MIT Scheme Reference Manual - College of Engineering, Forestry

MIT Scheme Reference Manual - College of Engineering, Forestry
MIT Scheme Reference Manual
Edition 1.96
for Scheme Release 7.7.0
13 March 2002
by Chris Hanson
the MIT Scheme Team
and a cast of thousands
Copyright c 1988-2002 Massachusetts Institute of Technology
Permission is granted to copy, distribute and/or modify this document under the terms of
the GNU Free Documentation License, Version 1.1 or any later version published by the
Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with
no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free
Documentation License".
Acknowledgements
1
Acknowledgements
While "a cast of thousands" may be an overstatement, it is certainly the case that this
document represents the work of many people. First and foremost, thanks go to the authors
of the Revised^4 Report on the Algorithmic Language Scheme, from which much of this
document is derived. Thanks also to BBN Advanced Computers Inc. for the use of parts of
their Butterfly Scheme Reference, and to Margaret O’Connell for translating it from BBN’s
text-formatting language to ours.
Special thanks to Richard Stallman, Bob Chassell, and Brian Fox, all of the Free Software
Foundation, for creating and maintaining the Texinfo formatting language in which this
document is written.
This report describes research done at the Artificial Intelligence Laboratory and the
Laboratory for Computer Science, both of the Massachusetts Institute of Technology. Support for this research is provided in part by the Advanced Research Projects Agency of the
Department of Defense and by the National Science Foundation.
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Chapter 1: Overview
3
1 Overview
This manual is a detailed description of the MIT Scheme runtime system. It is intended
to be a reference document for programmers. It does not describe how to run Scheme or
how to interact with it — that is the subject of the MIT Scheme User’s Manual.
This chapter summarizes the semantics of Scheme, briefly describes the MIT Scheme programming environment, and explains the syntactic and lexical conventions of the language.
Subsequent chapters describe special forms, numerous data abstractions, and facilities for
input and output.
Throughout this manual, we will make frequent references to standard Scheme, which
is the language defined by the document Revised^4 Report on the Algorithmic Language
Scheme, by William Clinger, Jonathan Rees, et al., or by ieee Std. 1178-1990, IEEE Standard for the Scheme Programming Language (in fact, several parts of this document are
copied from the Revised Report). MIT Scheme is an extension of standard Scheme.
These are the significant semantic characteristics of the Scheme language:
Variables are statically scoped
Scheme is a statically scoped programming language, which means that each
use of a variable is associated with a lexically apparent binding of that variable.
Algol is another statically scoped language.
Types are latent
Scheme has latent types as opposed to manifest types, which means that Scheme
associates types with values (or objects) rather than with variables. Other
languages with latent types (also referred to as weakly typed or dynamically
typed languages) include APL, Snobol, and other dialects of Lisp. Languages
with manifest types (sometimes referred to as strongly typed or statically typed
languages) include Algol 60, Pascal, and C.
Objects have unlimited extent
All objects created during a Scheme computation, including procedures and
continuations, have unlimited extent; no Scheme object is ever destroyed. The
system doesn’t run out of memory because the garbage collector reclaims the
storage occupied by an object when the object cannot possibly be needed by
a future computation. Other languages in which most objects have unlimited
extent include APL and other Lisp dialects.
Proper tail recursion
Scheme is properly tail-recursive, which means that iterative computation can
occur in constant space, even if the iterative computation is described by a syntactically recursive procedure. With a tail-recursive implementation, you can
express iteration using the ordinary procedure-call mechanics; special iteration
expressions are provided only for syntactic convenience.
Procedures are objects
Scheme procedures are objects, which means that you can create them dynamically, store them in data structures, return them as the results of other
procedures, and so on. Other languages with such procedure objects include
Common Lisp and ML.
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Continuations are explicit
In most other languages, continuations operate behind the scenes. In Scheme,
continuations are objects; you can use continuations for implementing a variety
of advanced control constructs, including non-local exits, backtracking, and
coroutines.
Arguments are passed by value
Arguments to Scheme procedures are passed by value, which means that Scheme
evaluates the argument expressions before the procedure gains control, whether
or not the procedure needs the result of the evaluations. ML, C, and APL
are three other languages that pass arguments by value. In languages such as
SASL and Algol 60, argument expressions are not evaluated unless the values
are needed by the procedure.
Scheme uses a parenthesized-list Polish notation to describe programs and (other) data.
The syntax of Scheme, like that of most Lisp dialects, provides for great expressive power,
largely due to its simplicity. An important consequence of this simplicity is the susceptibility
of Scheme programs and data to uniform treatment by other Scheme programs. As with
other Lisp dialects, the read primitive parses its input; that is, it performs syntactic as well
as lexical decomposition of what it reads.
1.1 Notational Conventions
This section details the notational conventions used throughout the rest of this document.
1.1.1 Errors
When this manual uses the phrase “an error will be signalled,” it means that Scheme
will call error, which normally halts execution of the program and prints an error message.
When this manual uses the phrase “it is an error,” it means that the specified action is
not valid in Scheme, but the system may or may not signal the error. When this manual
says that something “must be,” it means that violating the requirement is an error.
1.1.2 Examples
This manual gives many examples showing the evaluation of expressions. The examples
have a common format that shows the expression being evaluated on the left hand side, an
“arrow” in the middle, and the value of the expression written on the right. For example:
) 3
(+ 1 2)
Sometimes the arrow and value will be moved under the expression, due to lack of space.
Occasionally we will not care what the value is, in which case both the arrow and the value
are omitted.
If an example shows an evaluation that results in an error, an error message is shown,
prefaced by ‘ error ’:
(+ 1 ’foo)
error Illegal datum
An example that shows printed output marks it with ‘ a ’:
Chapter 1: Overview
5
(begin (write ’foo) ’bar)
a foo
) bar
When this manual indicates that the value returned by some expression is unspecified,
it means that the expression will evaluate to some object without signalling an error, but
that programs should not depend on the value in any way.
1.1.3 Entry Format
Each description of an MIT Scheme variable, special form, or procedure begins with one
or more header lines in this format:
template
category
where category specifies the kind of item (“variable”, “special form”, or “procedure”). The
form of template is interpreted depending on category.
Variable
Template consists of the variable’s name.
Special Form
Template starts with the syntactic keyword of the special form, followed by a
description of the special form’s syntax. The description is written using the
following conventions.
Named components are italicized in the printed manual, and uppercase in the
Info file. “Noise” keywords, such as the else keyword in the cond special form,
are set in a fixed width font in the printed manual; in the Info file they are not
distinguished. Parentheses indicate themselves.
A horizontal ellipsis (. . . ) is describes repeated components. Specifically,
thing . . .
indicates zero or more occurrences of thing, while
thing thing . . .
indicates one or more occurrences of thing.
Brackets, [ ], enclose optional components.
Several special forms (e.g. lambda) have an internal component consisting of a
series of expressions; usually these expressions are evaluated sequentially under conditions that are specified in the description of the special form. This
sequence of expressions is commonly referred to as the body of the special form.
Procedure Template starts with the name of the variable to which the procedure is bound,
followed by a description of the procedure’s arguments. The arguments are
described using “lambda list” notation (see Section 2.1 [Lambda Expressions],
page 15), except that brackets are used to denote optional arguments, and
ellipses are used to denote “rest” arguments.
The names of the procedure’s arguments are italicized in the printed manual,
and uppercase in the Info file.
When an argument names a Scheme data type, it indicates that the argument
must be that type of data object. For example,
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cdr pair
procedure
indicates that the standard Scheme procedure cdr takes one argument, which
must be a pair.
Many procedures signal an error when an argument is of the wrong type; usually
this error is a condition of type condition-type:wrong-type-argument.
In addition to the standard data-type names (pair, list, boolean, string, etc.),
the following names as arguments also imply type restrictions:
• object: any object
• thunk: a procedure of no arguments
• x, y: real numbers
• q, n: integers
• k: an exact non-negative integer
Some examples:
list object . . .
procedure
indicates that the standard Scheme procedure list takes zero or more arguments, each of
which may be any Scheme object.
write-char char [output-port]
procedure
indicates that the standard Scheme procedure write-char must be called with a character,
char, and may also be called with a character and an output port.
1.2 Scheme Concepts
1.2.1 Variable Bindings
Any identifier that is not a syntactic keyword may be used as a variable (see Section 1.3.3
[Identifiers], page 10). A variable may name a location where a value can be stored. A
variable that does so is said to be bound to the location. The value stored in the location
to which a variable is bound is called the variable’s value. (The variable is sometimes said
to name the value or to be bound to the value.)
A variable may be bound but still not have a value; such a variable is said to be unassigned. Referencing an unassigned variable is an error. When this error is signalled, it is
a condition of type condition-type:unassigned-variable; sometimes the compiler does
not generate code to signal the error. Unassigned variables are useful only in combination
with side e↵ects (see Section 2.5 [Assignments], page 22).
1.2.2 Environment Concepts
An environment is a set of variable bindings. If an environment has no binding for a
variable, that variable is said to be unbound in that environment. Referencing an unbound
variable signals a condition of type condition-type:unbound-variable.
A new environment can be created by extending an existing environment with a set of
new bindings. Note that “extending an environment” does not modify the environment;
Chapter 1: Overview
7
rather, it creates a new environment that contains the new bindings and the old ones. The
new bindings shadow the old ones; that is, if an environment that contains a binding for x
is extended with a new binding for x, then only the new binding is seen when x is looked
up in the extended environment. Sometimes we say that the original environment is the
parent of the new one, or that the new environment is a child of the old one, or that the
new environment inherits the bindings in the old one.
Procedure calls extend an environment, as do let, let*, letrec, and do expressions.
Internal definitions (see Section 2.4.2 [Internal Definitions], page 21) also extend an environment. (Actually, all the constructs that extend environments can be expressed in terms
of procedure calls, so there is really just one fundamental mechanism for environment extension.) A top-level definition (see Section 2.4.1 [Top-Level Definitions], page 21) may add
a binding to an existing environment.
1.2.3 Initial and Current Environments
MIT Scheme provides an initial environment that contains all of the variable bindings
described in this manual. Most environments are ultimately extensions of this initial environment. In Scheme, the environment in which your programs execute is actually a child
(extension) of the environment containing the system’s bindings. Thus, system names are
visible to your programs, but your names do not interfere with system programs.
The environment in e↵ect at some point in a program is called the current environment
at that point. In particular, every rep loop has a current environment. (rep stands for
“read-eval-print”; the rep loop is the Scheme program that reads your input, evaluates it,
and prints the result.) The environment of the top-level rep loop (the one you are in when
Scheme starts up) starts as user-initial-environment, although it can be changed by
the ge procedure. When a new rep loop is created, its environment is determined by the
program that creates it.
1.2.4 Static Scoping
Scheme is a statically scoped language with block structure. In this respect, it is like
Algol and Pascal, and unlike most other dialects of Lisp except for Common Lisp.
The fact that Scheme is statically scoped (rather than dynamically bound) means that
the environment that is extended (and becomes current) when a procedure is called is the
environment in which the procedure was created (i.e. in which the procedure’s defining
lambda expression was evaluated), not the environment in which the procedure is called.
Because all the other Scheme binding expressions can be expressed in terms of procedures,
this determines how all bindings behave.
Consider the following definitions, made at the top-level rep loop (in the initial environment):
(define x 1)
(define (f x) (g 2))
(define (g y) (+ x y))
) 3 ; not 7
(f 5)
Here f and g are bound to procedures created in the initial environment. Because Scheme
is statically scoped, the call to g from f extends the initial environment (the one in which
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g was created) with a binding of y to 2. In this extended environment, y is 2 and x is 1.
(In a dynamically bound Lisp, the call to g would extend the environment in e↵ect during
the call to f, in which x is bound to 5 by the call to f, and the answer would be 7.)
Note that with static scoping, you can tell what binding a variable reference refers
to just from looking at the text of the program; the referenced binding cannot depend
on how the program is used. That is, the nesting of environments (their parent-child
relationship) corresponds to the nesting of binding expressions in program text. (Because
of this connection to the text of the program, static scoping is also called lexical scoping.)
For each place where a variable is bound in a program there is a corresponding region
of the program text within which the binding is e↵ective. For example, the region of a
binding established by a lambda expression is the entire body of the lambda expression.
The documentation of each binding expression explains what the region of the bindings it
makes is. A use of a variable (that is, a reference to or assignment of a variable) refers to
the innermost binding of that variable whose region contains the variable use. If there is no
such region, the use refers to the binding of the variable in the global environment (which
is an ancestor of all other environments, and can be thought of as a region in which all your
programs are contained).
1.2.5 True and False
In Scheme, the boolean values true and false are denoted by #t and #f. However, any
Scheme value can be treated as a boolean for the purpose of a conditional test. This manual
uses the word true to refer to any Scheme value that counts as true, and the word false to
refer to any Scheme value that counts as false. In conditional tests, all values count as true
except for #f, which counts as false (see Section 2.7 [Conditionals], page 24).
Implementation note: In MIT Scheme, #f and the empty list are the same object, and
the printed representation of #f is always ‘()’. As this contradicts the Scheme standard,
MIT Scheme will be changed to make #f and the empty list di↵erent objects.
1.2.6 External Representations
An important concept in Scheme is that of the external representation of an object as
a sequence of characters. For example, an external representation of the integer 28 is the
sequence of characters ‘28’, and an external representation of a list consisting of the integers
8 and 13 is the sequence of characters ‘(8 13)’.
The external representation of an object is not necessarily unique. The integer 28 also
has representations ‘#e28.000’ and ‘#x1c’, and the list in the previous paragraph also has
the representations ‘( 08 13 )’ and ‘(8 . (13 . ( )))’.
Many objects have standard external representations, but some, such as procedures
and circular data structures, do not have standard representations (although particular
implementations may define representations for them).
An external representation may be written in a program to obtain the corresponding
object (see Section 2.6 [Quoting], page 22).
External representations can also be used for input and output. The procedure read
parses external representations, and the procedure write generates them. Together, they
provide an elegant and powerful input/output facility.
Chapter 1: Overview
9
Note that the sequence of characters ‘(+ 2 6)’ is not an external representation of the
integer 8, even though it is an expression that evaluates to the integer 8; rather, it is an
external representation of a three-element list, the elements of which are the symbol + and
the integers 2 and 6. Scheme’s syntax has the property that any sequence of characters
that is an expression is also the external representation of some object. This can lead to
confusion, since it may not be obvious out of context whether a given sequence of characters
is intended to denote data or program, but it is also a source of power, since it facilitates
writing programs such as interpreters and compilers that treat programs as data or data as
programs.
1.2.7 Disjointness of Types
Every object satisfies at most one of the following predicates (but see Section 1.2.5 [True
and False], page 8, for an exception):
bit-string?
boolean?
cell?
char?
condition?
environment?
null?
number?
pair?
port?
procedure?
promise?
string?
symbol?
vector?
weak-pair?
1.2.8 Storage Model
This section describes a model that can be used to understand Scheme’s use of storage.
Variables and objects such as pairs, vectors, and strings implicitly denote locations or
sequences of locations. A string, for example, denotes as many locations as there are
characters in the string. (These locations need not correspond to a full machine word.) A
new value may be stored into one of these locations using the string-set! procedure, but
the string continues to denote the same locations as before.
An object fetched from a location, by a variable reference or by a procedure such as car,
vector-ref, or string-ref, is equivalent in the sense of eqv? to the object last stored in
the location before the fetch.
Every location is marked to show whether it is in use. No variable or object ever refers
to a location that is not in use. Whenever this document speaks of storage being allocated
for a variable or object, what is meant is that an appropriate number of locations are chosen
from the set of locations that are not in use, and the chosen locations are marked to indicate
that they are now in use before the variable or object is made to denote them.
In many systems it is desirable for constants (i.e. the values of literal expressions) to
reside in read-only memory. To express this, it is convenient to imagine that every object
that denotes locations is associated with a flag telling whether that object is mutable or
immutable. The constants and the strings returned by symbol->string are then the immutable objects, while all objects created by other procedures are mutable. It is an error
to attempt to store a new value into a location that is denoted by an immutable object.
Note that the MIT Scheme compiler takes advantage of this property to share constants,
but that these constants are not immutable. Instead, two constants that are equal? may
be eq? in compiled code.
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1.3 Lexical Conventions
This section describes Scheme’s lexical conventions.
1.3.1 Whitespace
Whitespace characters are spaces, newlines, tabs, and page breaks. Whitespace is used
to improve the readability of your programs and to separate tokens from each other, when
necessary. (A token is an indivisible lexical unit such as an identifier or number.) Whitespace is otherwise insignificant. Whitespace may occur between any two tokens, but not
within a token. Whitespace may also occur inside a string, where it is significant.
1.3.2 Delimiters
All whitespace characters are delimiters. In addition, the following characters act as
delimiters:
(
)
;
"
’
‘
|
Finally, these next characters act as delimiters, despite the fact that Scheme does not
define any special meaning for them:
[
]
{
}
For example, if the value of the variable name is "max":
(list"Hi"name(+ 1 2))
)
("Hi" "max" 3)
1.3.3 Identifiers
An identifier is a sequence of one or more non-delimiter characters. Identifiers are used
in several ways in Scheme programs:
• An identifier can be used as a variable or as a syntactic keyword.
• When an identifier appears as a literal or within a literal, it denotes a symbol.
Scheme accepts most of the identifiers that other programming languages allow. MIT
Scheme allows all of the identifiers that standard Scheme does, plus many more.
MIT Scheme defines a potential identifier to be a sequence of non-delimiter characters
that does not begin with either of the characters ‘#’ or ‘,’. Any such sequence of characters
that is not a syntactically valid number (see Chapter 4 [Numbers], page 57) is considered to
be a valid identifier. Note that, although it is legal for ‘#’ and ‘,’ to appear in an identifier
(other than in the first character position), it is poor programming practice.
Here are some examples of identifiers:
lambda
q
list->vector
soup
+
V17a
<=?
a34kTMNs
the-word-recursion-has-many-meanings
Chapter 1: Overview
11
1.3.4 Uppercase and Lowercase
Scheme doesn’t distinguish uppercase and lowercase forms of a letter except within
character and string constants; in other words, Scheme is case-insensitive. For example,
‘Foo’ is the same identifier as ‘FOO’, and ‘#x1AB’ is the same number as ‘#X1ab’. But ‘#\a’
and ‘#\A’ are di↵erent characters.
1.3.5 Naming Conventions
A predicate is a procedure that always returns a boolean value (#t or #f). By convention,
predicates usually have names that end in ‘?’.
A mutation procedure is a procedure that alters a data structure. By convention, mutation procedures usually have names that end in ‘!’.
1.3.6 Comments
The beginning of a comment is indicated with a semicolon (;). Scheme ignores everything
on a line in which a semicolon appears, from the semicolon until the end of the line. The
entire comment, including the newline character that terminates it, is treated as whitespace.
An alternative form of comment (sometimes called an extended comment) begins with
the characters ‘#|’ and ends with the characters ‘|#’. This alternative form is an MIT
Scheme extension. As with ordinary comments, all of the characters in an extended comment, including the leading ‘#|’ and trailing ‘|#’, are treated as whitespace. Comments
of this form may extend over multiple lines, and additionally may be nested (unlike the
comments of the programming language C, which have a similar syntax).
;;; This is a comment about the FACT procedure. Scheme
;;; ignores all of this comment. The FACT procedure computes
;;; the factorial of a non-negative integer.
#|
This is an extended comment.
Such comments are useful for commenting out code fragments.
|#
(define fact
(lambda (n)
(if (= n 0)
1
(* n (fact (- n 1))))))
;This is another comment:
;Base case: return 1
1.3.7 Additional Notations
The following list describes additional notations used in Scheme. See Chapter 4 [Numbers], page 57, for a description of the notations used for numbers.
+-.
The plus sign, minus sign, and period are used in numbers, and may also occur
in an identifier. A delimited period (not occurring within a number or identifier)
is used in the notation for pairs and to indicate a “rest” parameter in a formal
parameter list (see Section 2.1 [Lambda Expressions], page 15).
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()
Parentheses are used for grouping and to notate lists (see Chapter 7 [Lists],
page 105).
"
The double quote delimits strings (see Chapter 6 [Strings], page 87).
\
The backslash is used in the syntax for character constants (see Chapter 5
[Characters], page 77) and as an escape character within string constants (see
Chapter 6 [Strings], page 87).
;
The semicolon starts a comment.
’
The single quote indicates literal data; it suppresses evaluation (see Section 2.6
[Quoting], page 22).
‘
The backquote indicates almost-constant data (see Section 2.6 [Quoting],
page 22).
,
The comma is used in conjunction with the backquote (see Section 2.6 [Quoting],
page 22).
,@
A comma followed by an at-sign is used in conjunction with the backquote (see
Section 2.6 [Quoting], page 22).
#
The sharp (or pound) sign has di↵erent uses, depending on the character that
immediately follows it:
#t #f
These character sequences denote the boolean constants (see Section 10.1
[Booleans], page 127).
#\
This character sequence introduces a character constant (see Chapter 5 [Characters], page 77).
#(
This character sequence introduces a vector constant (see Chapter 8 [Vectors],
page 119). A close parenthesis, ‘)’, terminates a vector constant.
#e #i #b #o #d #l #s #x
These character sequences are used in the notation for numbers (see Chapter 4
[Numbers], page 57).
#|
This character sequence introduces an extended comment. The comment is
terminated by the sequence ‘|#’. This notation is an MIT Scheme extension.
#!
This character sequence is used to denote a small set of named constants. Currently there are only two of these, #!optional and #!rest, both of which are
used in the lambda special form to mark certain parameters as being “optional”
or “rest” parameters. This notation is an MIT Scheme extension.
#*
This character sequence introduces a bit string (see Chapter 9 [Bit Strings],
page 123). This notation is an MIT Scheme extension.
#[
This character sequence is used to denote objects that do not have a readable
external representation (see Section 14.7 [Custom Output], page 193). A close
bracket, ‘]’, terminates the object’s notation. This notation is an MIT Scheme
extension.
#@
This character sequence is a convenient shorthand used to refer to objects by
their hash number (see Section 14.7 [Custom Output], page 193). This notation
is an MIT Scheme extension.
Chapter 1: Overview
#=
##
13
These character sequences introduce a notation used to show circular structures
in printed output, or to denote them in input. The notation works much like
that in Common Lisp, and is an MIT Scheme extension.
1.4 Expressions
A Scheme expression is a construct that returns a value. An expression may be a literal,
a variable reference, a special form, or a procedure call.
1.4.1 Literal Expressions
Literal constants may be written by using an external representation of the data. In
general, the external representation must be quoted (see Section 2.6 [Quoting], page 22);
but some external representations can be used without quotation.
) "abc"
"abc"
) 145932
145932
) #t
#t
) #\a
#\a
The external representation of numeric constants, string constants, character constants,
and boolean constants evaluate to the constants themselves. Symbols, pairs, lists, and
vectors require quoting.
1.4.2 Variable References
An expression consisting of an identifier (see Section 1.3.3 [Identifiers], page 10) is a
variable reference; the identifier is the name of the variable being referenced. The value of
the variable reference is the value stored in the location to which the variable is bound. An
error is signalled if the referenced variable is unbound or unassigned.
(define x 28)
) 28
x
1.4.3 Special Form Syntax
(keyword component ...)
A parenthesized expression that starts with a syntactic keyword is a special form. Each
special form has its own syntax, which is described later in the manual.
Note that syntactic keywords and variable bindings share the same namespace. A local
variable binding may shadow a syntactic keyword, and a local syntactic-keyword definition
may shadow a variable binding.
The following list contains all of the syntactic keywords that are defined when MIT
Scheme is initialized:
access
and
begin
case
cond
cons-stream
declare
default-object?
define
define-integrable
define-structure
define-syntax
delay
do
er-macro-transformer
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fluid-let
let
let-syntax
local-declare
if
let*
letrec
named-lambda
or
rsc-macro-transformer
syntax-rules
quasiquote
sc-macro-transformer
the-environment
lambda
let*-syntax
letrec-syntax
non-hygienic-macrotransformer
quote
set!
1.4.4 Procedure Call Syntax
(operator operand ...)
A procedure call is written by simply enclosing in parentheses expressions for the procedure to be called (the operator) and the arguments to be passed to it (the operands). The
operator and operand expressions are evaluated and the resulting procedure is passed the
resulting arguments. See Section 2.1 [Lambda Expressions], page 15, for a more complete
description of this.
Another name for the procedure call expression is combination. This word is more
specific in that it always refers to the expression; “procedure call” sometimes refers to the
process of calling a procedure.
Unlike some other dialects of Lisp, Scheme always evaluates the operator expression
and the operand expressions with the same evaluation rules, and the order of evaluation is
unspecified.
) 7
(+ 3 4)
) 12
((if #f = *) 3 4)
A number of procedures are available as the values of variables in the initial environment;
for example, the addition and multiplication procedures in the above examples are the values
of the variables + and *. New procedures are created by evaluating lambda expressions.
If the operator is a syntactic keyword, then the expression is not treated as a procedure
call: it is a special form.
Chapter 2: Special Forms
15
2 Special Forms
A special form is an expression that follows special evaluation rules. This chapter describes the basic Scheme special forms.
2.1 Lambda Expressions
lambda formals expression expression . . .
special form
A lambda expression evaluates to a procedure. The environment in e↵ect when the
lambda expression is evaluated is remembered as part of the procedure; it is called
the closing environment. When the procedure is later called with some arguments,
the closing environment is extended by binding the variables in the formal parameter
list to fresh locations, and the locations are filled with the arguments according to
rules about to be given. The new environment created by this process is referred to
as the invocation environment.
Once the invocation environment has been constructed, the expressions in the body
of the lambda expression are evaluated sequentially in it. This means that the region
of the variables bound by the lambda expression is all of the expressions in the body.
The result of evaluating the last expression in the body is returned as the result of
the procedure call.
Formals, the formal parameter list, is often referred to as a lambda list.
The process of matching up formal parameters with arguments is somewhat involved.
There are three types of parameters, and the matching treats each in sequence:
Required All of the required parameters are matched against the arguments first.
If there are fewer arguments than required parameters, an error of type
condition-type:wrong-number-of-arguments is signalled; this error is
also signalled if there are more arguments than required parameters and
there are no further parameters.
Optional
Once the required parameters have all been matched, the optional parameters are matched against the remaining arguments. If there are fewer arguments than optional parameters, the unmatched parameters are bound
to special objects called default objects. If there are more arguments
than optional parameters, and there are no further parameters, an error
of type condition-type:wrong-number-of-arguments is signalled.
The predicate default-object?, which is true only of default objects,
can be used to determine which optional parameters were supplied, and
which were defaulted.
Rest
Finally, if there is a rest parameter (there can only be one), any remaining
arguments are made into a list, and the list is bound to the rest parameter.
(If there are no remaining arguments, the rest parameter is bound to the
empty list.)
In Scheme, unlike some other Lisp implementations, the list to which a
rest parameter is bound is always freshly allocated. It has infinite extent
and may be modified without a↵ecting the procedure’s caller.
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MIT Scheme Reference
Specially recognized keywords divide the formals parameters into these three classes.
The keywords used here are ‘#!optional’, ‘.’, and ‘#!rest’. Note that only ‘.’
is defined by standard Scheme — the other keywords are MIT Scheme extensions.
‘#!rest’ has the same meaning as ‘.’ in formals.
The use of these keywords is best explained by means of examples. The following
are typical lambda lists, followed by descriptions of which parameters are required,
optional, and rest. We will use ‘#!rest’ in these examples, but anywhere it appears
‘.’ could be used instead.
(a b c)
a, b, and c are all required. The procedure must be passed exactly three
arguments.
(a b #!optional c)
a and b are required, c is optional. The procedure may be passed either
two or three arguments.
(#!optional a b c)
a, b, and c are all optional. The procedure may be passed any number
of arguments between zero and three, inclusive.
a
(#!rest a)
These two examples are equivalent. a is a rest parameter. The procedure
may be passed any number of arguments. Note: this is the only case in
which ‘.’ cannot be used in place of ‘#!rest’.
(a b #!optional c d #!rest e)
a and b are required, c and d are optional, and e is rest. The procedure
may be passed two or more arguments.
Some examples of lambda expressions:
(lambda (x) (+ x x))
)
((lambda (x) (+ x x)) 4)
(define reverse-subtract
(lambda (x y)
(- y x)))
(reverse-subtract 7 10)
(define foo
(let ((x 4))
(lambda (y) (+ x y))))
(foo 6)
named-lambda formals expression expression . . .
#[compound-procedure 53]
)
8
)
3
)
10
special form
The named-lambda special form is similar to lambda, except that the first “required
parameter” in formals is not a parameter but the name of the resulting procedure;
thus formals must have at least one required parameter. This name has no semantic
meaning, but is included in the external representation of the procedure, making it
Chapter 2: Special Forms
17
useful for debugging. In MIT Scheme, lambda is implemented as named-lambda, with
a special name that means “unnamed”.
) #[compound-procedure 53 f]
(named-lambda (f x) (+ x x))
) 8
((named-lambda (f x) (+ x x)) 4)
2.2 Lexical Binding
The three binding constructs let, let*, and letrec, give Scheme block structure. The
syntax of the three constructs is identical, but they di↵er in the regions they establish for
their variable bindings. In a let expression, the initial values are computed before any
of the variables become bound. In a let* expression, the evaluations and bindings are
sequentially interleaved. And in a letrec expression, all the bindings are in e↵ect while
the initial values are being computed (thus allowing mutually recursive definitions).
let ((variable init) . . . ) expression expression . . .
special form
The inits are evaluated in the current environment (in some unspecified order), the
variables are bound to fresh locations holding the results, the expressions are evaluated sequentially in the extended environment, and the value of the last expression is
returned. Each binding of a variable has the expressions as its region.
MIT Scheme allows any of the inits to be omitted, in which case the corresponding
variables are unassigned.
Note that the following are equivalent:
(let ((variable init) ...) expression expression ...)
((lambda (variable ...) expression expression ...) init ...)
Some examples:
(let ((x 2) (y 3))
) 6
(* x y))
(let ((x 2) (y 3))
(let ((foo (lambda (z) (+ x y z)))
(x 7))
) 9
(foo 4)))
See Section 2.9 [Iteration], page 27, for information on “named let”.
let* ((variable init) . . . ) expression expression . . .
special form
let* is similar to let, but the bindings are performed sequentially from left to right,
and the region of a binding is that part of the let* expression to the right of the
binding. Thus the second binding is done in an environment in which the first binding
is visible, and so on.
Note that the following are equivalent:
(let* ((variable1 init1)
(variable2 init2)
...
(variableN initN ))
expression
expression ...)
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MIT Scheme Reference
(let ((variable1 init1))
(let ((variable2 init2))
...
(let ((variableN initN ))
expression
expression ...)
...))
An example:
(let ((x 2) (y 3))
(let* ((x 7)
(z (+ x y)))
(* z x)))
)
70
letrec ((variable init) . . . ) expression expression . . .
special form
The variables are bound to fresh locations holding unassigned values, the inits are
evaluated in the extended environment (in some unspecified order), each variable is
assigned to the result of the corresponding init, the expressions are evaluated sequentially in the extended environment, and the value of the last expression is returned.
Each binding of a variable has the entire letrec expression as its region, making it
possible to define mutually recursive procedures.
MIT Scheme allows any of the inits to be omitted, in which case the corresponding
variables are unassigned.
(letrec ((even?
(lambda (n)
(if (zero? n)
#t
(odd? (- n 1)))))
(odd?
(lambda (n)
(if (zero? n)
#f
(even? (- n 1))))))
) #t
(even? 88))
One restriction on letrec is very important: it shall be possible to evaluated each
init without assigning or referring to the value of any variable. If this restriction
is violated, then it is an error. The restriction is necessary because Scheme passes
arguments by value rather than by name. In the most common uses of letrec, all the
inits are lambda or delay expressions and the restriction is satisfied automatically.
2.3 Dynamic Binding
fluid-let ((variable init) . . . ) expression expression . . .
special form
The inits are evaluated in the current environment (in some unspecified order), the
current values of the variables are saved, the results are assigned to the variables, the
expressions are evaluated sequentially in the current environment, the variables are
restored to their original values, and the value of the last expression is returned.
Chapter 2: Special Forms
19
The syntax of this special form is similar to that of let, but fluid-let temporarily
rebinds existing variables. Unlike let, fluid-let creates no new bindings; instead
it assigns the value of each init to the binding (determined by the rules of lexical
scoping) of its corresponding variable.
MIT Scheme allows any of the inits to be omitted, in which case the corresponding
variables are temporarily unassigned.
An error of type condition-type:unbound-variable is signalled if any of the variables are unbound. However, because fluid-let operates by means of side e↵ects,
it is valid for any variable to be unassigned when the form is entered.
Here is an example showing the di↵erence between fluid-let and let. First see how
let a↵ects the binding of a variable:
(define variable #t)
(define (access-variable) variable)
variable
(let ((variable #f))
(access-variable))
variable
)
#t
)
)
#t
#t
access-variable returns #t in this case because it is defined in an environment with
variable bound to #t. fluid-let, on the other hand, temporarily reuses an existing
variable:
variable
(fluid-let ((variable #f))
(access-variable))
variable
) #t
;reuses old binding
) #f
) #t
The extent of a dynamic binding is defined to be the time period during which the
variable contains the new value. Normally this time period begins when the body is
entered and ends when it is exited; on a sequential machine it is normally a contiguous
time period. However, because Scheme has first-class continuations, it is possible to
leave the body and then reenter it, as many times as desired. In this situation, the
extent becomes non-contiguous.
When the body is exited by invoking a continuation, the new value is saved, and
the variable is set to the old value. Then, if the body is reentered by invoking a
continuation, the old value is saved, and the variable is set to the new value. In
addition, side e↵ects to the variable that occur both inside and outside of body are
preserved, even if continuations are used to jump in and out of body repeatedly.
Here is a complicated example that shows the interaction between dynamic binding and
continuations:
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MIT Scheme Reference
(define (complicated-dynamic-binding)
(let ((variable 1)
(inside-continuation))
(write-line variable)
(call-with-current-continuation
(lambda (outside-continuation)
(fluid-let ((variable 2))
(write-line variable)
(set! variable 3)
(call-with-current-continuation
(lambda (k)
(set! inside-continuation k)
(outside-continuation #t)))
(write-line variable)
(set! inside-continuation #f))))
(write-line variable)
(if inside-continuation
(begin
(set! variable 4)
(inside-continuation #f)))))
Evaluating ‘(complicated-dynamic-binding)’ writes the following on the console:
1
2
1
3
4
Commentary: the first two values written are the initial binding of variable and its
new binding after the fluid-let’s body is entered. Immediately after they are written,
variable is set to ‘3’, and then outside-continuation is invoked, causing us to exit the
body. At this point, ‘1’ is written, demonstrating that the original value of variable has
been restored, because we have left the body. Then we set variable to ‘4’ and reenter the
body by invoking inside-continuation. At this point, ‘3’ is written, indicating that the
side e↵ect that previously occurred within the body has been preserved. Finally, we exit
body normally, and write ‘4’, demonstrating that the side e↵ect that occurred outside of
the body was also preserved.
2.4 Definitions
define variable [expression]
define formals expression expression . . .
special form
special form
Definitions are valid in some but not all contexts where expressions are allowed.
Definitions may only occur at the top level of a program and at the beginning of
a lambda body (that is, the body of a lambda, let, let*, letrec, fluid-let, or
“procedure define” expression). A definition that occurs at the top level of a program
is called a top-level definition, and a definition that occurs at the beginning of a body
is called an internal definition.
Chapter 2: Special Forms
21
In the second form of define (called “procedure define”), the component formals is
identical to the component of the same name in a named-lambda expression. In fact,
these two expressions are equivalent:
(define (name1 name2 ...)
expression
expression ...)
(define name1
(named-lambda (name1 name2 ...)
expression
expression ...))
2.4.1 Top-Level Definitions
A top-level definition,
(define variable expression)
has essentially the same e↵ect as this assignment expression, if variable is bound:
(set! variable expression)
If variable is not bound, however, define binds variable to a new location in the current
environment before performing the assignment (it is an error to perform a set! on an
unbound variable). If you omit expression, the variable becomes unassigned; an attempt to
reference such a variable is an error.
(define add3
(lambda (x) (+ x 3)))
(add3 3)
)
)
unspecified
6
(define first car)
(first ’(1 2))
)
)
unspecified
1
(define bar)
bar
) unspecified
error Unassigned variable
2.4.2 Internal Definitions
An internal definition is a definition that occurs at the beginning of a body (that is,
the body of a lambda, let, let*, letrec, fluid-let, or “procedure define” expression),
rather than at the top level of a program. The variable defined by an internal definition is
local to the body. That is, variable is bound rather than assigned, and the region of the
binding is the entire body. For example,
(let ((x 5))
(define foo (lambda (y) (bar x y)))
(define bar (lambda (a b) (+ (* a b) a)))
) 45
(foo (+ x 3)))
A body containing internal definitions can always be converted into a completely equivalent letrec expression. For example, the let expression in the above example is equivalent
to
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MIT Scheme Reference
(let ((x 5))
(letrec ((foo (lambda (y) (bar x y)))
(bar (lambda (a b) (+ (* a b) a))))
(foo (+ x 3))))
2.5 Assignments
set! variable [expression]
special form
If expression is specified, evaluates expression and stores the resulting value in the
location to which variable is bound. If expression is omitted, variable is altered to be
unassigned; a subsequent reference to such a variable is an error. In either case, the
value of the set! expression is unspecified.
Variable must be bound either in some region enclosing the set! expression, or at
the top level. However, variable is permitted to be unassigned when the set! form
is entered.
(define x 2)
(+ x 1)
(set! x 4)
(+ x 1)
)
)
)
)
unspecified
3
unspecified
5
Variable may be an access expression (see Chapter 13 [Environments], page 173).
This allows you to assign variables in an arbitrary environment. For example,
(define x (let ((y 0)) (the-environment)))
(define y ’a)
)
y
)
(access y x)
)
(set! (access y x) 1)
)
y
)
(access y x)
a
0
unspecified
a
1
2.6 Quoting
This section describes the expressions that are used to modify or prevent the evaluation
of objects.
quote datum
special form
(quote datum) evaluates to datum. Datum may be any external representation of a
Scheme object (see Section 1.2.6 [External Representations], page 8). Use quote to
include literal constants in Scheme code.
(quote a)
(quote #(a b c))
(quote (+ 1 2))
)
)
)
a
#(a b c)
(+ 1 2)
(quote datum) may be abbreviated as ’datum. The two notations are equivalent in
all respects.
Chapter 2: Special Forms
’a
’#(a b c)
’(+ 1 2)
’(quote a)
’’a
23
)
)
)
)
)
a
#(a b c)
(+ 1 2)
(quote a)
(quote a)
Numeric constants, string constants, character constants, and boolean constants evaluate to themselves, so they don’t need to be quoted.
’"abc"
"abc"
’145932
145932
’#t
#t
’#\a
#\a
)
)
)
)
)
)
)
)
"abc"
"abc"
145932
145932
#t
#t
#\a
#\a
quasiquote template
special form
“Backquote” or “quasiquote” expressions are useful for constructing a list or vector
structure when most but not all of the desired structure is known in advance. If no
commas appear within the template, the result of evaluating ‘template is equivalent
(in the sense of equal?) to the result of evaluating ’template. If a comma appears
within the template, however, the expression following the comma is evaluated (“unquoted”) and its result is inserted into the structure instead of the comma and the
expression. If a comma appears followed immediately by an at-sign (@), then the
following expression shall evaluate to a list; the opening and closing parentheses of
the list are then “stripped away” and the elements of the list are inserted in place of
the comma at-sign expression sequence.
‘(list ,(+ 1 2) 4)
)
(list 3 4)
(let ((name ’a)) ‘(list ,name ’,name))
)
(list a ’a)
‘(a ,(+ 1 2) ,@(map abs ’(4 -5 6)) b)
)
(a 3 4 5 6 b)
‘((foo ,(- 10 3)) ,@(cdr ’(c)) . ,(car ’(cons)))
) ((foo 7) . cons)
‘#(10 5 ,(sqrt 4) ,@(map sqrt ’(16 9)) 8)
‘,(+ 2 3)
)
#(10 5 2 4 3 8)
)
5
Quasiquote forms may be nested. Substitutions are made only for unquoted components appearing at the same nesting level as the outermost backquote. The nesting
level increases by one inside each successive quasiquotation, and decreases by one
inside each unquotation.
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MIT Scheme Reference
‘(a ‘(b ,(+ 1 2) ,(foo ,(+ 1 3) d) e) f)
) (a ‘(b ,(+ 1 2) ,(foo 4 d) e) f)
(let ((name1 ’x)
(name2 ’y))
‘(a ‘(b ,,name1 ,’,name2 d) e))
) (a ‘(b ,x ,’y d) e)
The notations ‘template and (quasiquote template) are identical in all respects.
,expression is identical to (unquote expression) and ,@expression is identical to
(unquote-splicing expression).
(quasiquote (list (unquote (+ 1 2)) 4))
) (list 3 4)
’(quasiquote (list (unquote (+ 1 2)) 4))
) ‘(list ,(+ 1 2) 4)
i.e., (quasiquote (list (unquote (+ 1 2)) 4))
Unpredictable behavior can result if any of the symbols quasiquote, unquote, or
unquote-splicing appear in a template in ways otherwise than as described above.
2.7 Conditionals
The behavior of the conditional expressions is determined by whether objects are true
or false. The conditional expressions count only #f as false. They count everything else,
including #t, pairs, symbols, numbers, strings, vectors, and procedures as true (but see
Section 1.2.5 [True and False], page 8).
In the descriptions that follow, we say that an object has “a true value” or “is true”
when the conditional expressions treat it as true, and we say that an object has “a false
value” or “is false” when the conditional expressions treat it as false.
if predicate consequent [alternative]
special form
Predicate, consequent, and alternative are expressions. An if expression is evaluated
as follows: first, predicate is evaluated. If it yields a true value, then consequent is
evaluated and its value is returned. Otherwise alternative is evaluated and its value
is returned. If predicate yields a false value and no alternative is specified, then the
result of the expression is unspecified.
An if expression evaluates either consequent or alternative, never both. Programs
should not depend on the value of an if expression that has no alternative.
) yes
(if (> 3 2) ’yes ’no)
) no
(if (> 2 3) ’yes ’no)
(if (> 3 2)
(- 3 2)
) 1
(+ 3 2))
cond clause clause . . .
Each clause has this form:
special form
Chapter 2: Special Forms
25
(predicate expression ...)
where predicate is any expression. The last clause may be an else clause, which has
the form:
(else expression expression ...)
A cond expression does the following:
1. Evaluates the predicate expressions of successive clauses in order, until one of
the predicates evaluates to a true value.
2. When a predicate evaluates to a true value, cond evaluates the expressions in
the associated clause in left to right order, and returns the result of evaluating
the last expression in the clause as the result of the entire cond expression.
If the selected clause contains only the predicate and no expressions, cond returns
the value of the predicate as the result.
3. If all predicates evaluate to false values, and there is no else clause, the result of
the conditional expression is unspecified; if there is an else clause, cond evaluates
its expressions (left to right) and returns the value of the last one.
(cond ((> 3 2) ’greater)
) greater
((< 3 2) ’less))
(cond ((> 3 3) ’greater)
((< 3 3) ’less)
) equal
(else ’equal))
Normally, programs should not depend on the value of a cond expression that has no
else clause. However, some Scheme programmers prefer to write cond expressions
in which at least one of the predicates is always true. In this style, the final clause is
equivalent to an else clause.
Scheme supports an alternative clause syntax:
(predicate => recipient)
where recipient is an expression. If predicate evaluates to a true value, then recipient
is evaluated. Its value must be a procedure of one argument; this procedure is then
invoked on the value of the predicate.
(cond ((assv ’b ’((a 1) (b 2))) => cadr)
) 2
(else #f))
case key clause clause . . .
special form
Key may be any expression. Each clause has this form:
((object ...) expression expression ...)
No object is evaluated, and all the objects must be distinct. The last clause may be
an else clause, which has the form:
(else expression expression ...)
A case expression does the following:
1. Evaluates key and compares the result with each object.
2. If the result of evaluating key is equivalent (in the sense of eqv?; see Chapter 3
[Equivalence Predicates], page 51) to an object, case evaluates the expressions
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MIT Scheme Reference
in the corresponding clause from left to right and returns the result of evaluating
the last expression in the clause as the result of the case expression.
3. If the result of evaluating key is di↵erent from every object, and if there’s an
else clause, case evaluates its expressions and returns the result of the last one
as the result of the case expression. If there’s no else clause, case returns an
unspecified result. Programs should not depend on the value of a case expression
that has no else clause.
For example,
(case (* 2 3)
((2 3 5 7) ’prime)
((1 4 6 8 9) ’composite))
(case (car ’(c d))
((a) ’a)
((b) ’b))
(case (car ’(c d))
((a e i o u) ’vowel)
((w y) ’semivowel)
(else ’consonant))
)
composite
)
unspecified
)
consonant
and expression . . .
special form
The expressions are evaluated from left to right, and the value of the first expression
that evaluates to a false value is returned. Any remaining expressions are not evaluated. If all the expressions evaluate to true values, the value of the last expression is
returned. If there are no expressions then #t is returned.
