

35.4.4.1 Evaluate a Tcl expression and get Prolog events

Another way for Prolog to communicate with Tcl is through the predicate tcl_event/3: tcl_event(+TclInterpreter, +Command, -Events)

This is similar to tcl_eval/3 in that the command Command is evaluated in the Tcl interpreter TclInterpreter but the call returns a list of events in Events rather than the result of the Tcl evaluation. Command is again a Tcl command represented as a Prolog term in the special Command Format described previously (

Section 35.4.3.1 [Command

Format], page 488

).

This begs the questions what are these events and where does the event list come from? The

Tcl interpreters in the SICStus Prolog Tcl/Tk library have been extended with the notion of a Prolog event queue. (This is not available in plain standalone Tcl interpreters.) The

Tcl interpreter can put events on the event queue by executing a prolog_event command.

Each event is a Prolog term. So a Tcl interpreter has a method of putting Prolog terms onto a queue, which can later be picked up by Prolog as a list as the result of a call to tcl_event/3. (It may be helpful to think of this as a way of passing messages as Prolog terms from Tcl to Prolog.)

A call to tcl_event/3 blocks until there is something on the event queue.

A second way of getting Prolog events from a Prolog event queue is through the tk_next_ event/[2,3] predicates. These have the form: tk_next_event(+TclInterpreter, -Event) tk_next_event(+ListOrBitMask, +TclInterpreter, -Event) where TclInterpreter reference to a Tcl interpreter and Event is the Prolog term at the head of the associated Prolog event queue. (The ListOrBitMask feature will be described below in the Housekeeping section when we talk about Tcl and Tk events; see

Section 35.4.7

[Housekeeping], page 497 .).

(We will meet tk_next_event/[2,3] again later when we discuss how it can be used to service Tk events; see

Section 35.4.5 [Servicing Tk Events], page 495 ).

If the interpreter has been deleted then the empty list [] is returned.

Chapter 35: How to use the Tcl/Tk library 493

35.4.4.2 Adding events to the Prolog event queue

The Tcl interpreters under the SICStus Prolog library are extended with a command, prolog_event, for adding events to a Prolog event queue.

The prolog_event command has the following form: prolog_event Terms ...

where Terms are strings that contain the printed representation of Prolog terms. These are stored in a queue and retrieved as Prolog terms by tcl_event/3 or tk_next_event/[2,3]

(described above).

An example of using the prolog_event command: test_event(Event) :tcl_new(Interp), tcl_event(Interp, [prolog_event, dq(write(zap(42)))], Event), tcl_delete(Interp).

with the query:

| ?- test_event(Event).

will succeed, binding Event to the list [zap(42)].

This is because tcl_event converts its argument using the special Command Format conversion (see

Section 35.4.3.1 [Command Format], page 488 ) which yields the Tcl command

prolog_event "zap(42)". Ths command is evaluated in the Tcl interpreter referenced by the variable Interp. The effect of the command is to take the string given as argument to prolog_event (in this case "zap(42)") and to place it on the Tcl to Prolog event queue.

The final action of a tcl_event/3 call is to pick up any strings on the Prolog queue from

Tcl, add a trailing full stop and space to each string, and parse them as Prolog terms, binding Event to the list of values, which in this case is the singleton list [zap(42)]. (The queue is a list the elements of which are terms put there through calls to prolog_event).

If any of the Term-s in the list of arguments to prolog_event is not a valid representation of a Prolog term, then an exception is raised in Prolog when it is converted from the

Tcl string to the Prolog term via the Command Format (see

Section 35.4.3.1 [Command

Format], page 488

).

35.4.4.3 An example

As an example of using the prolog event system supplied by the tcltk library, we will return to our 8-queens example but now approaching from the Prolog side rather than the Tcl/Tk side:

:- use_module(library(tcltk)).

setup :-

494 SICStus Prolog tk_new([name(’SICStus+Tcl/Tk - Queens’)], Tcl), tcl_eval(Tcl, ’source queens.tcl’, _), tk_next_event(Tcl, Event),

( Event = next -> go(Tcl),

; closedown(Tcl)

).

closedown(Tcl) :tcl_delete(Tcl).

go(Tcl) :tcl_eval(Tcl, ’clear_board’, _), queens(8, Qs), show_solution(Qs, Tcl), tk_next_event(Tcl, Event),

( Event = next -> fail

; closedown(Tcl)

).

go(Tcl) :tcl_eval(Tcl, ’disable_next’, _), tcl_eval(Tcl, ’clear_board’, _), tk_next_event(Tcl, _Event), closedown(Tcl).

This is the top-level fragement of the Prolog side of the 8-queens example. It has three predicates: setup/0, closedown/1, and go/1. setup/0 simply creates the Tcl interpreter, loads the Tcl code into the interpreter using a call to tcl_eval/3 (which also initialises the display) but then calls tk_next_event/2 to wait for a message from the Tk side.

The Tk part that sends prolog_event-s to Prolog looks like this: button .next -text next -command {prolog_event next} pack .next

button .stop -text stop -command {prolog_event stop} pack .stop

that is two buttons, one that sends the atom next, the other that sends the atom stop.

They are used to get the next solution and to stop the program respectively.

So if the user presses the next button in the Tk window, then the Prolog program will receive a next atom via a prolog_event/tk_next_event pair, and the program can proceed to execute go/1.

go/1 is a failure driven loop that generates 8-queens solutions and displays them. First it generates a solution in Prolog and displays it through a tcl_eval/3 call. Then it waits again for a Prolog events via tk_next_event/2. If the term received on the Prolog event queue is next, corresponding to the user pressing the “next solution” button, then fail is executed and the next solution found, thus driving the loop.

Chapter 35: How to use the Tcl/Tk library 495

If the stop button is pressed then the program does some tidying up (clearing the display and so on) and then executes closedown/1, which deletes the Tcl interpreter and the corresponding Tk windows altoegther, and the program terminates.

This example fragment show how it is possible for a Prolog program and a Tcl/Tk program to communicate via the Prolog event queue.

35.4.5 Servicing Tcl and Tk events

The notion of an event in the Prolog+Tcl/Tk system is overloaded. We have already come across the following kinds of events:

• Tk widget events captured in Tcl/Tk through the bind command

• Prolog queue events controlled through the tcl_event/3, tk_next_event(2,3), and prolog_event functions

It is further about to be overloaded with the notion of Tcl/Tk events. It is possible to create event handlers in Tcl/Tk for reacting to other kinds of events. We will not cover them here but describe them so that the library functions are understandable and in case the user needs these features in an advanced application.

There are the following kinds of Tcl/Tk events: idle events happen when the Tcl/Tk system is idle file events happen when input arrives on a file handle that has a file event handler attached to it timer events happen when a Tcl/Tk timer times out window events when something happens to a Tk window, such as being resized or destroyed

The problem is that in advanced Tcl/Tk applications it is possible to create event handlers for each of these kinds of event, but they are not normally serviced while in Prolog code.

This can result in unresponsive behavior in the application; for example, if window events are not serviced regularly then if the user tries to resize a Tk window, it will not resize in a timely fashion.

The solution to this is to introduce a Prolog predicate that passes control to Tk for a while so that it can process its events, tk_do_one_event/[0,1]. If an application is unresponsive because it is spending a lot of time in Prolog and is not servicing Tk events often enough, then critical sections of the Prolog code can be sprinkled with calls to tk_do_one_ event/[0,1] to alleviate the problem.

tk_do_one_event/[0,1] has the following forms: tk_do_one_event tk_do_one_event(+ListOrBitMask)

496 SICStus Prolog which passes control to Tk to handle a single event before passing control back to Prolog.

The type of events handled is passed through the ListOrBitMask variable. As indicated, this is either a list of atoms which are event types, or a bit mask as specified in the Tcl/Tk documentation. (The bit mask should be avoided for portability between Tcl/Tk versions.)

The ListOrBitMask list can contain the following atoms: tk_dont_wait don’t wait for new events, process only those that are ready tk_x_events tk_window_events process window events tk_file_events process file events tk_timer_events process timer events tk_idle_events process Tk_DoWhenIdle events tk_all_events process any event

Calling tk_do_one_event/0 is equivalent to a call to tk_do_one_event/1 with all flags set.

A call to either of these predicates succeeds only if an event of the appropriate type happens in the Tcl/Tk interpreter. If there are no such events, then tk_do_one_event/1 will fail if the tk_dont_wait wait flag is present, as will tk_do_one_event/0 which has that flag set implicitly.

If the tk_dont_wait flag is not set, then a call to tk_do_one_event/1 will block until an appropriate Tk event happens (in which case it will succeed).

It is straight forward to define a predicate which handles all Tk events and then returns: tk_do_all_events :tk_do_one_event, !, tk_do_all_events.

tk_do_all_events.