) #t
(and (= 2 2) (> 2 1))
) #f
(and (= 2 2) (< 2 1))
) (f g)
(and 1 2 ’c ’(f g))
) #t
(and)
or expression . . .
special form
The expressions are evaluated from left to right, and the value of the first expression
that evaluates to a true value is returned. Any remaining expressions are not evaluated. If all expressions evaluate to false values, the value of the last expression is
returned. If there are no expressions then #f is returned.
) #t
(or (= 2 2) (> 2 1))
) #t
(or (= 2 2) (< 2 1))
) #f
(or #f #f #f)
) (b c)
(or (memq ’b ’(a b c)) (/ 3 0))
2.8 Sequencing
The begin special form is used to evaluate expressions in a particular order.
Chapter 2: Special Forms
27
begin expression expression . . .
special form
The expressions are evaluated sequentially from left to right, and the value of the last
expression is returned. This expression type is used to sequence side e↵ects such as
input and output.
(define x 0)
(begin (set! x 5)
) 6
(+ x 1))
(begin (display "4 plus 1 equals ")
(display (+ 4 1)))
a 4 plus 1 equals 5
) unspecified
Often the use of begin is unnecessary, because many special forms already support
sequences of expressions (that is, they have an implicit begin). Some of these special
forms are:
case
cond
define
;“procedure define” only
do
fluid-let
lambda
let
let*
letrec
named-lambda
The obsolete special form sequence is identical to begin. It should not be used in
new code.
2.9 Iteration
The iteration expressions are: “named let” and do. They are also binding expressions,
but are more commonly referred to as iteration expressions. Because Scheme is properly
tail-recursive, you don’t need to use these special forms to express iteration; you can simply
use appropriately written “recursive” procedure calls.
let name ((variable init) . . . ) expression expression . . .
special form
MIT Scheme permits a variant on the syntax of let called “named let” which provides a more general looping construct than do, and may also be used to express
recursions.
Named let has the same syntax and semantics as ordinary let except that name is
bound within the expressions to a procedure whose formal arguments are the variables
and whose body is the expressions. Thus the execution of the expressions may be
repeated by invoking the procedure named by name.
MIT Scheme allows any of the inits to be omitted, in which case the corresponding
variables are unassigned.
Note: the following expressions are equivalent:
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MIT Scheme Reference
(let name ((variable init) ...)
expression
expression ...)
((letrec ((name
(named-lambda (name variable ...)
expression
expression ...)))
name)
init ...)
Here is an example:
(let loop
((numbers ’(3 -2 1 6 -5))
(nonneg ’())
(neg ’()))
(cond ((null? numbers)
(list nonneg neg))
((>= (car numbers) 0)
(loop (cdr numbers)
(cons (car numbers) nonneg)
neg))
(else
(loop (cdr numbers)
nonneg
(cons (car numbers) neg)))))
)
((6 1 3) (-5 -2))
do ((variable init step) . . . ) (test expression . . . ) command . . .
special form
do is an iteration construct. It specifies a set of variables to be bound, how they are
to be initialized at the start, and how they are to be updated on each iteration. When
a termination condition is met, the loop exits with a specified result value.
do expressions are evaluated as follows: The init expressions are evaluated (in some
unspecified order), the variables are bound to fresh locations, the results of the init
expressions are stored in the bindings of the variables, and then the iteration phase
begins.
Each iteration begins by evaluating test; if the result is false, then the command
expressions are evaluated in order for e↵ect, the step expressions are evaluated in
some unspecified order, the variables are bound to fresh locations, the results of the
steps are stored in the bindings of the variables, and the next iteration begins.
If test evaluates to a true value, then the expressions are evaluated from left to right
and the value of the last expression is returned as the value of the do expression. If no
expressions are present, then the value of the do expression is unspecified in standard
Scheme; in MIT Scheme, the value of test is returned.
The region of the binding of a variable consists of the entire do expression except
for the inits. It is an error for a variable to appear more than once in the list of do
variables.
Chapter 2: Special Forms
29
A step may be omitted, in which case the e↵ect is the same as if (variable init
variable) had been written instead of (variable init).
(do ((vec (make-vector 5))
(i 0 (+ i 1)))
((= i 5) vec)
) #(0 1 2 3 4)
(vector-set! vec i i))
(let ((x ’(1 3 5 7 9)))
(do ((x x (cdr x))
(sum 0 (+ sum (car x))))
((null? x) sum)))
)
25
2.10 Structure Definitions
This section provides examples and describes the options and syntax of definestructure, an MIT Scheme macro that is very similar to defstruct in Common Lisp.
The di↵erences between them are summarized at the end of this section. For more
information, see Steele’s Common Lisp book.
define-structure (name structure-option . . . ) slot-description . . .
special form
Each slot-description takes one of the following forms:
slot-name
(slot-name default-init [slot-option value]*)
The fields name and slot-name must both be symbols. The field default-init is an
expression for the initial value of the slot. It is evaluated each time a new instance
is constructed. If it is not specified, the initial content of the slot is undefined.
Default values are only useful with a boa constructor with argument list or a keyword
constructor (see below).
Evaluation of a define-structure expression defines a structure descriptor and a
set of procedures to manipulate instances of the structure. These instances are represented as records by default (see Section 10.4 [Records], page 132) but may alternately
be lists or vectors. The accessors and modifiers are marked with compiler declarations so that calls to them are automatically transformed into appropriate references.
Often, no options are required, so a simple call to define-structure looks like:
(define-structure foo a b c)
This defines a type descriptor foo, a constructor make-foo, a predicate foo?, accessors foo-a, foo-b, and foo-c, and modifiers set-foo-a!, set-foo-b!, and setfoo-c!.
In general, if no options are specified, define-structure defines the following (using
the simple call above as an example):
type descriptor
The name of the type descriptor is the same as the name of the structure,
e.g. ‘foo’. The type descriptor satisfies the predicate record-type?.
constructor
The name of the constructor is "make-" followed by the name of the
structure, e.g. ‘make-foo’. The number of arguments accepted by the
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constructor is the same as the number of slots; the arguments are the
initial values for the slots, and the order of the arguments matches the
order of the slot definitions.
predicate
The name of the predicate is the name of the structure followed by "?",
e.g. ‘foo?’. The predicate is a procedure of one argument, which returns #t if its argument is a record of the type defined by this structure
definition, and #f otherwise.
accessors
For each slot, an accessor is defined. The name of the accessor is formed
by appending the name of the structure, a hyphen, and the name of the
slot, e.g. ‘foo-a’. The accessor is a procedure of one argument, which
must be a record of the type defined by this structure definition. The
accessor extracts the contents of the corresponding slot in that record
and returns it.
modifiers
For each slot, a modifier is defined. The name of the modifier is formed by
appending "set-", the name of the accessor, and "!", e.g. ‘set-foo-a!’.
The modifier is a procedure of two arguments, the first of which must
be a record of the type defined by this structure definition, and the second of which may be any object. The modifier modifies the contents of
the corresponding slot in that record to be that object, and returns an
unspecified value.
When options are not supplied, (name) may be abbreviated to name. This convention
holds equally for structure-options and slot-options. Hence, these are equivalent:
(define-structure foo a b c)
(define-structure (foo) (a) b (c))
as are
(define-structure (foo keyword-constructor) a b c)
(define-structure (foo (keyword-constructor)) a b c)
When specified as option values, false and nil are equivalent to #f, and true and
t are equivalent to #t.
Possible slot-options are:
read-only value
slot option
When given a value other than #f, this specifies that no modifier should be created
for the slot.
type type-descriptor
This is accepted but not presently used.
slot option
Possible structure-options are:
predicate [name]
structure option
This option controls the definition of a predicate procedure for the structure. If name
is not given, the predicate is defined with the default name (see above). If name is
#f, the predicate is not defined at all. Otherwise, name must be a symbol, and the
predicate is defined with that symbol as its name.
Chapter 2: Special Forms
31
copier [name]
structure option
This option controls the definition of a procedure to copy instances of the structure. This is a procedure of one argument, a structure instance, that makes a newly
allocated copy of the structure and returns it. If name is not given, the copier is
defined, and the name of the copier is "copy-" followed by the structure name (e.g.
‘copy-foo’). If name is #f, the copier is not defined. Otherwise, name must be a
symbol, and the copier is defined with that symbol as its name.
print-procedure expression
structure option
Evaluating expression must yield a procedure of two arguments, which is used to print
instances of the structure. The procedure is an unparser method (see Section 14.7
[Custom Output], page 193). If the structure instances are records, this option has
the same e↵ect as calling set-record-type-unparser-method!.
constructor [name [argument-list]]
structure option
This option controls the definition of constructor procedures. These constructor procedures are called “boa constructors”, for “By Order of Arguments”, because the
arguments to the constructor specify the initial contents the structure’s slots by the
order in which they are given. This is as opposed to “keyword constructors”, which
specify the initial contents using keywords, and in which the order of arguments is
irrelevant.
If name is not given, a constructor is defined with the default name and arguments (see
above). If name is #f, no constructor is defined; argument-list may not be specified
in this case. Otherwise, name must be a symbol, and a constructor is defined with
that symbol as its name. If name is a symbol, argument-list is optionally allowed;
if it is omitted, the constructor accepts one argument for each slot in the structure
definition, in the same order in which the slots appear in the definition. Otherwise,
argument-list must be a lambda list (see Section 2.1 [Lambda Expressions], page 15),
and each of the parameters of the lambda list must be the name of a slot in the
structure. The arguments accepted by the constructor are defined by this lambda
list. Any slot that is not specified by the lambda list is initialized to the default-init
as specified above; likewise for any slot specified as an optional parameter when the
corresponding argument is not supplied.
If the constructor option is specified, the default constructor is not defined. Additionally, the constructor option may be specified multiple times to define multiple
constructors with di↵erent names and argument lists.
(define-structure (foo
(constructor make-foo (#!optional a b)))
(a 6 read-only #t)
(b 9))
keyword-constructor [name]
structure option
This option controls the definition of keyword constructor procedures. A keyword
constructor is a procedure that accepts arguments that are alternating slot names
and values. If name is omitted, a keyword constructor is defined, and the name of
the constructor is "make-" followed by the name of the structure (e.g. ‘make-foo’).
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Otherwise, name must be a symbol, and a keyword constructor is defined with this
symbol as its name.
If the keyword-constructor option is specified, the default constructor is not defined.
Additionally, the keyword-constructor option may be specified multiple times to
define multiple keyword constructors; this is usually not done since such constructors
would all be equivalent.
(define-structure (foo (keyword-constructor make-bar)) a b)
) 19
(foo-a (make-bar ’b 20 ’a 19))
type-descriptor name
This option cannot be used with the type or named options.
structure option
By default, structures are implemented as records. The name of the structure is
defined to hold the type descriptor of the record defined by the structure. The typedescriptor option specifies a di↵erent name to hold the type descriptor.
(define-structure foo a b)
) #[record-type 18]
foo
(define-structure (bar (type-descriptor bar-rtd)) a b)
bar
error Unbound variable: bar
) #[record-type 19]
bar-rtd
conc-name [name]
structure option
By default, the prefix for naming accessors and modifiers is the name of the structure
followed by a hyphen. The conc-name option can be used to specify an alternative.
If name is not given, the prefix is the name of the structure followed by a hyphen (the
default). If name is #f, the slot names are used directly, without prefix. Otherwise,
name must a symbol, and that symbol is used as the prefix.
(define-structure (foo (conc-name moby/)) a b)
defines accessors moby/a and moby/b, and modifiers set-moby/a! and set-moby/b!.
(define-structure (foo (conc-name #f)) a b)
defines accessors a and b, and modifiers set-a! and set-b!.
type representation-type
This option cannot be used with the type-descriptor option.
structure option
By default, structures are implemented as records. The type option overrides this
default, allowing the programmer to specify that the structure be implemented using
another data type. The option value representation-type specifies the alternate data
type; it is allowed to be one of the symbols vector or list, and the data type used
is the one corresponding to the symbol.
If this option is given, and the named option is not specified, the representation will
not be tagged, and neither a predicate nor a type descriptor will be defined; also, the
print-procedure option may not be given.
(define-structure (foo (type list)) a b)
) (1 2)
(make-foo 1 2)
Chapter 2: Special Forms
33
named [expression]
structure option
This is valid only in conjunction with the type option and specifies that the structure
instances be tagged to make them identifiable as instances of this structure type. This
option cannot be used with the type-descriptor option.
In the usual case, where expression is not given, the named option causes a type
descriptor and predicate to be defined for the structure (recall that the type option
without named suppresses their definition), and also defines a default unparser method
for the structure instances (which can be overridden by the print-procedure option).
If the default unparser method is not wanted then the print-procedure option should
be specified as #F. This causes the structure to be printed in its native representation,
as a list or vector, which includes the type descriptor. The type descriptor is a unique
object, not a record type, that describes the structure instances and is additionally
stored in the structure instances to identify them: if the representation type is vector,
the type descriptor is stored in the zero-th slot of the vector, and if the representation
type is list, it is stored as the first element of the list.
(define-structure (foo (type vector) named) a b c)
(vector-ref (make-foo 1 2 3) 0) ) #[structure-type 52]
If expression is specified, it is an expression that is evaluated to yield a tag object. The
expression is evaluated once when the structure definition is evaluated (to specify the
unparser method), and again whenever a predicate or constructor is called. Because
of this, expression is normally a variable reference or a constant. The value yielded by
expression may be any object at all. That object is stored in the structure instances
in the same place that the type descriptor is normally stored, as described above. If
expression is specified, no type descriptor is defined, only a predicate.
(define-structure (foo (type vector) (named ’foo)) a b c)
(vector-ref (make-foo 1 2 3) 0) ) foo
safe-accessors [boolean]
structure option
This option allows the programmer to have some control over the safety of the slot
accessors (and modifiers) generated by define-structure. If safe-accessors is not
specified, or if boolean is #f, then the accessors are optimized for speed at the expense
of safety; when compiled, the accessors will turn into very fast inline sequences, usually
one to three machine instructions in length. However, if safe-accessors is specified
and boolean is either omitted or #t, then the accessors are optimized for safety, will
check the type and structure of their argument, and will be close-coded.
(define-structure (foo safe-accessors) a b c)
initial-o↵set o↵set
structure option
This is valid only in conjunction with the type option. O↵set must be an exact nonnegative integer and specifies the number of slots to leave open at the beginning of
the structure instance before the specified slots are allocated. Specifying an o↵set of
zero is equivalent to omitting the initial-offset option.
If the named option is specified, the structure tag appears in the first slot, followed
by the “o↵set” slots, and then the regular slots. Otherwise, the “o↵set” slots come
first, followed by the regular slots.
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(define-structure (foo (type vector) (initial-offset 3))
a b c)
) #(() () () 1 2 3)
(make-foo 1 2 3)
The essential di↵erences between MIT Scheme’s define-structure and Common Lisp’s
defstruct are:
• The default constructor procedure takes positional arguments, in the same order as
specified in the definition of the structure. A keyword constructor may be specified by
giving the option keyword-constructor.
• boa constructors are described using Scheme lambda lists. Since there is nothing
corresponding to &aux in Scheme lambda lists, this functionality is not implemented.
• By default, no copier procedure is defined.
• The side-e↵ect procedure corresponding to the accessor foo is given the name setfoo!.
• Keywords are ordinary symbols – use foo instead of :foo.
• The option values false, nil, true, and t are treated as if the appropriate boolean
constant had been specified instead.
• The print-function option is named print-procedure. Its argument is a procedure
of two arguments (the unparser state and the structure instance) rather than three as
in Common Lisp.
• By default, named structures are tagged with a unique object of some kind. In Common
Lisp, the structures are tagged with symbols. This depends on the Common Lisp
package system to help generate unique tags; MIT Scheme has no such way to generate
unique symbols.
• The named option may optionally take an argument, which is normally the name of
a variable (any expression may be used, but it is evaluated whenever the tag name is
needed). If used, structure instances will be tagged with that variable’s value. The
variable must be defined when define-structure is evaluated.
• The type option is restricted to the values vector and list.
• The include option is not implemented.
2.11 Macros
(This section is largely taken from the Revised^4 Report on the Algorithmic Language
Scheme. The section on Syntactic Closures is derived from a document written by Chris
Hanson. The section on Explicit Renaming is derived from a document written by William
Clinger.)
Scheme programs can define and use new derived expression types, called macros.
Program-defined expression types have the syntax
(keyword datum ...)
where keyword is an identifier that uniquely determines the expression type. This identifier
is called the syntactic keyword, or simply keyword, of the macro. The number of the datums,
and their syntax, depends on the expression type.
Chapter 2: Special Forms
35
Each instance of a macro is called a use of the macro. The set of rules that specifies how
a use of a macro is transcribed into a more primitive expression is called the transformer
of the macro.
MIT Scheme also supports anonymous syntactic keywords. This means that it’s not
necessary to binding a macro transformer to a syntactic keyword before it is used. Instead,
any macro-transformer expression can appear as the first element of a form, and the form
will be expanded by the transformer.
The macro definition facility consists of these parts:
• A set of expressions used to establish that certain identifiers are macro keywords,
associate them with macro transformers, and control the scope within which a macro
is defined.
• A standard high-level pattern language for specifying macro transformers, introduced
by the syntax-rules special form.
• Two non-standard low-level languages for specifying macro transformers, syntactic closures and explicit renaming.
The syntactic keyword of a macro may shadow variable bindings, and local variable
bindings may shadow keyword bindings. All macros defined using the pattern language are
“hygienic” and “referentially transparent” and thus preserve Scheme’s lexical scoping:
• If a macro transformer inserts a binding for an identifier (variable or keyword), the
identifier will in e↵ect be renamed throughout its scope to avoid conflicts with other
identifiers.
• If a macro transformer inserts a free reference to an identifier, the reference refers to
the binding that was visible where the transformer was specified, regardless of any local
bindings that may surround the use of the macro.
2.11.1 Binding Constructs for Syntactic Keywords
let-syntax, letrec-syntax, let*-syntax and define-syntax are analogous to let,
letrec, let* and define, but they bind syntactic keywords to macro transformers instead
of binding variables to locations that contain values.
let-syntax bindings expression expression . . .
Bindings should have the form
special form
((keyword transformer-spec) ...)
Each keyword is an identifier, each transformer-spec is a a macro-transformer expression, and the body is a sequence of one or more expressions. It is an error for a
keyword to appear more than once in the list of keywords being bound.
The expressions are expanded in the syntactic environment obtained by extending the
syntactic environment of the let-syntax expression with macros whose keywords are
the keywords, bound to the specified transformers. Each binding of a keyword has
the expressions as its region.
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(let-syntax ((when (syntax-rules ()
((when test stmt1 stmt2 ...)
(if test
(begin stmt1
stmt2 ...))))))
(let ((if #t))
(when if (set! if ’now))
) now
if))
(let ((x ’outer))
(let-syntax ((m (syntax-rules () ((m) x))))
(let ((x ’inner))
) outer
(m))))
letrec-syntax bindings expression expression . . .
The syntax of letrec-syntax is the same as for let-syntax.
special form
The expressions are expanded in the syntactic environment obtained by extending the
syntactic environment of the letrec-syntax expression with macros whose keywords
are the keywords, bound to the specified transformers. Each binding of a keyword
has the bindings as well as the expressions within its region, so the transformers
can transcribe expressions into uses of the macros introduced by the letrec-syntax
expression.
(letrec-syntax
((my-or (syntax-rules ()
((my-or) #f)
((my-or e) e)
((my-or e1 e2 ...)
(let ((temp e1))
(if temp
temp
(my-or e2 ...)))))))
(let ((x #f)
(y 7)
(temp 8)
(let odd?)
(if even?))
(my-or x
(let temp)
(if y)
) 7
y)))
let*-syntax bindings expression expression . . .
The syntax of let*-syntax is the same as for let-syntax.
special form
The expressions are expanded in the syntactic environment obtained by extending the
syntactic environment of the letrec-syntax expression with macros whose keywords
are the keywords, bound to the specified transformers. Each binding of a keyword
has the subsequent bindings as well as the expressions within its region. Thus
Chapter 2: Special Forms
37
(let*-syntax
((a (syntax-rules ...))
(b (syntax-rules ...)))
...)
is equivalent to
(let-syntax ((a (syntax-rules ...)))
(let-syntax ((b (syntax-rules ...)))
...))
define-syntax keyword transformer-spec
special form
Keyword is an identifier, and transformer-spec is a macro transformer expression.
The syntactic environment is extended by binding the keyword to the specified transformer.
The region of the binding introduced by define-syntax is the entire block in which
it appears. However, the keyword may only be used after it has been defined.
MIT Scheme permits define-syntax to appear both at top level and within lambda
bodies. The Revised^4 Report permits only top-level uses of define-syntax.
When compiling a program, a top-level instance of define-syntax both defines the
syntactic keyword and generates code that will redefine the keyword when the program
is loaded. This means that the same syntax can be used for defining macros that will
be used during compilation and for defining macros to be used at run time.
Although macros may expand into definitions and syntax definitions in any context
that permits them, it is an error for a definition or syntax definition to shadow a
syntactic keyword whose meaning is needed to determine whether some form in the
group of forms that contains the shadowing definition is in fact a definition, or, for
internal definitions, is needed to determine the boundary between the group and the
expressions that follow the group. For example, the following are errors:
(define define 3)
(begin (define begin list))
(let-syntax
((foo (syntax-rules ()
((foo (proc args ...) body ...)
(define proc
(lambda (args ...)
body ...))))))
(let ((x 3))
(foo (plus x y) (+ x y))
(define foo x)
(plus foo x)))
2.11.2 Pattern Language
MIT Scheme supports a high-level pattern language for specifying macro transformers.
This pattern language is defined by the Revised^4 Report and is portable to other conform-
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ing Scheme implementations. To use the pattern language, specify a transformer-spec as a
syntax-rules form:
syntax-rules literals syntax-rule . . .
Literals is a list of identifiers and each syntax-rule should be of the form
(pattern template)
special form
The pattern in a syntax-rule is a list pattern that begins with the keyword for the
macro.
A pattern is either an identifier, a constant, or one of the following
(pattern ...)
(pattern pattern ... . pattern)
(pattern ... pattern ellipsis)
and a template is either an identifier, a constant, or one of the following
(element ...)
(element element ... . template)
where an element is a template optionally followed by an ellipsis and an ellipsis is
the identifier ‘...’ (which cannot be used as an identifier in either a template or a
pattern).
An instance of syntax-rules produces a new macro transformer by specifying a
sequence of hygienic rewrite rules. A use of a macro whose keyword is associated with
a transformer specified by syntax-rules is matched against the patterns contained
in the syntax-rules, beginning with the leftmost syntax-rule. When a match is found,
the macro use is transcribed hygienically according to the template.
An identifier that appears in the pattern of a syntax-rule is a pattern-variable, unless
it is the keyword that begins the pattern, is listed in literals, or is the identifier ‘...’.
Pattern variables match arbitrary input elements and are used to refer to elements
of the input in the template. It is an error for the same pattern variable to appear
more than once in a pattern.
The keyword at the beginning of the pattern in a syntax-rule is not involved in the
matching and is not considered a pattern variable or literal identifier.
Identifiers that appear in literals are interpreted as literal identifiers to be matched
against corresponding subforms of the input. A subform in the input matches a literal
identifier if and only if it is an identifier and either both its occurrence in the macro
expression and its occurrence in the macro definition have the same lexical binding,
or the two identifiers are equal and both have no lexical binding.
A subpattern followed by ... can match zero or more elements of the input. It is an
error for ... to appear in literals. Within a pattern the identifier ... must follow
the last element of a nonempty sequence of subpatterns.
More formally, an input form F matches a pattern P if and only if:
• P is a non-literal identifier; or
• P is a literal identifier and F is an identifier with the same binding; or
• P is a list (P 1 ... P n) and F is a list of n forms that match P 1 through P n,
respectively; or
Chapter 2: Special Forms
39
• P is an improper list (P 1 P 2 ... P n . P n+1) and F is a list or improper
list of n or more forms that match P 1 through P n, respectively, and whose nth
“cdr” matches P n+1; or
• P is of the form (P 1 ... P n P n+1 ellipsis) where ellipsis is the identifier ...
and F is a proper list of at least n forms, the first n of which match P 1 through
P n, respectively, and each remaining element of F matches P n+1; or
• P is a datum and F is equal to P in the sense of the equal? procedure.
It is an error to use a macro keyword, within the scope of its binding, in an expression
that does not match any of the patterns.
When a macro use is transcribed according to the template of the matching syntax
rule, pattern variables that occur in the template are replaced by the subforms they
match in the input. Pattern variables that occur in subpatterns followed by one or
more instances of the identifier ... are allowed only in subtemplates that are followed
by as many instances of .... They are replaced in the output by all of the subforms
they match in the input, distributed as indicated. It is an error if the output cannot
be built up as specified.
Identifiers that appear in the template but are not pattern variables or the identifier
... are inserted into the output as literal identifiers. If a literal identifier is inserted
as a free identifier then it refers to the binding of that identifier within whose scope
the instance of syntax-rules appears. If a literal identifier is inserted as a bound
identifier then it is in e↵ect renamed to prevent inadvertent captures of free identifiers.
(let ((=> #f))
(cond (#t => ’ok)))
) ok
The macro transformer for cond recognizes => as a local variable, and hence an
expression, and not as the top-level identifier =>, which the macro transformer treats
as a syntactic keyword. Thus the example expands into
(let ((=> #f))
(if #t (begin => ’ok)))
instead of
(let ((=> #f))
(let ((temp #t))
(if temp
(’ok temp))))
which would result in an invalid procedure call.
2.11.3 Syntactic Closures
MIT Scheme’s syntax-transformation engine is an implementation of syntactic closures,
a mechanism invented by Alan Bawden and Jonathan Rees. The main feature of the
syntactic-closures mechanism is its simplicity and its close relationship to the environment
models commonly used with Scheme. Using the mechanism to write macro transformers
is somewhat cumbersome and can be confusing for the newly initiated, but it is easily
mastered.
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2.11.3.1 Syntax Terminology
This section defines the concepts and data types used by the syntactic closures facility.
• Forms are the syntactic entities out of which programs are recursively constructed. A
form is any expression, any definition, any syntactic keyword, or any syntactic closure.
The variable name that appears in a set! special form is also a form. Examples of
forms:
17
#t
car
(+ x 4)
(lambda (x) x)
(define pi 3.14159)
if
define
• An alias is an alternate name for a given symbol. It can appear anywhere in a form that
the symbol could be used, and when quoted it is replaced by the symbol; however, it
does not satisfy the predicate symbol?. Macro transformers rarely distinguish symbols
from aliases, referring to both as identifiers. Another name for an alias is synthetic
identifier; this document uses both names.
• A syntactic environment maps identifiers to their meanings. More precisely, it determines whether an identifier is a syntactic keyword or a variable. If it is a keyword,
the meaning is an interpretation for the form in which that keyword appears. If it
is a variable, the meaning identifies which binding of that variable is referenced. In
short, syntactic environments contain all of the contextual information necessary for
interpreting the meaning of a particular form.
• A syntactic closure consists of a form, a syntactic environment, and a list of identifiers.
All identifiers in the form take their meaning from the syntactic environment, except
those in the given list. The identifiers in the list are to have their meanings determined
later.
A syntactic closure may be used in any context in which its form could have been used.
Since a syntactic closure is also a form, it may not be used in contexts where a form
would be illegal. For example, a form may not appear as a clause in the cond special
form.
A syntactic closure appearing in a quoted structure is replaced by its form.
2.11.3.2 Transformer Definition
This section describes the special forms for defining syntactic-closures macro transformers, and the associated procedures for manipulating syntactic closures and syntactic environments.
sc-macro-transformer expression
special form
The expression is expanded in the syntactic environment of the sc-macrotransformer expression, and the expanded expression is evaluated in the
transformer environment to yield a macro transformer as described below. This
Chapter 2: Special Forms
41
macro transformer is bound to a macro keyword by the special form in which the
transformer expression appears (for example, let-syntax).
In the syntactic closures facility, a macro transformer is a procedure that takes two
arguments, a form and a syntactic environment, and returns a new form. The first
argument, the input form, is the form in which the macro keyword occurred. The
second argument, the usage environment, is the syntactic environment in which the
input form occurred. The result of the transformer, the output form, is automatically
closed in the transformer environment, which is the syntactic environment in which
the transformer expression occurred.
For example, here is a definition of a push macro using syntax-rules:
(define-syntax push
(syntax-rules ()
((push item list)
(set! list (cons item list)))))
Here is an equivalent definition using sc-macro-transformer:
(define-syntax push
(sc-macro-transformer
(lambda (exp env)
(let ((item (make-syntactic-closure env ’() (cadr exp)))
(list (make-syntactic-closure env ’() (caddr exp))))
‘(set! ,list (cons ,item ,list))))))
In this example, the identifiers set! and cons are closed in the transformer environment, and thus will not be a↵ected by the meanings of those identifiers in the usage
environment env.
Some macros may be non-hygienic by design. For example, the following defines a
loop macro that implicitly binds exit to an escape procedure. The binding of exit
is intended to capture free references to exit in the body of the loop, so exit must
be left free when the body is closed:
(define-syntax loop
(sc-macro-transformer
(lambda (exp env)
(let ((body (cdr exp)))
‘(call-with-current-continuation
(lambda (exit)
(let f ()
,@(map (lambda (exp)
(make-syntactic-closure env ’(exit)
exp))
body)
(f))))))))
rsc-macro-transformer expression
special form
This form is an alternative way to define a syntactic-closures macro transformer. Its
syntax and usage are identical to sc-macro-transformer, except that the roles of the
usage environment and transformer environment are reversed. (Hence rsc stands for
Reversed Syntactic Closures.) In other words, the procedure specified by expression
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still accepts two arguments, but its second argument will be the transformer environment rather than the usage environment, and the returned expression is closed in the
usage environment rather than the transformer environment.
The advantage of this arrangement is that it allows a simpler definition style in some
situations. For example, here is the push macro from above, rewritten in this style:
(define-syntax push
(rsc-macro-transformer
(lambda (exp env)
‘(,(make-syntactic-closure env ’() ’SET!)
,(caddr exp)
(,(make-syntactic-closure env ’() ’CONS)
,(cadr exp)
,(caddr exp))))))
In this style only the introduced keywords are closed, while everything else remains
open.
Note that rsc-macro-transformer and sc-macro-transformer are easily
interchangeable.
Here is how to emulate rsc-macro-transformer using
sc-macro-transformer. (This technique can be used to e↵ect the opposite
emulation as well.)
(define-syntax push
(sc-macro-transformer
(lambda (exp usage-env)
(capture-syntactic-environment
(lambda (env)
(make-syntactic-closure usage-env ’()
‘(,(make-syntactic-closure env ’() ’SET!)
,(caddr exp)
(,(make-syntactic-closure env ’() ’CONS)
,(cadr exp)
,(caddr exp)))))))))
To assign meanings to the identifiers in a form, use make-syntactic-closure to close
the form in a syntactic environment.
make-syntactic-closure environment free-names form
procedure
Environment must be a syntactic environment, free-names must be a list of identifiers, and form must be a form. make-syntactic-closure constructs and returns a
syntactic closure of form in environment, which can be used anywhere that form could
have been used. All the identifiers used in form, except those explicitly excepted by
free-names, obtain their meanings from environment.
Here is an example where free-names is something other than the empty list. It is
instructive to compare the use of free-names in this example with its use in the loop
example above: the examples are similar except for the source of the identifier being
left free.
Chapter 2: Special Forms
43
(define-syntax let1
(sc-macro-transformer
(lambda (exp env)
(let ((id (cadr exp))
(init (caddr exp))
(exp (cadddr exp)))
‘((lambda (,id)
,(make-syntactic-closure env (list id) exp))
,(make-syntactic-closure env ’() init))))))
let1 is a simplified version of let that only binds a single identifier, and whose body
consists of a single expression. When the body expression is syntactically closed in
its original syntactic environment, the identifier that is to be bound by let1 must be
left free, so that it can be properly captured by the lambda in the output form.
In most situations, the free-names argument to make-syntactic-closure is the empty
list. In those cases, the more succinct close-syntax can be used:
close-syntax form environment
procedure
Environment must be a syntactic environment and form must be a form. Returns a
new syntactic closure of form in environment, with no free names. Entirely equivalent
to
(make-syntactic-closure environment ’() form)
To obtain a syntactic environment other than the usage environment, use capturesyntactic-environment.
capture-syntactic-environment procedure
procedure
capture-syntactic-environment returns a form that will, when transformed, call
procedure on the current syntactic environment. Procedure should compute and
return a new form to be transformed, in that same syntactic environment, in place of
the form.
An example will make this clear. Suppose we wanted to define a simple loop-until
keyword equivalent to
(define-syntax loop-until
(syntax-rules ()
((loop-until id init test return step)
(letrec ((loop
(lambda (id)
(if test return (loop step)))))
(loop init)))))
The following attempt at defining loop-until has a subtle bug:
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MIT Scheme Reference
(define-syntax loop-until
(sc-macro-transformer
(lambda (exp env)
(let ((id (cadr exp))
(init (caddr exp))
(test (cadddr exp))
(return (cadddr (cdr exp)))
(step (cadddr (cddr exp)))
(close
(lambda (exp free)
(make-syntactic-closure env free exp))))
‘(letrec ((loop
(lambda (,id)
(if ,(close test (list id))
,(close return (list id))
(loop ,(close step (list id)))))))
(loop ,(close init ’())))))))
This definition appears to take all of the proper precautions to prevent unintended
captures. It carefully closes the subexpressions in their original syntactic environment
and it leaves the id identifier free in the test, return, and step expressions, so that it
will be captured by the binding introduced by the lambda expression. Unfortunately
it uses the identifiers if and loop within that lambda expression, so if the user of
loop-until just happens to use, say, if for the identifier, it will be inadvertently
captured.
The syntactic environment that if and loop want to be exposed to is the one just
outside the lambda expression: before the user’s identifier is added to the syntactic
environment, but after the identifier loop has been added. capture-syntacticenvironment captures exactly that environment as follows:
Chapter 2: Special Forms
45
(define-syntax loop-until
(sc-macro-transformer
(lambda (exp env)
(let ((id (cadr exp))
(init (caddr exp))
(test (cadddr exp))
(return (cadddr (cdr exp)))
(step (cadddr (cddr exp)))
(close
(lambda (exp free)
(make-syntactic-closure env free exp))))
‘(letrec ((loop
,(capture-syntactic-environment
(lambda (env)
‘(lambda (,id)
(,(make-syntactic-closure env ’() ‘if)
,(close test (list id))
,(close return (list id))
(,(make-syntactic-closure env ’() ‘loop)
,(close step (list id)))))))))
(loop ,(close init ’())))))))
In this case, having captured the desired syntactic environment, it is convenient to
construct syntactic closures of the identifiers if and the loop and use them in the
body of the lambda.
A common use of capture-syntactic-environment is to get the transformer environment of a macro transformer:
(sc-macro-transformer
(lambda (exp env)
(capture-syntactic-environment
(lambda (transformer-env)
...))))
2.11.3.3 Identifiers
This section describes the procedures that create and manipulate identifiers. The identifier data type extends the syntactic closures facility to be compatible with the high-level
syntax-rules facility.
As discussed earlier, an identifier is either a symbol or an alias. An alias is implemented
as a syntactic closure whose form is an identifier:
(make-syntactic-closure env ’() ’a) ) an alias
Aliases are implemented as syntactic closures because they behave just like syntactic closures
most of the time. The di↵erence is that an alias may be bound to a new value (for example
by lambda or let-syntax); other syntactic closures may not be used this way. If an alias is
bound, then within the scope of that binding it is looked up in the syntactic environment
just like any other identifier.
Aliases are used in the implementation of the high-level facility syntax-rules. A macro
transformer created by syntax-rules uses a template to generate its output form, substi-
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MIT Scheme Reference
tuting subforms of the input form into the template. In a syntactic closures implementation,
all of the symbols in the template are replaced by aliases closed in the transformer environment, while the output form itself is closed in the usage environment. This guarantees
that the macro transformation is hygienic, without requiring the transformer to know the
syntactic roles of the substituted input subforms.
identifier? object
Returns #t if object is an identifier, otherwise returns #f. Examples:
procedure
) #t
(identifier? ’a)
(identifier? (make-syntactic-closure env ’() ’a))
) #t
(identifier?
(identifier?
(identifier?
(identifier?
(identifier?
(identifier?
"a")
#\a)
97)
#f)
’(a))
’#(a))
)
)
)
)
)
)
#f
#f
#f
#f
#f
#f
The predicate eq? is used to determine if two identifers are “the same”. Thus eq? can
be used to compare identifiers exactly as it would be used to compare symbols. Often,
though, it is useful to know whether two identifiers “mean the same thing”. For example,
the cond macro uses the symbol else to identify the final clause in the conditional. A
macro transformer for cond cannot just look for the symbol else, because the cond form
might be the output of another macro transformer that replaced the symbol else with an
alias. Instead the transformer must look for an identifier that “means the same thing” in
the usage environment as the symbol else means in the transformer environment.
identifier=? environment1 identifier1 environment2 identifier2
procedure
Environment1 and environment2 must be syntactic environments, and identifier1 and
identifier2 must be identifiers. identifier=? returns #t if the meaning of identifier1
in environment1 is the same as that of identifier2 in environment2, otherwise it returns
#f. Examples:
(let-syntax
((foo
(sc-macro-transformer
(lambda (form env)
(capture-syntactic-environment
(lambda (transformer-env)
(identifier=? transformer-env ’x env ’x)))))))
(list (foo)
(let ((x 3))
(foo))))
) (#t #f)
Chapter 2: Special Forms
47
(let-syntax ((bar foo))
(let-syntax
((foo
(sc-macro-transformer
(lambda (form env)
(capture-syntactic-environment
(lambda (transformer-env)
(identifier=? transformer-env ’foo
env (cadr form))))))))
(list (foo foo)
(foo bar))))
) (#f #t)
Sometimes it is useful to be able to introduce a new identifier that is guaranteed to
be di↵erent from any existing identifier, similarly to the way that generate-uninternedsymbol is used.
make-synthetic-identifier identifier
procedure
Creates and returns and new synthetic identifier (alias) that is guaranteed to be
di↵erent from all existing identifiers. Identifier is any existing identifier, which is
used in deriving the name of the new identifier.
This is implemented by syntactically closing identifier in a special empty environment.
2.11.4 Explicit Renaming
Explicit renaming is an alternative facility for defining macro transformers. In the MIT
Scheme implementation, explicit-renaming transformers are implemented as an abstraction
layer on top of syntactic closures. An explicit-renaming macro transformer is defined by an
instance of the er-macro-transformer keyword:
er-macro-transformer expression
special form
The expression is expanded in the syntactic environment of the er-macrotransformer expression, and the expanded expression is evaluated in the
transformer environment to yield a macro transformer as described below. This
macro transformer is bound to a macro keyword by the special form in which the
transformer expression appears (for example, let-syntax).
In the explicit-renaming facility, a macro transformer is a procedure that takes three
arguments, a form, a renaming procedure, and a comparison predicate, and returns
a new form. The first argument, the input form, is the form in which the macro
keyword occurred.
The second argument to a transformation procedure is a renaming procedure that
takes the representation of an identifier as its argument and returns the representation of a fresh identifier that occurs nowhere else in the program. For example, the
transformation procedure for a simplified version of the let macro might be written
as
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MIT Scheme Reference
(lambda (exp rename compare)
(let ((vars (map car (cadr exp)))
(inits (map cadr (cadr exp)))
(body (cddr exp)))
‘((lambda ,vars ,@body)
,@inits)))
This would not be hygienic, however. A hygienic let macro must rename the identifier
lambda to protect it from being captured by a local binding. The renaming e↵ectively
creates an fresh alias for lambda, one that cannot be captured by any subsequent
binding:
(lambda (exp rename compare)
(let ((vars (map car (cadr exp)))
(inits (map cadr (cadr exp)))
(body (cddr exp)))
‘((,(rename ’lambda) ,vars ,@body)
,@inits)))
The expression returned by the transformation procedure will be expanded in the
syntactic environment obtained from the syntactic environment of the macro application by binding any fresh identifiers generated by the renaming procedure to the
denotations of the original identifiers in the syntactic environment in which the macro
was defined. This means that a renamed identifier will denote the same thing as the
original identifier unless the transformation procedure that renamed the identifier
placed an occurrence of it in a binding position.
The renaming procedure acts as a mathematical function in the sense that the identifiers obtained from any two calls with the same argument will be the same in the sense
of eqv?. It is an error if the renaming procedure is called after the transformation
procedure has returned.
The third argument to a transformation procedure is a comparison predicate that
takes the representations of two identifiers as its arguments and returns true if and
only if they denote the same thing in the syntactic environment that will be used to
expand the transformed macro application. For example, the transformation procedure for a simplified version of the cond macro can be written as
(lambda (exp rename compare)
(let ((clauses (cdr exp)))
(if (null? clauses)
‘(,(rename ’quote) unspecified)
(let* ((first (car clauses))
(rest (cdr clauses))
(test (car first)))
(cond ((and (identifier? test)
(compare test (rename ’else)))
‘(,(rename ’begin) ,@(cdr first)))
(else ‘(,(rename ’if)
,test
(,(rename ’begin) ,@(cdr first))
(cond ,@rest))))))))))
Chapter 2: Special Forms
49
In this example the identifier else is renamed before being passed to the comparison
predicate, so the comparison will be true if and only if the test expression is an
identifier that denotes the same thing in the syntactic environment of the expression
being transformed as else denotes in the syntactic environment in which the cond
macro was defined. If else were not renamed before being passed to the comparison
predicate, then it would match a local variable that happened to be named else, and
the macro would not be hygienic.
Some macros are non-hygienic by design. For example, the following defines a loop
macro that implicitly binds exit to an escape procedure. The binding of exit is
intended to capture free references to exit in the body of the loop, so exit is not
renamed.
(define-syntax loop
(er-macro-transformer
(lambda (x r c)
(let ((body (cdr x)))
‘(,(r ’call-with-current-continuation)
(,(r ’lambda) (exit)
(,(r ’let) ,(r ’f) () ,@body (,(r ’f)))))))))
Suppose a while macro is implemented using loop, with the intent that exit may
be used to escape from the while loop. The while macro cannot be written as
(define-syntax while
(syntax-rules ()
((while test body ...)
(loop (if (not test) (exit #f))
body ...))))
because the reference to exit that is inserted by the while macro is intended to be
captured by the binding of exit that will be inserted by the loop macro. In other
words, this while macro is not hygienic. Like loop, it must be written using the
er-macro-transformer syntax:
(define-syntax while
(er-macro-transformer
(lambda (x r c)
(let ((test (cadr x))
(body (cddr x)))
‘(,(r ’loop)
(,(r ’if) (,(r ’not) ,test) (exit #f))
,@body)))))
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MIT Scheme Reference
Chapter 3: Equivalence Predicates
51
3 Equivalence Predicates
A predicate is a procedure that always returns a boolean value (#t or #f). An equivalence predicate is the computational analogue of a mathematical equivalence relation (it is
symmetric, reflexive, and transitive). Of the equivalence predicates described in this section, eq? is the finest or most discriminating, and equal? is the coarsest. eqv? is slightly
less discriminating than eq?.
eqv? obj1 obj2
procedure
The eqv? procedure defines a useful equivalence relation on objects. Briefly, it returns
#t if obj1 and obj2 should normally be regarded as the same object.
The eqv? procedure returns #t if:
• obj1 and obj2 are both #t or both #f.
• obj1 and obj2 are both interned symbols and
(string=? (symbol->string obj1)
(symbol->string obj2))
) #t
• obj1 and obj2 are both numbers, are numerically equal according to the = procedure, and are either both exact or both inexact (see Chapter 4 [Numbers],
page 57).
• obj1 and obj2 are both characters and are the same character according to the
char=? procedure (see Chapter 5 [Characters], page 77).
• both obj1 and obj2 are the empty list.
• obj1 and obj2 are procedures whose location tags are equal.
• obj1 and obj2 are pairs, vectors, strings, bit strings, records, cells, or weak pairs
that denote the same locations in the store.
The eqv? procedure returns #f if:
• obj1 and obj2 are of di↵erent types.
• one of obj1 and obj2 is #t but the other is #f.
• obj1 and obj2 are symbols but
(string=? (symbol->string obj1)
(symbol->string obj2))
) #f
• one of obj1 and obj2 is an exact number but the other is an inexact number.
• obj1 and obj2 are numbers for which the = procedure returns #f.
• obj1 and obj2 are characters for which the char=? procedure returns #f.
• one of obj1 and obj2 is the empty list but the other is not.
• obj1 and obj2 are procedures that would behave di↵erently (return a di↵erent
value or have di↵erent side e↵ects) for some arguments.
• obj1 and obj2 are pairs, vectors, strings, bit strings, records, cells, or weak pairs
that denote distinct locations.
Some examples:
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MIT Scheme Reference
(eqv?
(eqv?
(eqv?
(eqv?
(eqv?
(eqv?
(eqv?
’a ’a)
’a ’b)
2 2)
’() ’())
100000000 100000000)
(cons 1 2) (cons 1 2))
(lambda () 1)
(lambda () 2))
(eqv? #f ’nil)
(let ((p (lambda (x) x)))
(eqv? p p))
)
)
)
)
)
)
#t
#f
#t
#t
#t
#f
)
)
#f
#f
)
#t
The following examples illustrate cases in which the above rules do not fully specify
the behavior of eqv?. All that can be said about such cases is that the value returned
by eqv? must be a boolean.
(eqv? "" "")
(eqv? ’#() ’#())
(eqv? (lambda (x)
(lambda (x)
(eqv? (lambda (x)
(lambda (y)
x)
x))
x)
y))
)
)
unspecified
unspecified
)
unspecified
)
unspecified
The next set of examples shows the use of eqv? with procedures that have local state.
gen-counter must return a distinct procedure every time, since each procedure has
its own internal counter. gen-loser, however, returns equivalent procedures each
time, since the local state does not a↵ect the value or side e↵ects of the procedures.
(define gen-counter
(lambda ()
(let ((n 0))
(lambda () (set! n (+ n 1)) n))))
(let ((g (gen-counter)))
) #t
(eqv? g g))
(eqv? (gen-counter) (gen-counter))
) #f
(define gen-loser
(lambda ()
(let ((n 0))
(lambda () (set! n (+ n 1)) 27))))
(let ((g (gen-loser)))
) #t
(eqv? g g))
(eqv? (gen-loser) (gen-loser))
) unspecified
Chapter 3: Equivalence Predicates
53
(letrec ((f (lambda () (if (eqv? f g) ’both ’f)))
(g (lambda () (if (eqv? f g) ’both ’g)))
(eqv? f g))
) unspecified
(letrec ((f (lambda () (if (eqv? f g) ’f ’both)))
(g (lambda () (if (eqv? f g) ’g ’both)))
(eqv? f g))
) #f
Objects of distinct types must never be regarded as the same object.
Since it is an error to modify constant objects (those returned by literal expressions),
the implementation may share structure between constants where appropriate. Thus
the value of eqv? on constants is sometimes unspecified.
(let ((x ’(a)))
) #t
(eqv? x x))
) unspecified
(eqv? ’(a) ’(a))
) unspecified
(eqv? "a" "a")
) unspecified
(eqv? ’(b) (cdr ’(a b)))
Rationale: The above definition of eqv? allows implementations latitude in their
treatment of procedures and literals: implementations are free either to detect or to
fail to detect that two procedures or two literals are equivalent to each other, and
can decide whether or not to merge representations of equivalent objects by using the
same pointer or bit pattern to represent both.
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MIT Scheme Reference
eq? obj1 obj2
procedure
eq? is similar to eqv? except that in some cases it is capable of discerning distinctions
finer than those detectable by eqv?.
eq? and eqv? are guaranteed to have the same behavior on symbols, booleans, the
empty list, pairs, records, and non-empty strings and vectors. eq?’s behavior on
numbers and characters is implementation-dependent, but it will always return either
true or false, and will return true only when eqv? would also return true. eq? may
also behave di↵erently from eqv? on empty vectors and empty strings.
) #t
(eq? ’a ’a)
) unspecified
(eq? ’(a) ’(a))
) #f
(eq? (list ’a) (list ’a))
) unspecified
(eq? "a" "a")
) unspecified
(eq? "" "")
) #t
(eq? ’() ’())
) unspecified
(eq? 2 2)
) unspecified
(eq? #\A #\A)
) #t
(eq? car car)
(let ((n (+ 2 3)))
) unspecified
(eq? n n))
(let ((x ’(a)))
) #t
(eq? x x))
(let ((x ’#()))
) #t
(eq? x x))
(let ((p (lambda (x) x)))
) #t
(eq? p p))
Rationale: It will usually be possible to implement eq? much more efficiently than
eqv?, for example, as a simple pointer comparison instead of as some more complicated operation. One reason is that it may not be possible to compute eqv? of two
numbers in constant time, whereas eq? implemented as pointer comparison will always
finish in constant time. eq? may be used like eqv? in applications using procedures
to implement objects with state since it obeys the same constraints as eqv?.
Chapter 3: Equivalence Predicates
equal? obj1 obj2
55
procedure
equal? recursively compares the contents of pairs, vectors, and strings, applying
eqv? on other objects such as numbers, symbols, and records. A rule of thumb is
that objects are generally equal? if they print the same. equal? may fail to terminate
if its arguments are circular data structures.
) #t
(equal? ’a ’a)
) #t
(equal? ’(a) ’(a))
(equal? ’(a (b) c)
) #t
’(a (b) c))
) #t
(equal? "abc" "abc")
) #t
(equal? 2 2)
(equal? (make-vector 5 ’a)
) #t
(make-vector 5 ’a))
(equal? (lambda (x) x)
) unspecified
(lambda (y) y))
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MIT Scheme Reference
Chapter 4: Numbers
57
4 Numbers
(This section is largely taken from the Revised^4 Report on the Algorithmic Language
Scheme.)
Numerical computation has traditionally been neglected by the Lisp community. Until
Common Lisp there was no carefully thought out strategy for organizing numerical computation, and with the exception of the MacLisp system little e↵ort was made to execute
numerical code efficiently. This report recognizes the excellent work of the Common Lisp
committee and accepts many of their recommendations. In some ways this report simplifies
and generalizes their proposals in a manner consistent with the purposes of Scheme.
It is important to distinguish between the mathematical numbers, the Scheme numbers
that attempt to model them, the machine representations used to implement the Scheme
numbers, and notations used to write numbers. This report uses the types number, complex,
real, rational, and integer to refer to both mathematical numbers and Scheme numbers.
Machine representations such as fixed point and floating point are referred to by names
such as fixnum and flonum.
4.1 Numerical types
Mathematically, numbers may be arranged into a tower of subtypes in which each level
is a subset of the level above it:
number
complex
real
rational
integer
For example, 3 is an integer. Therefore 3 is also a rational, a real, and a complex. The
same is true of the Scheme numbers that model 3. For Scheme numbers, these types are
defined by the predicates number?, complex?, real?, rational?, and integer?.
There is no simple relationship between a number’s type and its representation inside
a computer. Although most implementations of Scheme will o↵er at least two di↵erent
representations of 3, these di↵erent representations denote the same integer.
Scheme’s numerical operations treat numbers as abstract data, as independent of their
representation as possible. Although an implementation of Scheme may use fixnum, flonum,
and perhaps other representations for numbers, this should not be apparent to a casual
programmer writing simple programs.
It is necessary, however, to distinguish between numbers that are represented exactly
and those that may not be. For example, indexes into data structures must be known
exactly, as must some polynomial coefficients in a symbolic algebra system. On the other
hand, the results of measurements are inherently inexact, and irrational numbers may be
approximated by rational and therefore inexact approximations. In order to catch uses of
inexact numbers where exact numbers are required, Scheme explicitly distinguishes exact
from inexact numbers. This distinction is orthogonal to the dimension of type.
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MIT Scheme Reference
4.2 Exactness
Scheme numbers are either exact or inexact. A number is exact if it was written as an
exact constant or was derived from exact numbers using only exact operations. A number is
inexact if it was written as an inexact constant, if it was derived using inexact ingredients,
or if it was derived using inexact operations. Thus inexactness is a contagious property of
a number.
If two implementations produce exact results for a computation that did not involve inexact intermediate results, the two ultimate results will be mathematically equivalent. This
is generally not true of computations involving inexact numbers since approximate methods
such as floating point arithmetic may be used, but it is the duty of each implementation to
make the result as close as practical to the mathematically ideal result.
Rational operations such as + should always produce exact results when given exact
arguments. If the operation is unable to produce an exact result, then it may either report
the violation of an implementation restriction or it may silently coerce its result to an
inexact value. See Section 4.3 [Implementation restrictions], page 58.
With the exception of inexact->exact, the operations described in this section must
generally return inexact results when given any inexact arguments. An operation may,
however, return an exact result if it can prove that the value of the result is una↵ected by
the inexactness of its arguments. For example, multiplication of any number by an exact
zero may produce an exact zero result, even if the other argument is inexact.
4.3 Implementation restrictions
Implementations of Scheme are not required to implement the whole tower of subtypes
(see Section 4.1 [Numerical types], page 57), but they must implement a coherent subset
consistent with both the purposes of the implementation and the spirit of the Scheme
language. For example, an implementation in which all numbers are real may still be quite
useful.1
Implementations may also support only a limited range of numbers of any type, subject
to the requirements of this section. The supported range for exact numbers of any type
may be di↵erent from the supported range for inexact numbers of that type. For example,
an implementation that uses flonums to represent all its inexact real numbers may support
a practically unbounded range of exact integers and rationals while limiting the range of
inexact reals (and therefore the range of inexact integers and rationals) to the dynamic range
of the flonum format. Furthermore the gaps between the representable inexact integers and
rationals are likely to be very large in such an implementation as the limits of this range
are approached.
An implementation of Scheme must support exact integers throughout the range of
numbers that may be used for indexes of lists, vectors, and strings or that may result from
computing the length of a list, vector, or string. The length, vector-length, and stringlength procedures must return an exact integer, and it is an error to use anything but an
1
MIT Scheme implements the whole tower of numerical types. It has unlimited-precision exact integers
and exact rationals. Flonums are used to implement all inexact reals; on machines that support ieee
floating-point arithmetic these are double-precision floating-point numbers.
Chapter 4: Numbers
59
exact integer as an index. Furthermore any integer constant within the index range, if
expressed by an exact integer syntax, will indeed be read as an exact integer, regardless of
any implementation restrictions that may apply outside this range. Finally, the procedures
listed below will always return an exact integer result provided all their arguments are exact
integers and the mathematically expected result is representable as an exact integer within
the implementation:
*
+
abs
angle
ceiling
denominator
expt
floor
gcd
imag-part
inexact->exact
lcm
magnitude
make-polar
make-rectangular
max
min
modulo
numerator
quotient
rationalize
real-part
remainder
round
truncate
Implementations are encouraged, but not required, to support exact integers and exact
rationals of practically unlimited size and precision, and to implement the above procedures
and the / procedure in such a way that they always return exact results when given exact
arguments. If one of these procedures is unable to deliver an exact result when given exact
arguments, then it may either report a violation of an implementation restriction or it may
silently coerce its result to an inexact number. Such a coercion may cause an error later.
An implementation may use floating point and other approximate representation strategies for inexact numbers. This report recommends, but does not require, that the ieee
32-bit and 64-bit floating point standards be followed by implementations that use flonum
representations, and that implementations using other representations should match or exceed the precision achievable using these floating point standards.
In particular, implementations that use flonum representations must follow these rules:
A flonum result must be represented with at least as much precision as is used to express any
of the inexact arguments to that operation. It is desirable (but not required) for potentially
inexact operations such as sqrt, when applied to exact arguments, to produce exact answers
whenever possible (for example the square root of an exact 4 ought to be an exact 2). If,
however, an exact number is operated upon so as to produce an inexact result (as by sqrt),
and if the result is represented as a flonum, then the most precise flonum format available
must be used; but if the result is represented in some other way then the representation
must have at least as much precision as the most precise flonum format available.
Although Scheme allows a variety of written notations for numbers, any particular implementation may support only some of them.2 For example, an implementation in which
all numbers are real need not support the rectangular and polar notations for complex numbers. If an implementation encounters an exact numerical constant that it cannot represent
as an exact number, then it may either report a violation of an implementation restriction
or it may silently represent the constant by an inexact number.
2
MIT Scheme implements all of the written notations for numbers.
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4.4 Syntax of numerical constants
A number may be written in binary, octal, decimal, or hexadecimal by the use of a radix
prefix. The radix prefixes are #b (binary), #o (octal), #d (decimal), and #x (hexadecimal).
With no radix prefix, a number is assumed to be expressed in decimal.
A numerical constant may be specified to be either exact or inexact by a prefix. The
prefixes are #e for exact, and #i for inexact. An exactness prefix may appear before or after
any radix prefix that is used. If the written representation of a number has no exactness
prefix, the constant may be either inexact or exact. It is inexact if it contains a decimal
point, an exponent, or a # character in the place of a digit, otherwise it is exact.
In systems with inexact numbers of varying precisions it may be useful to specify the
precision of a constant. For this purpose, numerical constants may be written with an
exponent marker that indicates the desired precision of the inexact representation. The
letters s, f, d, and l specify the use of short, single, double, and long precision, respectively.
(When fewer than four internal inexact representations exist, the four size specifications
are mapped onto those available. For example, an implementation with two internal representations may map short and single together and long and double together.) In addition,
the exponent marker e specifies the default precision for the implementation. The default
precision has at least as much precision as double, but implementations may wish to allow
this default to be set by the user.
3.14159265358979F0
Round to single — 3.141593
0.6L0
Extend to long — .600000000000000
4.5 Numerical operations
See Section 1.1.3 [Entry Format], page 5, for a summary of the naming conventions
used to specify restrictions on the types of arguments to numerical routines. The examples
used in this section assume that any numerical constant written using an exact notation is
indeed represented as an exact number. Some examples also assume that certain numerical
constants written using an inexact notation can be represented without loss of accuracy;
the inexact constants were chosen so that this is likely to be true in implementations that
use flonums to represent inexact numbers.
number? object
complex? object
real? object
rational? object
integer? object
procedure
procedure
procedure
procedure
procedure
These numerical type predicates can be applied to any kind of argument, including
non-numbers. They return #t if the object is of the named type, and otherwise they
return #f. In general, if a type predicate is true of a number then all higher type
predicates are also true of that number. Consequently, if a type predicate is false of
a number, then all lower type predicates are also false of that number.3
3
In MIT Scheme the rational? procedure is the same as real?, and the complex? procedure is the same
as number?.
Chapter 4: Numbers
61
If z is an inexact complex number, then (real? z) is true if and only if (zero?
(imag-part z)) is true. If x is an inexact real number, then (integer? x) is true if
and only if (= x (round x)).
(complex? 3+4i)
(complex? 3)
(real? 3)
(real? -2.5+0.0i)
(real? #e1e10)
(rational? 6/10)
(rational? 6/3)
(integer? 3+0i)
(integer? 3.0)
(integer? 8/4)
)
)
)
)
)
)
)
)
)
)
#t
#t
#t
#t
#t
#t
#t
#t
#t
#t
Note: The behavior of these type predicates on inexact numbers is unreliable, since
any inaccuracy may a↵ect the result.
exact? z
inexact? z
procedure
procedure
These numerical predicates provide tests for the exactness of a quantity. For any
Scheme number, precisely one of these predicates is true.
exact-integer? object
exact-nonnegative-integer? object
exact-rational? object
procedure
procedure
procedure
These procedures test for some very common types of numbers. These tests could be
written in terms of simpler predicates, but are more efficient.
= z1 z2 z3 . . .
< x1 x2 x3 . . .
> x1 x2 x3 . . .
<= x1 x2 x3 . . .
>= x1 x2 x3 . . .
procedure
procedure
procedure
procedure
procedure
These procedures return #t if their arguments are (respectively): equal, monotonically
increasing, monotonically decreasing, monotonically nondecreasing, or monotonically
nonincreasing.
These predicates are transitive. Note that the traditional implementations of these
predicates in Lisp-like languages are not transitive.
Note: While it is not an error to compare inexact numbers using these predicates,
the results may be unreliable because a small inaccuracy may a↵ect the result; this
is especially true of = and zero?. When in doubt, consult a numerical analyst.
62
MIT Scheme Reference
zero? z
positive? x
negative? x
odd? x
even? x
procedure
procedure
procedure
procedure
procedure
These numerical predicates test a number for a particular property, returning #t or
#f. See note above regarding inexact numbers.
max x1 x2 . . .
min x1 x2 . . .
These procedures return the maximum or minimum of their arguments.
) 4
(max 3 4)
; exact
) 4.0 ; inexact
(max 3.9 4)
procedure
procedure
Note: If any argument is inexact, then the result will also be inexact (unless the
procedure can prove that the inaccuracy is not large enough to a↵ect the result, which
is possible only in unusual implementations). If min or max is used to compare numbers
of mixed exactness, and the numerical value of the result cannot be represented as an
inexact number without loss of accuracy, then the procedure may report a violation
of an implementation restriction.4
+ z1 . . .
* z1 . . .
These procedures return the sum or product of their arguments.
) 7
(+ 3 4)
) 3
(+ 3)
) 0
(+)
) 4
(* 4)
) 1
(*)
procedure
procedure
- z1 z2 . . .
/ z1 z2 . . .
procedure
procedure
With two or more arguments, these procedures return the di↵erence or quotient of
their arguments, associating to the left. With one argument, however, they return
the additive or multiplicative inverse of their argument.
) -1
(- 3 4)
) -6
(- 3 4 5)
) -3
(- 3)
) 3/20
(/ 3 4 5)
) 1/3
(/ 3)
1+ z
-1+ z
4
(1+ z) is equivalent to (+ z 1); (-1+ z) is equivalent to (- z 1).
MIT Scheme signals an error of type condition-type:bad-range-argument in this case.
procedure
procedure
Chapter 4: Numbers
63
abs x
abs returns the magnitude of its argument.
) 7
(abs -7)
procedure
quotient n1 n2
remainder n1 n2
modulo n1 n2
procedure
procedure
procedure
These procedures implement number-theoretic (integer) division: for positive integers
n1 and n2, if n3 and n4 are integers such that
n1 = n2 n3 + n4
0  n4 < n2
then
) n3
(quotient n1 n2)
) n4
(remainder n1 n2)
) n4
(modulo n1 n2)
For integers n1 and n2 with n2 not equal to 0,
(= n1
(+ (* n2 (quotient n1 n2))
(remainder n1 n2)))
) #t
provided all numbers involved in that computation are exact.
The value returned by quotient always has the sign of the product of its arguments.
remainder and modulo di↵er on negative arguments — the remainder always has
the sign of the dividend, the modulo always has the sign of the divisor:
) 1
(modulo 13 4)
) 1
(remainder 13 4)
(modulo -13 4)
(remainder -13 4)
)
)
3
-1
(modulo 13 -4)
(remainder 13 -4)
)
)
-3
1
(modulo -13 -4)
(remainder -13 -4)
)
)
-1
-1
) -1.0 ; inexact
(remainder -13 -4.0)
Note that quotient is the same as integer-truncate.
integer-floor n1 n2
integer-ceiling n1 n2
integer-truncate n1 n2
integer-round n1 n2
procedure
procedure
procedure
procedure
These procedures combine integer division with rounding. For example, the following
are equivalent:
64
MIT Scheme Reference
(integer-floor n1 n2)
(floor (/ n1 n2))
However, the former is faster and does not produce an intermediate result.
Note that integer-truncate is the same as quotient.
integer-divide n1 n2
integer-divide-quotient qr
integer-divide-remainder qr
procedure
procedure
procedure
integer-divide is equivalent to performing both quotient and remainder at
once. The result of integer-divide is an object with two components; the
procedures integer-divide-quotient and integer-divide-remainder select
those components. These procedures are useful when both the quotient and
remainder are needed; often computing both of these numbers simultaneously is
much faster than computing them separately.
For example, the following are equivalent:
(lambda (n d)
(cons (quotient n d)
(remainder n d)))
(lambda (n d)
(let ((qr (integer-divide n d)))
(cons (integer-divide-quotient qr)
(integer-divide-remainder qr))))
gcd n1 . . .
lcm n1 . . .
procedure
procedure
These procedures return the greatest common divisor or least common multiple of
their arguments. The result is always non-negative.
(gcd 32 -36)
(gcd)
)
)
4
0
(lcm 32 -36)
(lcm 32.0 -36)
(lcm)
)
)
)
288
288.0
1
; inexact
numerator q
denominator q
procedure
procedure
These procedures return the numerator or denominator of their argument; the result
is computed as if the argument was represented as a fraction in lowest terms. The
denominator is always positive. The denominator of 0 is defined to be 1.
(numerator (/ 6 4)) ) 3
(denominator (/ 6 4)) ) 2
(denominator (exact->inexact (/ 6 4))) ) 2.0
Chapter 4: Numbers
65
floor x
ceiling x
truncate x
round x
procedure
procedure
procedure
procedure
These procedures return integers. floor returns the largest integer not larger than
x. ceiling returns the smallest integer not smaller than x. truncate returns the
integer closest to x whose absolute value is not larger than the absolute value of x.
round returns the closest integer to x, rounding to even when x is halfway between
two integers.
Rationale: round rounds to even for consistency with the rounding modes required
by the ieee floating point standard.
Note: If the argument to one of these procedures is inexact, then the result will also
be inexact. If an exact value is needed, the result should be passed to the inexact>exact procedure (or use one of the procedures below).
) -5.0
(floor -4.3)
) -4.0
(ceiling -4.3)
) -4.0
(truncate -4.3)
) -4.0
(round -4.3)
(floor 3.5)
(ceiling 3.5)
(truncate 3.5)
(round 3.5)
)
)
)
)
3.0
4.0
3.0
4.0
(round 7/2)
(round 7)
)
)
4
7
; inexact
; exact
floor->exact x
ceiling->exact x
truncate->exact x
round->exact x
procedure
procedure
procedure
procedure
These procedures are similar to the preceding procedures except that they always
return an exact result. For example, the following are equivalent
(floor->exact x)
(inexact->exact (floor x))
except that the former is faster and has fewer range restrictions.
rationalize x y
rationalize->exact x y
procedure
procedure
rationalize returns the simplest rational number di↵ering from x by no more than
y. A rational number r1 is simpler than another rational number r2 if r1=p1/q1
and r2=p2/q2 (both in lowest terms) and |p1|<=|p2| and |q1|<=|q2|. Thus 3/5 is
simpler than 4/7. Although not all rationals are comparable in this ordering (consider
2/7 and 3/5) any interval contains a rational number that is simpler than every other
rational number in that interval (the simpler 2/5 lies between 2/7 and 3/5). Note
that 0=0/1 is the simplest rational of all.
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MIT Scheme Reference
(rationalize (inexact->exact .3) 1/10) ) 1/3
; exact
)
(rationalize .3 1/10)
#i1/3 ; inexact
rationalize->exact is similar to rationalize except that it always returns an exact
result.
simplest-rational x y
simplest-exact-rational x y
procedure
procedure
simplest-rational returns the simplest rational number between x and y inclusive;
simplest-exact-rational is similar except that it always returns an exact result.
These procedures implement the same functionality as rationalize and
rationalize->exact, except that they specify the input range by its endpoints;
rationalize specifies the range by its center point and its (half-) width.
exp z
log z
sin z
cos z
tan z
asin z
acos z
atan z
atan y x
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
These procedures compute the usual transcendental functions. log computes the
natural logarithm of z (not the base ten logarithm). asin, acos, and atan compute
arcsine, arccosine, and arctangent, respectively. The two-argument variant of atan
computes (angle (make-rectangular x y)) (see below).
In general, the mathematical functions log, arcsine, arccosine, and arctangent are
multiply defined. For nonzero real x, the value of log x is defined to be the one
whose imaginary part lies in the range minus pi (exclusive) to pi (inclusive). log 0 is
undefined. The value of log z when z is complex is defined according to the formula
log z = log magnitude(z) + iangle(z)
With log defined this way, the values of arcsine, arccosine, and arctangent are according to the following formulae:
p
sin 1 z = i log(iz + 1 z 2 )
cos
tan
1
1
z = ⇡/2
z = (log(1 + iz)
sin
1
log(1
z
iz))/(2i)
The above specification follows Common Lisp: the Language, which in turn cites
Principal Values and Branch Cuts in Complex APL; refer to these sources for more
detailed discussion of branch cuts, boundary conditions, and implementation of these
functions. When it is possible these procedures produce a real result from a real
argument.
Chapter 4: Numbers
67
sqrt z
procedure
Returns the principal square root of z. The result will have either positive real part,
or zero real part and non-negative imaginary part.
expt z1 z2
Returns z1 raised to the power z2:
procedure
z1 z2 = ez2 log z1
00 is defined to be equal to 1.
make-rectangular x1 x2
make-polar x3 x4
real-part z
imag-part z
magnitude z
angle z
conjugate z
procedure
procedure
procedure
procedure
procedure
procedure
procedure
Suppose x1, x2, x3, and x4 are real numbers and z is a complex number such that
z = x1 + x2 i = x3 · eix4
Then make-rectangular and make-polar return z, real-part returns x1, imagpart returns x2, magnitude returns x3, and angle returns x4. In the case of angle,
whose value is not uniquely determined by the preceding rule, the value returned will
be the one in the range minus pi (exclusive) to pi (inclusive).
conjugate returns the complex conjugate of z.
exact->inexact z
inexact->exact z
procedure
procedure
exact->inexact returns an inexact representation of z. The value returned is the inexact number that is numerically closest to the argument. If an exact argument has no
reasonably close inexact equivalent, then a violation of an implementation restriction
may be reported; MIT Scheme signals an error of type condition-type:bad-rangeargument in this case.
inexact->exact returns an exact representation of z. The value returned is the exact
number that is numerically closest to the argument. If an inexact argument has no
reasonably close exact equivalent, then a violation of an implementation restriction
may be reported; in MIT Scheme this case does not occur because all inexact numbers
are representable as exact numbers.
These procedures implement the natural one-to-one correspondence between exact
and inexact integers throughout an implementation-dependent range. See Section 4.3
[Implementation restrictions], page 58.
68
MIT Scheme Reference
4.6 Numerical input and output
number->string number [radix]
procedure
Radix must be an exact integer, either 2, 8, 10, or 16. If omitted, radix defaults to
10. The procedure number->string takes a number and a radix and returns as a
string an external representation of the given number in the given radix such that
(let ((number number)
(radix radix))
(eqv? number
(string->number (number->string number radix)
radix)))
is true. It is an error if no possible result makes this expression true.
If number is inexact, the radix is 10, and the above expression can be satisfied by
a result that contains a decimal point, then the result contains a decimal point and
is expressed using the minimum number of digits (exclusive of exponent and trailing
zeroes) needed to make the above expression true; otherwise the format of the result
is unspecified.
The result returned by number->string never contains an explicit radix prefix.
Note: The error case can occur only when number is not a complex number or is a
complex number with an non-rational real or imaginary part.
Rationale: If number is an inexact number represented using flonums, and the radix
is 10, then the above expression is normally satisfied by a result containing a decimal
point. The unspecified case allows for infinities, NaNs, and non-flonum representations.
flonum-parser-fast?
variable
This variable controls the behavior of string->number when parsing inexact numbers.
Specifically, it allows the user to trade o↵ accuracy against speed.
When set to its default value, #f, the parser provides maximal accuracy, as required
by the Scheme standard. If set to #t, the parser uses faster algorithms that will
sometimes introduce small errors in the result. The errors a↵ect a few of the leastsignificant bits of the result, and consequently can be tolerated by many applications.
flonum-unparser-cuto↵
variable
This variable controls the action of number->string when number is a flonum (and
consequently controls all printing of flonums). The value of this variable is normally
a list of three items:
rounding-type
One of the following symbols: normal, relative, or absolute. The symbol normal means that the number should be printed with full precision.
The symbol relative means that the number should be rounded to a
specific number of digits. The symbol absolute means that the number
should be rounded so that there are a specific number of digits to the
right of the decimal point.
Chapter 4: Numbers
precision
format-type
69
An exact integer. If rounding-type is normal, precision is ignored. If
rounding-type is relative, precision must be positive, and it specifies
the number of digits to which the printed representation will be rounded.
If rounding-type is absolute, the printed representation will be rounded
precision digits to the right of the decimal point; if precision is negative,
the representation is rounded (- precision) digits to the left of the decimal
point.
One of the symbols: normal, scientific, or engineering. This specifies the format in which the number will be printed.
scientific specifies that the number will be printed using scientific notation: x.xxxeyyy. In other words, the number is printed as a mantissa
between zero inclusive and ten exclusive, and an exponent. engineering
is like scientific, except that the exponent is always a power of three,
and the mantissa is constrained to be between zero inclusive and 1000
exclusive. If normal is specified, the number will be printed in positional
notation if it is “small enough”, otherwise it is printed in scientific notation. A number is “small enough” when the number of digits that would
be printed using positional notation does not exceed the number of digits
of precision in the underlying floating-point number representation; ieee
double-precision floating-point numbers have 17 digits of precision.
This three-element list may be abbreviated in two ways. First, the symbol normal
may be used, which is equivalent to the list (normal 0 normal). Second, the third
element of the list, format-type, may be omitted, in which case it defaults to normal.
The default value for flonum-unparser-cutoff is normal. If it is bound to a value
di↵erent from those described here, number->string issues a warning and acts as
though the value had been normal.
Some examples of flonum-unparser-cutoff:
(number->string (* 4 (atan 1 1)))
) "3.141592653589793"
(fluid-let ((flonum-unparser-cutoff ’(relative 5)))
(number->string (* 4 (atan 1 1))))
) "3.1416"
(fluid-let ((flonum-unparser-cutoff ’(relative 5)))
(number->string (* 4000 (atan 1 1))))
) "3141.6"
(fluid-let ((flonum-unparser-cutoff ’(relative 5 scientific)))
(number->string (* 4000 (atan 1 1))))
) "3.1416e3"
(fluid-let ((flonum-unparser-cutoff ’(relative 5 scientific)))
(number->string (* 40000 (atan 1 1))))
) "3.1416e4"
(fluid-let ((flonum-unparser-cutoff ’(relative 5 engineering)))
(number->string (* 40000 (atan 1 1))))
) "31.416e3"
(fluid-let ((flonum-unparser-cutoff ’(absolute 5)))
70
MIT Scheme Reference
(number->string (* 4 (atan 1 1))))
) "3.14159"
(fluid-let ((flonum-unparser-cutoff ’(absolute 5)))
(number->string (* 4000 (atan 1 1))))
) "3141.59265"
(fluid-let ((flonum-unparser-cutoff ’(absolute -4)))
(number->string (* 4e10 (atan 1 1))))
) "31415930000."
(fluid-let ((flonum-unparser-cutoff ’(absolute -4 scientific)))
(number->string (* 4e10 (atan 1 1))))
) "3.141593e10"
(fluid-let ((flonum-unparser-cutoff ’(absolute -4 engineering)))
(number->string (* 4e10 (atan 1 1))))
) "31.41593e9"
(fluid-let ((flonum-unparser-cutoff ’(absolute -5)))
(number->string (* 4e10 (atan 1 1))))
) "31415900000."
string->number string [radix]
procedure
Returns a number of the maximally precise representation expressed by the given
string. Radix must be an exact integer, either 2, 8, 10, or 16. If supplied, radix
is a default radix that may be overridden by an explicit radix prefix in string (e.g.
"#o177"). If radix is not supplied, then the default radix is 10. If string is not a
syntactically valid notation for a number, then string->number returns #f.
) 100
(string->number "100")
) 256
(string->number "100" 16)
) 100.0
(string->number "1e2")
) 1500.0
(string->number "15##")
Note that a numeric representation using a decimal point or an exponent marker is
not recognized unless radix is 10.
4.7 Fixnum and Flonum Operations
This section describes numerical operations that are restricted forms of the operations
described above. These operations are useful because they compile very efficiently. However,
care should be exercised: if used improperly, these operations can return incorrect answers,
or even malformed objects that confuse the garbage collector.
4.7.1 Fixnum Operations
A fixnum is an exact integer that is small enough to fit in a machine word. In MIT
Scheme, fixnums are typically 24 or 26 bits, depending on the machine; it is reasonable to
assume that fixnums are at least 24 bits. Fixnums are signed; they are encoded using 2’s
complement.
All exact integers that are small enough to be encoded as fixnums are always encoded as
fixnums — in other words, any exact integer that is not a fixnum is too big to be encoded
as such. For this reason, small constants such as 0 or 1 are guaranteed to be fixnums.
Chapter 4: Numbers
71
fix:fixnum? object
procedure
Returns #t if object is a fixnum; otherwise returns #f.
Here is an expression that determines the largest fixnum:
(let loop ((n 1))
(if (fix:fixnum? n)
(loop (* n 2))
(- n 1)))
A similar expression determines the smallest fixnum.
fix:= fixnum fixnum
fix:< fixnum fixnum
fix:> fixnum fixnum
fix:<= fixnum fixnum
fix:>= fixnum fixnum
procedure
procedure
procedure
procedure
procedure
These are the standard order and equality predicates on fixnums. When compiled,
they do not check the types of their arguments.
fix:zero? fixnum
fix:positive? fixnum
fix:negative? fixnum
procedure
procedure
procedure
These procedures compare their argument to zero. When compiled, they do not
check the type of their argument. The code produced by the following expressions is
identical:
(fix:zero? fixnum)
(fix:= fixnum 0)
Similarly, fix:positive? and fix:negative? produce code identical to equivalent
expressions using fix:> and fix:<.
fix:+ fixnum fixnum
fix:- fixnum fixnum
fix:* fixnum fixnum
fix:quotient fixnum fixnum
fix:remainder fixnum fixnum
fix:gcd fixnum fixnum
fix:1+ fixnum
fix:-1+ fixnum
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
These procedures are the standard arithmetic operations on fixnums. When compiled,
they do not check the types of their arguments. Furthermore, they do not check to
see if the result can be encoded as a fixnum. If the result is too large to be encoded
as a fixnum, a malformed object is returned, with potentially disastrous e↵ect on the
garbage collector.
fix:divide fixnum fixnum
procedure
This procedure is like integer-divide, except that its arguments and its results must
be fixnums. It should be used in conjunction with integer-divide-quotient and
integer-divide-remainder.
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MIT Scheme Reference
The following are bitwise-logical operations on fixnums.
fix:not fixnum
procedure
This returns the bitwise-logical inverse of its argument. When compiled, it does not
check the type of its argument.
(fix:not
(fix:not
(fix:not
(fix:not
0)
-1)
1)
-34)
)
)
)
)
-1
0
-2
33
fix:and fixnum fixnum
procedure
This returns the bitwise-logical “and” of its arguments. When compiled, it does not
check the types of its arguments.
(fix:and #x43 #x0f)
(fix:and #x43 #xf0)
)
)
3
#x40
fix:andc fixnum fixnum
procedure
Returns the bitwise-logical “and” of the first argument with the bitwise-logical inverse
of the second argument. When compiled, it does not check the types of its arguments.
(fix:andc #x43 #x0f)
(fix:andc #x43 #xf0)
)
)
#x40
3
fix:or fixnum fixnum
procedure
This returns the bitwise-logical “inclusive or” of its arguments. When compiled, it
does not check the types of its arguments.
(fix:or #x40 3)
(fix:or #x41 3)
) #x43
) #x43
fix:xor fixnum fixnum
procedure
This returns the bitwise-logical “exclusive or” of its arguments. When compiled, it
does not check the types of its arguments.
(fix:xor #x40 3)
(fix:xor #x41 3)
) #x43
) #x42
fix:lsh fixnum1 fixnum2
procedure
This procedure returns the result of logically shifting fixnum1 by fixnum2 bits. If
fixnum2 is positive, fixnum1 is shifted left; if negative, it is shifted right. When
compiled, it does not check the types of its arguments, nor the validity of its result.
(fix:lsh
(fix:lsh
(fix:lsh
(fix:lsh
1 10)
#x432 -10)
-1 3)
-128 -4)
)
)
)
)
#x400
1
-8
#x3FFFF8
Chapter 4: Numbers
73
4.7.2 Flonum Operations
A flonum is an inexact real number that is implemented as a floating-point number. In
MIT Scheme, all inexact real numbers are flonums. For this reason, constants such as 0.
and 2.3 are guaranteed to be flonums.
flo:flonum? object
Returns #t if object is a flonum; otherwise returns #f.
procedure
flo:= flonum1 flonum2
flo:< flonum1 flonum2
flo:> flonum1 flonum2
procedure
procedure
procedure
These procedures are the standard order and equality predicates on flonums. When
compiled, they do not check the types of their arguments.
flo:zero? flonum
flo:positive? flonum
flo:negative? flonum
procedure
procedure
procedure
Each of these procedures compares its argument to zero. When compiled, they do
not check the type of their argument.
flo:+ flonum1 flonum2
flo:- flonum1 flonum2
flo:* flonum1 flonum2
flo:/ flonum1 flonum2
procedure
procedure
procedure
procedure
These procedures are the standard arithmetic operations on flonums. When compiled,
they do not check the types of their arguments.
flo:finite? flonum
procedure
The ieee floating-point number specification supports three special “numbers”: positive infinity (+inf), negative infinity (-inf), and not-a-number (NaN). This predicate
returns #f if flonum is one of these objects, and #t if it is any other floating-point
number.
flo:negate flonum
procedure
This procedure returns the negation of its argument. When compiled, it does not
check the type of its argument. Equivalent to (flo:- 0. flonum).
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MIT Scheme Reference
flo:abs flonum
flo:exp flonum
flo:log flonum
flo:sin flonum
flo:cos flonum
flo:tan flonum
flo:asin flonum
flo:acos flonum
flo:atan flonum
flo:sqrt flonum
flo:expt flonum1 flonum2
flo:floor flonum
flo:ceiling flonum
flo:truncate flonum
flo:round flonum
flo:floor->exact flonum
flo:ceiling->exact flonum
flo:truncate->exact flonum
flo:round->exact flonum
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
These procedures are flonum versions of the corresponding procedures. When compiled, they do not check the types of their arguments.
flo:atan2 flonum1 flonum2
procedure
This is the flonum version of atan with two arguments. When compiled, it does not
check the types of its arguments.
4.8 Random Numbers
MIT Scheme provides a facility for generating pseudo-random numbers. The current implementation is a “subtract-with-carry” random-number generator, based on the algorithm
from A New Class of Random Number Generators, George Marsaglia and Arif Zaman, The
Annals of Applied Probability, Vol. 1, No. 3, 1991. At the time it was implemented, this
was a good algorithm for general purposes, but the state of the art in random-number generation is constantly changing. If necessary, the implementation will be updated to use a
new algorithm while retaining the same interface.
The interface described here is very similar to that of Common Lisp.
random modulus [state]
procedure
Modulus must be a positive real number. random returns a pseudo-random number
between zero (inclusive) and modulus (exclusive). The exactness of the returned
number is the same as the exactness of modulus. Additionally, if modulus is an exact
integer, the returned number will be also. Usually, modulus is either an exact integer
or an inexact real; the current implementation has been tuned to make these two
cases fast.
If state is given and not #f, it must be a random-state object; otherwise, it defaults
to the value of the variable *random-state*. This object is used to maintain the
Chapter 4: Numbers
75
state of the pseudo-random-number generator and is altered as a side e↵ect of the
random procedure.
) .32744744667719056
(random 1.0)
) .01668326768172354
(random 1.0)
) 3
(random 10)
) 8
(random 10)
) 38
(random 100)
) 63
(random 100)
(random 100/3) ) 130501475769920525/6755399441055744
(random 100/3) ) 170571694016427575/13510798882111488
flo:random-unit state
procedure
State must be a random-state object. flo:random-unit returns a pseudo-random
number between zero inclusive and one exclusive; the returned number is always
a flonum and therefore an inexact real number. flo:random-unit is equivalent to
random with a modulus of 1.0, except that it is faster.
The next three definitions concern random-state objects. In addition to these definitions,
it is important to know that random-state objects are specifically designed so that they
can be saved to disk using the fasdump procedure, and later restored using the fasload
procedure. This allows a particular random-state object to be saved in order to replay a
particular pseudo-random sequence.
*random-state*
variable
This variable holds a data structure, a random-state object, that encodes the internal
state of the random-number generator that random uses by default. A call to random
will perform a side e↵ect on this data structure. This variable may be changed, using
set! or fluid-let, to hold a new random-state object.
make-random-state [state]
procedure
This procedure returns a new random-state object, suitable for use as the value of the
variable *random-state*, or as the state argument to random. If state is not given
or #f, make-random-state returns a copy of the current random-number state object
(the value of the variable *random-state*). If state is a random-state object, a copy
of that object is returned. If state is #t, then a new random-state object is returned
that has been “randomly” initialized by some means (such as by a time-of-day clock).
random-state? object
Returns #t if object is a random-state object, otherwise returns #f.
procedure
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MIT Scheme Reference
Chapter 5: Characters
77
5 Characters
Characters are objects that represent printed characters, such as letters and digits.1
5.1 External Representation of Characters
Characters are written using the notation #\character or #\character-name. For example:
;
;
;
;
;
#\a
#\A
#\(
#\space
#\newline
lowercase letter
uppercase letter
left parenthesis
the space character
the newline character
Case is significant in #\character, but not in #\character-name. If character in #\character
is a letter, character must be followed by a delimiter character such as a space or parenthesis.
Characters written in the #\ notation are self-evaluating; you don’t need to quote them.
A character name may include one or more bucky bit prefixes to indicate that the
character includes one or more of the keyboard shift keys Control, Meta, Super, Hyper, or
Top (note that the Control bucky bit prefix is not the same as the ascii control key). The
bucky bit prefixes and their meanings are as follows (case is not significant):
Key
---
Bucky bit prefix
----------------
Meta
Control
Super
Hyper
Top
MCSHT-
or
or
or
or
or
MetaControlSuperHyperTop-
Bucky bit
--------1
2
4
8
16
For example,
#\c-a
#\meta-b
#\c-s-m-h-a
; Control-a
; Meta-b
; Control-Meta-Super-Hyper-A
The following character-names are supported, shown here with their ascii equivalents:
1
Some of the details in this section depend on the fact that the underlying operating system uses the
ascii character set. This may change when someone ports MIT Scheme to a non-ascii operating system.
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MIT Scheme Reference
Character Name
--------------
ASCII Name
----------
altmode
backnext
backspace
call
linefeed
page
return
rubout
space
tab
ESC
US
BS
SUB
LF
FF
CR
DEL
HT
In addition, #\newline is the same as #\linefeed (but this may change in the future, so
you should not depend on it). All of the standard ascii names for non-printing characters
are supported:
NUL
BS
DLE
CAN
DEL
SOH
HT
DC1
EM
STX
LF
DC2
SUB
ETX
VT
DC3
ESC
EOT
FF
DC4
FS
ENQ
CR
NAK
GS
ACK
SO
SYN
RS
BEL
SI
ETB
US
char->name char [slashify?]
procedure
Returns a string corresponding to the printed representation of char. This is the
character or character-name component of the external representation, combined with
the appropriate bucky bit prefixes.
(char->name
(char->name
(char->name
(char->name
#\a)
#\space)
#\c-a)
#\control-a)
)
)
)
)
"a"
"Space"
"C-a"
"C-a"
Slashify?, if specified and true, says to insert the necessary backslash characters in
the result so that read will parse it correctly. In other words, the following generates
the external representation of char:
(string-append "#\\" (char->name char #t))
If slashify? is not specified, it defaults to #f.
name->char string
procedure
Converts a string that names a character into the character specified. If string does
not name any character, name->char signals an error.
(name->char
(name->char
(name->char
(name->char
"a")
"space")
"c-a")
"control-a")
)
)
)
)
#\a
#\Space
#\C-a
#\C-a
Chapter 5: Characters
79
5.2 Comparison of Characters
char=? char1 char2
char<? char1 char2
char>? char1 char2
char<=? char1 char2
char>=? char1 char2
char-ci=? char1 char2
char-ci<? char1 char2
char-ci>? char1 char2
char-ci<=? char1 char2
char-ci>=? char1 char2
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
Returns #t if the specified characters are have the appropriate order relationship to
one another; otherwise returns #f. The -ci procedures don’t distinguish uppercase
and lowercase letters.
Character ordering follows these rules:
• The digits are in order; for example, (char<? #\0 #\9) returns #t.
• The uppercase characters are in order; for example, (char<? #\A #\B) returns
#t.
• The lowercase characters are in order; for example, (char<? #\a #\b) returns
#t.
In addition, MIT Scheme orders those characters that satisfy char-standard? the
same way that iso-8859-1 does.
Characters are ordered by first comparing their bucky bits part and then their code
part. In particular, characters without bucky bits come before characters with bucky
bits.
5.3 Miscellaneous Character Operations
char? object
Returns #t if object is a character; otherwise returns #f.
procedure
char-upcase char
char-downcase char
procedure
procedure
Returns the uppercase or lowercase equivalent of char if char is a letter; otherwise
returns char. These procedures return a character char2 such that (char-ci=? char
char2).
char->digit char [radix]
procedure
If char is a character representing a digit in the given radix, returns the corresponding
integer value. If you specify radix (which must be an exact integer between 2 and 36
inclusive), the conversion is done in that base, otherwise it is done in base 10. If char
doesn’t represent a digit in base radix, char->digit returns #f.
Note that this procedure is insensitive to the alphabetic case of char.
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MIT Scheme Reference
(char->digit #\8)
(char->digit #\e 16)
(char->digit #\e)
)
)
)
8
14
#f
digit->char digit [radix]
procedure
Returns a character that represents digit in the radix given by radix. Radix must be
an exact integer between 2 and 36 (inclusive), and defaults to 10. Digit, which must
be an exact non-negative integer, should be less than radix; if digit is greater than or
equal to radix, digit->char returns #f.