The predicate tk_next_event/[2,3] is similar to tk_do_one_event/[0,1] except that it processes Tk events until at least one Prolog event happens. (We came across this predicate before when discussing Prolog event queue predicates. This shows the overloading of the notion event where we have a predicate that handles both Tcl/Tk events and Prolog queue events.)

It has the following forms: tk_next_event(+TclInterpreter, -Event)

Chapter 35: How to use the Tcl/Tk library 497 tk_next_event(+ListOrBitMask, +TclInterpreter, -Event)

The Prolog event is returned in the variable Event and is the first term on the Prolog event queue associated with the interpreter TclInterpreter. (Prolog events are initiated on the

Tcl side through the new Tcl command prolog_event, covered earlier; see Section 35.4.4.2

[prolog event], page 493 ).

35.4.6 Passing control to Tk

There is a predicate for passing control completely over to Tk, the tk_main_loop/0 command. This passes control to Tk until all windows in all the Tcl/Tk interpreters in the

Prolog have have been destroyed: tk_main_loop

35.4.7 Housekeeping functions

Here we will described the functions that do not fit into any of the above categories and are essentially housekeeping functions.

There is a predicate that returns a reference to the main window of a Tcl/Tk interpreter: tk_main_window(+TclInterpreter, -TkWindow ) which given a reference to a Tcl interpreter Tclnterpreter, returns a reference to its main window in TkWindow.

The window reference can then be used in tk_destroy_window/1: tk_destroy_window(+TkWindow ) which destroys the window or widget referenced by TkWindow and all of its sub-widgets.

The predicate tk_make_window_exist/1 also takes a window reference: tk_make_window_exist(+TkWindow ) which causes the window referenced by TkWindow in the Tcl interpreter TclInterpreter to be immediately mapped to the display. This is useful because normally Tk delays displaying new information for a long as possible (waiting until the machine is idle, for example), but using this call causes Tk to display the window immediately.

There is a predicate for determining how many main windows, and hence Tcl/Tk interpreters (excluding simple Tcl interpreters), are currently in use: tk_num_main_windows(-NumberOfWindows) which returns an integer in the variable NumberOfWindows.

498 SICStus Prolog

35.4.8 Summary

The functions provided by the SICStus Prolog Tcl/Tk library can be grouped in two ways: by function, and by package.

By function, we can group them like this:

• basic functions tcl_new/1 create a Tcl interpreter tcl_delete/1 remove a Tcl interpreter tk_new/2 create a Tcl interpreter with Tk extensions

• evaluation functions tcl_eval/3 evaluate a Tcl expression from Prolog prolog evaluate a Prolog expression from Tcl

• Prolog event queue functions tcl_event/3 evaluate a Tcl expression and return a Prolog queue event list tk_next_event/[2,3] pass control to Tk until a Prolog queue event happens and return the head of the queue prolog_event place a Prolog term on the Prolog event queue from Tcl

• servicing Tcl and Tk events tk_do_one_event/[0,1] pass control to Tk until one Tk event is serviced tk_next_event/[2,3] also services Tk events but returns when a Prolog queue event happens and returns the head of the queue

• passing control completely to Tk tk_main_loop/0 control passed to Tk until all windows in all Tcl/Tk interpreters are gone

• housekeeping tk_main_window/2 return reference to main in of a Tcl/Tk interpreter tk_destroy_window/1 destroy a window or widget tk_make_window_exist/1 force display of a window or widget

Chapter 35: How to use the Tcl/Tk library 499 tk_num_main_windows/1 return a count of the total number of Tk main windows existing in the system

By package, we can group them like this:

• predicates for Prolog to interact with Tcl interpreters tcl_new/1 create a Tcl interpreter tcl_delete/1 remove a Tcl interpreter tcl_eval/3 evaluate a Tcl expression from Prolog tcl_event/3 evaluate a Tcl expression and return a Prolog event list

• predicates for Prolog to interact with Tcl interpreters with Tk extensions tk_new/2 create a Tcl interpreter with Tk extensions tk_do_one_event/[0,1] pass control to Tk until one Tk event is serviced tk_next_event/[2,3] also services Tk events but returns when a Prolog queue event happens and returns the head of the queue tk_main_loop/0 control passed to Tk until all windows in all Tcl/Tk interpreters are gone tk_main_window/2 return reference to main in of a Tcl/Tk interpreter tk_destroy_window/1 destroy a window or widget tk_make_window_exist/1 force display of a window or widget tk_num_main_windows/1 return a count of the total number of Tk main windows existing in the system

• commands for the Tcl interpreters to interact with the Prolog system prolog evaluate a Prolog expression from Tcl prolog_event place a Prolog term on the Prolog event queue from Tcl

In the next section we will discuss how to use the tcltk library to build graphical user interfaces to Prolog applications. More specifically we will discuss the ways in which cooperation between Prolog and Tcl/Tk can be arranged: how to achieve them, and their benefits.