) #\8
(digit->char 8)
) #\E
(digit->char 14 16)
5.4 Internal Representation of Characters
An MIT Scheme character consists of a code part and a bucky bits part. The MIT
Scheme set of characters can represent more characters than ascii can; it includes characters
with Super, Hyper, and Top bucky bits, as well as Control and Meta. Every ascii character
corresponds to some MIT Scheme character, but not vice versa.2
MIT Scheme uses a 16-bit character code with 5 bucky bits. Normally, Scheme uses the
least significant 8 bits of the character code to contain the iso-8859-1 representation for
the character. The representation is expanded in order to allow for the use of utf-16 in
the future.
make-char code bucky-bits
procedure
Builds a character from code and bucky-bits. Both code and bucky-bits must be
exact non-negative integers in the appropriate range. Use char-code and char-bits
to extract the code and bucky bits from the character. If 0 is specified for bucky-bits,
make-char produces an ordinary character; otherwise, the appropriate bits are turned
on as follows:
1
Meta
2
Control
4
Super
8
Hyper
16
Top
For example,
) #\a
(make-char 97 0)
) #\M-a
(make-char 97 1)
) #\C-a
(make-char 97 2)
) #\C-M-a
(make-char 97 3)
char-bits char
Returns the exact integer representation of char’s bucky bits. For example,
2
procedure
Note that the Control bucky bit is di↵erent from the ascii control key. This means that #\SOH (ascii
ctrl-A) is di↵erent from #\C-A. In fact, the Control bucky bit is completely orthogonal to the ascii
control key, making possible such characters as #\C-SOH.
Chapter 5: Characters
(char-bits
(char-bits
(char-bits
(char-bits
81
)
)
)
)
#\a)
#\m-a)
#\c-a)
#\c-m-a)
0
1
2
3
char-code char
Returns the character code of char, an exact integer. For example,
)
)
(char-code #\a)
(char-code #\c-a)
procedure
97
97
char-code-limit
char-bits-limit
variable
variable
These variables define the (exclusive) upper limits for the character code and bucky
bits (respectively). The character code and bucky bits are always exact non-negative
integers, and are strictly less than the value of their respective limit variable.
char->integer char
integer->char k
procedure
procedure
char->integer returns the character code representation for char. integer->char
returns the character whose character code representation is k.
In MIT Scheme, if (char-ascii? char) is true, then
(eqv? (char->ascii char) (char->integer char))
However, this behavior is not required by the Scheme standard, and code that depends
on it is not portable to other implementations.
These procedures implement order isomorphisms between the set of characters under
the char<=? ordering and some subset of the integers under the <= ordering. That
is, if
(char<=? a b)
)
#t
and
(<= x y)
and x and y are in the range of char->integer, then
(<= (char->integer a)
(char->integer b))
(char<=? (integer->char x)
(integer->char y))
)
)
#t
)
#t
#t
Note: If the argument to char->integer or integer->char is a constant, the compiler will constant-fold the call, replacing it with the corresponding result. This is a
very useful way to denote unusual character constants or ascii codes.
char-integer-limit
variable
The range of char->integer is defined to be the exact non-negative integers that are
less than the value of this variable (exclusive).
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MIT Scheme Reference
5.5 ISO-8859-1 Characters
MIT Scheme internally uses iso-8859-1 codes for i/o, and stores character objects in
a fashion that makes it convenient to convert between iso-8859-1 codes and characters.
Also, character strings are implemented as byte vectors whose elements are iso-8859-1
codes; these codes are converted to character objects when accessed. For these reasons it is
sometimes desirable to be able to convert between iso-8859-1 codes and characters.
Not all characters can be represented as iso-8859-1 codes. A character that has an
equivalent iso-8859-1 representation is called an ISO-8859-1 character.
For historical reasons, the procedures that manipulate iso-8859-1 characters use the
word “ascii” rather than “iso-8859-1”.
char-ascii? char
procedure
Returns the iso-8859-1 code for char if char has an iso-8859-1 representation; otherwise returns #f.
In the current implementation, the characters that satisfy this predicate are those in
which the bucky bits are turned o↵, and for which the character code is less than 256.
char->ascii char
procedure
Returns the iso-8859-1 code for char. An error condition-type:bad-rangeargument is signalled if char doesn’t have an iso-8859-1 representation.
ascii->char code
procedure
Code must be the exact integer representation of an iso-8859-1 code. This procedure
returns the character corresponding to code.
5.6 Character Sets
MIT Scheme’s character-set abstraction is used to represent groups of characters, such as
the letters or digits. Character sets may contain only iso-8859-1 characters; in the future
this may be changed to allow the full range of characters.
There is no meaningful external representation for character sets; use char-set-members
to examine their contents. There is (at present) no specific equivalence predicate for character sets; use equal? for this purpose.
char-set? object
Returns #t if object is a character set; otherwise returns #f.
procedure
Chapter 5: Characters
83
char-set:upper-case
char-set:lower-case
char-set:alphabetic
char-set:numeric
char-set:alphanumeric
char-set:whitespace
char-set:not-whitespace
char-set:graphic
char-set:not-graphic
char-set:standard
variable
variable
variable
variable
variable
variable
variable
variable
variable
variable
These variables contain predefined character sets. To see the contents of one of these
sets, use char-set-members.
Alphabetic characters are the 52 upper and lower case letters. Numeric characters
are the 10 decimal digits. Alphanumeric characters are those in the union of these two
sets. Whitespace characters are #\space, #\tab, #\page, #\linefeed, and #\return.
Graphic characters are the printing characters and #\space. Standard characters are
the printing characters, #\space, and #\newline. These are the printing characters:
! " # $ % & ’ ( ) * + , - . /
0 1 2 3 4 5 6 7 8 9
: ; < = > ? @
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
[ \ ] ^ _ ‘
a b c d e f g h i j k l m n o p q r s t u v w x y z
{ | } ~
char-upper-case? char
char-lower-case? char
char-alphabetic? char
char-numeric? char
char-alphanumeric? char
char-whitespace? char
char-graphic? char
char-standard? object
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
These predicates are defined in terms of the respective character sets defined above.
char-set-members char-set
procedure
char-set-member? char-set char
procedure
Returns a newly allocated list of the characters in char-set.
Returns #t if char is in char-set; otherwise returns #f.
char-set char . . .
procedure
Returns a character set consisting of the specified iso-8859-1 characters. With no
arguments, char-set returns an empty character set.
chars->char-set chars
procedure
Returns a character set consisting of chars, which must be a list of iso-8859-1 characters. This is equivalent to (apply char-set chars).
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MIT Scheme Reference
string->char-set string
Returns a character set consisting of all the characters that occur in string.
procedure
ascii-range->char-set lower upper
procedure
Lower and upper must be exact non-negative integers representing iso-8859-1 character codes, and lower must be less than or equal to upper. This procedure creates
and returns a new character set consisting of the characters whose iso-8859-1 codes
are between lower (inclusive) and upper (exclusive).
For historical reasons, the name of this procedure refers to “ascii” rather than “iso8859-1”.
predicate->char-set predicate
procedure
Predicate must be a procedure of one argument. predicate->char-set creates and
returns a character set consisting of the iso-8859-1 characters for which predicate is
true.
char-set-di↵erence char-set1 char-set2
procedure
Returns a character set consisting of the characters that are in char-set1 but aren’t
in char-set2.
char-set-intersection char-set . . .
procedure
Returns a character set consisting of the characters that are in all of the char-sets.
char-set-union char-set . . .
procedure
Returns a character set consisting of the characters that are in at least one o the
char-sets.
char-set-invert char-set
procedure
Returns a character set consisting of the iso-8859-1 characters that are not in charset.
5.7 Unicode
MIT Scheme provides rudimentary support for Unicode characters. In an ideal world,
Unicode would be the base character set for MIT Scheme, but this implementation predates
the invention of Unicode. And converting an application of this size is a considerable
undertaking. So for the time being, the base character set is iso-8859-1 and Unicode
support is grafted on.
This Unicode support was implemented as a part of the xml parser (see Section 14.12
[XML Parser], page 217) implementation. xml uses Unicode as its base character set, and
any xml implementation must support Unicode.
The Unicode implementation consists of two parts: i/o procedures that read and write
utf-8 characters, and an alphabet abstraction, which is an efficient implementation of sets
of Unicode code points (similar to the char-set abstraction).
The basic unit in a Unicode implementation is the code point.
Chapter 5: Characters
85
unicode-code-point? object
procedure
Returns #t if object is a Unicode code point. Code points are implemented as exact
non-negative integers. Code points are further limited, by the Unicode standard, to
be strictly less than #x80000000.
The next few procedures do i/o on code points.
read-utf8-code-point port
procedure
Reads and returns a utf-8-encoded code point from port. Returns an end-of-file
object if there are no more characters available from port. Signals an error if the
input stream isn’t a valid utf-8 encoding.
write-utf8-code-point code-point port
procedure
utf8-string->code-point string
procedure
Writes code-point to port in the utf-8 encoding.
Reads and returns a utf-8-encoded code point from string. Equivalent to
(read-utf8-code-point (string->input-port string))
code-point->utf8-string code-point
procedure
Returns a newly-allocated string containing the utf-8 encoding of code-point. Equivalent to
(with-string-output-port
(lambda (port)
(write-utf8-code-point code-point port)))
Applications often need to manipulate sets of characters, such as the set of alphabetic
characters or the set of whitespace characters. The alphabet abstraction provides an efficient
implementation of sets of Unicode code points.
alphabet? object
Returns #t if object is a Unicode alphabet, otherwise returns #f.
procedure
code-points->alphabet items
procedure
Returns a Unicode alphabet containing the code points described by items. Items
must satisfy well-formed-code-points-list?.
alphabet->code-points alphabet
procedure
Returns a well-formed code-points list that describes the code points represented by
alphabet.
well-formed-code-points-list? object
procedure
Returns #t if object is a well-formed code-points list, otherwise returns #f. A wellformed code-points list is a proper list, each element of which is either a code point
or a pair of code points. A pair of code points represents a contiguous range of code
points. The car of the pair is the lower limit, and the cdr is the upper limit. Both
limits are inclusive, and the lower limit must be strictly less than the upper limit.
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MIT Scheme Reference
code-point-in-alphabet? code-point alphabet
Returns #t if code-point is a member of alphabet, otherwise returns #f.
procedure
char-in-alphabet? char alphabet
procedure
Returns #t if char is a member of alphabet, otherwise returns #f. Equivalent to
(code-point-in-alphabet? (char-code char) alphabet)
Character sets and alphabets can be converted to one another, provided that the alphabet
contains only 8-bit code points. This is true because 8-bit code points in Unicode map
directly to iso-8859-1 characters, which is what character sets contain.
char-set->alphabet char-set
procedure
Returns a Unicode alphabet containing the code points that correspond to characters
that are members of char-set.
alphabet->char-set alphabet
procedure
Returns a character set containing the characters that correspond to 8-bit code points
that are members of alphabet. (Code points outside the 8-bit range are ignored.)
string->alphabet string
procedure
Returns a Unicode alphabet containing the code points corresponding to the characters in string. Equivalent to
(char-set->alphabet (string->char-set string))
alphabet->string alphabet
procedure
Returns a newly-allocated string containing the characters corresponding to the 8-bit
code points in alphabet. (Code points outside the 8-bit range are ignored.)
8-bit-alphabet? alphabet
Returns #t if alphabet contains only 8-bit code points, otherwise returns #f.
procedure
alphabet+ alphabet . . .
procedure
Returns a Unicode alphabet that contains each code point that is a member of any
of the alphabet arguments.
alphabet- alphabet1 alphabet2
procedure
Returns a Unicode alphabet that contains each code point that is a member of alphabet1 and is not a member of alphabet2.
Chapter 6: Strings
87
6 Strings
A string is a mutable sequence of characters. In the current implementation of MIT
Scheme, the elements of a string must all satisfy the predicate char-ascii?; if someone
ports MIT Scheme to a non-ascii operating system this requirement will change.
A string is written as a sequence of characters enclosed within double quotes " ". To
include a double quote inside a string, precede the double quote with a backslash \ (escape
it), as in
"The word \"recursion\" has many meanings."
The printed representation of this string is
The word "recursion" has many meanings.
To include a backslash inside a string, precede it with another backslash; for example,
"Use #\\Control-q to quit."
The printed representation of this string is
Use #\Control-q to quit.
The e↵ect of a backslash that doesn’t precede a double quote or backslash is unspecified
in standard Scheme, but MIT Scheme specifies the e↵ect for three other characters: \t, \n,
and \f. These escape sequences are respectively translated into the following characters:
#\tab, #\newline, and #\page. Finally, a backslash followed by exactly three octal digits
is translated into the character whose iso-8859-1 code is those digits.
If a string literal is continued from one line to another, the string will contain the newline
character (#\newline) at the line break. Standard Scheme does not specify what appears
in a string literal at a line break.
The length of a string is the number of characters that it contains. This number is an
exact non-negative integer that is established when the string is created (but see Section 6.10
[Variable-Length Strings], page 102). Each character in a string has an index, which is a
number that indicates the character’s position in the string. The index of the first (leftmost)
character in a string is 0, and the index of the last character is one less than the length of
the string. The valid indexes of a string are the exact non-negative integers less than the
length of the string.
A number of the string procedures operate on substrings. A substring is a segment of a
string, which is specified by two integers start and end satisfying these relationships:
0 <= start <= end <= (string-length string)
Start is the index of the first character in the substring, and end is one greater than the
index of the last character in the substring. Thus if start and end are equal, they refer to
an empty substring, and if start is zero and end is the length of string, they refer to all of
string.
Some of the procedures that operate on strings ignore the di↵erence between uppercase
and lowercase. The versions that ignore case include ‘-ci’ (for “case insensitive”) in their
names.
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MIT Scheme Reference
6.1 Construction of Strings
make-string k [char]
procedure
Returns a newly allocated string of length k. If you specify char, all elements of
the string are initialized to char, otherwise the contents of the string are unspecified.
Char must satisfy the predicate char-ascii?.
) "xxxxxxxxxx"
(make-string 10 #\x)
string char . . .
procedure
Returns a newly allocated string consisting of the specified characters. The arguments
must all satisfy char-ascii?.
) "a"
(string #\a)
) "abc"
(string #\a #\b #\c)
) "a b c"
(string #\a #\space #\b #\space #\c)
) ""
(string)
list->string char-list
procedure
Char-list must be a list of iso-8859-1 characters. list->string returns a newly
allocated string formed from the elements of char-list. This is equivalent to (apply
string char-list). The inverse of this operation is string->list.
) "ab"
(list->string ’(#\a #\b))
) (#\H #\e #\l #\l #\o)
(string->list "Hello")
string-copy string
Returns a newly allocated copy of string.
procedure
Note regarding variable-length strings: the maximum length of the result depends
only on the length of string, not its maximum length. If you wish to copy a string
and preserve its maximum length, do the following:
(define (string-copy-preserving-max-length string)
(let ((length))
(dynamic-wind
(lambda ()
(set! length (string-length string))
(set-string-length! string
(string-maximum-length string)))
(lambda ()
(string-copy string))
(lambda ()
(set-string-length! string length)))))
6.2 Selecting String Components
string? object
Returns #t if object is a string; otherwise returns #f.
) #t
(string? "Hi")
) #f
(string? ’Hi)
procedure
Chapter 6: Strings
89
string-length string
Returns the length of string as an exact non-negative integer.
(string-length "")
(string-length "The length")
)
)
0
10
string-null? string
procedure
Returns #t if string has zero length; otherwise returns #f.
(string-null? "")
(string-null? "Hi")
)
)
#t
#f
string-ref string k
Returns character k of string. K must be a valid index of string.
(string-ref "Hello" 1)
(string-ref "Hello" 5)
procedure
procedure
) #\e
error 5 not in correct range
string-set! string k char
procedure
Stores char in element k of string and returns an unspecified value. K must be a
valid index of string, and char must satisfy the predicate char-ascii?.
(define str "Dog")
(string-set! str 0 #\L)
str
(string-set! str 3 #\t)
) unspecified
) unspecified
) "Log"
error 3 not in correct range
6.3 Comparison of Strings
string=? string1 string2
substring=? string1 start end string2 start end
string-ci=? string1 string2
substring-ci=? string1 start end string2 start end
procedure
procedure
procedure
procedure
Returns #t if the two strings (substrings) are the same length and contain the same
characters in the same (relative) positions; otherwise returns #f. string-ci=? and
substring-ci=? don’t distinguish uppercase and lowercase letters, but string=? and
substring=? do.
(string=? "PIE" "PIE")
(string=? "PIE" "pie")
(string-ci=? "PIE" "pie")
(substring=? "Alamo" 1 3 "cola" 2 4)
)
)
)
)
#t
#f
#t
#t ; compares "la"
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MIT Scheme Reference
string<? string1 string2
substring<? string1 start1 end1 string2 start2 end2
string>? string1 string2
string<=? string1 string2
string>=? string1 string2
string-ci<? string1 string2
substring-ci<? string1 start1 end1 string2 start2 end2
string-ci>? string1 string2
string-ci<=? string1 string2
string-ci>=? string1 string2
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
These procedures compare strings (substrings) according to the order of the characters they contain (also see Section 5.2 [Comparison of Characters], page 79). The
arguments are compared using a lexicographic (or dictionary) order. If two strings
di↵er in length but are the same up to the length of the shorter string, the shorter
string is considered to be less than the longer string.
(string<? "cat" "dog")
(string<? "cat" "DOG")
(string-ci<? "cat" "DOG")
(string>? "catkin" "cat")
)
)
)
)
#t
#f
#t
#t ; shorter is lesser
string-compare string1 string2 if-eq if-lt if-gt
string-compare-ci string1 string2 if-eq if-lt if-gt
procedure
procedure
If-eq, if-lt, and if-gt are procedures of no arguments (thunks). The two strings are
compared; if they are equal, if-eq is applied, if string1 is less than string2, if-lt is
applied, else if string1 is greater than string2, if-gt is applied. The value of the
procedure is the value of the thunk that is applied.
string-compare distinguishes uppercase and lowercase letters;
string-compare-ci does not.
(define (cheer) (display "Hooray!"))
(define (boo)
(display "Boo-hiss!"))
(string-compare "a" "b" cheer (lambda() ’ignore)
a Hooray!
) unspecified
boo)
string-hash string
string-hash-mod string k
procedure
procedure
string-hash returns an exact non-negative integer that can be used for storing the
specified string in a hash table. Equal strings (in the sense of string=?) return equal
(=) hash codes, and non-equal but similar strings are usually mapped to distinct hash
codes.
string-hash-mod is like string-hash, except that it limits the result to a particular
range based on the exact non-negative integer k. The following are equivalent:
(string-hash-mod string k)
(modulo (string-hash string) k)
Chapter 6: Strings
91
6.4 Alphabetic Case in Strings
string-capitalized? string
substring-capitalized? string start end
procedure
procedure
These procedures return #t if the first word in the string (substring) is capitalized,
and any subsequent words are either lower case or capitalized. Otherwise, they return
#f. A word is defined as a non-null contiguous sequence of alphabetic characters,
delimited by non-alphabetic characters or the limits of the string (substring). A word
is capitalized if its first letter is upper case and all its remaining letters are lower case.
(map string-capitalized? ’(""
"A"
"art" "Art" "ART"))
) (#f
#t
#f
#t
#f)
string-upper-case? string
substring-upper-case? string start end
string-lower-case? string
substring-lower-case? string start end
procedure
procedure
procedure
procedure
These procedures return #t if all the letters in the string (substring) are of the correct
case, otherwise they return #f. The string (substring) must contain at least one letter
or the procedures return #f.
(map string-upper-case? ’(""
"A"
"art" "Art" "ART"))
) (#f
#t
#f
#f
#t)
string-capitalize string
string-capitalize! string
substring-capitalize! string start end
procedure
procedure
procedure
string-capitalize returns a newly allocated copy of string in which the first alphabetic character is uppercase and the remaining alphabetic characters are lowercase.
For example, "abcDEF" becomes "Abcdef". string-capitalize! is the destructive
version of string-capitalize: it alters string and returns an unspecified value.
substring-capitalize! destructively capitalizes the specified part of string.
string-downcase string
string-downcase! string
substring-downcase! string start end
procedure
procedure
procedure
string-downcase returns a newly allocated copy of string in which all uppercase
letters are changed to lowercase. string-downcase! is the destructive version of
string-downcase: it alters string and returns an unspecified value. substringdowncase! destructively changes the case of the specified part of string.
) unspecified
(define str "ABCDEFG")
) unspecified
(substring-downcase! str 3 5)
) "ABCdeFG"
str
string-upcase string
string-upcase! string
substring-upcase! string start end
procedure
procedure
procedure
string-upcase returns a newly allocated copy of string in which all lowercase letters
are changed to uppercase. string-upcase! is the destructive version of string-
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MIT Scheme Reference
upcase: it alters string and returns an unspecified value. substring-upcase! destructively changes the case of the specified part of string.
6.5 Cutting and Pasting Strings
string-append string . . .
procedure
Returns a newly allocated string made from the concatenation of the given strings.
With no arguments, string-append returns the empty string ("").
) ""
(string-append)
) "*ace*"
(string-append "*" "ace" "*")
) ""
(string-append "" "" "")
) #f ; newly allocated
(eq? str (string-append str))
substring string start end
procedure
Returns a newly allocated string formed from the characters of string beginning with
index start (inclusive) and ending with end (exclusive).
) ""
(substring "" 0 0)
) "duo"
(substring "arduous" 2 5)
(substring "arduous" 2 8)
error 8 not in correct range
(define (string-copy s)
(substring s 0 (string-length s)))
string-head string end
procedure
Returns a newly allocated copy of the initial substring of string, up to but excluding
end. It could have been defined by:
(define (string-head string end)
(substring string 0 end))
string-tail string start
procedure
Returns a newly allocated copy of the final substring of string, starting at index start
and going to the end of string. It could have been defined by:
(define (string-tail string start)
(substring string start (string-length string)))
(string-tail "uncommon" 2)
string-pad-left string k [char]
string-pad-right string k [char]
)
"common"
procedure
procedure
These procedures return a newly allocated string created by padding string out to
length k, using char. If char is not given, it defaults to #\space. If k is less than the
length of string, the resulting string is a truncated form of string. string-pad-left
adds padding characters or truncates from the beginning of the string (lowest indices),
while string-pad-right does so at the end of the string (highest indices).
Chapter 6: Strings
(string-pad-left "hello" 4)
(string-pad-left "hello" 8)
(string-pad-left "hello" 8 #\*)
(string-pad-right "hello" 4)
(string-pad-right "hello" 8)
93
)
)
)
)
)
"ello"
"
hello"
"***hello"
"hell"
"hello
"
string-trim string [char-set]
string-trim-left string [char-set]
string-trim-right string [char-set]
procedure
procedure
procedure
Returns a newly allocated string created by removing all characters that are not
in char-set from: (string-trim) both ends of string; (string-trim-left) the beginning of string; or (string-trim-right) the end of string. Char-set defaults to
char-set:not-whitespace.
)
(string-trim " in the end ")
)
(string-trim "
")
(string-trim "100th" char-set:numeric) )
(string-trim-left "-.-+-=-" (char-set #\+))
)
(string-trim "but (+ x y) is" (char-set #\(
)
"in the end"
""
"100"
"+-=-"
#\)))
"(+ x y)"
6.6 Searching Strings
The first few procedures in this section perform string search, in which a given string
(the text) is searched to see if it contains another given string (the pattern) as a proper
substring. At present these procedures are implemented using a hybrid strategy. For short
patterns of less than 4 characters, the naive string-search algorithm is used. For longer
patterns, the Boyer-Moore string-search algorithm is used.
string-search-forward pattern string
substring-search-forward pattern string start end
procedure
procedure
Pattern must be a string. Searches string for the leftmost occurrence of the substring
pattern. If successful, the index of the first character of the matched substring is
returned; otherwise, #f is returned.
substring-search-forward limits its search to the specified substring of string;
string-search-forward searches all of string.
(string-search-forward "rat" "pirate")
) 2
(string-search-forward "rat" "pirate rating")
) 2
(substring-search-forward "rat" "pirate rating" 4 13)
) 7
(substring-search-forward "rat" "pirate rating" 9 13)
) #f
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MIT Scheme Reference
string-search-backward pattern string
substring-search-backward pattern string start end
procedure
procedure
Pattern must be a string. Searches string for the rightmost occurrence of the substring
pattern. If successful, the index to the right of the last character of the matched
substring is returned; otherwise, #f is returned.
substring-search-backward limits its search to the specified substring of string;
string-search-backward searches all of string.
(string-search-backward "rat" "pirate")
) 5
(string-search-backward "rat" "pirate rating")
) 10
(substring-search-backward "rat" "pirate rating" 1 8)
) 5
(substring-search-backward "rat" "pirate rating" 9 13)
) #f
string-search-all pattern string
substring-search-all pattern string start end
procedure
procedure
Pattern must be a string. Searches string to find all occurrences of the substring
pattern. Returns a list of the occurrences; each element of the list is an index pointing
to the first character of an occurrence.
substring-search-all limits its search to the specified substring of string; stringsearch-all searches all of string.
(string-search-all "rat" "pirate")
) (2)
(string-search-all "rat" "pirate rating")
) (2 7)
(substring-search-all "rat" "pirate rating" 4 13)
) (7)
(substring-search-all "rat" "pirate rating" 9 13)
) ()
substring? pattern string
procedure
Pattern must be a string. Searches string to see if it contains the substring pattern.
Returns #t if pattern is a substring of string, otherwise returns #f.
) #t
(substring? "rat" "pirate")
) #f
(substring? "rat" "outrage")
) #t
(substring? "" any-string)
(if (substring? "moon" text)
(process-lunar text)
’no-moon)
string-find-next-char string char
substring-find-next-char string start end char
string-find-next-char-ci string char
substring-find-next-char-ci string start end char
procedure
procedure
procedure
procedure
Returns the index of the first occurrence of char in the string (substring); returns
#f if char does not appear in the string. For the substring procedures, the index
Chapter 6: Strings
95
returned is relative to the entire string, not just the substring. The -ci procedures
don’t distinguish uppercase and lowercase letters.
) 0
(string-find-next-char "Adam" #\A)
) #f
(substring-find-next-char "Adam" 1 4 #\A)
) 2
(substring-find-next-char-ci "Adam" 1 4 #\A)
string-find-next-char-in-set string char-set
substring-find-next-char-in-set string start end char-set
procedure
procedure
Returns the index of the first character in the string (or substring) that is also in
char-set, or returns #f if none of the characters in char-set occur in string. For the
substring procedure, only the substring is searched, but the index returned is relative
to the entire string, not just the substring.
(string-find-next-char-in-set my-string char-set:alphabetic)
) start position of the first word in my-string
; Can be used as a predicate:
(if (string-find-next-char-in-set my-string
(char-set #\( #\) ))
’contains-parentheses
’no-parentheses)
string-find-previous-char string char
substring-find-previous-char string start end char
string-find-previous-char-ci string char
substring-find-previous-char-ci string start end char
procedure
procedure
procedure
procedure
Returns the index of the last occurrence of char in the string (substring); returns #f
if char doesn’t appear in the string. For the substring procedures, the index returned
is relative to the entire string, not just the substring. The -ci procedures don’t
distinguish uppercase and lowercase letters.
string-find-previous-char-in-set string char-set
substring-find-previous-char-in-set string start end char-set
procedure
procedure
Returns the index of the last character in the string (substring) that is also in charset. For the substring procedure, the index returned is relative to the entire string,
not just the substring.
6.7 Matching Strings
string-match-forward string1 string2
substring-match-forward string1 start end string2 start end
string-match-forward-ci string1 string2
substring-match-forward-ci string1 start end string2 start end
procedure
procedure
procedure
procedure
Compares the two strings (substrings), starting from the beginning, and returns the
number of characters that are the same. If the two strings (substrings) start differently, returns 0. The -ci procedures don’t distinguish uppercase and lowercase
letters.
(string-match-forward "mirror" "micro") ) 2 ; matches "mi"
) 0 ; no match
(string-match-forward "a" "b")
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MIT Scheme Reference
string-match-backward string1 string2
substring-match-backward string1 start end string2 start end
string-match-backward-ci string1 string2
substring-match-backward-ci string1 start end string2 start end
procedure
procedure
procedure
procedure
Compares the two strings (substrings), starting from the end and matching toward the
front, returning the number of characters that are the same. If the two strings (substrings) end di↵erently, returns 0. The -ci procedures don’t distinguish uppercase
and lowercase letters.
(string-match-backward-ci "BULBOUS" "fractious")
) 3 ; matches "ous"
string-prefix? string1 string2
substring-prefix? string1 start1 end1 string2 start2 end2
string-prefix-ci? string1 string2
substring-prefix-ci? string1 start1 end1 string2 start2 end2
procedure
procedure
procedure
procedure
These procedures return #t if the first string (substring) forms the prefix of the second;
otherwise returns #f. The -ci procedures don’t distinguish uppercase and lowercase
letters.
(string-prefix? "abc" "abcdef")
(string-prefix? "" any-string)
)
)
#t
#t
string-suffix? string1 string2
substring-suffix? string1 start1 end1 string2 start2 end2
string-suffix-ci? string1 string2
substring-suffix-ci? string1 start1 end1 string2 start2 end2
procedure
procedure
procedure
procedure
These procedures return #t if the first string (substring) forms the suffix of the second;
otherwise returns #f. The -ci procedures don’t distinguish uppercase and lowercase
letters.
(string-suffix? "ous" "bulbous")
(string-suffix? "" any-string)
)
)
#t
#t
6.8 Regular Expressions
MIT Scheme provides support for using regular expressions to search and match strings.
This manual does not define regular expressions; instead see section “Syntax of Regular
Expressions” in The Emacs Editor.
In addition to providing standard regular-expression support, MIT Scheme also provides
the rexp abstraction. This is an alternative way to write regular expressions that is easier
to read and understand than the standard notation. Regular expressions written in this
notation can be translated into the standard notation.
The regular-expression support is a run-time-loadable option. To use it, execute
(load-option ’regular-expression)
once before calling any of the procedures defined here.
Chapter 6: Strings
97
6.8.1 Regular-expression procedures
Procedures that perform regular-expression match and search accept standardized arguments. Regexp is the regular expression; it is a string. String is the string being matched or
searched. Procedures that operate on substrings also accept start and end index arguments
with the usual meaning. The optional argument case-fold? says whether the match/search
is case-sensitive; if case-fold? is #f, it is case-sensitive, otherwise it is case-insensitive. The
optional argument syntax-table is a character syntax table that defines the character syntax,
such as which characters are legal word constituents. This feature is primarily for Edwin,
so character syntax tables will not be documented here. Supplying #f for (or omitting)
syntax-table will select the default character syntax, equivalent to Edwin’s fundamental
mode.
re-string-match regexp string [case-fold? [syntax-table]]
re-substring-match regexp string start end [case-fold? [syntax-table]]
procedure
procedure
These procedures match regexp against the respective string or substring, returning
#f for no match, or a set of match registers (see below) if the match succeeds. Here
is an example showing how to extract the matched substring:
(let ((r (re-substring-match regexp string start end)))
(and r
(substring string start (re-match-end-index 0 r))))
re-string-search-forward regexp string [case-fold? [syntax-table]]
re-substring-search-forward regexp string start end [case-fold?
procedure
procedure
[syntax-table]]
Searches string for the leftmost substring matching regexp. Returns a set of match
registers (see below) if the search is successful, or #f if it is unsuccessful.
re-substring-search-forward limits its search to the specified substring of string;
re-string-search-forward searches all of string.
re-string-search-backward regexp string [case-fold? [syntax-table]]
re-substring-search-backward regexp string start end [case-fold?
procedure
procedure
[syntax-table]]
Searches string for the rightmost substring matching regexp. Returns a set of match
registers (see below) if the search is successful, or #f if it is unsuccessful.
re-substring-search-backward limits its search to the specified substring of string;
re-string-search-backward searches all of string.
When a successful match or search occurs, the above procedures return a set of match
registers. The match registers are a set of index registers that record indexes into the
matched string. Each index register corresponds to an instance of the regular-expression
grouping operator ‘\(’, and records the start index (inclusive) and end index (exclusive) of
the matched group. These registers are numbered from 1 to 9, corresponding left-to-right to
the grouping operators in the expression. Additionally, register 0 corresponds to the entire
substring matching the regular expression.
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MIT Scheme Reference
re-match-start-index n registers
re-match-end-index n registers
procedure
procedure
N must be an exact integer between 0 and 9 inclusive. Registers must be a matchregisters object as returned by one of the regular-expression match or search procedures above. re-match-start-index returns the start index of the corresponding
regular-expression register, and re-match-end-index returns the corresponding end
index.
re-match-extract string registers n
procedure
Registers must be a match-registers object as returned by one of the regular-expression
match or search procedures above. String must be the string that was passed as an
argument to the procedure that returned registers. N must be an exact integer
between 0 and 9 inclusive. If the matched regular expression contained m grouping
operators, then the value of this procedure is undefined for n strictly greater than m.
This procedure extracts the substring corresponding to the match register specified
by registers and n. This is equivalent to the following expression:
(substring string
(re-match-start-index n registers)
(re-match-end-index n registers))
regexp-group alternative . . .
procedure
Each alternative must be a regular expression. The returned value is a new regular
expression that consists of the alternatives combined by a grouping operator. For
example:
(regexp-group "foo" "bar" "baz")
) "\\(foo\\|bar\\|baz\\)"
6.8.2 REXP abstraction
In addition to providing standard regular-expression support, MIT Scheme also provides
the rexp abstraction. This is an alternative way to write regular expressions that is easier
to read and understand than the standard notation. Regular expressions written in this
notation can be translated into the standard notation.
The rexp abstraction is a set of combinators that are composed into a complete regular
expression. Each combinator directly corresponds to a particular piece of regular-expression
notation. For example, the expression (rexp-any-char) corresponds to the . character in
standard regular-expression notation, while (rexp* rexp) corresponds to the * character.
The primary advantages of rexp are that it makes the nesting structure of regular
expressions explicit, and that it simplifies the description of complex regular expressions by
allowing them to be built up using straightforward combinators.
rexp? object
procedure
Returns #t if object is a rexp expression, or #f otherwise. A rexp is one of: a string,
which represents the pattern matching that string; a character set, which represents
the pattern matching a character in that set; or an object returned by calling one of
the procedures defined here.
Chapter 6: Strings
99
rexp->regexp rexp
procedure
Converts rexp to standard regular-expression notation, returning a newly-allocated
string.
rexp-compile rexp
procedure
Converts rexp to standard regular-expression notation, then compiles it and returns
the compiled result. Equivalent to
(re-compile-pattern (rexp->regexp rexp) #f)
rexp-any-char
procedure
Returns a rexp that matches any single character except a newline. This is equivalent
to the . construct.
rexp-line-start
procedure
Returns a rexp that matches the start of a line. This is equivalent to the ^ construct.
rexp-line-end
procedure
Returns a rexp that matches the end of a line. This is equivalent to the $ construct.
rexp-string-start
procedure
Returns a rexp that matches the start of the text being matched. This is equivalent
to the \‘ construct.
rexp-string-end
procedure
Returns a rexp that matches the end of the text being matched. This is equivalent
to the \’ construct.
rexp-word-edge
procedure
Returns a rexp that matches the start or end of a word. This is equivalent to the \b
construct.
rexp-not-word-edge
procedure
Returns a rexp that matches anywhere that is not the start or end of a word. This
is equivalent to the \B construct.
rexp-word-start
procedure
Returns a rexp that matches the start of a word. This is equivalent to the \<
construct.
rexp-word-end
procedure
Returns a rexp that matches the end of a word. This is equivalent to the \> construct.
rexp-word-char
procedure
Returns a rexp that matches any word-constituent character. This is equivalent to
the \w construct.
100
MIT Scheme Reference
rexp-not-word-char
procedure
Returns a rexp that matches any character that isn’t a word constituent. This is
equivalent to the \W construct.
The next two procedures accept a syntax-type argument specifying the syntax class
to be matched against. This argument is a symbol selected from the following list. Each
symbol is followed by the equivalent character used in standard regular-expression notation.
whitespace (space character), punctuation (.), word (w), symbol (_), open ((), close
()), quote (’), string-delimiter ("), math-delimiter ($), escape (\), char-quote (/),
comment-start (<), comment-end (>).
rexp-syntax-char syntax-type
procedure
Returns a rexp that matches any character of type syntax-type. This is equivalent
to the \s construct.
rexp-not-syntax-char syntax-type
procedure
Returns a rexp that matches any character not of type syntax-type. This is equivalent
to the \S construct.
rexp-sequence rexp . . .
procedure
Returns a rexp that matches each rexp argument in sequence. If no rexp argument
is supplied, the result matches the null string. This is equivalent to concatenating the
regular expressions corresponding to each rexp argument.
rexp-alternatives rexp . . .
procedure
Returns a rexp that matches any of the rexp arguments. This is equivalent to concatenating the regular expressions corresponding to each rexp argument, separating
them by the \| construct.
rexp-group rexp . . .
procedure
rexp-group is like rexp-sequence, except that the result is marked as a match group.
This is equivalent to the \( ... \) construct.
The next three procedures in principal accept a single rexp argument. For convenience,
they accept multiple arguments, which are converted into a single argument by rexp-group.
Note, however, that if only one rexp argument is supplied, and it’s very simple, no grouping
occurs.
rexp* rexp . . .
procedure
Returns a rexp that matches zero or more instances of the pattern matched by the
rexp arguments. This is equivalent to the * construct.
rexp+ rexp . . .
procedure
Returns a rexp that matches one or more instances of the pattern matched by the
rexp arguments. This is equivalent to the + construct.
Chapter 6: Strings
101
rexp-optional rexp . . .
procedure
Returns a rexp that matches zero or one instances of the pattern matched by the
rexp arguments. This is equivalent to the ? construct.
rexp-case-fold rexp
procedure
Returns a rexp that matches the same pattern as rexp, but is insensitive to character
case. This has no equivalent in standard regular-expression notation.
6.9 Modification of Strings
string-replace string char1 char2
substring-replace string start end char1 char2
string-replace! string char1 char2
substring-replace! string start end char1 char2
procedure
procedure
procedure
procedure
These procedures replace all occurrences of char1 with char2 in the original string
(substring). string-replace and substring-replace return a newly allocated
string containing the result. string-replace! and substring-replace! destructively modify string and return an unspecified value.
) unspecified
(define str "a few words")
) "a-few-words"
(string-replace str #\space #\-)
(substring-replace str 2 9 #\space #\-) ) "a few-words"
) "a few words"
str
) unspecified
(string-replace! str #\space #\-)
) "a-few-words"
str
string-fill! string char
Stores char in every element of string and returns an unspecified value.
procedure
substring-fill! string start end char
procedure
Stores char in elements start (inclusive) to end (exclusive) of string and returns an
unspecified value.
) unspecified
(define s (make-string 10 #\space))
) unspecified
(substring-fill! s 2 8 #\*)
) " ****** "
s
substring-move-left! string1 start1 end1 string2 start2
substring-move-right! string1 start1 end1 string2 start2
procedure
procedure
Copies the characters from start1 to end1 of string1 into string2 at the start2-th
position. The characters are copied as follows (note that this is only important when
string1 and string2 are eqv?):
substring-move-left!
The copy starts at the left end and moves toward the right (from smaller
indices to larger). Thus if string1 and string2 are the same, this procedure
moves the characters toward the left inside the string.
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MIT Scheme Reference
substring-move-right!
The copy starts at the right end and moves toward the left (from larger
indices to smaller). Thus if string1 and string2 are the same, this procedure moves the characters toward the right inside the string.
The following example shows how these procedures can be used to build up a string
(it would have been easier to use string-append):
) unspecified
(define answer (make-string 9 #\*))
) "*********"
answer
(substring-move-left! "start" 0 5 answer 0) ) unspecified
) "start****"
answer
(substring-move-left! "-end" 0 4 answer 5) ) unspecified
) "start-end"
answer
reverse-string string
reverse-substring string start end
reverse-string! string
reverse-substring! string start end
procedure
procedure
procedure
procedure
Reverses the order of the characters in the given string or substring. reversestring and reverse-substring return newly allocated strings; reverse-string!
and reverse-substring! modify their argument strings and return an unspecified
value.
) "zab rab oof"
(reverse-string "foo bar baz")
) "rab"
(reverse-substring "foo bar baz" 4 7)
(let ((foo "foo bar baz"))
(reverse-string! foo)
) "zab rab oof"
foo)
(let ((foo "foo bar baz"))
(reverse-substring! foo 4 7)
) "foo rab baz"
foo)
6.10 Variable-Length Strings
MIT Scheme allows the length of a string to be dynamically adjusted in a limited way.
When a new string is allocated, by whatever method, it has a specific length. At the time of
allocation, it is also given a maximum length, which is guaranteed to be at least as large as
the string’s length. (Sometimes the maximum length will be slightly larger than the length,
but it is a bad idea to count on this. Programs should assume that the maximum length is
the same as the length at the time of the string’s allocation.) After the string is allocated,
the operation set-string-length! can be used to alter the string’s length to any value
between 0 and the string’s maximum length, inclusive.
string-maximum-length string
Returns the maximum length of string. The following is guaranteed:
(<= (string-length string)
) #t
(string-maximum-length string))
The maximum length of a string never changes.
procedure
Chapter 6: Strings
103
set-string-length! string k
procedure
Alters the length of string to be k, and returns an unspecified value. K must be
less than or equal to the maximum length of string. set-string-length! does not
change the maximum length of string.
6.11 Byte Vectors
MIT Scheme implements strings as packed vectors of 8-bit iso-8859-1 bytes. Most of
the string operations, such as string-ref, coerce these 8-bit codes into character objects.
However, some lower-level operations are made available for use.
vector-8b-ref string k
procedure
Returns character k of string as an iso-8859-1 code. K must be a valid index of
string.
) 99 ;c
(vector-8b-ref "abcde" 2)
vector-8b-set! string k code
procedure
Stores code in element k of string and returns an unspecified value. K must be a
valid index of string, and code must be a valid iso-8859-1 code.
vector-8b-fill! string start end code
procedure
Stores code in elements start (inclusive) to end (exclusive) of string and returns an
unspecified value. Code must be a valid iso-8859-1 code.
vector-8b-find-next-char string start end code
vector-8b-find-next-char-ci string start end code
procedure
procedure
Returns the index of the first occurrence of code in the given substring; returns #f
if code does not appear. The index returned is relative to the entire string, not just
the substring. Code must be a valid iso-8859-1 code.
vector-8b-find-next-char-ci doesn’t distinguish uppercase and lowercase letters.
vector-8b-find-previous-char string start end code
vector-8b-find-previous-char-ci string start end code
procedure
procedure
Returns the index of the last occurrence of code in the given substring; returns #f if
code does not appear. The index returned is relative to the entire string, not just the
substring. Code must be a valid iso-8859-1 code.
vector-8b-find-previous-char-ci doesn’t distinguish uppercase and lowercase letters.
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MIT Scheme Reference
Chapter 7: Lists
105
7 Lists
A pair (sometimes called a dotted pair) is a data structure with two fields called the
car and cdr fields (for historical reasons). Pairs are created by the procedure cons. The
car and cdr fields are accessed by the procedures car and cdr. The car and cdr fields are
assigned by the procedures set-car! and set-cdr!.
Pairs are used primarily to represent lists. A list can be defined recursively as either
the empty list or a pair whose cdr is a list. More precisely, the set of lists is defined as the
smallest set X such that
• The empty list is in X.
• If list is in X, then any pair whose cdr field contains list is also in X.
The objects in the car fields of successive pairs of a list are the elements of the list. For
example, a two-element list is a pair whose car is the first element and whose cdr is a pair
whose car is the second element and whose cdr is the empty list. The length of a list is the
number of elements, which is the same as the number of pairs. The empty list is a special
object of its own type (it is not a pair); it has no elements and its length is zero.1
The most general notation (external representation) for Scheme pairs is the “dotted”
notation (c1 . c2) where c1 is the value of the car field and c2 is the value of the cdr field.
For example, (4 . 5) is a pair whose car is 4 and whose cdr is 5. Note that (4 . 5) is the
external representation of a pair, not an expression that evaluates to a pair.
A more streamlined notation can be used for lists: the elements of the list are simply
enclosed in parentheses and separated by spaces. The empty list is written (). For example,
the following are equivalent notations for a list of symbols:
(a b c d e)
(a . (b . (c . (d . (e . ())))))
Whether a given pair is a list depends upon what is stored in the cdr field. When the
set-cdr! procedure is used, an object can be a list one moment and not the next:
(define x (list ’a ’b ’c))
(define y x)
) (a b c)
y
) #t
(list? y)
) unspecified
(set-cdr! x 4)
) (a . 4)
x
) #t
(eqv? x y)
) (a . 4)
y
) #f
(list? y)
) unspecified
(set-cdr! x x)
) #f
(list? y)
A chain of pairs that doesn’t end in the empty list is called an improper list. Note that
an improper list is not a list. The list and dotted notations can be combined to represent
improper lists, as the following equivalent notations show:
(a b c . d)
(a . (b . (c . d)))
1
The above definitions imply that all lists have finite length and are terminated by the empty list.
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Within literal expressions and representations of objects read by the read procedure, the
forms ’datum, ‘datum, ,datum, and ,@datum denote two-element lists whose first elements
are the symbols quote, quasiquote, unquote, and unquote-splicing, respectively. The
second element in each case is datum. This convention is supported so that arbitrary Scheme
programs may be represented as lists. Among other things, this permits the use of the read
procedure to parse Scheme programs.
7.1 Pairs
This section describes the simple operations that are available for constructing and
manipulating arbitrary graphs constructed from pairs.
pair? object
Returns #t if
(pair?
(pair?
(pair?
(pair?
object is a pair; otherwise returns #f.
’(a . b))
’(a b c))
’())
’#(a b))
procedure
)
)
)
)
#t
#t
#f
#f
cons obj1 obj2
procedure
Returns a newly allocated pair whose car is obj1 and whose cdr is obj2. The pair
is guaranteed to be di↵erent (in the sense of eqv?) from every previously existing
object.
) (a)
(cons ’a ’())
) ((a) b c d)
(cons ’(a) ’(b c d))
) ("a" b c)
(cons "a" ’(b c))
) (a . 3)
(cons ’a 3)
) ((a b) . c)
(cons ’(a b) ’c)
car pair
procedure
Returns the contents of the car field of pair. Note that it is an error to take the car
of the empty list.
) a
(car ’(a b c))
) (a)
(car ’((a) b c d))
) 1
(car ’(1 . 2))
(car ’())
error Illegal datum
cdr pair
procedure
Returns the contents of the cdr field of pair. Note that it is an error to take the cdr
of the empty list.
) (b c d)
(cdr ’((a) b c d))
) 2
(cdr ’(1 . 2))
(cdr ’())
error Illegal datum
set-car! pair object
procedure
Stores object in the car field of pair. The value returned by set-car! is unspecified.
Chapter 7: Lists
107
(define (f) (list ’not-a-constant-list))
(define (g) ’(constant-list))
) unspecified
(set-car! (f) 3)
(set-car! (g) 3)
error Illegal datum
set-cdr! pair object
procedure
Stores object in the cdr field of pair. The value returned by set-cdr! is unspecified.
caar pair
cadr pair
cdar pair
cddr pair
caaar pair
caadr pair
cadar pair
caddr pair
cdaar pair
cdadr pair
cddar pair
cdddr pair
caaaar pair
caaadr pair
caadar pair
caaddr pair
cadaar pair
cadadr pair
caddar pair
cadddr pair
cdaaar pair
cdaadr pair
cdadar pair
cdaddr pair
cddaar pair
cddadr pair
cdddar pair
cddddr pair
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
These procedures are compositions of car and cdr; for example, caddr could be
defined by
(define caddr (lambda (x) (car (cdr (cdr x)))))
general-car-cdr object path
procedure
This procedure is a generalization of car and cdr. Path encodes a particular sequence
of car and cdr operations, which general-car-cdr executes on object. Path is an
exact non-negative integer that encodes the operations in a bitwise fashion: a zero
bit represents a cdr operation, and a one bit represents a car. The bits are executed
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MIT Scheme Reference
LSB to MSB, and the most significant one bit, rather than being interpreted as an
operation, signals the end of the sequence.2
For example, the following are equivalent:
(general-car-cdr object #b1011)
(cdr (car (car object)))
Here is a partial table of path/operation equivalents:
#b10
cdr
#b11
car
#b100
cddr
#b101
cdar
#b110
cadr
#b111
caar
#b1000 cdddr
tree-copy tree
procedure
This copies an arbitrary tree constructed from pairs, copying both the car and cdr
elements of every pair. This could have been defined by
(define (tree-copy tree)
(let loop ((tree tree))
(if (pair? tree)
(cons (loop (car tree)) (loop (cdr tree)))
tree)))
7.2 Construction of Lists
list object . . .
Returns a list of its arguments.
) (a 7 c)
(list ’a (+ 3 4) ’c)
) ()
(list)
These expressions are equivalent:
(list obj1 obj2 ... objN )
(cons obj1 (cons obj2 ... (cons objN ’()) ...))
procedure
make-list k [element]
procedure
This procedure returns a newly allocated list of length k, whose elements are all
element. If element is not supplied, it defaults to the empty list.
cons* object object . . .
procedure
cons* is similar to list, except that cons* conses together the last two arguments
rather than consing the last argument with the empty list. If the last argument is
not a list the result is an improper list. If the last argument is a list, the result is a
list consisting of the initial arguments and all of the items in the final argument. If
there is only one argument, the result is the argument.
2
Note that path is restricted to a machine-dependent range, usually the size of a machine word. On many
machines, this means that the maximum length of path will be 30 operations (32 bits, less the sign bit
and the “end-of-sequence” bit).
Chapter 7: Lists
(cons* ’a ’b ’c)
(cons* ’a ’b ’(c d))
(cons* ’a)
These expressions are equivalent:
109
)
)
)
(a b . c)
(a b c d)
a
(cons* obj1 obj2 ... objN-1 objN )
(cons obj1 (cons obj2 ... (cons objN-1 objN ) ...))
list-copy list
procedure
Returns a newly allocated copy of list. This copies each of the pairs comprising list.
This could have been defined by
(define (list-copy list)
(if (null? list)
’()
(cons (car list)
(list-copy (cdr list)))))
vector->list vector
subvector->list vector start end
procedure
procedure
vector->list returns a newly allocated list of the elements of vector.
subvector->list returns a newly allocated list of the elements of the given subvector.
The inverse of vector->list is list->vector.
) (dah dah didah)
(vector->list ’#(dah dah didah))
string->list string
substring->list string start end
procedure
procedure
string->list returns a newly allocated list of the character elements of string.
substring->list returns a newly allocated list of the character elements of the given
substring. The inverse of string->list is list->string.
) (#\a #\b #\c #\d)
(string->list "abcd")
) (#\b #\c)
(substring->list "abcdef" 1 3)
7.3 Selecting List Components
list? object
procedure
Returns #t if object is a list, otherwise returns #f. By definition, all lists have finite
length and are terminated by the empty list. This procedure returns an answer even
for circular structures.
Any object satisfying this predicate will also satisfy exactly one of pair? or null?.
) #t
(list? ’(a b c))
) #t
(list? ’())
) #f
(list? ’(a . b))
(let ((x (list ’a)))
(set-cdr! x x)
) #f
(list? x))
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MIT Scheme Reference
length list
Returns the length of list. Signals an error if list isn’t a proper list.
) 3
(length ’(a b c))
) 3
(length ’(a (b) (c d e)))
) 0
(length ’())
null? object
Returns #t if object is the empty list; otherwise returns
[True and False], page 8).
)
(null? ’(a . b))
)
(null? ’(a b c))
)
(null? ’())
procedure
procedure
#f (but see Section 1.2.5
#f
#f
#t
list-ref list k
procedure
Returns the kth element of list, using zero-origin indexing. The valid indexes of a list
are the exact non-negative integers less than the length of the list. The first element
of a list has index 0, the second has index 1, and so on.
) c
(list-ref ’(a b c d) 2)
(list-ref ’(a b c d)
(inexact->exact (round 1.8)))
) c
(list-ref list k) is equivalent to (car (list-tail list k)).
first list
second list
third list
fourth list
fifth list
sixth list
seventh list
eighth list
ninth list
tenth list
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
Returns the specified element of list. It is an error if list is not long enough to contain
the specified element (for example, if the argument to seventh is a list that contains
only six elements).
7.4 Cutting and Pasting Lists
sublist list start end
Start and end must be exact integers satisfying
procedure
0 <= start <= end <= (length list)
sublist returns a newly allocated list formed from the elements of list beginning at
index start (inclusive) and ending at end (exclusive).
Chapter 7: Lists
111
list-head list k
procedure
Returns a newly allocated list consisting of the first k elements of list. K must not
be greater than the length of list.
We could have defined list-head this way:
(define (list-head list k)
(sublist list 0 k))
list-tail list k
procedure
Returns the sublist of list obtained by omitting the first k elements. The result, if
it is not the empty list, shares structure with list. K must not be greater than the
length of list.
append list . . .
procedure
Returns a list consisting of the elements of the first list followed by the elements of
the other lists.
) (x y)
(append ’(x) ’(y))
) (a b c d)
(append ’(a) ’(b c d))
) (a (b) (c))
(append ’(a (b)) ’((c)))
) ()
(append)
The resulting list is always newly allocated, except that it shares structure with the
last list argument. The last argument may actually be any object; an improper list
results if the last argument is not a proper list.
) (a b c . d)
(append ’(a b) ’(c . d))
) a
(append ’() ’a)
append! list . . .
procedure
Returns a list that is the argument lists concatenated together. The arguments are
changed rather than copied. (Compare this with append, which copies arguments
rather than destroying them.) For example:
(define x ’(a b c))
(define y ’(d e f))
(define z ’(g h))
) (a b c d e f g h)
(append! x y z)
) (a b c d e f g h)
x
) (d e f g h)
y
) (g h)
z
last-pair list
procedure
Returns the last pair in list, which may be an improper list. last-pair could have
been defined this way:
(define last-pair
(lambda (x)
(if (pair? (cdr x))
(last-pair (cdr x))
x)))
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MIT Scheme Reference
except-last-pair list
except-last-pair! list
procedure
procedure
These procedures remove the last pair from list. List may be an improper list, except
that it must consist of at least one pair. except-last-pair returns a newly allocated
copy of list that omits the last pair. except-last-pair! destructively removes the
last pair from list and returns list. If the cdr of list is not a pair, the empty list is
returned by either procedure.
7.5 Filtering Lists
keep-matching-items list predicate
delete-matching-items list predicate
procedure
procedure
These procedures return a newly allocated copy of list containing only the elements
for which predicate is (respectively) true or false. Predicate must be a procedure of
one argument.
(keep-matching-items ’(1 2 3 4 5) odd?) ) (1 3 5)
(delete-matching-items ’(1 2 3 4 5) odd?) ) (2 4)
For compatibility, the procedure list-transform-positive is an alias for keepmatching-items, and list-transform-negative is an alias for delete-matchingitems.
keep-matching-items! list predicate
delete-matching-items! list predicate
procedure
procedure
These procedures are exactly like keep-matching-items and delete-matchingitems, respectively, except that they destructively modify the list argument rather
than allocating a new result.
delq element list
delv element list
delete element list
procedure
procedure
procedure
Returns a newly allocated copy of list with all entries equal to element removed. delq
uses eq? to compare element with the entries in list, delv uses eqv?, and delete uses
equal?.
delq! element list
delv! element list
delete! element list
procedure
procedure
procedure
Returns a list consisting of the top-level elements of list with all entries equal to
element removed. These procedures are like delq, delv, and delete except that
they destructively modify list. delq! uses eq? to compare element with the entries
in list, delv! uses eqv?, and delete! uses equal?. Because the result may not be
eq? to list, it is desirable to do something like (set! x (delete! x)).
Chapter 7: Lists
(define x ’(a b c b))
(delete ’b x)
x
(define x ’(a b c b))
(delete! ’b x)
x
;; Returns correct result:
(delete! ’a x)
;; Didn’t modify what x points to:
x
113
)
)
(a c)
(a b c b)
)
)
(a c)
(a c)
)
(c)
)
(a c)
delete-member-procedure deletor predicate
procedure
Returns a deletion procedure similar to delv or delete!. Deletor should be one of
the procedures list-deletor or list-deletor!. Predicate must be an equivalence
predicate. The returned procedure accepts exactly two arguments: first, an object to
be deleted, and second, a list of objects from which it is to be deleted. If deletor is
list-deletor, the procedure returns a newly allocated copy of the given list in which
all entries equal to the given object have been removed. If deletor is list-deletor!,
the procedure returns a list consisting of the top-level elements of the given list with
all entries equal to the given object removed; the given list is destructively modified
to produce the result. In either case predicate is used to compare the given object to
the elements of the given list.
Here are some examples that demonstrate how delete-member-procedure could have
been used to implement delv and delete!:
(define delv
(delete-member-procedure list-deletor eqv?))
(define delete!
(delete-member-procedure list-deletor! equal?))
list-deletor predicate
list-deletor! predicate
procedure
procedure
These procedures each return a procedure that deletes elements from lists. Predicate
must be a procedure of one argument. The returned procedure accepts exactly one
argument, which must be a proper list, and applies predicate to each of the elements
of the argument, deleting those for which it is true.
The procedure returned by list-deletor deletes elements non-destructively, by returning a newly allocated copy of the argument with the appropriate elements removed. The procedure returned by list-deletor! performs a destructive deletion.
7.6 Searching Lists
find-matching-item list predicate
procedure
Returns the first element in list for which predicate is true; returns #f if it doesn’t find
such an element. (This means that if predicate is true for #f, it may be impossible
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MIT Scheme Reference
to distinguish a successful result from an unsuccessful one.) Predicate must be a
procedure of one argument.
For compatibility, list-search-positive is an alias for find-matching-item.
list-search-negative is similar but the sense of the predicate is reversed.
memq object list
memv object list
member object list
procedure
procedure
procedure
These procedures return the first pair of list whose car is object; the returned pair
is always one from which list is composed. If object does not occur in list, #f (n.b.:
not the empty list) is returned. memq uses eq? to compare object with the elements
of list, while memv uses eqv? and member uses equal?.3
) (a b c)
(memq ’a ’(a b c))
) (b c)
(memq ’b ’(a b c))
) #f
(memq ’a ’(b c d))
) #f
(memq (list ’a) ’(b (a) c))
) ((a) c)
(member (list ’a) ’(b (a) c))
) unspecified
(memq 101 ’(100 101 102))
) (101 102)
(memv 101 ’(100 101 102))
member-procedure predicate
procedure
Returns a procedure similar to memq, except that predicate, which must be an equivalence predicate, is used instead of eq?. This could be used to define memv as follows:
(define memv (member-procedure eqv?))
7.7 Mapping of Lists
map procedure list list . . .
procedure
Procedure must be a procedure taking as many arguments as there are lists. If more
than one list is given, then they must all be the same length. map applies procedure
element-wise to the elements of the lists and returns a list of the results, in order from
left to right. The dynamic order in which procedure is applied to the elements of the
lists is unspecified; use for-each to sequence side e↵ects.
) (b e h)
(map cadr ’((a b) (d e) (g h)))
(map (lambda (n) (expt n n)) ’(1 2 3 4)) ) (1 4 27 256)
) (5 7 9)
(map + ’(1 2 3) ’(4 5 6))
(let ((count 0))
(map (lambda (ignored)
(set! count (+ count 1))
count)
) unspecified
’(a b c)))
map* initial-value procedure list1 list2 . . .
procedure
Similar to map, except that the resulting list is terminated by initial-value rather than
the empty list. The following are equivalent:
3
Although they are often used as predicates, memq, memv, and member do not have question marks in their
names because they return useful values rather than just #t or #f.
Chapter 7: Lists
115
(map procedure list list ...)
(map* ’() procedure list list ...)
append-map procedure list list . . .
append-map* initial-value procedure list list . . .
procedure
procedure
Similar to map and map*, respectively, except that the results of applying procedure
to the elements of lists are concatenated together by append rather than by cons.
The following are equivalent, except that the former is more efficient:
(append-map procedure list list ...)
(apply append (map procedure list list ...))
append-map! procedure list list . . .
append-map*! initial-value procedure list list . . .
procedure
procedure
Similar to map and map*, respectively, except that the results of applying procedure
to the elements of lists are concatenated together by append! rather than by cons.
The following are equivalent, except that the former is more efficient:
(append-map! procedure list list ...)
(apply append! (map procedure list list ...))
for-each procedure list list . . .
procedure
The arguments to for-each are like the arguments to map, but for-each calls procedure for its side e↵ects rather than for its values. Unlike map, for-each is guaranteed
to call procedure on the elements of the lists in order from the first element to the
last, and the value returned by for-each is unspecified.
(let ((v (make-vector 5)))
(for-each (lambda (i)
(vector-set! v i (* i i)))
’(0 1 2 3 4))
) #(0 1 4 9 16)
v)
7.8 Reduction of Lists
reduce procedure initial list
procedure
Combines all the elements of list using the binary operation procedure. For example,
using + one can add up all the elements:
(reduce + 0 list-of-numbers)
The argument initial is used only if list is empty; in this case initial is the result of the
call to reduce. If list has a single argument, it is returned. Otherwise, the arguments
are reduced in a left-associative fashion. For example:
) 10
(reduce + 0 ’(1 2 3 4))
) 3
(reduce + 0 ’(1 2))
) 1
(reduce + 0 ’(1))
) 0
(reduce + 0 ’())
) foo
(reduce + 0 ’(foo))
) (((1 2) 3) 4)
(reduce list ’() ’(1 2 3 4))
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MIT Scheme Reference
reduce-right procedure initial list
Like reduce except that it is right-associative.
(reduce-right list ’() ’(1 2 3 4))
procedure
)
(1 (2 (3 4)))
fold-right procedure initial list
procedure
Combines all of the elements of list using the binary operation procedure. Unlike
reduce and reduce-right, initial is always used:
) 10
(fold-right + 0 ’(1 2 3 4))
(fold-right + 0 ’(foo))
error Illegal datum
) (1 (2 (3 (4 ()))))
(fold-right list ’() ’(1 2 3 4))
Fold-right has interesting properties because it establishes a homomorphism between (cons, ()) and (procedure, initial). It can be thought of as replacing the
pairs in the spine of the list with procedure and replacing the () at the end with
initial. Many of the classical list-processing procedures can be expressed in terms of
fold-right, at least for the simple versions that take a fixed number of arguments:
(define (copy-list list)
(fold-right cons ’() list))
(define (append list1 list2)
(fold-right cons list2 list1))
(define (map p list)
(fold-right (lambda (x r) (cons (p x) r)) ’() list))
(define (reverse items)
(fold-right (lambda (x r) (append r (list x))) ’() items))
fold-left procedure initial list
procedure
Combines all the elements of list using the binary operation procedure. Elements are
combined starting with initial and then the elements of list from left to right. Whereas
fold-right is recursive in nature, capturing the essence of cdr-ing down a list and
then computing a result, fold-left is iterative in nature, combining the elements as
the list is traversed.
) ((((() 1) 2) 3) 4)
(fold-left list ’() ’(1 2 3 4))
(define (length list)
(fold-left (lambda (sum element) (+ sum 1)) 0 list))
(define (reverse items)
(fold-left (lambda (x y) (cons y x)) () items))
there-exists? list predicate
procedure
Predicate must be a procedure of one argument. Applies predicate to each element
of list, in order from left to right. If predicate is true for any element of list, the
value yielded by predicate is immediately returned as the value of there-exists?;
predicate will not be applied to the remaining elements of list. If predicate returns
#f for all of the elements of list, then #f is returned.
Chapter 7: Lists
117
for-all? list predicate
procedure
Predicate must be a procedure of one argument. Applies predicate to each element
of list, in order from left to right. If predicate returns #f for any element of list, #f is
immediately returned as the value of for-all?; predicate will not be applied to the
remaining elements of list. If predicate is true for all of the elements of list, then #t
is returned.
7.9 Miscellaneous List Operations
circular-list object . . .
make-circular-list k [element]
procedure
procedure
These procedures are like list and make-list, respectively, except that the returned
lists are circular. circular-list could have been defined like this:
(define (circular-list . objects)
(append! objects objects))
reverse list
procedure
Returns a newly allocated list consisting of the top-level elements of list in reverse
order.
) (c b a)
(reverse ’(a b c))
) ((e (f)) d (b c) a)
(reverse ’(a (b c) d (e (f))))
reverse! list
procedure
Returns a list consisting of the top-level elements of list in reverse order. reverse!
is like reverse, except that it destructively modifies list. Because the result may not
be eqv? to list, it is desirable to do something like (set! x (reverse! x)).
sort sequence procedure
merge-sort sequence procedure
quick-sort sequence procedure
procedure
procedure
procedure
Sequence must be either a list or a vector. Procedure must be a procedure of two
arguments that defines a total ordering on the elements of sequence. In other words,
if x and y are two distinct elements of sequence, then it must be the case that
(and (procedure x y)
(procedure y x))
) #f
If sequence is a list (vector), sort returns a newly allocated list (vector) whose elements are those of sequence, except that they are rearranged to be sorted in the order
defined by procedure. So, for example, if the elements of sequence are numbers, and
procedure is <, then the resulting elements are sorted in monotonically nondecreasing
order. Likewise, if procedure is >, the resulting elements are sorted in monotonically
nonincreasing order. To be precise, if x and y are any two adjacent elements in the
result, where x precedes y, it is the case that
(procedure y x)
) #f
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Two sorting algorithms are implemented: merge-sort and quick-sort. The procedure sort is an alias for merge-sort.
See also the definition of sort!.
Chapter 8: Vectors
119
8 Vectors
Vectors are heterogenous structures whose elements are indexed by exact non-negative
integers. A vector typically occupies less space than a list of the same length, and the
average time required to access a randomly chosen element is typically less for the vector
than for the list.
The length of a vector is the number of elements that it contains. This number is an
exact non-negative integer that is fixed when the vector is created. The valid indexes of
a vector are the exact non-negative integers less than the length of the vector. The first
element in a vector is indexed by zero, and the last element is indexed by one less than the
length of the vector.
Vectors are written using the notation #(object ...). For example, a vector of length
3 containing the number zero in element 0, the list (2 2 2 2) in element 1, and the string
"Anna" in element 2 can be written as
#(0 (2 2 2 2) "Anna")
Note that this is the external representation of a vector, not an expression evaluating to a
vector. Like list constants, vector constants must be quoted:
) #(0 (2 2 2 2) "Anna")
’#(0 (2 2 2 2) "Anna")
A number of the vector procedures operate on subvectors. A subvector is a segment of a
vector that is specified by two exact non-negative integers, start and end. Start is the index
of the first element that is included in the subvector, and end is one greater than the index
of the last element that is included in the subvector. Thus if start and end are the same,
they refer to a null subvector, and if start is zero and end is the length of the vector, they
refer to the entire vector. The valid indexes of a subvector are the exact integers between
start inclusive and end exclusive.
8.1 Construction of Vectors
make-vector k [object]
procedure
Returns a newly allocated vector of k elements. If object is specified, make-vector
initializes each element of the vector to object. Otherwise the initial elements of the
result are unspecified.
vector object . . .
procedure
Returns a newly allocated vector whose elements are the given arguments. vector is
analogous to list.
) #(a b c)
(vector ’a ’b ’c)
vector-copy vector
Returns a newly allocated vector that is a copy of vector.
list->vector list
procedure
procedure
Returns a newly allocated vector initialized to the elements of list. The inverse of
list->vector is vector->list.
) #(dididit dah)
(list->vector ’(dididit dah))
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make-initialized-vector k initialization
procedure
Similar to make-vector, except that the elements of the result are determined by
calling the procedure initialization on the indices. For example:
(make-initialized-vector 5 (lambda (x) (* x x)))
) #(0 1 4 9 16)
vector-grow vector k
procedure
K must be greater than or equal to the length of vector. Returns a newly allocated
vector of length k. The first (vector-length vector) elements of the result are
initialized from the corresponding elements of vector. The remaining elements of the
result are unspecified.
vector-map procedure vector
procedure
Procedure must be a procedure of one argument. vector-map applies procedure
element-wise to the elements of vector and returns a newly allocated vector of the
results, in order from left to right. The dynamic order in which procedure is applied
to the elements of vector is unspecified.
) #(b e h)
(vector-map cadr ’#((a b) (d e) (g h)))
(vector-map (lambda (n) (expt n n)) ’#(1 2 3 4))
) #(1 4 27 256)
) #(5 7 9)
(vector-map + ’#(5 7 9))
8.2 Selecting Vector Components
vector? object
procedure
vector-length vector
procedure
Returns #t if object is a vector; otherwise returns #f.
Returns the number of elements in vector.
vector-ref vector k
procedure
Returns the contents of element k of vector. K must be a valid index of vector.
(vector-ref ’#(1 1 2 3 5 8 13 21) 5)
)
vector-set! vector k object
8
procedure
Stores object in element k of vector and returns an unspecified value. K must be a
valid index of vector.
(let ((vec (vector 0 ’(2 2 2 2) "Anna")))
(vector-set! vec 1 ’("Sue" "Sue"))
vec)
) #(0 ("Sue" "Sue") "Anna")
Chapter 8: Vectors
121
vector-first vector
vector-second vector
vector-third vector
vector-fourth vector
vector-fifth vector
vector-sixth vector
vector-seventh vector
vector-eighth vector
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
These procedures access the first several elements of vector in the obvious way. It is
an error if the implicit index of one of these procedurs is not a valid index of vector.
vector-binary-search vector key<? unwrap-key key
procedure
Searches vector for an element with a key matching key, returning the element if one
is found or #f if none. The search operation takes time proportional to the logarithm
of the length of vector. Unwrap-key must be a procedure that maps each element of
vector to a key. Key<? must be a procedure that implements a total ordering on the
keys of the elements.
(define (translate number)
(vector-binary-search ’#((1 . i)
(2 . ii)
(3 . iii)
(6 . vi))
< car number))
(translate 2) ) (2 . ii)
(translate 4) ) #F
8.3 Cutting Vectors
subvector vector start end
procedure
Returns a newly allocated vector that contains the elements of vector between index
start (inclusive) and end (exclusive).
vector-head vector end
procedure
vector-tail vector start
procedure
Equivalent to
(subvector vector 0 end)
Equivalent to
(subvector vector start (vector-length vector))
8.4 Modifying Vectors
vector-fill! vector object
subvector-fill! vector start end object
procedure
procedure
Stores object in every element of the vector (subvector) and returns an unspecified
value.
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subvector-move-left! vector1 start1 end1 vector2 start2
subvector-move-right! vector1 start1 end1 vector2 start2
procedure
procedure
Destructively copies the elements of vector1, starting with index start1 (inclusive)
and ending with end1 (exclusive), into vector2 starting at index start2 (inclusive).
Vector1, start1, and end1 must specify a valid subvector, and start2 must be a valid
index for vector2. The length of the source subvector must not exceed the length of
vector2 minus the index start2.
The elements are copied as follows (note that this is only important when vector1
and vector2 are eqv?):
subvector-move-left!
The copy starts at the left end and moves toward the right (from smaller
indices to larger). Thus if vector1 and vector2 are the same, this procedure moves the elements toward the left inside the vector.
subvector-move-right!
The copy starts at the right end and moves toward the left (from larger
indices to smaller). Thus if vector1 and vector2 are the same, this procedure moves the elements toward the right inside the vector.
sort! vector procedure
merge-sort! vector procedure
quick-sort! vector procedure
procedure
procedure
procedure
Procedure must be a procedure of two arguments that defines a total ordering on the
elements of vector. The elements of vector are rearranged so that they are sorted in
the order defined by procedure. The elements are rearranged in place, that is, vector
is destructively modified so that its elements are in the new order.
sort! returns vector as its value.
Two sorting algorithms are implemented: merge-sort! and quick-sort!. The procedure sort! is an alias for merge-sort!.
See also the definition of sort.
Chapter 9: Bit Strings
123
9 Bit Strings
A bit string is a sequence of bits. Bit strings can be used to represent sets or to
manipulate binary data. The elements of a bit string are numbered from zero up to the
number of bits in the string less one, in right to left order, (the rightmost bit is numbered
zero). When you convert from a bit string to an integer, the zero-th bit is associated with
the zero-th power of two, the first bit is associated with the first power, and so on.
Bit strings are encoded very densely in memory. Each bit occupies exactly one bit of
storage, and the overhead for the entire bit string is bounded by a small constant. However,
accessing a bit in a bit string is slow compared to accessing an element of a vector or
character string. If performance is of overriding concern, it is better to use character strings
to store sets of boolean values even though they occupy more space.
The length of a bit string is the number of bits that it contains. This number is an exact
non-negative integer that is fixed when the bit string is created. The valid indexes of a bit
string are the exact non-negative integers less than the length of the bit string.
Bit strings may contain zero or more bits. They are not limited by the length of a
machine word. In the printed representation of a bit string, the contents of the bit string
are preceded by ‘#*’. The contents are printed starting with the most significant bit (highest
index).
Note that the external representation of bit strings uses a bit ordering that is the reverse
of the representation for bit strings in Common Lisp. It is likely that MIT Scheme’s representation will be changed in the future, to be compatible with Common Lisp. For the time
being this representation should be considered a convenience for viewing bit strings rather
than a means of entering them as data.
#*11111
#*1010
#*00000000
#*
All of the bit-string procedures are MIT Scheme extensions.
9.1 Construction of Bit Strings
make-bit-string k initialization
procedure
Returns a newly allocated bit string of length k. If initialization is #f, the bit string
is filled with 0 bits; otherwise, the bit string is filled with 1 bits.
(make-bit-string 7 #f)
)
#*0000000
bit-string-allocate k
procedure
bit-string-copy bit-string
procedure
Returns a newly allocated bit string of length k, but does not initialize it.
Returns a newly allocated copy of bit-string.
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9.2 Selecting Bit String Components
bit-string? object
procedure
bit-string-length bit-string
procedure
Returns #t if object is a bit string; otherwise returns #f.
Returns the length of bit-string.
bit-string-ref bit-string k
procedure
Returns #t if the kth bit is 1; otherwise returns #f. K must be a valid index of
bit-string.
bit-string-set! bit-string k
procedure
Sets the kth bit in bit-string to 1 and returns an unspecified value. K must be a valid
index of bit-string.
bit-string-clear! bit-string k
procedure
Sets the kth bit in bit-string to 0 and returns an unspecified value. K must be a valid
index of bit-string.
bit-substring-find-next-set-bit bit-string start end
procedure
Returns the index of the first occurrence of a set bit in the substring of bit-string
from start (inclusive) to end (exclusive). If none of the bits in the substring are set
#f is returned. The index returned is relative to the whole bit string, not substring.
The following procedure uses bit-substring-find-next-set-bit to find all the set
bits and display their indexes:
(define (scan-bitstring bs)
(let ((end (bit-string-length bs)))
(let loop ((start 0))
(let ((next
(bit-substring-find-next-set-bit bs start end)))
(if next
(begin
(write-line next)
(if (< next end)
(loop (+ next 1)))))))))
9.3 Cutting and Pasting Bit Strings
bit-string-append bit-string-1 bit-string-2
procedure
Appends the two bit string arguments, returning a newly allocated bit string as its
result. In the result, the bits copied from bit-string-1 are less significant (smaller
indices) than those copied from bit-string-2.
bit-substring bit-string start end
procedure
Returns a newly allocated bit string whose bits are copied from bit-string, starting
at index start (inclusive) and ending at end (exclusive).
Chapter 9: Bit Strings
125
9.4 Bitwise Operations on Bit Strings
bit-string-zero? bit-string
Returns #t if bit-string contains only 0 bits; otherwise returns #f.
procedure
bit-string=? bit-string-1 bit-string-2
procedure
Compares the two bit string arguments and returns #t if they are the same length
and contain the same bits; otherwise returns #f.
bit-string-not bit-string
procedure
Returns a newly allocated bit string that is the bitwise-logical negation of bit-string.
bit-string-movec! target-bit-string bit-string
procedure
The destructive version of bit-string-not. The arguments target-bit-string and
bit-string must be bit strings of the same length. The bitwise-logical negation of
bit-string is computed and the result placed in target-bit-string. The value of this
procedure is unspecified.
bit-string-and bit-string-1 bit-string-2
procedure
Returns a newly allocated bit string that is the bitwise-logical “and” of the arguments.
The arguments must be bit strings of identical length.
bit-string-andc bit-string-1 bit-string-2
procedure
Returns a newly allocated bit string that is the bitwise-logical “and” of bit-string-1
with the bitwise-logical negation of bit-string-2. The arguments must be bit strings
of identical length.
bit-string-or bit-string-1 bit-string-2
procedure
Returns a newly allocated bit string that is the bitwise-logical “inclusive or” of the
arguments. The arguments must be bit strings of identical length.
bit-string-xor bit-string-1 bit-string-2
procedure
Returns a newly allocated bit string that is the bitwise-logical “exclusive or” of the
arguments. The arguments must be bit strings of identical length.
bit-string-and! target-bit-string bit-string
bit-string-or! target-bit-string bit-string
bit-string-xor! target-bit-string bit-string
bit-string-andc! target-bit-string bit-string
procedure
procedure
procedure
procedure
These are destructive versions of the above operations. The arguments target-bitstring and bit-string must be bit strings of the same length. Each of these procedures
performs the corresponding bitwise-logical operation on its arguments, places the
result into target-bit-string, and returns an unspecified result.
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9.5 Modification of Bit Strings
bit-string-fill! bit-string initialization
procedure
Fills bit-string with zeroes if initialization is #f; otherwise fills bit-string with ones.
Returns an unspecified value.
bit-string-move! target-bit-string bit-string
procedure
Moves the contents of bit-string into target-bit-string. Both arguments must be bit
strings of the same length. The results of the operation are undefined if the arguments
are the same bit string.
bit-substring-move-right! bit-string-1 start1 end1 bit-string-2 start2
procedure
Destructively copies the bits of bit-string-1, starting at index start1 (inclusive) and
ending at end1 (exclusive), into bit-string-2 starting at index start2 (inclusive). Start1
and end1 must be valid substring indices for bit-string-1, and start2 must be a valid
index for bit-string-2. The length of the source substring must not exceed the length
of bit-string-2 minus the index start2.
The bits are copied starting from the MSB and working towards the LSB; the direction
of copying only matters when bit-string-1 and bit-string-2 are eqv?.
9.6 Integer Conversions of Bit Strings
unsigned-integer->bit-string length integer
procedure
Both length and integer must be exact non-negative integers. Converts integer into
a newly allocated bit string of length bits. Signals an error of type conditiontype:bad-range-argument if integer is too large to be represented in length bits.
signed-integer->bit-string length integer
procedure
Length must be an exact non-negative integer, and integer may be any exact integer. Converts integer into a newly allocated bit string of length bits, using two’s
complement encoding for negative numbers. Signals an error of type conditiontype:bad-range-argument if integer is too large to be represented in length bits.
bit-string->unsigned-integer bit-string
bit-string->signed-integer bit-string
procedure
procedure
Converts bit-string into an exact integer. bit-string->signed-integer regards
bit-string as a two’s complement representation of a signed integer, and produces
an integer of like sign and absolute value. bit-string->unsigned-integer regards
bit-string as an unsigned quantity and converts to an integer accordingly.
Chapter 10: Miscellaneous Datatypes
127
10 Miscellaneous Datatypes
10.1 Booleans
The boolean objects are true and false. The boolean constant true is written as ‘#t’,
and the boolean constant false is written as ‘#f’.
The primary use for boolean objects is in the conditional expressions if, cond, and, and
or; the behavior of these expressions is determined by whether objects are true or false.
These expressions count only #f as false. They count everything else, including #t, pairs,
symbols, numbers, strings, vectors, and procedures as true (but see Section 1.2.5 [True and
False], page 8).
Programmers accustomed to other dialects of Lisp should note that Scheme distinguishes
#f and the empty list from the symbol nil. Similarly, #t is distinguished from the symbol
t. In fact, the boolean objects (and the empty list) are not symbols at all.
Boolean constants evaluate to themselves, so you don’t need to quote them.
) #t
#t
) #f
#f
) #f
’#f
t
error Unbound variable
false
true
variable
variable
These variables are bound to the objects #f and #t respectively. The compiler, given
the usual-integrations declaration, replaces references to these variables with their
respective values.
Note that the symbol true is not equivalent to #t, and the symbol false is not
equivalent to #f.
boolean? object
Returns #t if object is either #t or #f; otherwise returns #f.
) #t
(boolean? #f)
) #f
(boolean? 0)
procedure
not object
false? object
procedure
procedure
These procedures return #t if object is false; otherwise they return #f. In other
words they invert boolean values. These two procedures have identical semantics;
their names are di↵erent to give di↵erent connotations to the test.
) #f
(not #t)
) #f
(not 3)
) #f
(not (list 3))
) #t
(not #f)
boolean=? obj1 obj2
This predicate is true i↵ obj1 and obj2 are either both true or both false.
procedure
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MIT Scheme Reference
boolean/and object . . .
procedure
This procedure returns #t if none of its arguments are #f. Otherwise it returns #f.
boolean/or object . . .
procedure
This procedure returns #f if all of its arguments are #f. Otherwise it returns #t.
10.2 Symbols
MIT Scheme provides two types of symbols: interned and uninterned. Interned symbols
are far more common than uninterned symbols, and there are more ways to create them.
Interned symbols have an external representation that is recognized by the procedure read;
uninterned symbols do not.1
Interned symbols have an extremely useful property: any two interned symbols whose
names are the same, in the sense of string=?, are the same object (i.e. they are eq? to one
another). The term interned refers to the process of interning by which this is accomplished.
Uninterned symbols do not share this property.
The names of interned symbols are not distinguished by their alphabetic case. Because
of this, MIT Scheme converts all alphabetic characters in the name of an interned symbol
to a specific case (lower case) when the symbol is created. When the name of an interned
symbol is referenced (using symbol->string) or written (using write) it appears in this
case. It is a bad idea to depend on the name being lower case. In fact, it is preferable to
take this one step further: don’t depend on the name of a symbol being in a uniform case.
The rules for writing an interned symbol are the same as the rules for writing an identifier
(see Section 1.3.3 [Identifiers], page 10). Any interned symbol that has been returned as
part of a literal expression, or read using the read procedure and subsequently written out
using the write procedure, will read back in as the identical symbol (in the sense of eq?).
Usually it is also true that reading in an interned symbol that was previously written out
produces the same symbol. An exception are symbols created by the procedures string>symbol and intern; they can create symbols for which this write/read invariance may not
hold because the symbols’ names contain special characters or letters in the non-standard
case.2
The external representation for uninterned symbols is special, to distinguish them from
interned symbols and prevent them from being recognized by the read procedure:
1
2
In older dialects of Lisp, uninterned symbols were fairly important. This was true because symbols were
complicated data structures: in addition to having value cells (and sometimes, function cells), these
structures contained property lists. Because of this, uninterned symbols were often used merely for
their property lists — sometimes an uninterned symbol used this way was referred to as a disembodied
property list. In MIT Scheme, symbols do not have property lists, or any other components besides their
names. There is a di↵erent data structure similar to disembodied property lists: one-dimensional tables
(see Section 11.2 [1D Tables], page 141). For these reasons, uninterned symbols are not very useful in
MIT Scheme. In fact, their primary purpose is to simplify the generation of unique variable names in
programs that generate Scheme code.
MIT Scheme reserves a specific set of interned symbols for its own use. If you use these reserved symbols
it is possible that you could break specific pieces of software that depend on them. The reserved symbols
all have names beginning with the characters ‘#[’ and ending with the character ‘]’; thus none of these
symbols can be read by the procedure read and hence are not likely to be used by accident. For example,
(intern "#[unnamed-procedure]") produces a reserved symbol.
Chapter 10: Miscellaneous Datatypes
129
(string->uninterned-symbol "foo")
) #[uninterned-symbol 30 foo]
In this section, the procedures that return symbols as values will either always return
interned symbols, or always return uninterned symbols. The procedures that accept symbols as arguments will always accept either interned or uninterned symbols, and do not
distinguish the two.
symbol? object
Returns #t if object is a symbol, otherwise returns #f.
(symbol? ’foo)
(symbol? (car ’(a b)))
(symbol? "bar")
procedure
)
)
)
#t
#t
#f
symbol->string symbol
procedure
Returns the name of symbol as a string. If symbol was returned by string->symbol,
the value of this procedure will be identical (in the sense of string=?) to the string
that was passed to string->symbol. It is an error to apply mutation procedures such
as string-set! to strings returned by this procedure.
) "flying-fish"
(symbol->string ’flying-fish)
) "martin"
(symbol->string ’Martin)
(symbol->string (string->symbol "Malvina"))
) "Malvina"
Note that two distinct uninterned symbols can have the same name.
intern string
procedure
Returns the interned symbol whose name is string. Converts string to the standard
alphabetic case before generating the symbol. This is the preferred way to create
interned symbols, as it guarantees the following independent of which case the implementation uses for symbols’ names:
(eq? ’bitBlt (intern "bitBlt")) )
#t
The user should take care that string obeys the rules for identifiers (see Section 1.3.3
[Identifiers], page 10), otherwise the resulting symbol cannot be read as itself.
intern-soft string
procedure
Returns the interned symbol whose name is string. Converts string to the standard
alphabetic case before generating the symbol. If no such interned symbol exists,
returns #f.
This is exactly like intern, except that it will not create an interned symbol, but
only returns symbols that already exist.
string->symbol string
procedure
Returns the interned symbol whose name is string. Although you can use this procedure to create symbols with names containing special characters or lowercase letters,
it’s usually a bad idea to create such symbols because they cannot be read as themselves. See symbol->string.
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) #t
(eq? ’mISSISSIppi ’mississippi)
(string->symbol "mISSISSIppi")
) the symbol with the name "mISSISSIppi"
(eq? ’bitBlt (string->symbol "bitBlt")) ) #f
(eq? ’JollyWog
(string->symbol
) #t
(symbol->string ’JollyWog)))
(string=? "K. Harper, M.D."
(symbol->string
(string->symbol
) #t
"K. Harper, M.D.")))
string->uninterned-symbol string
procedure
Returns a newly allocated uninterned symbol whose name is string. It is unimportant
what case or characters are used in string.
Note: this is the fastest way to make a symbol.
generate-uninterned-symbol [object]
procedure
Returns a newly allocated uninterned symbol that is guaranteed to be di↵erent from
any other object. The symbol’s name consists of a prefix string followed by the (exact
non-negative integer) value of an internal counter. The counter is initially zero, and
is incremented after each call to this procedure.
The optional argument object is used to control how the symbol is generated. It may
take one of the following values:
• If object is omitted or #f, the prefix is "G".
• If object is an exact non-negative integer, the internal counter is set to that
integer prior to generating the result.
• If object is a string, it is used as the prefix.
• If object is a symbol, its name is used as the prefix.
(generate-uninterned-symbol)
) #[uninterned-symbol 31 G0]
(generate-uninterned-symbol)
) #[uninterned-symbol 32 G1]
(generate-uninterned-symbol ’this)
) #[uninterned-symbol 33 this2]
(generate-uninterned-symbol)
) #[uninterned-symbol 34 G3]
(generate-uninterned-symbol 100)
) #[uninterned-symbol 35 G100]
(generate-uninterned-symbol)
) #[uninterned-symbol 36 G101]
symbol-append symbol . . .
procedure
Returns the interned symbol whose name is formed by concatenating the names of
the given symbols. This procedure preserves the case of the names of its arguments,
so if one or more of the arguments’ names has non-standard case, the result will also
have non-standard case.
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131
) foo-bar
(symbol-append ’foo- ’bar)
;; the arguments may be uninterned:
(symbol-append ’foo- (string->uninterned-symbol "baz"))
) foo-baz
;; the result has the same case as the arguments:
) foo-BAZ
(symbol-append ’foo- (string->symbol "BAZ"))
symbol-hash symbol
procedure
Returns a hash number for symbol, which is computed by calling string-hash on
symbol’s name. The hash number is an exact non-negative integer.
symbol-hash-mod symbol modulus
procedure
Modulus must be an exact positive integer. Equivalent to
(modulo (symbol-hash symbol) modulus)
This procedure is provided for convenience in constructing hash tables. However,
it is normally preferable to use make-eq-hash-table to build hash tables keyed by
symbols, because eq? hash tables are much faster.
symbol<? symbol1 symbol2
This procedure computes a total order on symbols. It is equivalent to
(string<? (symbol->string symbol1)
(symbol->string symbol2))
procedure
10.3 Cells
Cells are data structures similar to pairs except that they have only one element. They
are useful for managing state.
cell? object
procedure
make-cell object
procedure
cell-contents cell
procedure
set-cell-contents! cell object
procedure
Returns #t if object is a cell; otherwise returns #f.
Returns a newly allocated cell whose contents is object.
Returns the current contents of cell.
Alters the contents of cell to be object. Returns an unspecified value.
bind-cell-contents! cell object thunk
procedure
Alters the contents of cell to be object, calls thunk with no arguments, then restores the original contents of cell and returns the value returned by thunk. This is
completely equivalent to dynamic binding of a variable, including the behavior when
continuations are used (see Section 2.3 [Dynamic Binding], page 18).
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10.4 Records
MIT Scheme provides a record abstraction, which is a simple and flexible mechanism
for building structures with named components. Records can be defined and accessed using
the procedures defined in this section. A less flexible but more concise way to manipulate
records is to use the define-structure special form (see Section 2.10 [Structure Definitions], page 29).
make-record-type type-name field-names
procedure
Returns a record-type descriptor, a value representing a new data type, disjoint from
all others. The type-name argument must be a string, but is only used for debugging
purposes (such as the printed representation of a record of the new type). The fieldnames argument is a list of symbols naming the fields of a record of the new type.
It is an error if the list contains any duplicates. It is unspecified how record-type
descriptors are represented.
record-constructor record-type [field-names]
procedure
Returns a procedure for constructing new members of the type represented by recordtype. The returned procedure accepts exactly as many arguments as there are symbols
in the given list, field-names; these are used, in order, as the initial values of those
fields in a new record, which is returned by the constructor procedure. The values
of any fields not named in the list of field-names are unspecified. The field-names
argument defaults to the list of field-names in the call to make-record-type that
created the type represented by record-type; if the field-names argument is provided,
it is an error if it contains any duplicates or any symbols not in the default list.
record-predicate record-type
procedure
Returns a procedure for testing membership in the type represented by record-type.