500 SICStus Prolog

35.5 Putting it all together

At this point we now know Tcl, the Tk extensions, and how they can be integrated into

SICStus Prolog through the tcltk library module. The next problem is how to get all this to work together to produce a coherent application. Because Tcl can make Prolog calls and

Prolog can make Tcl calls it is easy to create programming spaghetti. In this section we will discuss some general principles of organizing the Prolog and Tcl code to make writing applications easier.

The first thing to do is to review the tools that we have. We have two programming systems:

Prolog and Tcl/Tk. They can interact in the following ways:

• Prolog evaluates a Tcl expression in a Tcl interpreter, using tcl_eval

• Tcl evaluates a Prolog expression in the Prolog interpreter, using prolog

• Prolog evaluates a Tcl expression in a Tcl interpreter and waits for a Prolog event, using tcl_event

• Prolog waits for a Prolog event from a Tcl interpreter, using tk_next_event

• Tcl sends a Prolog predicate to Prolog on a Prolog event queue using prolog_event

With these interaction primitives there are three basic ways in which Prolog and Tcl/Tk can be organized:

1. Tcl the master, Prolog the slave: program control is with Tcl, which makes occasional calls to Prolog, through the prolog function.

2. Prolog the master, Tcl the slave: program control is with Prolog which makes occasional call to Tcl through the tcl_eval function

3. Prolog and Tcl share control: program control is shared with Tcl and Prolog interacting via the Prolog event queue, through tcl_event, tk_next_event, and prolog_event.

These are three ways of organizing cooperation between Tcl/Tk and Prolog to produce an application. In practice an application my use only one of these methods throughout, or may use a combination of them where appropriate. We describe them here so that the developer can see the different patterns of organization and can pick those relevant to their application.

35.5.1 Tcl the master, Prolog the slave

This is the classical way that GUIs are bolted onto applications. The slave (in this case

Prolog) sits mostly idle while the user interacts with the GUI, for example filling in a form.

When some action happens in the GUI that requires information from the slave (a form submit, for example), the slave is called, performs a calculation, and the GUI retrieves the result and updates its display accordingly.

In our Prolog+Tcl/Tk setting this involves the following steps:

• start Prolog and load the tcltk library

• load Prolog application code

Chapter 35: How to use the Tcl/Tk library 501

• start a Tcl/Tk interpreter through tk_new/2

• setup the Tk GUI through calls to tcl_eval/3

• pass control to Tcl/Tk through tk_main_loop

Some of The Tk widgets in the GUI will have “callbacks” to Prolog, i.e. they will call the prolog Tcl command. When the Prolog call returns, the values stored in the prolog_ variables array in the Tcl interpreter can then be used by Tcl to update the display.

Here is a simple example of a callback. The Prolog part is this:

:- use_module(library(tcltk)).

hello(’world’).

go :tk_new([], Tcl), tcl_eval(Tcl, ’source simple.tcl’, _), tk_main_loop.

which just loads the tcltk library module, defines a hello/1 data clause, and go/0 which starts a new Tcl/Tk interpreter, loads the code simple.tcl into it, and passes control to

Tcl/Tk.

The Tcl part, simple.tcl, is this: label .l -textvariable tvar button .b -text "push me" -command { call_and_display } pack .l .b -side top proc call_and_display { } { global tvar prolog "hello(X)" set tvar $prolog_variables(X)

} which creates a label, with an associated text variable, and a button, that has a call back procedure, call_and_display, attached to it. When the button is pressed, call_and_ display is executed, which simply evaluates the goal hello(X) in Prolog and the text variable of the label .l to whatever X becomes bound to, which happens to be ‘world’. In short, pressing the button causes the word ‘world’ to be displayed in the label.