The returned procedure accepts exactly one argument and returns #t if the argument
is a member of the indicated record type; it returns #f otherwise.
record-accessor record-type field-name
procedure
Returns a procedure for reading the value of a particular field of a member of the type
represented by record-type. The returned procedure accepts exactly one argument
which must be a record of the appropriate type; it returns the current value of the
field named by the symbol field-name in that record. The symbol field-name must
be a member of the list of field names in the call to make-record-type that created
the type represented by record-type.
record-modifier record-type field-name
procedure
Returns a procedure for writing the value of a particular field of a member of the type
represented by record-type. The returned procedure accepts exactly two arguments:
first, a record of the appropriate type, and second, an arbitrary Scheme value; it
modifies the field named by the symbol field-name in that record to contain the given
value. The returned value of the modifier procedure is unspecified. The symbol fieldname must be a member of the list of field names in the call to make-record-type
that created the type represented by record-type.
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133
record? object
procedure
Returns #t if object is a record of any type and #f otherwise. Note that record?
may be true of any Scheme value; of course, if it returns #t for some particular value,
then record-type-descriptor is applicable to that value and returns an appropriate
descriptor.
record-type-descriptor record
procedure
Returns the record-type descriptor representing the type of record. That is, for example, if the returned descriptor were passed to record-predicate, the resulting
predicate would return #t when passed record. Note that it is not necessarily the
case that the returned descriptor is the one that was passed to record-constructor
in the call that created the constructor procedure that created record.
record-type? object
Returns #t if object is a record-type descriptor; otherwise returns #f.
procedure
record-type-name record-type
procedure
Returns the type name associated with the type represented by record-type. The
returned value is eqv? to the type-name argument given in the call to make-recordtype that created the type represented by record-type.
record-type-field-names record-type
procedure
Returns a list of the symbols naming the fields in members of the type represented
by record-type. The returned value is equal? to the field-names argument given in
the call to make-record-type that created the type represented by record-type.3
10.5 Promises
delay expression
special form
The delay construct is used together with the procedure force to implement lazy
evaluation or call by need. (delay expression) returns an object called a promise
which at some point in the future may be asked (by the force procedure) to evaluate
expression and deliver the resulting value.
force promise
procedure
Forces the value of promise. If no value has been computed for the promise, then a
value is computed and returned. The value of the promise is cached (or “memoized”)
so that if it is forced a second time, the previously computed value is returned without
any recomputation.
) 3
(force (delay (+ 1 2)))
(let ((p (delay (+ 1 2))))
(list (force p) (force p)))
3
In MIT Scheme, the returned list is always newly allocated.
)
(3 3)
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(define head car)
(define tail
(lambda (stream)
(force (cdr stream))))
(define a-stream
(letrec ((next
(lambda (n)
(cons n (delay (next (+ n 1)))))))
(next 0)))
(head (tail (tail a-stream)))
promise? object
)
Returns #t if object is a promise; otherwise returns #f.
2
procedure
promise-forced? promise
procedure
Returns #t if promise has been forced and its value cached; otherwise returns #f.
promise-value promise
procedure
If promise has been forced and its value cached, this procedure returns the cached
value. Otherwise, an error is signalled.
force and delay are mainly intended for programs written in functional style. The
following examples should not be considered to illustrate good programming style, but they
illustrate the property that the value of a promise is computed at most once.
(define count 0)
(define p
(delay
(begin
(set! count (+ count 1))
(* x 3))))
(define x 5)
)
count
)
p
)
(force p)
)
p
)
count
)
(force p)
)
count
Here is a possible implementation of delay and force.
(delay expression)
to have the same meaning as the procedure call
(make-promise (lambda () expression))
where make-promise is defined as follows:
0
#[promise 54]
15
#[promise 54]
1
15
1
We define the expression
Chapter 10: Miscellaneous Datatypes
135
(define make-promise
(lambda (proc)
(let ((already-run? #f)
(result #f))
(lambda ()
(cond ((not already-run?)
(set! result (proc))
(set! already-run? #t)))
result))))
Promises are implemented here as procedures of no arguments, and force simply calls
its argument.
(define force
(lambda (promise)
(promise)))
Various extensions to this semantics of delay and force are supported in some implementations (none of these are currently supported in MIT Scheme):
• Calling force on an object that is not a promise may simply return the object.
• It may be the case that there is no means by which a promise can be operationally
distinguished from its forced value. That is, expressions like the following may evaluate
to either #t or #f, depending on the implementation:
) unspecified
(eqv? (delay 1) 1)
) unspecified
(pair? (delay (cons 1 2)))
• Some implementations will implement “implicit forcing”, where the value of a promise
is forced by primitive procedures like car and +:
) 34
(+ (delay (* 3 7)) 13)
10.6 Streams
In addition to promises, MIT Scheme supports a higher-level abstraction called streams.
Streams are similar to lists, except that the tail of a stream is not computed until it is
referred to. This allows streams to be used to represent infinitely long lists.
stream object . . .
procedure
Returns a newly allocated stream whose elements are the arguments. Note that the
expression (stream) returns the empty stream, or end-of-stream marker.
list->stream list
procedure
Returns a newly allocated stream whose elements are the elements of list. Equivalent
to (apply stream list).
stream->list stream
procedure
Returns a newly allocated list whose elements are the elements of stream. If stream
has infinite length this procedure will not terminate. This could have been defined
by
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(define (stream->list stream)
(if (stream-null? stream)
’()
(cons (stream-car stream)
(stream->list (stream-cdr stream)))))
cons-stream object expression
special form
Returns a newly allocated stream pair. Equivalent to (cons object (delay expression)).
stream-pair? object
procedure
Returns #t if object is a pair whose cdr contains a promise. Otherwise returns #f.
This could have been defined by
(define (stream-pair? object)
(and (pair? object)
(promise? (cdr object))))
stream-car stream
stream-first stream
procedure
procedure
Returns the first element in stream. stream-car is equivalent to car. stream-first
is a synonym for stream-car.
stream-cdr stream
stream-rest stream
procedure
procedure
Returns the first tail of stream. Equivalent to (force (cdr stream)). stream-rest
is a synonym for stream-cdr.
stream-null? stream
procedure
Returns #t if stream is the end-of-stream marker; otherwise returns #f. This is
equivalent to null?, but should be used whenever testing for the end of a stream.
stream-length stream
procedure
Returns the number of elements in stream. If stream has an infinite number of
elements this procedure will not terminate. Note that this procedure forces all of the
promises that comprise stream.
stream-ref stream k
procedure
Returns the element of stream that is indexed by k; that is, the kth element. K must
be an exact non-negative integer strictly less than the length of stream.
stream-head stream k
procedure
Returns the first k elements of stream as a list. K must be an exact non-negative
integer strictly less than the length of stream.
stream-tail stream k
procedure
Returns the tail of stream that is indexed by k; that is, the kth tail. This is equivalent
to performing stream-cdr k times. K must be an exact non-negative integer strictly
less than the length of stream.
Chapter 10: Miscellaneous Datatypes
137
stream-map procedure stream stream . . .
procedure
Returns a newly allocated stream, each element being the result of invoking procedure
with the corresponding elements of the streams as its arguments.
10.7 Weak Pairs
Weak pairs are a mechanism for building data structures that point at objects without
protecting them from garbage collection. The car of a weak pair holds its pointer weakly,
while the cdr holds its pointer in the normal way. If the object in the car of a weak pair is
not held normally by any other data structure, it will be garbage-collected.
Note: weak pairs are not pairs; that is, they do not satisfy the predicate pair?.
weak-pair? object
Returns #t if object is a weak pair; otherwise returns #f.
procedure
weak-cons car cdr
procedure
Allocates and returns a new weak pair, with components car and cdr. The car
component is held weakly.
weak-pair/car? weak-pair
procedure
This predicate returns #f if the car of weak-pair has been garbage-collected; otherwise
returns #t. In other words, it is true if weak-pair has a valid car component.
weak-car weak-pair
procedure
Returns the car component of weak-pair. If the car component has been garbagecollected, this operation returns #f, but it can also return #f if that is the value that
was stored in the car.
Normally, weak-pair/car? is used to determine if weak-car would return a valid value.
An obvious way of doing this would be:
(if (weak-pair/car? x)
(weak-car x)
...)
However, since a garbage collection could occur between the call to weak-pair/car? and
weak-car, this would not always work correctly. Instead, the following should be used,
which always works:
(or (weak-car x)
(and (not (weak-pair/car? x))
...))
The reason that the latter expression works is that weak-car returns #f in just two
instances: when the car component is #f, and when the car component has been garbagecollected. In the former case, if a garbage collection happens between the two calls, it won’t
matter, because #f will never be garbage-collected. And in the latter case, it also won’t
matter, because the car component no longer exists and cannot be a↵ected by the garbage
collector.
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weak-set-car! weak-pair object
procedure
Sets the car component of weak-pair to object and returns an unspecified result.
weak-cdr weak-pair
Returns the cdr component of weak-cdr.
weak-set-cdr! weak-pair object
procedure
procedure
Sets the cdr component of weak-pair to object and returns an unspecified result.
Chapter 11: Associations
139
11 Associations
MIT Scheme provides several mechanisms for associating objects with one another. Each
of these mechanisms creates a link between one or more objects, called keys, and some other
object, called a datum. Beyond this common idea, however, each of the mechanisms has
various di↵erent properties that make it appropriate in di↵erent situations:
• Association lists are one of Lisp’s oldest association mechanisms. Because they are
made from ordinary pairs, they are easy to build and manipulate, and very flexible in
use. However, the average lookup time for an association list is linear in the number
of associations.
• 1D tables have a very simple interface, making them easy to use, and o↵er the feature
that they do not prevent their keys from being reclaimed by the garbage collector. Like
association lists, their average lookup time is linear in the number of associations; but
1D tables aren’t as flexible.
• The association table is MIT Scheme’s equivalent to the property lists of Lisp. It has
the advantages that the keys may be any type of object and that it does not prevent the
keys from being reclaimed by the garbage collector. However, two linear-time lookups
must be performed, one for each key, whereas for traditional property lists only one
lookup is required for both keys.
• Hash tables are a powerful mechanism with constant-time access to large amounts of
data. Hash tables are not as flexible as association lists, but because their access times
are independent of the number of associations in the table, for most applications they
are the mechanism of choice.
• Balanced binary trees are another association mechanism that is useful for applications
in which the keys are ordered. Binary trees have access times that are proportional to
the logarithm of the number of associations in the tree. While they aren’t as fast as
hash tables, they o↵er the advantage that the contents of the tree can be converted to
a sorted alist in linear time. Additionally, two trees can be compared for equality in
worst-case linear time.
• Red-Black trees are a kind of balanced binary tree. The implementation supports
destructive insertion and deletion operations with a good constant factor.
• Weight-Balanced trees are a kind of balanced binary tree. The implementation provides
non-destructive operations. There is a comprehensive set of operations, including: a
constant-time size operation; many high-level operations such as the set operations
union, intersection and di↵erence; and indexing of elements by position.
11.1 Association Lists
An association list, or alist, is a data structure used very frequently in Scheme. An alist
is a list of pairs, each of which is called an association. The car of an association is called
the key.
An advantage of the alist representation is that an alist can be incrementally augmented
simply by adding new entries to the front. Moreover, because the searching procedures
assv et al. search the alist in order, new entries can “shadow” old entries. If an alist is
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viewed as a mapping from keys to data, then the mapping can be not only augmented but
also altered in a non-destructive manner by adding new entries to the front of the alist.1
alist? object
procedure
Returns #t if object is an association list (including the empty list); otherwise returns
#f. Any object satisfying this predicate also satisfies list?.
assq object alist
assv object alist
assoc object alist
procedure
procedure
procedure
These procedures find the first pair in alist whose car field is object, and return that
pair; the returned pair is always an element of alist, not one of the pairs from which
alist is composed. If no pair in alist has object as its car, #f (n.b.: not the empty
list) is returned. assq uses eq? to compare object with the car fields of the pairs in
alist, while assv uses eqv? and assoc uses equal?.2
(define e ’((a 1) (b 2) (c 3)))
) (a 1)
(assq ’a e)
) (b 2)
(assq ’b e)
) #f
(assq ’d e)
) #f
(assq (list ’a) ’(((a)) ((b)) ((c))))
(assoc (list ’a) ’(((a)) ((b)) ((c)))) ) ((a))
) unspecified
(assq 5 ’((2 3) (5 7) (11 13)))
) (5 7)
(assv 5 ’((2 3) (5 7) (11 13)))
association-procedure predicate selector
procedure
Returns an association procedure that is similar to assv, except that selector (a procedure of one argument) is used to select the key from the association, and predicate
(an equivalence predicate) is used to compare the key to the given item. This can be
used to make association lists whose elements are, say, vectors instead of pairs (also
see Section 7.6 [Searching Lists], page 113).
For example, here is how assv could be implemented:
(define assv (association-procedure eqv? car))
Another example is a “reverse association” procedure:
(define rassv (association-procedure eqv? cdr))
del-assq object alist
del-assv object alist
del-assoc object alist
procedure
procedure
procedure
These procedures return a newly allocated copy of alist in which all associations with
keys equal to object have been removed. Note that while the returned copy is a newly
allocated list, the association pairs that are the elements of the list are shared with
alist, not copied. del-assq uses eq? to compare object with the keys, while del-assv
uses eqv? and del-assoc uses equal?.
1
2
This introduction is taken from Common Lisp, The Language, second edition, p. 431.
Although they are often used as predicates, assq, assv, and assoc do not have question marks in their
names because they return useful values rather than just #t or #f.
Chapter 11: Associations
141
(define a
’((butcher . "231 e22nd St.")
(baker . "515 w23rd St.")
(hardware . "988 Lexington Ave.")))
(del-assq ’baker a)
)
((butcher . "231 e22nd St.")
(hardware . "988 Lexington Ave."))
del-assq! object alist
del-assv! object alist
del-assoc! object alist
procedure
procedure
procedure
These procedures remove from alist all associations with keys equal to object. They
return the resulting list. del-assq! uses eq? to compare object with the keys, while
del-assv! uses eqv? and del-assoc! uses equal?. These procedures are like delassq, del-assv, and del-assoc, respectively, except that they destructively modify
alist.
delete-association-procedure deletor predicate selector
procedure
This returns a deletion procedure similar to del-assv or del-assq!. The predicate
and selector arguments are the same as those for association-procedure, while the
deletor argument should be either the procedure list-deletor (for non-destructive
deletions), or the procedure list-deletor! (for destructive deletions).
For example, here is a possible implementation of del-assv:
(define del-assv
(delete-association-procedure list-deletor eqv? car))
alist-copy alist
procedure
Returns a newly allocated copy of alist. This is similar to list-copy except that the
“association” pairs, i.e. the elements of the list alist, are also copied. alist-copy
could have been implemented like this:
(define (alist-copy alist)
(if (null? alist)
’()
(cons (cons (car (car alist)) (cdr (car alist)))
(alist-copy (cdr alist)))))
11.2 1D Tables
1D tables (“one-dimensional” tables) are similar to association lists. In a 1D table, unlike
an association list, the keys of the table are held weakly: if a key is garbage-collected, its
associated value in the table is removed. 1D tables compare their keys for equality using
eq?.
1D tables can often be used as a higher-performance alternative to the two-dimensional
association table (see Section 11.3 [The Association Table], page 142). If one of the keys
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being associated is a compound object such as a vector, a 1D table can be stored in one
of the vector’s slots. Under these circumstances, accessing items in a 1D table will be
comparable in performance to using a property list in a conventional Lisp.
make-1d-table
Returns a newly allocated empty 1D table.
procedure
1d-table? object
procedure
Returns #t if object is a 1D table, otherwise returns #f. Any object that satisfies this
predicate also satisfies list?.
1d-table/put! 1d-table key datum
procedure
Creates an association between key and datum in 1d-table. Returns an unspecified
value.
1d-table/remove! 1d-table key
procedure
Removes any association for key in 1d-table and returns an unspecified value.
1d-table/get 1d-table key default
procedure
Returns the datum associated with key in 1d-table. If there is no association for key,
default is returned.
1d-table/lookup 1d-table key if-found if-not-found
procedure
If-found must be a procedure of one argument, and if-not-found must be a procedure
of no arguments. If 1d-table contains an association for key, if-found is invoked on
the datum of the association. Otherwise, if-not-found is invoked with no arguments.
In either case, the result of the invoked procedure is returned as the result of 1dtable/lookup.
1d-table/alist 1d-table
procedure
Returns a newly allocated association list that contains the same information as 1dtable.
11.3 The Association Table
MIT Scheme provides a generalization of the property-list mechanism found in most
other implementations of Lisp: a global two-dimensional association table. This table
is indexed by two keys, called x-key and y-key in the following procedure descriptions.
These keys and the datum associated with them can be arbitrary objects. eq? is used to
discriminate keys.
Think of the association table as a matrix: a single datum can be accessed using both
keys, a column using x-key only, and a row using y-key only.
2d-put! x-key y-key datum
procedure
Makes an entry in the association table that associates datum with x-key and y-key.
Returns an unspecified result.
Chapter 11: Associations
143
2d-remove! x-key y-key
procedure
If the association table has an entry for x-key and y-key, it is removed. Returns an
unspecified result.
2d-get x-key y-key
procedure
Returns the datum associated with x-key and y-key. Returns #f if no such association
exists.
2d-get-alist-x x-key
procedure
Returns an association list of all entries in the association table that are associated
with x-key. The result is a list of (y-key . datum) pairs. Returns the empty list if
no entries for x-key exist.
(2d-put! ’foo ’bar 5)
(2d-put! ’foo ’baz 6)
) ((baz . 6) (bar . 5))
(2d-get-alist-x ’foo)
2d-get-alist-y y-key
procedure
Returns an association list of all entries in the association table that are associated
with y-key. The result is a list of (x-key . datum) pairs. Returns the empty list if
no entries for y-key exist.
(2d-put! ’bar ’foo 5)
(2d-put! ’baz ’foo 6)
) ((baz . 6) (bar . 5))
(2d-get-alist-y ’foo)
11.4 Hash Tables
Hash tables are a fast, powerful mechanism for storing large numbers of associations.
MIT Scheme’s hash tables feature automatic resizing, customizable growth parameters, and
customizable hash procedures.
The average times for the insertion, deletion, and lookup operations on a hash table are
bounded by a constant. The space required by the table is proportional to the number of associations in the table; the constant of proportionality is described below (see Section 11.4.3
[Resizing of Hash Tables], page 146).
(Previously, the hash-table implementation was a run-time-loadable option, but as of
release 7.7.0 it is loaded by default. It’s no longer necessary to call load-option prior to
using hash tables.)
11.4.1 Construction of Hash Tables
The next few procedures are hash-table constructors. All hash table constructors are
procedures that accept one optional argument, initial-size, and return a newly allocated
hash table. If initial-size is given, it must be an exact non-negative integer or #f. The
meaning of initial-size is discussed below (see Section 11.4.3 [Resizing of Hash Tables],
page 146).
Hash tables are normally characterized by two things: the equivalence predicate that is
used to compare keys, and whether or not the table allows its keys to be reclaimed by the
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MIT Scheme Reference
garbage collector. If a table prevents its keys from being reclaimed by the garbage collector,
it is said to hold its keys strongly; otherwise it holds its keys weakly (see Section 10.7 [Weak
Pairs], page 137).
make-eq-hash-table [initial-size]
procedure
Returns a newly allocated hash table that accepts arbitrary objects as keys, and
compares those keys with eq?. The keys are held weakly. These are the fastest of the
standard hash tables.
make-eqv-hash-table [initial-size]
procedure
Returns a newly allocated hash table that accepts arbitrary objects as keys, and
compares those keys with eqv?. The keys are held weakly, except that booleans,
characters, and numbers are held strongly. These hash tables are a little slower than
those made by make-eq-hash-table.
make-equal-hash-table [initial-size]
procedure
Returns a newly allocated hash table that accepts arbitrary objects as keys, and
compares those keys with equal?. The keys are held strongly. These hash tables are
quite a bit slower than those made by make-eq-hash-table.
make-string-hash-table [initial-size]
procedure
Returns a newly allocated hash table that accepts character strings as keys, and
compares them with string=?. The keys are held strongly.
The next two procedures are used to create new hash-table constructors. All of the
above hash table constructors, with the exception of make-eqv-hash-table, could have
been created by calls to these “constructor-constructors”; see the examples below.
strong-hash-table/constructor key-hash key=? [rehash-after-gc?]
weak-hash-table/constructor key-hash key=? [rehash-after-gc?]
procedure
procedure
Each of these procedures accepts two arguments and returns a hash-table constructor.
The key=? argument is an equivalence predicate for the keys of the hash table. The
key-hash argument is a procedure that computes a hash number. Specifically, keyhash accepts two arguments, a key and an exact positive integer (the modulus), and
returns an exact non-negative integer that is less than the modulus.
The optional argument rehash-after-gc?, if true, says that the values returned by keyhash might change after a garbage collection. If so, the hash-table implementation
arranges for the table to be rehashed when necessary. (See Section 11.4.4 [Address
Hashing], page 149, for information about hash procedures that have this property.)
Otherwise, it is assumed that key-hash always returns the same value for the same
arguments. The default value of this argument is #f.
The
constructors
returned
by
strong-hash-table/constructor
make
hash tables that hold their keys strongly.
The constructors returned by
weak-hash-table/constructor make hash tables that hold their keys weakly.
Chapter 11: Associations
145
Some examples showing how some standard hash-table constructors could have been
defined:
(define make-eq-hash-table
(weak-hash-table/constructor eq-hash-mod eq? #t))
(define make-equal-hash-table
(strong-hash-table/constructor equal-hash-mod equal? #t))
(define make-string-hash-table
(strong-hash-table/constructor string-hash-mod string=? #f))
The following procedure is sometimes useful in conjunction with weak hash tables. Normally it is not needed, because such hash tables clean themselves automatically as they are
used.
hash-table/clean! hash-table
procedure
If hash-table is a type of hash table that holds its keys weakly, this procedure recovers
any space that was being used to record associations for objects that have been
reclaimed by the garbage collector. Otherwise, this procedure does nothing. In either
case, it returns an unspecified result.
11.4.2 Basic Hash Table Operations
The procedures described in this section are the basic operations on hash tables. They
provide the functionality most often needed by programmers. Subsequent sections describe
other operations that provide additional functionality needed by some applications.
hash-table? object
Returns #t if object is a hash table, otherwise returns #f.
procedure
hash-table/put! hash-table key datum
procedure
Associates datum with key in hash-table and returns an unspecified result. The
average time required by this operation is bounded by a constant.
hash-table/get hash-table key default
procedure
Returns the datum associated with key in hash-table. If there is no association for
key, default is returned. The average time required by this operation is bounded by
a constant.
hash-table/remove! hash-table key
procedure
If hash-table has an association for key, removes it. Returns an unspecified result.
The average time required by this operation is bounded by a constant.
hash-table/clear! hash-table
procedure
Removes all associations in hash-table and returns an unspecified result. The average
and worst-case times required by this operation are bounded by a constant.
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hash-table/count hash-table
procedure
Returns the number of associations in hash-table as an exact non-negative integer.
If hash-table holds its keys weakly, this is a conservative upper bound that may
count some associations whose keys have recently been reclaimed by the garbage
collector. The average and worst-case times required by this operation are bounded
by a constant.
hash-table->alist hash-table
procedure
Returns the contents of hash-table as a newly allocated alist. Each element of the
alist is a pair (key . datum) where key is one of the keys of hash-table, and datum
is its associated datum. The average and worst-case times required by this operation
are linear in the number of associations in the table.
hash-table/key-list hash-table
procedure
Returns a newly allocated list of the keys in hash-table. The average and worst-case
times required by this operation are linear in the number of associations in the table.
hash-table/datum-list hash-table
procedure
Returns a newly allocated list of the datums in hash-table. Each element of the list
corresponds to one of the associations in hash-table; if the table contains multiple
associations with the same datum, so will this list. The average and worst-case times
required by this operation are linear in the number of associations in the table.
hash-table/for-each hash-table procedure
procedure
Procedure must be a procedure of two arguments. Invokes procedure once for each
association in hash-table, passing the association’s key and datum as arguments,
in that order. Returns an unspecified result. Procedure must not modify hashtable, with one exception: it is permitted to call hash-table/remove! to remove the
association being processed.
The following procedure is an alternate form of hash-table/get that is useful in some
situations. Usually, hash-table/get is preferable because it is faster.
hash-table/lookup hash-table key if-found if-not-found
procedure
If-found must be a procedure of one argument, and if-not-found must be a procedure
of no arguments. If hash-table contains an association for key, if-found is invoked on
the datum of the association. Otherwise, if-not-found is invoked with no arguments.
In either case, the result yielded by the invoked procedure is returned as the result
of hash-table/lookup (hash-table/lookup reduces into the invoked procedure, i.e.
calls it tail-recursively). The average time required by this operation is bounded by
a constant.
11.4.3 Resizing of Hash Tables
Normally, hash tables automatically resize themselves according to need. Because of
this, the programmer need not be concerned with management of the table’s size. However,
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147
some limited control over the table’s size is provided, which will be discussed below. This
discussion involves two concepts, usable size and physical size, which we will now define.
The usable size of a hash table is the number of associations that the table can hold at
a given time. If the number of associations in the table exceeds the usable size, the table
will automatically grow, increasing the usable size to a new value that is sufficient to hold
the associations.
The physical size is an abstract measure of a hash table that specifies how much space
is allocated to hold the associations of the table. The physical size is always greater than
or equal to the usable size. The physical size is not interesting in itself; it is interesting
only for its e↵ect on the performance of the hash table. While the average performance
of a hash-table lookup is bounded by a constant, the worst-case performance is not. For
a table containing a given number of associations, increasing the physical size of the table
decreases the probability that worse-than-average performance will occur.
The physical size of a hash table is statistically related to the number of associations.
However, it is possible to place bounds on the physical size, and from this to estimate the
amount of space used by the table:
(define (hash-table-space-bounds count rehash-size rehash-threshold)
(let ((tf (/ 1 rehash-threshold)))
(values (if (exact-integer? rehash-size)
(- (* count (+ 4 tf))
(* tf (+ rehash-size rehash-size)))
(* count (+ 4 (/ tf (* rehash-size rehash-size)))))
(* count (+ 4 tf)))))
What this formula shows is that, for a “normal” rehash size (that is, not an exact integer),
the amount of space used by the hash table is proportional to the number of associations
in the table. The constant of proportionality varies statistically, with the low bound being
(+ 4 (/ (/ 1 rehash-threshold) (* rehash-size rehash-size)))
and the high bound being
(+ 4 (/ 1 rehash-threshold))
which, for the default values of these parameters, are 4.25 and 5, respectively. Reducing
the rehash size will tighten these bounds, but increases the amount of time spent resizing,
so you can see that the rehash size gives some control over the time-space tradeo↵ of the
table.
The programmer can control the size of a hash table by means of three parameters:
• Each table’s initial-size may be specified when the table is created.
• Each table has a rehash size that specifies how the size of the table is changed when it
is necessary to grow or shrink the table.
• Each table has a rehash threshold that specifies the relationship of the table’s physical
size to its usable size.
If the programmer knows that the table will initially contain a specific number of items,
initial-size can be given when the table is created. If initial-size is an exact non-negative
integer, it specifies the initial usable size of the hash table; the table will not change size
until the number of items in the table exceeds initial-size, after which automatic resizing is
enabled and initial-size no longer has any e↵ect. Otherwise, if initial-size is not given or
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is #f, the table is initialized to an unspecified size and automatic resizing is immediately
enabled.
The rehash size specifies how much to increase the usable size of the hash table when it
becomes full. It is either an exact positive integer, or a real number greater than one. If it
is an integer, the new size is the sum of the old size and the rehash size. Otherwise, it is a
real number, and the new size is the product of the old size and the rehash size. Increasing
the rehash size decreases the average cost of an insertion, but increases the average amount
of space used by the table. The rehash size of a table may be altered dynamically by the
application in order to optimize the resizing of the table; for example, if the table will
grow quickly for a known period and afterwards will not change size, performance might be
improved by using a large rehash size during the growth phase and a small one during the
static phase. The default rehash size of a newly constructed hash table is 2.0.
Warning: The use of an exact positive integer for a rehash size is almost always undesirable; this option is provided solely for compatibility with the Common Lisp hash-table
mechanism. The reason for this has to do with the time penalty for resizing the hash table.
The time needed to resize a hash table is proportional to the number of associations in
the table. This resizing cost is amortized across the insertions required to fill the table
to the point where it needs to grow again. If the table grows by an amount proportional
to the number of associations, then the cost of resizing and the increase in size are both
proportional to the number of associations, so the amortized cost of an insertion operation
is still bounded by a constant. However, if the table grows by a constant amount, this is
not true: the amortized cost of an insertion is not bounded by a constant. Thus, using a
constant rehash size means that the average cost of an insertion increases proportionally to
the number of associations in the hash table.
The rehash threshold is a real number, between zero exclusive and one inclusive, that
specifies the ratio between a hash table’s usable size and its physical size. Decreasing the
rehash threshold decreases the probability of worse-than-average insertion, deletion, and
lookup times, but increases the physical size of the table for a given usable size. The
default rehash threshold of a newly constructed hash table is 1.
hash-table/size hash-table
procedure
Returns the usable size of hash-table as an exact positive integer. This is the number
of associations that hash-table can hold before it will grow.
hash-table/rehash-size hash-table
Returns the rehash size of hash-table.
procedure
set-hash-table/rehash-size! hash-table x
procedure
X must be either an exact positive integer, or a real number that is greater than
one. Sets the rehash size of hash-table to x and returns an unspecified result. This
operation adjusts the “shrink threshold” of the table; the table might shrink if the
number of associations is less than the new threshold.
hash-table/rehash-threshold hash-table
Returns the rehash threshold of hash-table.
procedure
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set-hash-table/rehash-threshold! hash-table x
procedure
X must be a real number between zero exclusive and one inclusive. Sets the rehash
threshold of hash-table to x and returns an unspecified result. This operation does
not change the usable size of the table, but it usually changes the physical size of the
table, which causes the table to be rehashed.
11.4.4 Address Hashing
The procedures described in this section may be used to make very efficient key-hashing
procedures for arbitrary objects. All of these procedures are based on address hashing,
which uses the address of an object as its hash number. The great advantage of address
hashing is that converting an arbitrary object to a hash number is extremely fast and takes
the same amount of time for any object.
The disadvantage of address hashing is that the garbage collector changes the addresses
of most objects. The hash-table implementation compensates for this disadvantage by
automatically rehashing tables that use address hashing when garbage collections occur.
Thus, in order to use these procedures for key hashing, it is necessary to tell the hashtable implementation (by means of the rehash-after-gc? argument to the “constructorconstructor” procedure) that the hash numbers computed by your key-hashing procedure
must be recomputed after a garbage collection.
eq-hash object
eqv-hash object
equal-hash object
procedure
procedure
procedure
These procedures return a hash number for object. The result is always a nonnegative integer, and in the case of eq-hash, a non-negative fixnum. Two objects
that are equivalent according to eq?, eqv?, or equal?, respectively, will produce the
same hash number when passed as arguments to these procedures, provided that the
garbage collector does not run during or between the two calls.
The following procedures are the key-hashing procedures used by the standard addresshash-based hash tables.
eq-hash-mod object modulus
procedure
eqv-hash-mod object modulus
procedure
This procedure is the key-hashing procedure used by make-eq-hash-table.
This procedure is the key-hashing procedure used by make-eqv-hash-table.
equal-hash-mod object modulus
procedure
This procedure is the key-hashing procedure used by make-equal-hash-table.
11.4.5 Low-Level Hash Table Operations
The procedures in this section allow the programmer to control some of the internal
structure of a hash table. Normally, hash tables maintain associations between keys and
datums using pairs or weak pairs. These procedures allow the programmer to specify the
use of some other data structure to maintain the association. In this section, the data
structure that represents an association in a hash table is called an entry.
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hash-table/constructor key-hash key=? make-entry entry-valid?
procedure
entry-key entry-datum set-entry-datum! [rehash-after-gc?]
Creates and returns a hash-table constructor procedure (see Section 11.4.1 [Construction of Hash Tables], page 143). The arguments define the characteristics of the hash
table as follows:
key-hash
The hashing procedure. A procedure that accepts two arguments, a key
and an exact positive integer (the modulus), and returns an exact nonnegative integer that is less than the modulus.
key=?
A equivalence predicate that accepts two keys and is true i↵ they are the
same key. If this predicate is true of two keys, then key-hash must return
the same value for each of these keys (given the same modulus in both
cases).
make-entry
entry-valid?
entry-key
A procedure that accepts a key and a datum as arguments and returns a
newly allocated entry.
A procedure that accepts an entry and returns #f i↵ the entry’s key has
been reclaimed by the garbage collector. Instead of a procedure, this may
be #t, which is equivalent to (lambda (entry) #t).
A procedure that accepts an entry as an argument and returns the entry’s
key.
entry-datum
A procedure that accepts an entry as an argument and returns the entry’s
datum.
set-entry-datum!
A procedure that accepts an entry and an object as arguments, modifies
the entry’s datum to be the object, and returns an unspecified result.
rehash-after-gc?
An optional argument that, if true, says the values returned by key-hash
might change after a garbage collection. If so, the hash-table implementation arranges for the table to be rehashed when necessary. (See
Section 11.4.4 [Address Hashing], page 149, for information about hash
procedures that have this property.) Otherwise, it is assumed that keyhash always returns the same value for the same arguments. The default
value of this argument is #f.
For example, here is how the constructors for ordinary hash tables could be defined:
(define (strong-hash-table/constructor key-hash key=?
#!optional rehash-after-gc?)
(hash-table/constructor key-hash key=?
cons #t car cdr set-cdr!
(if (default-object? rehash-after-gc?)
#f
rehash-after-gc?)))
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151
(define (weak-hash-table/constructor key-hash key=?
#!optional rehash-after-gc?)
(hash-table/constructor key-hash key=? weak-cons weak-pair/car?
weak-car weak-cdr weak-set-cdr!
(if (default-object? rehash-after-gc?)
#f
rehash-after-gc?)))
hash-table/key-hash hash-table
hash-table/key=? hash-table
hash-table/make-entry hash-table
hash-table/entry-valid? hash-table
hash-table/entry-key hash-table
hash-table/entry-datum hash-table
hash-table/set-entry-datum! hash-table
procedure
procedure
procedure
procedure
procedure
procedure
procedure
Each procedure returns the value of the corresponding argument that was used to
construct hash-table.
The following procedures return the contents of a hash table as a collection of entries.
While the data structure holding the entries is newly allocated, the entries themselves are
not copied. Since hash table operations can modify these entries, the entries should be
copied if it is desired to keep them while continuing to modify the table.
hash-table/entries-list hash-table
procedure
hash-table/entries-vector hash-table
procedure
Returns a newly allocated list of the entries in hash-table.
Returns a newly allocated vector of the entries in hash-table. Equivalent to
(list->vector (hash-table/entries-list hash-table))
11.5 Object Hashing
The MIT Scheme object-hashing facility provides a mechanism for generating a unique
hash number for an arbitrary object. This hash number, unlike an object’s address, is
unchanged by garbage collection. The object-hashing facility is useful in conjunction with
hash tables, but it may be used for other things as well. In particular, it is used in the
generation of the written representation for many objects (see Section 14.7 [Custom Output],
page 193).
All of these procedures accept an optional argument called table; this table contains
the object-integer associations. If given, this argument must be an object-hash table as
constructed by hash-table/make (see below). If not given, a default table is used.
hash object [table]
procedure
hash associates an exact non-negative integer with object and returns that integer.
If hash was previously called with object as its argument, the integer returned is the
same as was returned by the previous call. hash guarantees that distinct objects (in
the sense of eq?) are associated with distinct integers.
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unhash k [table]
procedure
unhash takes an exact non-negative integer k and returns the object associated with
that integer. If there is no object associated with k, or if the object previously associated with k has been reclaimed by the garbage collector, an error of type conditiontype:bad-range-argument is signalled. In other words, if hash previously returned
k for some object, and that object has not been reclaimed, it is the value of the call
to unhash.
An object that is passed to hash as an argument is not protected from being reclaimed
by the garbage collector. If all other references to that object are eliminated, the object will
be reclaimed. Subsequently calling unhash with the hash number of the (now reclaimed)
object will signal an error.
) unspecified
(define x (cons 0 0))
) 77
(hash x)
) #t
(eqv? (hash x) (hash x))
) unspecified
(define x 0)
(gc-flip)
;force a garbage collection
(unhash 77)
error
object-hashed? object [table]
procedure
This predicate is true if object has an associated hash number. Otherwise it is false.
valid-hash-number? k [table]
procedure
This predicate is true if k is the hash number associated with some object. Otherwise
it is false.
The following two procedures provide a lower-level interface to the object-hashing mechanism.
object-hash object [table [insert?]]
procedure
object-hash is like hash, except that it accepts an additional optional argument,
insert?. If insert? is supplied and is #f, object-hash will return an integer for object
only if there is already an association in the table; otherwise, it will return #f. If
insert? is not supplied, or is not #f, object-hash always returns an integer, creating
an association in the table if necessary.
object-hash additionally treats #f di↵erently than does hash. Calling object-hash
with #f as its argument will return an integer that, when passed to unhash, will signal
an error rather than returning #f. Likewise, valid-hash-number? will return #f for
this integer.
object-unhash k [table]
procedure
object-unhash is like unhash, except that when k is not associated with any object
or was previously associated with an object that has been reclaimed, object-unhash
returns #f. This means that there is an ambiguity in the value returned by objectunhash: if #f is returned, there is no way to tell if k is associated with #f or is not
associated with any object at all.
Finally, this procedure makes new object-hash tables:
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153
hash-table/make
procedure
This procedure creates and returns a new, empty object-hash table that is suitable
for use as the optional table argument to the above procedures. The returned table
contains no associations.
11.6 Red-Black Trees
Balanced binary trees are a useful data structure for maintaining large sets of associations
whose keys are ordered. While most applications involving large association sets should use
hash tables, some applications can benefit from the use of binary trees. Binary trees have
two advantages over hash tables:
• The contents of a binary tree can be converted to an alist, sorted by key, in time
proportional to the number of associations in the tree. A hash table can be converted
into an unsorted alist in linear time; sorting it requires additional time.
• Two binary trees can be compared for equality in linear time. Hash tables, on the other
hand, cannot be compared at all; they must be converted to alists before comparison
can be done, and alist comparison is quadratic unless the alists are sorted.
MIT Scheme provides an implementation of red-black trees.
The red-black
tree-balancing algorithm provides generally good performance because it doesn’t try to
keep the tree very closely balanced. At any given node in the tree, one side of the node
can be twice as high as the other in the worst case. With typical data the tree will remain
fairly well balanced anyway.
A red-black tree takes space that is proportional to the number of associations in the
tree. For the current implementation, the constant of proportionality is eight words per
association.
Red-black trees hold their keys strongly. In other words, if a red-black tree contains an
association for a given key, that key cannot be reclaimed by the garbage collector.
The red-black tree implementation is a run-time-loadable option. To use red-black trees,
execute
(load-option ’rb-tree)
once before calling any of the procedures defined here.
make-rb-tree key=? key<?
procedure
This procedure creates and returns a newly allocated red-black tree. The tree contains no associations. Key=? and key<? are predicates that compare two keys and
determine whether they are equal to or less than one another, respectively. For any
two keys, at most one of these predicates is true.
rb-tree? object
Returns #t if object is a red-black tree, otherwise returns #f.
rb-tree/insert! rb-tree key datum
procedure
procedure
Associates datum with key in rb-tree and returns an unspecified value. If rb-tree
already has an association for key, that association is replaced. The average and
worst-case times required by this operation are proportional to the logarithm of the
number of assocations in rb-tree.
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rb-tree/lookup rb-tree key default
procedure
Returns the datum associated with key in rb-tree. If rb-tree doesn’t contain an
association for key, default is returned. The average and worst-case times required
by this operation are proportional to the logarithm of the number of assocations in
rb-tree.
rb-tree/delete! rb-tree key
procedure
If rb-tree contains an association for key, removes it. Returns an unspecified value.
The average and worst-case times required by this operation are proportional to the
logarithm of the number of assocations in rb-tree.
rb-tree->alist rb-tree
procedure
Returns the contents of rb-tree as a newly allocated alist. Each element of the alist
is a pair (key . datum) where key is one of the keys of rb-tree, and datum is its
associated datum. The alist is sorted by key according to the key<? argument used
to construct rb-tree. The time required by this operation is proportional to the
number of associations in the tree.
rb-tree/key-list rb-tree
procedure
Returns a newly allocated list of the keys in rb-tree. The list is sorted by key according
to the key<? argument used to construct rb-tree. The time required by this operation
is proportional to the number of associations in the tree.
rb-tree/datum-list rb-tree
procedure
Returns a newly allocated list of the datums in rb-tree. Each element of the list
corresponds to one of the associations in rb-tree, so if the tree contains multiple
associations with the same datum, so will this list. The list is sorted by the keys of
the associations, even though they do not appear in the result. The time required by
this operation is proportional to the number of associations in the tree.
This procedure is equivalent to:
(lambda (rb-tree) (map cdr (rb-tree->alist rb-tree)))
rb-tree/equal? rb-tree-1 rb-tree-2 datum=?
procedure
Compares rb-tree-1 and rb-tree-2 for equality, returning #t i↵ they are equal and
#f otherwise. The trees must have been constructed with the same equality and
order predicates (same in the sense of eq?). The keys of the trees are compared
using the key=? predicate used to build the trees, while the datums of the trees are
compared using the equivalence predicate datum=?. The worst-case time required by
this operation is proportional to the number of associations in the tree.
rb-tree/empty? rb-tree
Returns #t i↵ rb-tree contains no associations. Otherwise returns #f.
rb-tree/size rb-tree
procedure
procedure
Returns the number of associations in rb-tree, an exact non-negative integer. The average and worst-case times required by this operation are proportional to the number
of associations in the tree.
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155
rb-tree/height rb-tree
procedure
Returns the height of rb-tree, an exact non-negative integer. This is the length of the
longest path from a leaf of the tree to the root. The average and worst-case times
required by this operation are proportional to the number of associations in the tree.
The returned value satisfies the following:
(lambda (rb-tree)
(let ((size (rb-tree/size rb-tree))
(lg (lambda (x) (/ (log x) (log 2)))))
(<= (lg size)
(rb-tree/height rb-tree)
(* 2 (lg (+ size 1))))))
rb-tree/copy rb-tree
procedure
Returns a newly allocated copy of rb-tree. The copy is identical to rb-tree in all
respects, except that changes to rb-tree do not a↵ect the copy, and vice versa. The
time required by this operation is proportional to the number of associations in the
tree.
alist->rb-tree alist key=? key<?
procedure
Returns a newly allocated red-black tree that contains the same associations as alist.
This procedure is equivalent to:
(lambda (alist key=? key<?)
(let ((tree (make-rb-tree key=? key<?)))
(for-each (lambda (association)
(rb-tree/insert! tree
(car association)
(cdr association)))
alist)
tree))
The following operations provide access to the smallest and largest members in a
red/black tree. They are useful for implementing priority queues.
rb-tree/min rb-tree default
Returns the smallest key in rb-tree, or default if the tree is empty.
procedure
rb-tree/min-datum rb-tree default
procedure
Returns the datum associated with the smallest key in rb-tree, or default if the tree
is empty.
rb-tree/min-pair rb-tree
procedure
Finds the smallest key in rb-tree and returns a pair containing that key and its
associated datum. If the tree is empty, returns #f.
rb-tree/max rb-tree default
Returns the largest key in rb-tree, or default if the tree is empty.
procedure
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rb-tree/max-datum rb-tree default
procedure
Returns the datum associated with the largest key in rb-tree, or default if the tree is
empty.
rb-tree/max-pair rb-tree
procedure
Finds the largest key in rb-tree and returns a pair containing that key and its associated datum. If the tree is empty, returns #f.
rb-tree/delete-min! rb-tree default
rb-tree/delete-min-datum! rb-tree default
rb-tree/delete-min-pair! rb-tree
rb-tree/delete-max! rb-tree default
rb-tree/delete-max-datum! rb-tree default
rb-tree/delete-max-pair! rb-tree
procedure
procedure
procedure
procedure
procedure
procedure
These operations are exactly like the accessors above, in that they return information
associated with the smallest or largest key, except that they simultaneously delete
that key.
11.7 Weight-Balanced Trees
Balanced binary trees are a useful data structure for maintaining large sets of ordered
objects or sets of associations whose keys are ordered. MIT Scheme has a comprehensive
implementation of weight-balanced binary trees which has several advantages over the other
data structures for large aggregates:
• In addition to the usual element-level operations like insertion, deletion and lookup,
there is a full complement of collection-level operations, like set intersection, set union
and subset test, all of which are implemented with good orders of growth in time and
space. This makes weight-balanced trees ideal for rapid prototyping of functionally
derived specifications.
• An element in a tree may be indexed by its position under the ordering of the keys, and
the ordinal position of an element may be determined, both with reasonable efficiency.
• Operations to find and remove minimum element make weight-balanced trees simple
to use for priority queues.
• The implementation is functional rather than imperative. This means that operations
like ‘inserting’ an association in a tree do not destroy the old tree, in much the same way
that (+ 1 x) modifies neither the constant 1 nor the value bound to x. The trees are
referentially transparent thus the programmer need not worry about copying the trees.
Referential transparency allows space efficiency to be achieved by sharing subtrees.
These features make weight-balanced trees suitable for a wide range of applications,
especially those that require large numbers of sets or discrete maps. Applications that have
a few global databases and/or concentrate on element-level operations like insertion and
lookup are probably better o↵ using hash tables or red-black trees.
The size of a tree is the number of associations that it contains. Weight-balanced
binary trees are balanced to keep the sizes of the subtrees of each node within a constant
factor of each other. This ensures logarithmic times for single-path operations (like lookup
Chapter 11: Associations
157
and insertion). A weight-balanced tree takes space that is proportional to the number of
associations in the tree. For the current implementation, the constant of proportionality is
six words per association.
Weight-balanced trees can be used as an implementation for either discrete sets or discrete maps (associations). Sets are implemented by ignoring the datum that is associated
with the key. Under this scheme if an association exists in the tree this indicates that the
key of the association is a member of the set. Typically a value such as (), #t or #f is
associated with the key.
Many operations can be viewed as computing a result that, depending on whether the
tree arguments are thought of as sets or maps, is known by two di↵erent names. An example
is wt-tree/member?, which, when regarding the tree argument as a set, computes the set
membership operation, but, when regarding the tree as a discrete map, wt-tree/member?
is the predicate testing if the map is defined at an element in its domain. Most names
in this package have been chosen based on interpreting the trees as sets, hence the name
wt-tree/member? rather than wt-tree/defined-at?.
The weight-balanced tree implementation is a run-time-loadable option. To use weightbalanced trees, execute
(load-option ’wt-tree)
once before calling any of the procedures defined here.
11.7.1 Construction of Weight-Balanced Trees
Binary trees require there to be a total order on the keys used to arrange the elements
in the tree. Weight-balanced trees are organized by types, where the type is an object
encapsulating the ordering relation. Creating a tree is a two-stage process. First a tree
type must be created from the predicate that gives the ordering. The tree type is then used
for making trees, either empty or singleton trees or trees from other aggregate structures
like association lists. Once created, a tree ‘knows’ its type and the type is used to test
compatibility between trees in operations taking two trees. Usually a small number of
tree types are created at the beginning of a program and used many times throughout the
program’s execution.
make-wt-tree-type key<?
procedure
This procedure creates and returns a new tree type based on the ordering predicate
key<?. Key<? must be a total ordering, having the property that for all key values
a, b and c:
) #f
(key<? a a)
) #f
(and (key<? a b) (key<? b a))
(if (and (key<? a b) (key<? b c))
(key<? a c)
) #t
#t)
Two key values are assumed to be equal if neither is less than the other by key<?.
Each call to make-wt-tree-type returns a distinct value, and trees are only compatible if their tree types are eq?. A consequence is that trees that are intended to be
used in binary-tree operations must all be created with a tree type originating from
the same call to make-wt-tree-type.
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number-wt-type
variable
A standard tree type for trees with numeric keys. Number-wt-type could have been
defined by
(define number-wt-type (make-wt-tree-type <))
string-wt-type
variable
A standard tree type for trees with string keys. String-wt-type could have been
defined by
(define string-wt-type (make-wt-tree-type string<?))
make-wt-tree wt-tree-type
procedure
This procedure creates and returns a newly allocated weight-balanced tree. The tree
is empty, i.e. it contains no associations. Wt-tree-type is a weight-balanced tree type
obtained by calling make-wt-tree-type; the returned tree has this type.
singleton-wt-tree wt-tree-type key datum
procedure
This procedure creates and returns a newly allocated weight-balanced tree. The tree
contains a single association, that of datum with key. Wt-tree-type is a weightbalanced tree type obtained by calling make-wt-tree-type; the returned tree has
this type.
alist->wt-tree tree-type alist
procedure
Returns a newly allocated weight-balanced tree that contains the same associations
as alist. This procedure is equivalent to:
(lambda (type alist)
(let ((tree (make-wt-tree type)))
(for-each (lambda (association)
(wt-tree/add! tree
(car association)
(cdr association)))
alist)
tree))
11.7.2 Basic Operations on Weight-Balanced Trees
This section describes the basic tree operations on weight-balanced trees. These operations are the usual tree operations for insertion, deletion and lookup, some predicates and
a procedure for determining the number of associations in a tree.
wt-tree? object
procedure
wt-tree/empty? wt-tree
procedure
Returns #t if object is a weight-balanced tree, otherwise returns #f.
Returns #t if wt-tree contains no associations, otherwise returns #f.
wt-tree/size wt-tree
procedure
Returns the number of associations in wt-tree, an exact non-negative integer. This
operation takes constant time.
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159
wt-tree/add wt-tree key datum
procedure
Returns a new tree containing all the associations in wt-tree and the association of
datum with key. If wt-tree already had an association for key, the new association
overrides the old. The average and worst-case times required by this operation are
proportional to the logarithm of the number of associations in wt-tree.
wt-tree/add! wt-tree key datum
procedure
Associates datum with key in wt-tree and returns an unspecified value. If wt-tree
already has an association for key, that association is replaced. The average and
worst-case times required by this operation are proportional to the logarithm of the
number of associations in wt-tree.
wt-tree/member? key wt-tree
procedure
Returns #t if wt-tree contains an association for key, otherwise returns #f. The average and worst-case times required by this operation are proportional to the logarithm
of the number of associations in wt-tree.
wt-tree/lookup wt-tree key default
procedure
Returns the datum associated with key in wt-tree. If wt-tree doesn’t contain an
association for key, default is returned. The average and worst-case times required
by this operation are proportional to the logarithm of the number of associations in
wt-tree.
wt-tree/delete wt-tree key
procedure
Returns a new tree containing all the associations in wt-tree, except that if wt-tree
contains an association for key, it is removed from the result. The average and worstcase times required by this operation are proportional to the logarithm of the number
of associations in wt-tree.
wt-tree/delete! wt-tree key
procedure
If wt-tree contains an association for key the association is removed. Returns an
unspecified value. The average and worst-case times required by this operation are
proportional to the logarithm of the number of associations in wt-tree.
11.7.3 Advanced Operations on Weight-Balanced Trees
In the following the size of a tree is the number of associations that the tree contains,
and a smaller tree contains fewer associations.
wt-tree/split< wt-tree bound
procedure
Returns a new tree containing all and only the associations in wt-tree that have a key
that is less than bound in the ordering relation of the tree type of wt-tree. The average
and worst-case times required by this operation are proportional to the logarithm of
the size of wt-tree.
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wt-tree/split> wt-tree bound
procedure
Returns a new tree containing all and only the associations in wt-tree that have a
key that is greater than bound in the ordering relation of the tree type of wt-tree.
The average and worst-case times required by this operation are proportional to the
logarithm of the size of wt-tree.
wt-tree/union wt-tree-1 wt-tree-2
procedure
Returns a new tree containing all the associations from both trees. This operation
is asymmetric: when both trees have an association for the same key, the returned
tree associates the datum from wt-tree-2 with the key. Thus if the trees are viewed
as discrete maps then wt-tree/union computes the map override of wt-tree-1 by
wt-tree-2. If the trees are viewed as sets the result is the set union of the arguments.
The worst-case time required by this operation is proportional to the sum of the sizes
of both trees. If the minimum key of one tree is greater than the maximum key of
the other tree then the worst-case time required is proportional to the logarithm of
the size of the larger tree.
wt-tree/intersection wt-tree-1 wt-tree-2
procedure
Returns a new tree containing all and only those associations from wt-tree-1 that
have keys appearing as the key of an association in wt-tree-2. Thus the associated
data in the result are those from wt-tree-1. If the trees are being used as sets the
result is the set intersection of the arguments. As a discrete map operation, wttree/intersection computes the domain restriction of wt-tree-1 to (the domain of)
wt-tree-2. The worst-case time required by this operation is proportional to the sum
of the sizes of the trees.
wt-tree/di↵erence wt-tree-1 wt-tree-2
procedure
Returns a new tree containing all and only those associations from wt-tree-1 that
have keys that do not appear as the key of an association in wt-tree-2. If the trees
are viewed as sets the result is the asymmetric set di↵erence of the arguments. As a
discrete map operation, it computes the domain restriction of wt-tree-1 to the complement of (the domain of) wt-tree-2. The worst-case time required by this operation
is proportional to the sum of the sizes of the trees.
wt-tree/subset? wt-tree-1 wt-tree-2
procedure
Returns #t i↵ the key of each association in wt-tree-1 is the key of some association
in wt-tree-2, otherwise returns #f. Viewed as a set operation, wt-tree/subset? is
the improper subset predicate. A proper subset predicate can be constructed:
(define (proper-subset? s1 s2)
(and (wt-tree/subset? s1 s2)
(< (wt-tree/size s1) (wt-tree/size s2))))
As a discrete map operation, wt-tree/subset? is the subset test on the domain(s)
of the map(s). In the worst-case the time required by this operation is proportional
to the size of wt-tree-1.
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161
wt-tree/set-equal? wt-tree-1 wt-tree-2
procedure
Returns #t i↵ for every association in wt-tree-1 there is an association in wt-tree-2
that has the same key, and vice versa.
Viewing the arguments as sets, wt-tree/set-equal? is the set equality predicate.
As a map operation it determines if two maps are defined on the same domain.
This procedure is equivalent to
(lambda (wt-tree-1 wt-tree-2)
(and (wt-tree/subset? wt-tree-1 wt-tree-2
(wt-tree/subset? wt-tree-2 wt-tree-1)))
In the worst case the time required by this operation is proportional to the size of the
smaller tree.
wt-tree/fold combiner initial wt-tree
procedure
This procedure reduces wt-tree by combining all the associations, using an reverse
in-order traversal, so the associations are visited in reverse order. Combiner is a procedure of three arguments: a key, a datum and the accumulated result so far. Provided
combiner takes time bounded by a constant, wt-tree/fold takes time proportional
to the size of wt-tree.
A sorted association list can be derived simply:
(wt-tree/fold (lambda (key datum list)
(cons (cons key datum) list))
’()
wt-tree))
The data in the associations can be summed like this:
(wt-tree/fold (lambda (key datum sum) (+ sum datum))
0
wt-tree)
wt-tree/for-each action wt-tree
procedure
This procedure traverses wt-tree in order, applying action to each association. The
associations are processed in increasing order of their keys. Action is a procedure of
two arguments that takes the key and datum respectively of the association. Provided
action takes time bounded by a constant, wt-tree/for-each takes time proportional
to the size of wt-tree. The example prints the tree:
(wt-tree/for-each (lambda (key value)
(display (list key value)))
wt-tree))
wt-tree/union-merge wt-tree-1 wt-tree-2 merge
procedure
Returns a new tree containing all the associations from both trees. If both trees have
an association for the same key, the datum associated with that key in the result tree
is computed by applying the procedure merge to the key, the value from wt-tree-1
and the value from wt-tree-2. Merge is of the form
(lambda (key datum-1 datum-2) ...)
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If some key occurs only in one tree, that association will appear in the result tree
without being processed by merge, so for this operation to make sense, either merge
must have both a right and left identity that correspond to the association being
absent in one of the trees, or some guarantee must be made, for example, all the keys
in one tree are known to occur in the other.
These are all reasonable procedures for merge
(lambda (key val1 val2) (+ val1 val2))
(lambda (key val1 val2) (append val1 val2))
(lambda (key val1 val2) (wt-tree/union val1 val2))
However, a procedure like
(lambda (key val1 val2) (- val1 val2))
would result in a subtraction of the data for all associations with keys occuring in both
trees but associations with keys occuring in only the second tree would be copied, not
negated, as is presumably be intent. The programmer might ensure that this never
happens.
This procedure has the same time behavior as wt-tree/union but with a slightly
worse constant factor. Indeed, wt-tree/union might have been defined like this:
(define (wt-tree/union tree1 tree2)
(wt-tree/union-merge tree1 tree2
(lambda (key val1 val2) val2)))
The merge procedure takes the key as a parameter in case the data are not independent
of the key.
11.7.4 Indexing Operations on Weight-Balanced Trees
Weight-balanced trees support operations that view the tree as sorted sequence of associations. Elements of the sequence can be accessed by position, and the position of an
element in the sequence can be determined, both in logarthmic time.
wt-tree/index wt-tree index
wt-tree/index-datum wt-tree index
wt-tree/index-pair wt-tree index
procedure
procedure
procedure
Returns the 0-based indexth association of wt-tree in the sorted sequence under
the tree’s ordering relation on the keys. wt-tree/index returns the indexth key,
wt-tree/index-datum returns the datum associated with the indexth key and wttree/index-pair returns a new pair (key . datum) which is the cons of the indexth
key and its datum. The average and worst-case times required by this operation are
proportional to the logarithm of the number of associations in the tree.
These operations signal a condition of type condition-type:bad-range-argument
if index<0 or if index is greater than or equal to the number of associations in the
tree. If the tree is empty, they signal an anonymous error.
Indexing can be used to find the median and maximum keys in the tree as follows:
Chapter 11: Associations
median:
maximum:
163
(wt-tree/index wt-tree
(quotient (wt-tree/size wt-tree)
2))
(wt-tree/index wt-tree
(- (wt-tree/size wt-tree)
1))
wt-tree/rank wt-tree key
procedure
Determines the 0-based position of key in the sorted sequence of the keys under
the tree’s ordering relation, or #f if the tree has no association with for key. This
procedure returns either an exact non-negative integer or #f. The average and worstcase times required by this operation are proportional to the logarithm of the number
of associations in the tree.
wt-tree/min wt-tree
wt-tree/min-datum wt-tree
wt-tree/min-pair wt-tree
procedure
procedure
procedure
Returns the association of wt-tree that has the least key under the tree’s ordering
relation. wt-tree/min returns the least key, wt-tree/min-datum returns the datum associated with the least key and wt-tree/min-pair returns a new pair (key
. datum) which is the cons of the minimum key and its datum. The average and
worst-case times required by this operation are proportional to the logarithm of the
number of associations in the tree.
These operations signal an error if the tree is empty. They could have been written
(define (wt-tree/min tree)
(wt-tree/index tree 0))
(define (wt-tree/min-datum tree)
(wt-tree/index-datum tree 0))
(define (wt-tree/min-pair tree)
(wt-tree/index-pair tree 0))
wt-tree/delete-min wt-tree
procedure
Returns a new tree containing all of the associations in wt-tree except the association
with the least key under the wt-tree’s ordering relation. An error is signalled if
the tree is empty. The average and worst-case times required by this operation are
proportional to the logarithm of the number of associations in the tree. This operation
is equivalent to
(wt-tree/delete wt-tree (wt-tree/min wt-tree))
wt-tree/delete-min! wt-tree
procedure
Removes the association with the least key under the wt-tree’s ordering relation. An
error is signalled if the tree is empty. The average and worst-case times required by
this operation are proportional to the logarithm of the number of associations in the
tree. This operation is equivalent to
(wt-tree/delete! wt-tree (wt-tree/min wt-tree))
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Chapter 12: Procedures
165
12 Procedures
Procedures are created by evaluating lambda expressions (see Section 2.1 [Lambda Expressions], page 15); the lambda may either be explicit or may be implicit as in a “procedure
define” (see Section 2.4 [Definitions], page 20). Also there are special built-in procedures,
called primitive procedures, such as car; these procedures are not written in Scheme but in
the language used to implement the Scheme system. MIT Scheme also provides application
hooks, which support the construction of data structures that act like procedures.
In MIT Scheme, the written representation of a procedure tells you the type of the
procedure (compiled, interpreted, or primitive):
pp
) #[compiled-procedure 56 ("pp" #x2) #x10 #x307578]
(lambda (x) x)
) #[compound-procedure 57]
(define (foo x) x)
foo
) #[compound-procedure 58 foo]
car
) #[primitive-procedure car]
(call-with-current-continuation (lambda (x) x))
) #[continuation 59]
Note that interpreted procedures are called “compound” procedures (strictly speaking, compiled procedures are also compound procedures). The written representation makes this
distinction for historical reasons, and may eventually change.
12.1 Procedure Operations
apply procedure object object . . .
Calls procedure with the elements of the following list as arguments:
procedure
(cons* object object ...)
The initial objects may be any objects, but the last object (there must be at least
one object) must be a list.
) 18
(apply + (list 3 4 5 6))
) 18
(apply + 3 4 ’(5 6))
(define compose
(lambda (f g)
(lambda args
(f (apply g args)))))
((compose sqrt *) 12 75)
procedure? object
)
30
procedure
Returns #t if object is a procedure; otherwise returns #f. If #t is returned, exactly one
of the following predicates is satisfied by object: compiled-procedure?, compoundprocedure?, or primitive-procedure?.
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compiled-procedure? object
Returns #t if object is a compiled procedure; otherwise returns #f.
procedure
compound-procedure? object
procedure
Returns #t if object is a compound (i.e. interpreted) procedure; otherwise returns #f.
primitive-procedure? object
Returns #t if object is a primitive procedure; otherwise returns #f.
procedure
The following two procedures test the arity of a procedure, that is, the number of
arguments that the procedure accepts. The results of the test may be less restrictive than
the e↵ect of calling the procedure. In other words, these procedures may indicate that
the procedure will accept a given number of arguments, but if you call the procedure it
may signal a condition-type:wrong-number-of-arguments error. This is because these
procedures examine the apparent arity of a procedure. For example, here is a procedure
that appears to accept any number of arguments, but when called will signal an error if the
number of arguments is not one:
(lambda arguments (apply car arguments))
procedure-arity-valid? procedure k
Returns #t if procedure accepts k arguments; otherwise returns #f.
procedure
procedure-arity procedure
procedure
Returns a description of the number of arguments that procedure accepts. The result
is a newly allocated pair whose car field is the minimum number of arguments, and
whose cdr field is the maximum number of arguments. The minimum is an exact
non-negative integer. The maximum is either an exact non-negative integer, or #f
meaning that the procedure has no maximum number of arguments.
) (0 . 0)
(procedure-arity (lambda () 3))
) (1 . 1)
(procedure-arity (lambda (x) x))
) (1 . 1)
(procedure-arity car)
) (0 . #f)
(procedure-arity (lambda x x))
) (1 . #f)
(procedure-arity (lambda (x . y) x))
(procedure-arity (lambda (x #!optional y) x))
) (1 . 2)
procedure-environment procedure
procedure
Returns the closing environment of procedure. Signals an error if procedure is a
primitive procedure, or if procedure is a compiled procedure for which the debugging
information is unavailable.
12.2 Primitive Procedures
make-primitive-procedure name [arity]
procedure
Name must be a symbol. Arity must be an exact non-negative integer, -1, #f, or #t;
if not supplied it defaults to #f. Returns the primitive procedure called name. May
perform further actions depending on arity:
Chapter 12: Procedures
167
#f
If the primitive procedure is not implemented, signals an error.
#t
If the primitive procedure is not implemented, returns #f.
integer
If the primitive procedure is implemented, signals an error if its arity
is not equal to arity. If the primitive procedure is not implemented,
returns an unimplemented primitive procedure object that accepts arity
arguments. An arity of -1 means it accepts any number of arguments.
primitive-procedure-name primitive-procedure
procedure
Returns the name of primitive-procedure, a symbol.
(primitive-procedure-name car)
)
car
implemented-primitive-procedure? primitive-procedure
procedure
Returns #t if primitive-procedure is implemented; otherwise returns #f. Useful because the code that implements a particular primitive procedure is not necessarily
linked into the executable Scheme program.
12.3 Continuations
call-with-current-continuation procedure
procedure
Procedure must be a procedure of one argument. Packages up the current continuation (see below) as an escape procedure and passes it as an argument to procedure.
The escape procedure is a Scheme procedure of one argument that, if it is later passed
a value, will ignore whatever continuation is in e↵ect at that later time and will give
the value instead to the continuation that was in e↵ect when the escape procedure
was created. The escape procedure created by call-with-current-continuation
has unlimited extent just like any other procedure in Scheme. It may be stored in
variables or data structures and may be called as many times as desired.
The following examples show only the most common uses of this procedure. If all real
programs were as simple as these examples, there would be no need for a procedure
with the power of call-with-current-continuation.
(call-with-current-continuation
(lambda (exit)
(for-each (lambda (x)
(if (negative? x)
(exit x)))
’(54 0 37 -3 245 19))
#t))
)
-3
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(define list-length
(lambda (obj)
(call-with-current-continuation
(lambda (return)
(letrec ((r
(lambda (obj)
(cond ((null? obj) 0)
((pair? obj) (+ (r (cdr obj)) 1))
(else (return #f))))))
(r obj))))))
) 4
(list-length ’(1 2 3 4))
) #f
(list-length ’(a b . c))
A common use of call-with-current-continuation is for structured, non-local
exits from loops or procedure bodies, but in fact call-with-current-continuation
is quite useful for implementing a wide variety of advanced control structures.
Whenever a Scheme expression is evaluated a continuation exists that wants the
result of the expression. The continuation represents an entire (default) future for the
computation. If the expression is evaluated at top level, for example, the continuation
will take the result, print it on the screen, prompt for the next input, evaluate it, and
so on forever. Most of the time the continuation includes actions specified by user
code, as in a continuation that will take the result, multiply it by the value stored in
a local variable, add seven, and give the answer to the top-level continuation to be
printed. Normally these ubiquitous continuations are hidden behind the scenes and
programmers don’t think much about them. On the rare occasions that you may need
to deal explicitly with continuations, call-with-current-continuation lets you do
so by creating a procedure that acts just like the current continuation.
continuation? object
procedure
within-continuation continuation thunk
procedure
Returns #t if object is a continuation; otherwise returns #f.
Thunk must be a procedure of no arguments. Conceptually,
within-continuation invokes continuation on the result of invoking thunk, but
thunk is executed in the dynamic context of continuation. In other words, the “current” continuation is abandoned before thunk is invoked.
dynamic-wind before thunk after
procedure
Calls thunk without arguments, returning the result(s) of this call. Before and after
are called, also without arguments, as required by the following rules (note that in the
absence of calls to continuations captured using call-with-current-continuation
the three arguments are called once each, in order). Before is called whenever execution enters the dynamic extent of the call to thunk and after is called whenever it exits
that dynamic extent. The dynamic extent of a procedure call is the period between
when the call is initiated and when it returns. In Scheme, because of call-withcurrent-continuation, the dynamic extent of a call may not be a single, connected
time period. It is defined as follows:
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169
• The dynamic extent is entered when execution of the body of the called procedure
begins.
• The dynamic extent is also entered when execution is not within the dynamic extent and a continuation is invoked that was captured (using call-with-currentcontinuation) during the dynamic extent.
• It is exited when the called procedure returns.
• It is also exited when execution is within the dynamic extent and a continuation
is invoked that was captured while not within the dynamic extent.
If a second call to dynamic-wind occurs within the dynamic extent of the call to
thunk and then a continuation is invoked in such a way that the afters from these
two invocations of dynamic-wind are both to be called, then the after associated with
the second (inner) call to dynamic-wind is called first.
If a second call to dynamic-wind occurs within the dynamic extent of the call to
thunk and then a continuation is invoked in such a way that the befores from these
two invocations of dynamic-wind are both to be called, then the before associated
with the first (outer) call to dynamic-wind is called first.
If invoking a continuation requires calling the before from one call to dynamic-wind
and the after from another, then the after is called first.
The e↵ect of using a captured continuation to enter or exit the dynamic extent of a
call to before or after is undefined.
(let ((path ’())
(c #f))
(let ((add (lambda (s)
(set! path (cons s path)))))
(dynamic-wind
(lambda () (add ’connect))
(lambda ()
(add (call-with-current-continuation
(lambda (c0)
(set! c c0)
’talk1))))
(lambda () (add ’disconnect)))
(if (< (length path) 4)
(c ’talk2)
(reverse path))))
) (connect talk1 disconnect connect talk2 disconnect)
The following two procedures support multiple values.
call-with-values thunk procedure
procedure
Thunk must be a procedure of no arguments, and procedure must be a procedure.
Thunk is invoked with a continuation that expects to receive multiple values; specifically, the continuation expects to receive the same number of values that procedure
accepts as arguments. Thunk must return multiple values using the values procedure. Then procedure is called with the multiple values as its arguments. The result
yielded by procedure is returned as the result of call-with-values.
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values object . . .
procedure
Returns multiple values. The continuation in e↵ect when this procedure is called
must be a multiple-value continuation that was created by call-with-values. Furthermore it must accept as many values as there are objects.
12.4 Application Hooks
Application hooks are objects that can be applied like procedures. Each application
hook has two parts: a procedure that specifies what to do when the application hook is
applied, and an arbitrary object, called extra. Often the procedure uses the extra object to
determine what to do.
There are two kinds of application hooks, which di↵er in what arguments are passed to
the procedure. When an apply hook is applied, the procedure is passed exactly the same
arguments that were passed to the apply hook. When an entity is applied, the entity itself
is passed as the first argument, followed by the other arguments that were passed to the
entity.
Both apply hooks and entities satisfy the predicate procedure?. Each satisfies either
compiled-procedure?, compound-procedure?, or primitive-procedure?, depending on
its procedure component. An apply hook is considered to accept the same number of
arguments as its procedure, while an entity is considered to accept one less argument than
its procedure.
make-apply-hook procedure object
procedure
Returns a newly allocated apply hook with a procedure component of procedure and
an extra component of object.
apply-hook? object
procedure
apply-hook-procedure apply-hook
procedure
Returns #t if object is an apply hook; otherwise returns #f.
Returns the procedure component of apply-hook.
set-apply-hook-procedure! apply-hook procedure
procedure
Changes the procedure component of apply-hook to be procedure. Returns an unspecified value.
apply-hook-extra apply-hook
Returns the extra component of apply-hook.
procedure
set-apply-hook-extra! apply-hook object
procedure
Changes the extra component of apply-hook to be object. Returns an unspecified
value.
make-entity procedure object
procedure
Returns a newly allocated entity with a procedure component of procedure and an
extra component of object.
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171
entity? object
procedure
entity-procedure entity
procedure
Returns #t if object is an entity; otherwise returns #f.
Returns the procedure component of entity.
set-entity-procedure! entity procedure
procedure
Changes the procedure component of entity to be procedure. Returns an unspecified
value.
entity-extra entity
Returns the extra component of entity.
set-entity-extra! entity object
procedure
procedure
Changes the extra component of entity to be object. Returns an unspecified value.
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Chapter 13: Environments
173
13 Environments
13.1 Environment Operations
Environments are first-class objects in MIT Scheme. An environment consists of some
bindings and possibly a parent environment, from which other bindings are inherited. The
operations in this section reveal the frame-like structure of environments by permitting you
to examine the bindings of a particular environment separately from those of its parent.
There are several types of bindings that can occur in an environment. The most common is the simple variable binding, which associates a value (any Scheme object) with an
identifier (a symbol). A variable binding can also be unassigned, which means that it has
no value. An unassigned variable is bound, in that is will shadow other bindings of the
same name in ancestor environments, but a reference to that variable will signal an error
of type condition-type:unassigned-variable. An unassigned variable can be assigned
(using set! or environment-assign!) to give it a value.
In addition to variable bindings, an environment can also have keyword bindings. A
keyword binding associates a syntactic keyword (usually a macro transformer) with an
identifier. Keyword bindings are special in that they are considered “bound”, but ordinary
variable references don’t work on them. So an attempt to reference or assign a keyword
binding results in an error of type condition-type:macro-binding. However, keyword
bindings can be redefined using define or environment-define.
environment? object
procedure
environment-has-parent? environment
procedure
Returns #t if object is an environment; otherwise returns #f.
Returns #t if environment has a parent environment; otherwise returns #f.
environment-parent environment
procedure
Returns the parent environment of environment. It is an error if environment has no
parent.
environment-bound-names environment
procedure
Returns a newly allocated list of the names (symbols) that are bound by environment. This does not include the names that are bound by the parent environment of
environment. It does include names that are unassigned or keywords in environment.
environment-macro-names environment
procedure
Returns a newly allocated list of the names (symbols) that are bound to syntactic
keywords in environment.
environment-bindings environment
procedure
Returns a newly allocated list of the bindings of environment; does not include the
bindings of the parent environment. Each element of this list takes one of two forms:
(symbol) indicates that symbol is bound but unassigned, while (symbol object)
indicates that symbol is bound, and its value is object.
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environment-reference-type environment symbol
procedure
Returns a symbol describing the reference type of symbol in environment or one of
its ancestor environments. The result is one of the following:
normal
means symbol is a variable binding with a normal value.
unassigned
means symbol is a variable binding with no value.
macro
means symbol is a keyword binding.
unbound
means symbol has no associated binding.
environment-bound? environment symbol
procedure
Returns #t if symbol is bound in environment or one of its ancestor environments;
otherwise returns #f. This is equivalent to
(not (eq? ’unbound
(environment-reference-type environment symbol)))
environment-assigned? environment symbol
procedure
Returns #t if symbol is bound in environment or one of its ancestor environments,
and has a normal value. Returns #f if it is bound but unassigned. Signals an error if
it is unbound or is bound to a keyword.
environment-lookup environment symbol
procedure
Symbol must be bound to a normal value in environment or one of its ancestor
environments. Returns the value to which it is bound. Signals an error if unbound,
unassigned, or a keyword.
environment-lookup-macro environment symbol
procedure
If symbol is a keyword binding in environment or one of its ancestor environments,
returns the value of the binding. Otherwise, returns #f. Does not signal any errors
other than argument-type errors.
environment-assignable? environment symbol
procedure
Symbol must be bound in environment or one of its ancestor environments. Returns
#t if the binding may be modified by side e↵ect.
environment-assign! environment symbol object
procedure
Symbol must be bound in environment or one of its ancestor environments, and
must be assignable. Modifies the binding to have object as its value, and returns an
unspecified result.
environment-definable? environment symbol
procedure
Returns #t if symbol is definable in environment, and #f otherwise. At present, this
is false for environments generated by application of compiled procedures, and true
for all other environments.
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175
environment-define environment symbol object
procedure
Defines symbol to be bound to object in environment, and returns an unspecified
value. Signals an error if symbol isn’t definable in environment.
environment-define-macro environment symbol transformer
procedure
Defines symbol to be a keyword bound to transformer in environment, and returns an
unspecified value. Signals an error if symbol isn’t definable in environment. The type
of transformer is defined by the syntax engine and is not checked by this procedure.
If the type is incorrect this will subsequently signal an error during syntax expansion.
eval expression environment
procedure
Evaluates expression, a list-structure representation (sometimes called s-expression
representation) of a Scheme expression, in environment. You rarely need eval in
ordinary programs; it is useful mostly for evaluating expressions that have been created “on the fly” by a program. eval is relatively expensive because it must convert
expression to an internal form before it is executed.
(define foo (list ’+ 1 2))
) 3
(eval foo (the-environment))
13.2 Environment Variables
The user-initial-environment is where the top-level read-eval-print (rep) loop
evaluates expressions and binds definitions. It is a child of system-global-environment,
which is where all of the Scheme system definitions are bound. All of the bindings
in system-global-environment are available when the current environment is
user-initial-environment.
However, any new bindings that you create in the
rep loop (with define forms or by loading files containing define forms) occur in
user-initial-environment.
system-global-environment
variable
The variable system-global-environment is bound to the distinguished environment
that’s the ancestor of most other environments (except for those created by makeroot-top-level-environment). It is the parent environment of user-initialenvironment. Primitives, system procedures, and most syntactic keywords are bound
(and sometimes closed) in this environment.
user-initial-environment
variable
The variable user-initial-environment is bound to the default environment in
which typed expressions are evaluated by the top-level rep loop.
Although all bindings in system-global-environment are visible to the rep loop,
definitions that are typed at, or loaded by, the rep loop occur in the user-initialenvironment. This is partly a safety measure: if you enter a definition that happens to
have the same name as a critical system procedure, your definition will be visible only
to the procedures you define in the user-initial-environment; the MIT Scheme
system procedures, which are defined in system-global-environment, will continue
to see the original definition.
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13.3 REPL Environment
nearest-repl/environment
procedure
Returns the current rep loop environment (i.e. the current environment of the closest
enclosing rep loop). When Scheme first starts up, this is the same as user-initialenvironment.
ge environment
procedure
Changes the current rep loop environment to environment. Environment can be
either an environment or a procedure object. If it’s a procedure, the environment in
which that procedure was closed is the new environment.
13.4 Top-level Environments
The operations in this section manipulate top-level environments, as opposed to environments created by the application of procedures. For historical reasons, top-level environments are referred to as interpreter environments.
the-environment
special form
Returns the current environment. This form may only be evaluated in a top-level
environment. An error is signalled if it appears elsewhere.
top-level-environment? object
interpreter-environment? object
Returns #t if object is an top-level environment; otherwise returns #f.
interpreter-environment? is an alias for top-level-environment?.
procedure
procedure
extend-top-level-environment environment [names [values]]
make-root-top-level-environment [names [values]]
procedure
procedure
Returns a newly allocated top-level environment. extend-top-level-environment
creates an environment that has parent environment, while make-root-top-levelenvironment creates an environment that has no parent.
The optional arguments names and values are used to specify initial bindings in the
new environment. If specified, names must be a list of symbols, and values must be
a list of objects. If only names is specified, each name in names will be bound in the
environment, but unassigned. If names and values are both specified, they must be
the same length, and each name in names will be bound to the corresponding value in
values. If neither names nor values is specified, the environment will have no initial
bindings.
link-variables environment1 symbol1 environment2 symbol2
procedure
Defines symbol1 in environment1 to have the same binding as symbol2 in environment2, and returns an unspecified value. Prior to the call, symbol2 must be bound in
environment2, but the type of binding is irrelevant; it may be a normal binding, an
unassigned binding, or a keyword binding. Signals an error if symbol1 isn’t definable
in environment1, or if symbol2 is unbound in environment2.
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177
By “the same binding”, we mean that the value cell is shared between the two environments. If a value is assigned to symbol1 in environment1, a subsequent reference
to symbol2 in environment2 will see that value, and vice versa.
unbind-variable environment symbol
procedure
If symbol is bound in environment or one of its ancestor environments, removes the
binding, so that subsequent accesses to that symbol behave as if the binding never
existed. Returns #t if there was a binding prior to the call, and #f if there wasn’t.
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Chapter 14: Input/Output
179
14 Input/Output
This chapter describes the procedures that are used for input and output (i/o). The
chapter first describes ports and how they are manipulated, then describes the i/o operations. Finally, some low-level procedures are described that permit the implementation of
custom ports and high-performance i/o.
14.1 Ports
Scheme uses ports for i/o. A port, which can be treated like any other Scheme object,
serves as a source or sink for data. A port must be open before it can be read from or
written to. The standard i/o port, console-i/o-port, is opened automatically when you
start Scheme. When you use a file for input or output, you need to explicitly open and
close a port to the file (with procedures described in this chapter). Additional procedures
let you open ports to strings.
Many input procedures, such as read-char and read, read data from the current input
port by default, or from a port that you specify. The current input port is initially consolei/o-port, but Scheme provides procedures that let you change the current input port to
be a file or string.
Similarly, many output procedures, such as write-char and display, write data to the
current output port by default, or to a port that you specify. The current output port
is initially console-i/o-port, but Scheme provides procedures that let you change the
current output port to be a file or string.
All ports read or write only iso-8859-1 characters.
Every port is either an input port, an output port, or both. The following predicates
distinguish all of the possible cases.
port? object
Returns #t if object is a port, otherwise returns #f.
procedure
input-port? object
procedure
Returns #t if object is an input port, otherwise returns #f. Any object satisfying this
predicate also satisfies port?.
output-port? object
procedure
Returns #t if object is an output port, otherwise returns #f. Any object satisfying
this predicate also satisfies port?.
i/o-port? object
procedure
Returns #t if object is both an input port and an output port, otherwise returns #f.
Any object satisfying this predicate also satisfies port?, input-port?, and outputport?.
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guarantee-port object
guarantee-input-port object
guarantee-output-port object
guarantee-i/o-port object
procedure
procedure
procedure
procedure
These procedures check the type of object, signalling an error of type
condition-type:wrong-type-argument if it is not a port, input port, output port,
or i/o port, respectively. Otherwise they return object.
The next five procedures return the runtime system’s standard ports. All of the standard
ports are dynamically bound by the rep loop; this means that when a new rep loop is
started, for example by an error, each of these ports is dynamically bound to the i/o port
of the rep loop. When the rep loop exits, the ports revert to their original values.
current-input-port
procedure
Returns the current input port. This is the default port used by many input procedures. Initially, current-input-port returns the value of console-i/o-port.
current-output-port
procedure
Returns the current output port. This is the default port used by many output
procedures. Initially, current-output-port returns the value of console-i/o-port.
notification-output-port
procedure
Returns an output port suitable for generating “notifications”, that is, messages to
the user that supply interesting information about the execution of a program. For
example, the load procedure writes messages to this port informing the user that
a file is being loaded. Initially, notification-output-port returns the value of
console-i/o-port.
trace-output-port
procedure
Returns an output port suitable for generating “tracing” information about a program’s execution. The output generated by the trace procedure is sent to this port.
Initially, trace-output-port returns the value of console-i/o-port.
interaction-i/o-port
procedure
Returns an i/o port suitable for querying or prompting the user. The standard
prompting procedures use this port by default (see Section 14.8 [Prompting],
page 194). Initially, interaction-i/o-port returns the value of console-i/o-port.
with-input-from-port input-port thunk
with-output-to-port output-port thunk
with-notification-output-port output-port thunk
with-trace-output-port output-port thunk
with-interaction-i/o-port i/o-port thunk
procedure
procedure
procedure
procedure
procedure
Thunk must be a procedure of no arguments. Each of these procedures binds one
of the standard ports to its first argument, calls thunk with no arguments, restores
the port to its original value, and returns the result that was yielded by thunk. This
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temporary binding is performed the same way as dynamic binding of a variable,
including the behavior in the presence of continuations (see Section 2.3 [Dynamic
Binding], page 18).
with-input-from-port binds the current input port, with-output-to-port binds
the current output port, with-notification-output-port binds the “notification”
output port, with-trace-output-port binds the “trace” output port, and withinteraction-i/o-port binds the “interaction” i/o port.
set-current-input-port! input-port
set-current-output-port! output-port
set-notification-output-port! output-port
set-trace-output-port! output-port
set-interaction-i/o-port! i/o-port
procedure
procedure
procedure
procedure
procedure
Each of these procedures alters the binding of one of the standard ports and returns
an unspecified value. The binding that is modified corresponds to the name of the
procedure.
console-i/o-port
variable
console-i/o-port is an i/o port that communicates with the “console”. Under unix,
the console is the controlling terminal of the Scheme process. Under Windows and
OS/2, the console is the window that is created when Scheme starts up.
This variable is rarely used; instead programs should use one of the standard ports
defined above. This variable should not be modified.
close-port port
procedure
Closes port and returns an unspecified value. If port is a file port, the file is closed.
close-input-port port
procedure
Closes port and returns an unspecified value. Port must be an input port or an i/o
port; if it is an i/o port, then only the input side of the port is closed.
close-output-port port
procedure
Closes port and returns an unspecified value. Port must be an output port or an i/o
port; if it is an i/o port, then only the output side of the port is closed.
14.2 File Ports
Before Scheme can access a file for reading or writing, it is necessary to open a port to
the file. This section describes procedures used to open ports to files. Such ports are closed
(like any other port) by close-port. File ports are automatically closed if and when they
are reclaimed by the garbage collector.
Before opening a file for input or output, by whatever method, the filename argument
is converted to canonical form by calling the procedure merge-pathnames with filename as
its sole argument. Thus, filename can be either a string or a pathname, and it is merged
with the current pathname defaults to produce the pathname that is then opened.
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Any file can be opened in one of two modes, normal or binary. Normal mode is for
accessing text files, and binary mode is for accessing other files. Unix does not distinguish
these modes, but Windows and OS/2 do: in normal mode, their file ports perform newline
translation, mapping between the carriage-return/linefeed sequence that terminates text
lines in files, and the #\newline that terminates lines in Scheme. In binary mode, such
ports do not perform newline translation. Unless otherwise mentioned, the procedures in
this section open files in normal mode.
open-input-file filename
procedure
Takes a filename referring to an existing file and returns an input port capable of
delivering characters from the file. If the file cannot be opened, an error of type
condition-type:file-operation-error is signalled.
open-output-file filename [append?]
procedure
Takes a filename referring to an output file to be created and returns an output port
capable of writing characters to a new file by that name. If the file cannot be opened,
an error of type condition-type:file-operation-error is signalled.
The optional argument append? is an MIT Scheme extension. If append? is given
and not #f, the file is opened in append mode. In this mode, the contents of the file
are not overwritten; instead any characters written to the file are appended to the
end of the existing contents. If the file does not exist, append mode creates the file
and writes to it in the normal way.
open-i/o-file filename
procedure
Takes a filename referring to an existing file and returns an i/o port capable of both
reading and writing the file. If the file cannot be opened, an error of type conditiontype:file-operation-error is signalled.
This procedure is often used to open special files. For example, under unix this
procedure can be used to open terminal device files, pty device files, and named
pipes.
open-binary-input-file filename
open-binary-output-file filename [append?]
open-binary-i/o-file filename
procedure
procedure
procedure
These procedures open files in binary mode. In all other respects they are identical
to open-input-file, open-output-file, and open-i/o-file, respectively.
close-all-open-files
procedure
This procedure closes all file ports that are open at the time that it is called, and
returns an unspecified value.
call-with-input-file filename procedure
call-with-output-file filename procedure
procedure
procedure
These procedures call procedure with one argument: the port obtained by opening
the named file for input or output, respectively. If the file cannot be opened, an error
of type condition-type:file-operation-error is signalled. If procedure returns,
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then the port is closed automatically and the value yielded by procedure is returned.
If procedure does not return, then the port will not be closed automatically unless it
is reclaimed by the garbage collector.1
call-with-binary-input-file filename procedure
call-with-binary-output-file filename procedure
procedure
procedure
These procedures open files in binary mode. In all other respects they are identical
to call-with-input-file and call-with-output-file, respectively.
with-input-from-file filename thunk
with-output-to-file filename thunk
procedure
procedure
Thunk must be a procedure of no arguments. The file is opened for input or output, an
input or output port connected to it is made the default value returned by currentinput-port or current-output-port, and the thunk is called with no arguments.
When the thunk returns, the port is closed and the previous default is restored. withinput-from-file and with-output-to-file return the value yielded by thunk. If
an escape procedure is used to escape from the continuation of these procedures, their
behavior is implementation-dependent; in that situation MIT Scheme leaves the files
open.
with-input-from-binary-file filename thunk
with-output-to-binary-file filename thunk
procedure
procedure
These procedures open files in binary mode. In all other respects they are identical
to with-input-from-file and with-output-to-file, respectively.
14.3 String Ports
This section describes the simplest kinds of ports: input ports that read their input from
given strings, and output ports that accumulate their output and return it as a string. It
also describes “truncating” output ports, which can limit the length of the resulting string
to a given value.
string->input-port string [start [end]]
procedure
Returns a new string port that delivers characters from string. The optional arguments start and end may be used to specify that the string port delivers characters from a substring of string; if not given, start defaults to 0 and end defaults to
(string-length string).
with-input-from-string string thunk
procedure
Thunk must be a procedure of no arguments. with-input-from-string creates a
new input port that reads from string, makes that port the current input port, and
calls thunk. When thunk returns, with-input-from-string restores the previous
current input port and returns the result yielded by thunk.
1
Because Scheme’s escape procedures have unlimited extent, it is possible to escape from the current
continuation but later to escape back in. If implementations were permitted to close the port on any
escape from the current continuation, then it would be impossible to write portable code using both
call-with-current-continuation and call-with-input-file or call-with-output-file.
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(with-input-from-string "(a b c) (d e f)" read)
Note: this procedure is equivalent to:
)
(a b c)
(with-input-from-port (string->input-port string) thunk)
with-string-output-port procedure
procedure
Procedure is called with one argument, an output port. The value yielded by procedure is ignored. When procedure returns, with-string-output-port returns the
port’s accumulated output as a newly allocated string.
with-output-to-string thunk
procedure
Thunk must be a procedure of no arguments. with-output-to-string creates a new
output port that accumulates output, makes that port the default value returned by
current-output-port, and calls thunk with no arguments. When thunk returns,
with-output-to-string restores the previous default and returns the accumulated
output as a newly allocated string.
(with-output-to-string
(lambda ()
(write ’abc)))
Note: this procedure is equivalent to:
)
"abc"
(with-string-output-port
(lambda (port)
(with-output-to-port port thunk)))
with-output-to-truncated-string k thunk
procedure
Similar to with-output-to-string, except that the output is limited to k characters.
If thunk attempts to write more than k characters, it will be aborted by invoking an
escape procedure that returns from with-output-to-truncated-string.
The value of this procedure is a pair; the car of the pair is #t if thunk attempted
to write more than k characters, and #f otherwise. The cdr of the pair is a newly
allocated string containing the accumulated output.