Having Tcl as the master and Prolog as the slave, although a simple model to understand and implement, does have disadvantages. The Tcl command prolog is deterministic, i.e.

it can return only one result with no backtracking. If more than one result is needed it means either performing some kind of all-solutions search and returning a list of results for

Tcl to process, or asserting a clause into the Prolog clause store reflecting the state of the computation.

502 SICStus Prolog

Here is an example of how an all-solutions search can be done. It is a program that calculates the outcome of certain ancestor relationships; i.e. enter the name of a person, click on a button and it will tell you the mother, father, parents or ancestors of that person.

The Prolog portion looks like this (see also library(’tcltk/examples/ancestors.pl’)):

:- use_module(library(tcltk)).

go :- tk_new([name(’ancestors’)], X), tcl_eval(X, ’source ancestors.tcl’, _), tk_main_loop, tcl_delete(X).

father(ann, fred).

father(fred, jim).

mother(ann, lynn).

mother(fred, lucy).

father(jim, sam).

parent(X, Y) :- mother(X, Y).

parent(X, Y) :- father(X, Y).

ancestor(X, Y) :- parent(X, Y).

ancestor(X, Y) :- parent(X, Z), ancestor(Z, Y).

all_ancestors(X, Z) :- findall(Y, ancestor(X, Y), Z).

all_parents(X, Z) :findall(Y, parent(X, Y), Z).

This program consists of three parts. The first part is defined by go/0, the now familiar way in which a Prolog program can create a Tcl/Tk interpreter, load a Tcl file into that interpreter, and pass control over to the interpreter.

The second part is a small database of mother/father relationships between certain people through the clauses mother/2 and father/2.

The third part is a set of “rules” for determining certain relationships between people: parent/2, ancestor/2, all_ancestors/2 and all_parents/2.

The Tcl part looks like this (see also library(’tcltk/examples/ancestors.tcl’)):

#!/usr/bin/wish

# set up the tk display

# construct text filler labels label .search_for -text "SEARCHING FOR THE" -anchor w label .of

label .gives

-text "OF"

-text "GIVES"

-anchor w

-anchor w

Chapter 35: How to use the Tcl/Tk library 503

# construct frame to hold buttons frame .button_frame

# construct radio button group radiobutton .mother

radiobutton .father

-text mother

-text father

-variable type -value mother

-variable type -value father radiobutton .parents

-text parents -variable type -value parents radiobutton .ancestors -text ancestors -variable type -value ancestors

# add behaviors to radio buttons

.mother

.father

config -command { one_solution mother $name} config -command { one_solution father $name}

.parents

config -command { all_solutions all_parents $name}

.ancestors config -command { all_solutions all_ancestors $name}

# create entry box and result display widgets entry .name -textvariable name label .result -text ">>> result <<<" -relief sunken -anchor nw -justify left

# pack buttons into button frame pack .mother .father .parents .ancestors -fill x -side left -in .button_frame

# pack everything together into the main window pack .search_for .button_frame .of .name .gives .result -side top -fill x

# now everything is set up

# defined the callback procedures

# called for one solution results proc one_solution { type name } { if [prolog "${type}(’$name’, R)"] { display_result $prolog_variables(R)

} else { display_result ""

}

}

# called for all solution results proc all_solutions { type name } { prolog "${type}(’$name’, R)" display_result $prolog_variables(R)

}

# display the result of the search in the results box proc display_result { result } { if { $result != "" } {

# create a multiline result

.result config -text $result

} else {

504

}

}

.result config -text "*** no result ***"

SICStus Prolog

Ancestors Calculator

This program is in two parts. The first part sets up the Tk display which consists of four radiobuttons to choose the kind of relationship we want to calculate, an entry box to put the name of the person we want to calculate the relationship over, and a label in which to display the result.

Each radio buttons has an associated callback. Clicking on the radio button will invoke the appropriate callback, apply the appropriate relationship to the name entered in the text entry box, and display the result in the results label.

The second part consists of the callback procedures themselves. There are actually just two of them: one for a single solution calculation, and one for an all-solutions calculation. The single solution callback is used when we want to know the mother or father as we know that a person can have only one of each. The all-solutions callback is used when we want to know the parents or ancestors as we know that these can return more than one results. (We could have used the all-solutions callback for the single solutions cases too, but we would like to illustrate the difference in the two approaches.) There is little difference between the two approaches, except that in the single solution callback, it is possible that the call to

Prolog will fail, so we wrap it in an if ... else construct to catch this case. An all-solutions search, however, cannot fail, and so the if ... else is not needed.

But there are some technical problems too with this approach. During a callback Tk events are not serviced until the callback returns. For Prolog callbacks that take a very short time to complete this is not a problem, but in other cases, for example during a long search call when the callback takes a significant time to complete, this can cause problems. Imagine that, in our example, we had a vast database describing the parent relationships of millions of people. Performing an all-solutions ancestors search could take a long time. The classic problem is that the GUI no longer reacts to the user until the callback completes.

The solution to this is to sprinkle tk_do_one_event/[0,1] calls throughout the critical parts of the Prolog code, to keep various kinds of Tk events serviced.

Chapter 35: How to use the Tcl/Tk library 505

If this method is used in its purest form, then it is recommended that after initialization and passing of control to Tcl, Prolog does not make calls to Tcl through tcl_eval/3. This is to avoid programming spaghetti. In the pure master/slave relationship it is a general principle that the master only call the slave, and not the other way around.

35.5.2 Prolog the master, Tk the slave

The second approach is to have Prolog be the master and Tk the slave. This is suitable when heavy processing is done in the Prolog code and Tk is used mostly to display the state of the computation in some way rather than as a traditional GUI; i.e. during computation

Prolog often makes calls to Tk to show some state but the user rarely interacts with the application.

In our Prolog+Tcl/Tk setting this involves the following steps:

• start Prolog and load the tcltk library

• load Prolog application code

• start a Tcl/Tk interpreter through tk_new/2

• setup the Tk GUI through calls to tcl_eval/3

• Prolog calls tcl_eval to update the Tk display

• values are passed to Prolog through the Result string of tcl_eval

Again it its purest form, Prolog makes calls to Tcl but Tcl does not make calls to Prolog.

The result of a call to Tcl is either passed back through the Result variable of a tcl_eval/3 call.

A good example of this is the Tcl/Tk display for our 8-queens problem, that we saw earlier; see

Section 35.3.9 [Queens Display], page 481 .

We will now fill out the example by presenting the Prolog master part. The Prolog program calculates a solution to the 8-queens problem and then makes calls Tcl/Tk to display the solution. In this way Tcl/Tk is the slave, just being used as a simple display.

We have already seen the Tcl/Tk part, but here is the Prolog part for generating a solution and displaying it:

:- use_module(library(tcltk)).

:- use_module(library(lists)).

go :tk_new([name(’SICStus+Tcl/Tk - Queens’)], Tcl), tcl_eval(Tcl, ’source queens.tcl’, _), tk_next_event(Tcl, Event), queens(8, Qs), reverse(L, LR), tcl_eval(Tcl, [show_solution, br(LR)], _), fail.

go.

506 SICStus Prolog queens(N, Qs) :range(1, N, Ns), queens(Ns, [], Qs).

queens(UnplacedQs, SafeQs, Qs) :select(Q, UnplacedQs, UnplacedQs1),

\+ attack(Q, SafeQs), queens(UnplacedQs1, [Q|SafeQs], Qs).

queens([], Qs, Qs).

attack(X, Xs) :- attack(X, 1, Xs).

attack(X, N, [Y|_Ys]) :- X is Y + N.

attack(X, N, [Y|_Ys]) :- X is Y - N.

attack(X, N, [_Y|Ys]) :-

N1 is N + 1, attack(X, N1, Ys).

range(M, N, [M|Ns]) :-

M < N,

M1 is M + 1, range(M1, N, Ns).

range(N, N, [N]).

:- go.

All this simply does it to create a Tcl/Tk interpreter, load the Tcl code for displaying queens into it, generate a solution to the 8-queens problem as a list of integers, and then calls show_solution/2 in the Tcl interpreter to display the solution. At the end of first clause for go/0 is a fail clause that turns go/0 into a failure driven loop. The result of this is that the program will calculate all the solutions to the 8-queens problem, displaying them rapidly one after the other, until there are none left.

35.5.3 Prolog and Tcl interact through Prolog event queue

In the previous two methods, one of the language systems was the master and the other slave, the master called the slave to perform some action or calculation, the slave sits waiting until the master calls it. We have seen that this has disadvantages when Prolog is the slave in that the state of the Prolog call is lost. Each Prolog call starts from the beginning unless we save the state using message database manipulation through calls to assert and retract.

Using the Prolog event queue, however, it is possible to get a more balanced model where the two language systems cooperate without either really being the master or the slave.

One way to do this is the following:

• Prolog is started

Chapter 35: How to use the Tcl/Tk libra