This procedure is helpful for displaying circular lists, as shown in this example:
(define inf (list ’inf))
(with-output-to-truncated-string 40
(lambda ()
) (#f . "(inf)")
(write inf)))
(set-cdr! inf inf)
(with-output-to-truncated-string 40
(lambda ()
(write inf)))
) (#t . "(inf inf inf inf inf inf inf inf inf inf")
write-to-string object [k]
procedure
Writes object to a string output port, and returns the resulting newly allocated string.
If k is supplied and not #f, this procedure is equivalent to
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185
(with-output-to-truncated-string k
(lambda ()
(write object)))
otherwise it is equivalent to
(with-output-to-string
(lambda ()
(write object)))
14.4 Input Procedures
This section describes the procedures that read input. Input procedures can read either
from the current input port or from a given port. Remember that to read from a file, you
must first open a port to the file.
Input ports can be divided into two types, called interactive and non-interactive. Interactive input ports are ports that read input from a source that is time-dependent; for example,
a port that reads input from a terminal or from another program. Non-interactive input
ports read input from a time-independent source, such as an ordinary file or a character
string.
All optional arguments called input-port, if not supplied, default to the current input
port.
read-char [input-port]
procedure
Returns the next character available from input-port, updating input-port to point
to the following character. If no more characters are available, an end-of-file object
is returned.
In MIT Scheme, if input-port is an interactive input port and no characters are
immediately available, read-char will hang waiting for input, even if the port is in
non-blocking mode.
peek-char [input-port]
procedure
Returns the next character available from input-port, without updating input-port to
point to the following character. If no more characters are available, an end-of-file
object is returned.2
In MIT Scheme, if input-port is an interactive input port and no characters are
immediately available, peek-char will hang waiting for input, even if the port is in
non-blocking mode.
char-ready? [input-port]
procedure
Returns #t if a character is ready on input-port and returns #f otherwise. If charready? returns #t then the next read-char operation on input-port is guaranteed
not to hang. If input-port is a file port at end of file then char-ready? returns #t.3
2
3
The value returned by a call to peek-char is the same as the value that would have been returned by
a call to read-char on the same port. The only di↵erence is that the very next call to read-char or
peek-char on that input-port will return the value returned by the preceding call to peek-char. In
particular, a call to peek-char on an interactive port will hang waiting for input whenever a call to
read-char would have hung.
char-ready? exists to make it possible for a program to accept characters from interactive ports without
getting stuck waiting for input. Any input editors associated with such ports must make sure that
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read [input-port]
procedure
Converts external representations of Scheme objects into the objects themselves. read
returns the next object parsable from input-port, updating input-port to point to the
first character past the end of the written representation of the object. If an end
of file is encountered in the input before any characters are found that can begin
an object, read returns an end-of-file object. The input-port remains open, and
further attempts to read will also return an end-of-file object. If an end of file is
encountered after the beginning of an object’s written representation, but the written
representation is incomplete and therefore not parsable, an error is signalled.
eof-object? object
Returns #t if object is an end-of-file object; otherwise returns #f.
procedure
read-char-no-hang [input-port]
procedure
If input-port can deliver a character without blocking, this procedure acts exactly like
read-char, immediately returning that character. Otherwise, #f is returned, unless
input-port is a file port at end of file, in which case an end-of-file object is returned.
In no case will this procedure block waiting for input.
read-string char-set [input-port]
procedure
Reads characters from input-port until it finds a terminating character that is a
member of char-set (see Section 5.6 [Character Sets], page 82) or encounters end of
file. The port is updated to point to the terminating character, or to end of file if
no terminating character was found. read-string returns the characters, up to but
excluding the terminating character, as a newly allocated string.
This procedure ignores the blocking mode of the port, blocking unconditionally until
it sees either a delimiter or eof of file. If end of file is encountered before any characters
are read, an end-of-file object is returned.
On many input ports, this operation is significantly faster than the following equivalent code using peek-char and read-char:
(define (read-string char-set input-port)
(let ((char (peek-char input-port)))
(if (eof-object? char)
char
(list->string
(let loop ((char char))
(if (or (eof-object? char)
(char-set-member? char-set char))
’()
(begin
(read-char input-port)
(cons char
(loop (peek-char input-port))))))))))
characters whose existence has been asserted by char-ready? cannot be rubbed out. If char-ready?
were to return #f at end of file, a port at end of file would be indistinguishable from an interactive port
that has no ready characters.
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187
read-line [input-port]
procedure
read-line reads a single line of text from input-port, and returns that line as a newly
allocated string. The #\newline terminating the line, if any, is discarded and does
not appear in the returned string.
This procedure ignores the blocking mode of the port, blocking unconditionally until
it has read an entire line. If end of file is encountered before any characters are read,
an end-of-file object is returned.
read-string! string [input-port]
read-substring! string start end [input-port]
procedure
procedure
read-string! and read-substring! fill the specified region of string with characters
read from input-port until the region is full or else there are no more characters
available from the port. For read-string!, the region is all of string, and for readsubstring!, the region is that part of string specified by start and end.
The returned value is the number of characters filled into the region. However, there
are several interesting cases to consider:
• If read-string! (read-substring!) is called when input-port is at “end-of-file”,
then the returned value is 0. Note that “end-of-file” can mean a file port that is
at the file’s end, a string port that is at the string’s end, or any other port that
will never produce more characters.
• If input-port is an interactive port (e.g. a terminal), and one or more characters
are immediately available, the region is filled using the available characters. The
procedure then returns immediately, without waiting for further characters, even
if the number of available characters is less than the size of the region. The
returned value is the number of characters actually filled in.
• If input-port is an interactive port and no characters are immediately available,
the result of the operation depends on the blocking mode of the port. If the port
is in non-blocking mode, read-string! (read-substring!) immediately returns
the value #f. Otherwise, the operation blocks until a character is available. As
soon as at least one character is available, the region is filled using the available
characters. The procedure then returns immediately, without waiting for further
characters, even if the number of available characters is less than the size of the
region. The returned value is the number of characters actually filled in.
The importance of read-string! and read-substring! are that they are both flexible and extremely fast, especially for large amounts of data.
The following variables may be dynamically bound to change the behavior of the read
procedure.
*parser-radix*
variable
This variable defines the radix used by the reader when it parses numbers. This is
similar to passing a radix argument to string->number. The value of this variable
must be one of 2, 8, 10, or 16; any other value is ignored, and the reader uses radix
10.
Note that much of the number syntax is invalid for radixes other than 10. The
reader detects cases where such invalid syntax is used and signals an error. However,
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problems can still occur when *parser-radix* is set to 16, because syntax that
normally denotes symbols can now denote numbers (e.g. abc). Because of this, it is
usually undesirable to set this variable to anything other than the default.
The default value of this variable is 10.
*parser-canonicalize-symbols?*
variable
This variable controls how the parser handles case-sensitivity of symbols. If it is
bound to its default value of #t, symbols read by the parser are converted to lower
case before being interned. Otherwise, symbols are interned without case conversion.
In general, it is a bad idea to use this feature, as it doesn’t really make Scheme casesensitive, and therefore can break features of the Scheme runtime that depend on
case-insensitive symbols.
14.5 Output Procedures
Output ports may or may not support bu↵ering of output, in which output characters
are collected together in a bu↵er and then sent to the output device all at once. (Most of
the output ports implemented by the runtime system support bu↵ering.) Sending all of the
characters in the bu↵er to the output device is called flushing the bu↵er. In general, output
procedures do not flush the bu↵er of an output port unless the bu↵er is full.
However, the standard output procedures described in this section perform what is
called discretionary flushing of the bu↵er. Discretionary output flushing works as follows.
After a procedure performs its output (writing characters to the output bu↵er), it checks
to see if the port implements an operation called discretionary-flush-output. If so,
then that operation is invoked to flush the bu↵er. At present, only the console port defines
discretionary-flush-output; this is used to guarantee that output to the console appears
immediately after it is written, without requiring calls to flush-output.
All optional arguments called output-port, if not supplied, default to the current output
port.
write-char char [output-port]
procedure
Writes char (the character itself, not a written representation of the character) to
output-port, performs discretionary output flushing, and returns an unspecified value.
write-string string [output-port]
procedure
Writes string to output-port, performs discretionary output flushing, and returns an
unspecified value. This is equivalent to writing the contents of string, one character
at a time using write-char, except that it is usually much faster.
write-substring string start end [output-port]
procedure
Writes the substring defined by string, start, and end to output-port, performs discretionary output flushing, and returns an unspecified value. This is equivalent to
writing the contents of the substring, one character at a time using write-char,
except that it is usually much faster.
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189
write object [output-port]
procedure
Writes a written representation of object to output-port, and returns an unspecified
value. If object has a standard external representation, then the written representation generated by write shall be parsable by read into an equivalent object. Thus
strings that appear in the written representation are enclosed in doublequotes, and
within those strings backslash and doublequote are escaped by backslashes. write
performs discretionary output flushing and returns an unspecified value.
display object [output-port]
procedure
Writes a representation of object to output-port. Strings appear in the written representation as if written by write-string instead of by write. Character objects appear in the representation as if written by write-char instead of by write. display
performs discretionary output flushing and returns an unspecified value.4
newline [output-port]
procedure
Writes an end-of-line to output-port, performs discretionary output flushing, and
returns an unspecified value. Equivalent to (write-char #\newline output-port).
fresh-line [output-port]
procedure
Most output ports are able to tell whether or not they are at the beginning of a line of
output. If output-port is such a port, this procedure writes an end-of-line to the port
only if the port is not already at the beginning of a line. If output-port is not such
a port, this procedure is identical to newline. In either case, fresh-line performs
discretionary output flushing and returns an unspecified value.
write-line object [output-port]
procedure
Like write, except that it writes an end-of-line to output-port after writing object’s
representation. This procedure performs discretionary output flushing and returns
an unspecified value.
flush-output [output-port]
procedure
If output-port is bu↵ered, this causes the contents of its bu↵er to be written to the
output device. Otherwise it has no e↵ect. Returns an unspecified value.
beep [output-port]
procedure
Performs a “beep” operation on output-port, performs discretionary output flushing,
and returns an unspecified value. On the console port, this usually causes the console
bell to beep, but more sophisticated interactive ports may take other actions, such as
flashing the screen. On most output ports, e.g. file and string output ports, this does
nothing.
clear [output-port]
procedure
“Clears the screen” of output-port, performs discretionary output flushing, and returns an unspecified value. On a terminal or window, this has a well-defined e↵ect.
On other output ports, e.g. file and string output ports, this does nothing.
4
write is intended for producing machine-readable output and display is for producing human-readable
output.
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pp object [output-port [as-code?]]
procedure
pp prints object in a visually appealing and structurally revealing manner on outputport. If object is a procedure, pp attempts to print the source text. If the optional
argument as-code? is true, pp prints lists as Scheme code, providing appropriate
indentation; by default this argument is false. pp performs discretionary output
flushing and returns an unspecified value.
The following variables may be dynamically bound to change the behavior of the write
and display procedures.
*unparser-radix*
variable
This variable specifies the default radix used to print numbers. Its value must be one
of the exact integers 2, 8, 10, or 16; the default is 10. If *unparser-radix* is not
10, numbers are prefixed to indicate their radix.
*unparser-list-breadth-limit*
variable
This variable specifies a limit on the length of the printed representation of a list or
vector; for example, if the limit is 4, only the first four elements of any list are printed,
followed by ellipses to indicate any additional elements. The value of this variable
must be an exact non-negative integer, or #f meaning no limit; the default is #f.
(fluid-let ((*unparser-list-breadth-limit*
(write-to-string ’(a b c d)))
) "(a b c
(fluid-let ((*unparser-list-breadth-limit*
(write-to-string ’(a b c d e)))
) "(a b c
4))
d)"
4))
d ...)"
*unparser-list-depth-limit*
variable
This variable specifies a limit on the nesting of lists and vectors in the printed representation. If lists (or vectors) are more deeply nested than the limit, the part of the
representation that exceeds the limit is replaced by ellipses. The value of this variable
must be an exact non-negative integer, or #f meaning no limit; the default is #f.
(fluid-let ((*unparser-list-depth-limit* 4))
(write-to-string ’((((a))) b c d)))
) "((((a))) b c d)"
(fluid-let ((*unparser-list-depth-limit* 4))
(write-to-string ’(((((a)))) b c d)))
) "((((...))) b c d)"
*unparser-string-length-limit*
variable
This variable specifies a limit on the length of the printed representation of strings.
If a string’s length exceeds this limit, the part of the printed representation for the
characters exceeding the limit is replaced by ellipses. The value of this variable must
be an exact non-negative integer, or #f meaning no limit; the default is #f.
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191
(fluid-let ((*unparser-string-length-limit* 4))
(write-to-string "abcd"))
) "\"abcd\""
(fluid-let ((*unparser-string-length-limit* 4))
(write-to-string "abcde"))
) "\"abcd...\""
*unparse-with-maximum-readability?*
variable
This variable, which takes a boolean value, tells the printer to use a special printed
representation for objects that normally print in a form that cannot be recognized by
read. These objects are printed using the representation #@n, where n is the result
of calling hash on the object to be printed. The reader recognizes this syntax, calling
unhash on n to get back the original object. Note that this printed representation
can only be recognized by the Scheme program in which it was generated, because
these hash numbers are di↵erent for each invocation of Scheme.
14.6 Format
The procedure format is very useful for producing nicely formatted text, producing
good-looking messages, and so on. MIT Scheme’s implementation of format is similar to
that of Common Lisp, except that Common Lisp defines many more directives.5
format is a run-time-loadable option. To use it, execute
(load-option ’format)
once before calling it.
format destination control-string argument . . .
procedure
Writes the characters of control-string to destination, except that a tilde (~) introduces a format directive. The character after the tilde, possibly preceded by prefix
parameters and modifiers, specifies what kind of formatting is desired. Most directives use one or more arguments to create their output; the typical directive puts the
next argument into the output, formatted in some special way. It is an error if no
argument remains for a directive requiring an argument, but it is not an error if one
or more arguments remain unprocessed by a directive.
The output is sent to destination. If destination is #f, a string is created that contains
the output; this string is returned as the value of the call to format. In all other cases
format returns an unspecified value. If destination is #t, the output is sent to the
current output port. Otherwise, destination must be an output port, and the output
is sent there.
This procedure performs discretionary output flushing (see Section 14.5 [Output Procedures], page 188).
A format directive consists of a tilde (~), optional prefix parameters separated by
commas, optional colon (:) and at-sign (@) modifiers, and a single character indicating
what kind of directive this is. The alphabetic case of the directive character is ignored.
The prefix parameters are generally integers, notated as optionally signed decimal
5
This description of format is adapted from Common Lisp, The Language, second edition, section 22.3.3.
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numbers. If both the colon and at-sign modifiers are given, they may appear in either
order.
In place of a prefix parameter to a directive, you can put the letter ‘V’ (or ‘v’), which
takes an argument for use as a parameter to the directive. Normally this should be
an exact integer. This feature allows variable-width fields and the like. You can also
use the character ‘#’ in place of a parameter; it represents the number of arguments
remaining to be processed.
It is an error to give a format directive more parameters than it is described here
as accepting. It is also an error to give colon or at-sign modifiers to a directive in a
combination not specifically described here as being meaningful.
~A
The next argument, which may be any object, is printed as if by display.
~mincolA inserts spaces on the right, if necessary, to make the width at
least mincol columns. The @ modifier causes the spaces to be inserted on
the left rather than the right.
~S
The next argument, which may be any object, is printed as if by write.
~mincolS inserts spaces on the right, if necessary, to make the width at
least mincol columns. The @ modifier causes the spaces to be inserted on
the left rather than the right.
~%
This outputs a #\newline character. ~n% outputs n newlines. No argument is used. Simply putting a newline in control-string would work,
but ~% is often used because it makes the control string look nicer in the
middle of a program.
~~
This outputs a tilde. ~n~ outputs n tildes.
~newline
Tilde immediately followed by a newline ignores the newline and any
following non-newline whitespace characters. With an @, the newline is
left in place, but any following whitespace is ignored. This directive is
typically used when control-string is too long to fit nicely into one line of
the program:
(define (type-clash-error procedure arg spec actual)
(format
#t
"~%Procedure ~S~%requires its %A argument ~
to be of type ~S,~%but it was called with ~
an argument of type ~S.~%"
procedure arg spec actual))
(type-clash-error ’vector-ref
"first"
’integer
’vector)
prints
Procedure vector-ref
requires its first argument to be of type integer,
but it was called with an argument of type vector.
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Note that in this example newlines appear in the output only as specified
by the ~% directives; the actual newline characters in the control string
are suppressed because each is preceded by a tilde.
14.7 Custom Output
MIT Scheme provides hooks for specifying that certain kinds of objects have special
written representations. There are no restrictions on the written representations, but only
a few kinds of objects may have custom representation specified for them, specifically:
records (see Section 10.4 [Records], page 132), vectors that have special tags in their zeroth elements (see Chapter 8 [Vectors], page 119), and pairs that have special tags in their
car fields (see Chapter 7 [Lists], page 105). There is a di↵erent procedure for specifying the
written representation of each of these types.
set-record-type-unparser-method! record-type unparser-method
procedure
Changes the unparser method of the type represented by record-type to be unparsermethod, and returns an unspecified value. Subsequently, when the unparser encounters a record of this type, it will invoke unparser-method to generate the written
representation.
unparser/set-tagged-vector-method! tag unparser-method
Changes
method,
counters
generate
procedure
the unparser method of the vector type represented by tag to be unparserand returns an unspecified value. Subsequently, when the unparser ena vector with tag as its zero-th element, it will invoke unparser-method to
the written representation.
unparser/set-tagged-pair-method! tag unparser-method
procedure
Changes the unparser method of the pair type represented by tag to be unparsermethod, and returns an unspecified value. Subsequently, when the unparser encounters a pair with tag in its car field, it will invoke unparser-method to generate the
written representation.
An unparser method is a procedure that is invoked with two arguments: an unparser
state and an object. An unparser method generates a written representation for the object,
writing it to the output port specified by the unparser state. The value yielded by an
unparser method is ignored. Note that an unparser state is not an output port, rather it
is an object that contains an output port as one of its components. Application programs
generally do not construct or examine unparser state objects, but just pass them along.
There are two ways to create an unparser method (which is then registered by one of
the above procedures). The first, and easiest, is to use standard-unparser-method. The
second is to define your own method using the procedure with-current-unparser-state.
We encourage the use of the first method, as it results in a more uniform appearance for
objects. Many predefined datatypes, for example procedures and environments, already
have this appearance.
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standard-unparser-method name procedure
procedure
Returns a standard unparser method. Name may be any object, and is used as the
name of the type with which the unparser method is associated; name is usually a
symbol. Procedure must be #f or a procedure of two arguments.
If procedure is #f, the returned method generates an external representation of this
form:
#[name hash]
Here name is the external representation of the argument name, as generated by
write,6 and hash is the external representation of an exact non-negative integer
unique to the object being printed (specifically, it is the result of calling hash on the
object). Subsequently, the expression
#@hash
is notation for the object.
If procedure is supplied, the returned method generates a slightly di↵erent external
representation:
#[name hash output]
Here name and hash are as above, and output is the output generated by procedure.
The representation is constructed in three stages:
1. The first part of the format (up to output) is written to the output port specified
by the unparser state. This is "#[", name, " ", and hash.
2. Procedure is invoked on two arguments: the object and an output port.
3. The closing bracket is written to the output port.
The following procedure is useful for writing more general kinds of unparser methods.
with-current-unparser-state unparser-state procedure
procedure
This procedure calls procedure with one argument, the output port from unparserstate. Additionally, it arranges for the remaining components of unparser-state to be
given to the printer when they are needed. The procedure generates some output by
writing to the output port using the usual output operations, and the value yielded
by procedure is returned from with-current-unparser-state.
The port passed to procedure should only be used within the dynamic extent of
procedure.
14.8 Prompting
This section describes procedures that prompt the user for input. Why should the
programmer use these procedures when it is possible to do prompting using ordinary input
and output procedures? One reason is that the prompting procedures are more succinct.
However, a second and better reason is that the prompting procedures can be separately
customized for each user interface, providing more natural interaction. The interfaces for
Edwin and for GNU Emacs have already been customized in this fashion; because Edwin
and Emacs are very similar editors, their customizations provide very similar behavior.
6
Except that if the argument name is a string, its external representation is generated by write-string.
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Each of these procedure accepts an optional argument called port, which if given must
be an i/o port. If not given, this port defaults to the value of (interaction-i/o-port);
this is initially the console i/o port.
prompt-for-command-expression prompt [port]
procedure
Prompts the user for an expression that is to be executed as a command. This is the
procedure called by the rep loop to read the user’s expressions.
If prompt is a string, it is used verbatim as the prompt string. Otherwise, it must
be a pair whose car is standard and whose cdr is a string; in this case the prompt
string is formed by prepending to the string the current rep loop “level number” and
a space. Also, a space is appended to the string, unless it already ends in a space or
is an empty string.
The default behavior of this procedure is to print a fresh line, a newline, and the
prompt string; flush the output bu↵er; then read an object and return it.
Under Edwin and Emacs, before the object is read, the interaction bu↵er is put into
a mode that allows expressions to be edited and submitted for input using specific
editor commands. The first expression that is submitted is returned as the value of
this procedure.
prompt-for-command-char prompt [port]
procedure
Prompts the user for a single character that is to be executed as a command; the
returned character is guaranteed to satisfy char-graphic?. If at all possible, the
character is read from the user interface using a mode that reads the character as a
single keystroke; in other words, it should not be necessary for the user to follow the
character with a carriage return or something similar.
This is the procedure called by debug and where to read the user’s commands.
If prompt is a string, it is used verbatim as the prompt string. Otherwise, it must
be a pair whose car is standard and whose cdr is a string; in this case the prompt
string is formed by prepending to the string the current rep loop “level number” and
a space. Also, a space is appended to the string, unless it already ends in a space or
is an empty string.
The default behavior of this procedure is to print a fresh line, a newline, and the
prompt string; flush the output bu↵er; read a character in raw mode, echo that
character, and return it.
Under Edwin and Emacs, instead of reading a character, the interaction bu↵er is put
into a mode in which graphic characters submit themselves as input. After this mode
change, the first such character submitted is returned as the value of this procedure.
prompt-for-expression prompt [port]
procedure
Prompts the user for an expression.
The prompt string is formed by appending a colon and a space to prompt, unless
prompt already ends in a space or is the null string.
The default behavior of this procedure is to print a fresh line, a newline, and the
prompt string; flush the output bu↵er; then read an object and return it.
Under Edwin and Emacs, the expression is read in the minibu↵er.
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prompt-for-evaluated-expression prompt [environment [port]]
procedure
Prompts the user for an evaluated expression. Calls prompt-for-expression to read
an expression, then evaluates the expression using environment; if environment is not
given, the rep loop environment is used.
prompt-for-confirmation prompt [port]
procedure
Prompts the user for confirmation. The result yielded by this procedure is a boolean.
The prompt string is formed by appending the string " (y or n)? " to prompt, unless
prompt already ends in a space or is the null string.
The default behavior of this procedure is to print a fresh line, a newline, and the
prompt string; flush the output bu↵er; then read a character in raw mode. If the
character is #\y, #\Y, or #\space, the procedure returns #t; If the character is #\n,
#\N, or #\rubout, the procedure returns #f. Otherwise the prompt is repeated.
Under Edwin or Emacs, the confirmation is read in the minibu↵er.
14.9 Port Primitives
This section describes the low-level operations that can be used to build and manipulate
i/o ports.
The purpose of these operations is twofold: to allow programmers to construct new kinds
of i/o ports, and to provide faster i/o operations than those supplied by the standard
high level procedures. The latter is useful because the standard i/o operations provide
defaulting and error checking, and sometimes other features, which are often unnecessary.
This interface provides the means to bypass such features, thus improving performance.
The abstract model of an i/o port, as implemented here, is a combination of a set of
named operations and a state. The state is an arbitrary object, the meaning of which is
determined by the operations. The operations are defined by a mapping from names to
procedures.
The set of named operations is represented by an object called a port type. A port
type is constructed from a set of named operations, and is subsequently used to construct a
port. The port type completely specifies the behavior of the port. Port types also support
a simple form of inheritance, allowing you to create new ports that are similar to existing
ports.
The port operations are divided into two classes:
Standard operations
There is a specific set of standard operations for input ports, and a di↵erent set
for output ports. Applications can assume that the standard input operations
are implemented for all input ports, and likewise the standard output operations
are implemented for all output ports.
Custom operations
Some ports support additional operations. For example, ports that implement
output to terminals (or windows) may define an operation named y-size that
returns the height of the terminal in characters. Because only some ports will
implement these operations, programs that use custom operations must test
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197
each port for their existence, and be prepared to deal with ports that do not
implement them.
14.9.1 Port Types
The procedures in this section provide means for constructing port types with standard
and custom operations, and accessing their operations.
make-port-type operations port-type
procedure
Creates and returns a new port type. Operations must be a list; each element is a
list of two elements, the name of the operation (a symbol) and the procedure that
implements it. Port-type is either #f or a port type; if it is a port type, any operations
implemented by port-type but not specified in operations will be implemented by the
resulting port type.
Operations need not contain definitions for all of the standard operations; the procedure will provide defaults for any standard operations that are not defined. At
a minimum, the following operations must be defined: for input ports, read-char
and peek-char; for output ports, either write-char or write-substring. i/o ports
must supply the minimum operations for both input and output.
If an operation in operations is defined to be #f, then the corresponding operation in
port-type is not inherited.
If read-char is defined in operations, then any standard input operations defined
in port-type are ignored. Likewise, if write-char or write-substring is defined in
operations, then any standard output operations defined in port-type are ignored.
This feature allows overriding the standard operations without having to enumerate
them.
port-type? object
input-port-type? object
output-port-type? object
i/o-port-type? object
procedure
procedure
procedure
procedure
These predicates return #t if object is a port type, input-port type, output-port type,
or i/o-port type, respectively. Otherwise, they return #f.
port-type/operations port-type
procedure
Returns a newly allocated list containing all of the operations implemented by porttype. Each element of the list is a list of two elements — the name and its associated
operation.
port-type/operation-names port-type
procedure
Returns a newly allocated list whose elements are the names of the operations implemented by port-type.
port-type/operation port-type symbol
procedure
Returns the operation named symbol in port-type. If port-type has no such operation,
returns #f.
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14.9.2 Constructors and Accessors for Ports
The procedures in this section provide means for constructing ports, accessing the type
of a port, and manipulating the state of a port.
make-port port-type state
procedure
Returns a new port with type port-type and the given state. The port will be an
input, output, or i/o port according to port-type.
port/type port
procedure
port/state port
procedure
Returns the port type of port.
Returns the state component of port.
set-port/state! port object
procedure
Changes the state component of port to be object. Returns an unspecified value.
port/operation port symbol
procedure
port/operation-names port
procedure
Equivalent to
(port-type/operation (port/type port) symbol)
Equivalent to
(port-type/operation-names (port/type port))
make-eof-object input-port
procedure
Returns an object that satisfies the predicate eof-object?. This is sometimes useful
when building input ports.
14.9.3 Input Port Operations
This section describes the standard operations on input ports. Following that, some
useful custom operations are described.
read-char input-port
operation on input port
Removes the next character available from input-port and returns it. If input-port
has no more characters and will never have any (e.g. at the end of an input file), this
operation returns an end-of-file object. If input-port has no more characters but will
eventually have some more (e.g. a terminal where nothing has been typed recently),
and it is in non-blocking mode, #f is returned; otherwise the operation hangs until
input is available.
peek-char input-port
operation on input port
Reads the next character available from input-port and returns it. The character is
not removed from input-port, and a subsequent attempt to read from the port will
get that character again. In other respects this operation behaves like read-char.
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199
discard-char input-port
operation on input port
Discards the next character available from input-port and returns an unspecified
value. In other respects this operation behaves like read-char.
char-ready? input-port k
operation on input port
char-ready? returns #t if at least one character is available to be read from inputport. If no characters are available, the operation waits up to k milliseconds before
returning #f, returning immediately if any characters become available while it is
waiting.
read-string input-port char-set
discard-chars input-port char-set
operation on input port
operation on input port
These operations are like read-char and discard-char, except that they read or
discard multiple characters at once. This can have a marked performance improvement on bu↵ered input ports. All characters up to, but excluding, the first character
in char-set (or end of file) are read from input-port. read-string returns these characters as a newly allocated string, while discard-chars discards them and returns
an unspecified value. These operations hang until sufficient input is available, even
if input-port is in non-blocking mode. If end of file is encountered before any input
characters, read-string returns an end-of-file object.
read-substring input-port string start end
operation on input port
Reads characters from input-port into the substring defined by string, start, and end
until either the substring has been filled or there are no more characters available.
Returns the number of characters written to the substring.
If input-port is an interactive port, and at least one character is immediately available, the available characters are written to the substring and this operation returns
immediately. If no characters are available, and input-port is in blocking mode, the
operation blocks until at least one character is available. Otherwise, the operation
returns #f immediately.
This is an extremely fast way to read characters from a port.
input-port/read-char input-port
input-port/peek-char input-port
input-port/discard-char input-port
input-port/char-ready? input-port k
input-port/read-string input-port char-set
input-port/discard-chars input-port char-set
input-port/read-substring input-port string start end
procedure
procedure
procedure
procedure
procedure
procedure
procedure
Each of these procedures invokes the respective operation on input-port. For example,
the following are equivalent:
(input-port/read-char input-port)
((input-port/operation input-port ’read-char) input-port)
The following custom operations are implemented for input ports to files, and will also
work with some other kinds of input ports:
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eof ? input-port
operation on input port
Returns #t if input-port is known to be at end of file, otherwise it returns #f.
chars-remaining input-port
operation on input port
Returns an estimate of the number of characters remaining to be read from inputport. This is useful only when input-port is a file port in binary mode; in other cases,
it returns #f.
bu↵ered-input-chars input-port
operation on input port
Returns the number of unread characters that are stored in input-port’s bu↵er. This
will always be less than or equal to the bu↵er’s size.
input-bu↵er-size input-port
operation on input port
Returns the maximum number of characters that input-port’s bu↵er can hold.
set-input-bu↵er-size input-port size
operation on input port
Resizes input-port’s bu↵er so that it can hold at most size characters. Characters in
the bu↵er are discarded. Size must be an exact non-negative integer.
14.9.4 Output Port Operations
This section describes the standard operations on output ports. Following that, some
useful custom operations are described.
write-char output-port char
Writes char to output-port and returns an unspecified value.
operation on output port
write-substring output-port string start end
operation on output port
Writes the substring specified by string, start, and end to output-port and returns an
unspecified value. Equivalent to writing the characters of the substring, one by one,
to output-port, but is implemented very efficiently.
fresh-line output-port
operation on output port
Most output ports are able to tell whether or not they are at the beginning of a line
of output. If output-port is such a port, end-of-line is written to the port only if the
port is not already at the beginning of a line. If output-port is not such a port, and
end-of-line is unconditionally written to the port. Returns an unspecified value.
flush-output output-port
operation on output port
If output-port is bu↵ered, this causes its bu↵er to be written out. Otherwise it has
no e↵ect. Returns an unspecified value.
discretionary-flush-output output-port
operation on output port
Normally, this operation does nothing. However, ports that support discretionary
output flushing implement this operation identically to flush-output.
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output-port/write-char output-port char
output-port/write-substring output-port string start end
output-port/fresh-line output-port
output-port/flush-output output-port
output-port/discretionary-flush-output output-port
procedure
procedure
procedure
procedure
procedure
Each of these procedures invokes the respective operation on output-port. For example, the following are equivalent:
(output-port/write-char output-port char)
((output-port/operation output-port ’write-char)
output-port char)
output-port/write-string output-port string
Writes string to output-port. Equivalent to
(output-port/write-substring output-port
string
0
(string-length string))
procedure
The following custom operations are generally useful.
bu↵ered-output-chars output-port
operation on output port
Returns the number of unwritten characters that are stored in output-port’s bu↵er.
This will always be less than or equal to the bu↵er’s size.
output-bu↵er-size output-port
operation on output port
Returns the maximum number of characters that output-port’s bu↵er can hold.
set-output-bu↵er-size output-port size
operation on output port
Resizes output-port’s bu↵er so that it can hold at most size characters. Characters
in the bu↵er are discarded. Size must be an exact non-negative integer.
x-size output-port
operation on output port
Returns an exact positive integer that is the width of output-port in characters. If
output-port has no natural width, e.g. if it is a file port, #f is returned.
y-size output-port
operation on output port
Returns an exact positive integer that is the height of output-port in characters. If
output-port has no natural height, e.g. if it is a file port, #f is returned.
output-port/x-size output-port
procedure
This procedure invokes the custom operation whose name is the symbol x-size, if it
exists. If the x-size operation is both defined and returns a value other than #f, that
value is returned as the result of this procedure. Otherwise, output-port/x-size
returns a default value (currently 80).
output-port/x-size is useful for programs that tailor their output to the width
of the display (a fairly common practice). If the output device is not a display,
such programs normally want some reasonable default width to work with, and this
procedure provides exactly that.
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output-port/y-size output-port
procedure
This procedure invokes the custom operation whose name is the symbol y-size, if
it exists. If the y-size operation is defined, the value it returns is returned as the
result of this procedure; otherwise, #f is returned.
14.9.5 Blocking Mode
An interactive port is always in one of two modes: blocking or non-blocking. This
mode is independent of the terminal mode: each can be changed independent of the other.
Furthermore, if it is an interactive i/o port, there are separate blocking modes for input
and for output.
If an input port is in blocking mode, attempting to read from it when no input is available
will cause Scheme to “block”, i.e. suspend itself, until input is available. If an input port is
in non-blocking mode, attempting to read from it when no input is available will cause the
reading procedure to return immediately, indicating the lack of input in some way (exactly
how this situation is indicated is separately specified for each procedure or operation).
An output port in blocking mode will block if the output device is not ready to accept
output. In non-blocking mode it will return immediately after performing as much output
as the device will allow (again, each procedure or operation reports this situation in its own
way).
Interactive ports are initially in blocking mode; this can be changed at any time with
the procedures defined in this section.
These procedures represent blocking mode by the symbol blocking, and non-blocking
mode by the symbol nonblocking. An argument called mode must be one of these symbols.
A port argument to any of these procedures may be any port, even if that port does not
support blocking mode; in that case, the port is not modified in any way.
port/input-blocking-mode port
Returns the input blocking mode of port.
procedure
port/set-input-blocking-mode port mode
procedure
Changes the input blocking mode of port to be mode. Returns an unspecified value.
port/with-input-blocking-mode port mode thunk
procedure
Thunk must be a procedure of no arguments. port/with-input-blocking-mode
binds the input blocking mode of port to be mode, executes thunk, restores the input
blocking mode of port to what it was when port/with-input-blocking-mode was
called, and returns the value that was yielded by thunk. This binding is performed by
dynamic-wind, which guarantees that the input blocking mode is restored if thunk
escapes from its continuation.
port/output-blocking-mode port
Returns the output blocking mode of port.
port/set-output-blocking-mode port mode
procedure
procedure
Changes the output blocking mode of port to be mode. Returns an unspecified value.
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port/with-output-blocking-mode port mode thunk
procedure
Thunk must be a procedure of no arguments. port/with-output-blocking-mode
binds the output blocking mode of port to be mode, executes thunk, restores the
output blocking mode of port to what it was when port/with-output-blockingmode was called, and returns the value that was yielded by thunk. This binding
is performed by dynamic-wind, which guarantees that the output blocking mode is
restored if thunk escapes from its continuation.
14.9.6 Terminal Mode
A port that reads from or writes to a terminal has a terminal mode; this is either cooked
or raw. This mode is independent of the blocking mode: each can be changed independent
of the other. Furthermore, a terminal i/o port has independent terminal modes both for
input and for output.
A terminal port in cooked mode provides some standard processing to make the terminal
easy to communicate with. For example, under unix, cooked mode on input reads from the
terminal a line at a time and provides rubout processing within the line, while cooked
mode on output might translate linefeeds to carriage-return/linefeed pairs. In general, the
precise meaning of cooked mode is operating-system dependent, and furthermore might
be customizable by means of operating system utilities. The basic idea is that cooked
mode does whatever is necessary to make the terminal handle all of the usual user-interface
conventions for the operating system, while keeping the program’s interaction with the port
as normal as possible.
A terminal port in raw mode disables all of that processing. In raw mode, characters are
directly read from and written to the device without any translation or interpretation by
the operating system. On input, characters are available as soon as they are typed, and are
not echoed on the terminal by the operating system. In general, programs that put ports
in raw mode have to know the details of interacting with the terminal. In particular, raw
mode is used for writing programs such as text editors.
Terminal ports are initially in cooked mode; this can be changed at any time with the
procedures defined in this section.
These procedures represent cooked mode by the symbol cooked, and raw mode by the
symbol raw. Additionally, the value #f represents “no mode”; it is the terminal mode of a
port that is not a terminal. An argument called mode must be one of these three values.
A port argument to any of these procedures may be any port, even if that port does not
support terminal mode; in that case, the port is not modified in any way.
port/input-terminal-mode port
Returns the input terminal mode of port.
procedure
port/set-input-terminal-mode port mode
procedure
Changes the input terminal mode of port to be mode. Returns an unspecified value.
port/with-input-terminal-mode port mode thunk
procedure
Thunk must be a procedure of no arguments. port/with-input-terminal-mode
binds the input terminal mode of port to be mode, executes thunk, restores the input
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terminal mode of port to what it was when port/with-input-terminal-mode was
called, and returns the value that was yielded by thunk. This binding is performed by
dynamic-wind, which guarantees that the input terminal mode is restored if thunk
escapes from its continuation.
port/output-terminal-mode port
Returns the output terminal mode of port.
procedure
port/set-output-terminal-mode port mode
procedure
Changes the output terminal mode of port to be mode. Returns an unspecified value.
port/with-output-terminal-mode port mode thunk
procedure
Thunk must be a procedure of no arguments. port/with-output-terminal-mode
binds the output terminal mode of port to be mode, executes thunk, restores the
output terminal mode of port to what it was when port/with-output-terminalmode was called, and returns the value that was yielded by thunk. This binding
is performed by dynamic-wind, which guarantees that the output terminal mode is
restored if thunk escapes from its continuation.
14.10 Parser Bu↵ers
The parser bu↵er mechanism facilitates construction of parsers for complex grammars.
It does this by providing an input stream with unbounded bu↵ering and backtracking. The
amount of bu↵ering is under program control. The stream can backtrack to any position
in the bu↵er.
The mechanism defines two data types: the parser bu↵er and the parser-bu↵er pointer.
A parser bu↵er is like an input port with bu↵ering and backtracking. A parser-bu↵er pointer
is a pointer into the stream of characters provided by a parser bu↵er.
Note that all of the procedures defined here consider a parser bu↵er to contain a stream of
8-bit characters in the iso-8859-1 character set, except for match-utf8-char-in-alphabet
which treats it as a stream of Unicode characters encoded as 8-bit bytes in the utf-8
encoding.
There are several constructors for parser bu↵ers:
input-port->parser-bu↵er port
Returns a parser bu↵er that bu↵ers characters read from port.
procedure
substring->parser-bu↵er string start end
procedure
Returns a parser bu↵er that bu↵ers the characters in the argument substring. This is
equivalent to creating a string input port and calling input-port->parser-buffer,
but it runs faster and uses less memory.
string->parser-bu↵er string
Like substring->parser-buffer but bu↵ers the entire string.
procedure
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source->parser-bu↵er source
procedure
Returns a parser bu↵er that bu↵ers the characters returned by calling source. Source
is a procedure of three arguments: a string, a start index, and an end index (in other
words, a substring specifier). Each time source is called, it writes some characters
in the substring, and returns the number of characters written. When there are
no more characters available, it returns zero. It must not return zero in any other
circumstance.
Parser bu↵ers and parser-bu↵er pointers may be distinguished from other objects:
parser-bu↵er? object
procedure
parser-bu↵er-pointer? object
procedure
Returns #t if object is a parser bu↵er, otherwise returns #f.
Returns #t if object is a parser-bu↵er pointer, otherwise returns #f.
Characters can be read from a parser bu↵er much as they can be read from an input
port. The parser bu↵er maintains an internal pointer indicating its current position in the
input stream. Additionally, the bu↵er remembers all characters that were previously read,
and can look at characters arbitrarily far ahead in the stream. It is this bu↵ering capability
that facilitates complex matching and backtracking.
read-parser-bu↵er-char bu↵er
procedure
Returns the next character in bu↵er, advancing the internal pointer past that character. If there are no more characters available, returns #f and leaves the internal
pointer unchanged.
peek-parser-bu↵er-char bu↵er
procedure
Returns the next character in bu↵er, or #f if no characters are available. Leaves the
internal pointer unchanged.
parser-bu↵er-ref bu↵er index
procedure
Returns a character in bu↵er. Index is a non-negative integer specifying the character
to be returned. If index is zero, returns the next available character; if it is one,
returns the character after that, and so on. If index specifies a position after the last
character in bu↵er, returns #f. Leaves the internal pointer unchanged.
The internal pointer of a parser bu↵er can be read or written:
get-parser-bu↵er-pointer bu↵er
procedure
Returns a parser-bu↵er pointer object corresponding to the internal pointer of bu↵er.
set-parser-bu↵er-pointer! bu↵er pointer
procedure
Sets the internal pointer of bu↵er to the position specified by pointer. Pointer must
have been returned from a previous call of get-parser-buffer-pointer on bu↵er.
Additionally, if some of bu↵er’s characters have been discarded by discard-parserbuffer-head!, pointer must be outside the range that was discarded.
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get-parser-bu↵er-tail bu↵er pointer
procedure
Returns a newly-allocated string consisting of all of the characters in bu↵er that fall
between pointer and bu↵er’s internal pointer. Pointer must have been returned from
a previous call of get-parser-buffer-pointer on bu↵er. Additionally, if some of
bu↵er’s characters have been discarded by discard-parser-buffer-head!, pointer
must be outside the range that was discarded.
discard-parser-bu↵er-head! bu↵er
procedure
Discards all characters in bu↵er that have already been read; in other words, all
characters prior to the internal pointer. After this operation has completed, it is no
longer possible to move the internal pointer backwards past the current position by
calling set-parser-buffer-pointer!.
The next rather large set of procedures does conditional matching against the contents
of a parser bu↵er. All matching is performed relative to the bu↵er’s internal pointer, so
the first character to be matched against is the next character that would be returned
by peek-parser-buffer-char. The returned value is always #t for a successful match,
and #f otherwise. For procedures whose names do not end in ‘-no-advance’, a successful
match also moves the internal pointer of the bu↵er forward to the end of the matched text;
otherwise the internal pointer is unchanged.
match-parser-bu↵er-char bu↵er char
match-parser-bu↵er-char-ci bu↵er char
match-parser-bu↵er-not-char bu↵er char
match-parser-bu↵er-not-char-ci bu↵er char
match-parser-bu↵er-char-no-advance bu↵er char
match-parser-bu↵er-char-ci-no-advance bu↵er char
match-parser-bu↵er-not-char-no-advance bu↵er char
match-parser-bu↵er-not-char-ci-no-advance bu↵er char
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
Each of these procedures compares a single character in bu↵er to char. The basic comparison match-parser-buffer-char compares the character to char using
char=?. The procedures whose names contain the ‘-ci’ modifier do case-insensitive
comparison (i.e. they use char-ci=?). The procedures whose names contain the
‘not-’ modifier are successful if the character doesn’t match char.
match-parser-bu↵er-char-in-set bu↵er char-set
match-parser-bu↵er-char-in-set-no-advance bu↵er char-set
procedure
procedure
These procedures compare the next character in bu↵er against char-set using charset-member?.
match-parser-bu↵er-string bu↵er string
match-parser-bu↵er-string-ci bu↵er string
match-parser-bu↵er-string-no-advance bu↵er string
match-parser-bu↵er-string-ci-no-advance bu↵er string
procedure
procedure
procedure
procedure
These procedures match string against bu↵er’s contents. The ‘-ci’ procedures do
case-insensitive matching.
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match-parser-bu↵er-substring bu↵er string start end
match-parser-bu↵er-substring-ci bu↵er string start end
match-parser-bu↵er-substring-no-advance bu↵er string start
207
procedure
procedure
procedure
end
match-parser-bu↵er-substring-ci-no-advance bu↵er string start
procedure
end
These procedures match the specified substring against bu↵er’s contents. The ‘-ci’
procedures do case-insensitive matching.
match-utf8-char-in-alphabet bu↵er alphabet
procedure
This procedure treats bu↵er’s contents as utf-8 encoded Unicode characters and
matches the next such character against alphabet, which must be a Unicode alphabet
(see Section 5.7 [Unicode], page 84). utf-8 represents characters with 1 to 6 bytes,
so a successful match can move the internal pointer forward by as many as 6 bytes.
The remaining procedures provide information that can be used to identify locations in
a parser bu↵er’s stream.
parser-bu↵er-position-string pointer
procedure
Returns a string describing the location of pointer in terms of its character and line
indexes. This resulting string is meant to be presented to an end user in order to
direct their attention to a feature in the input stream. In this string, the indexes are
presented as one-based numbers.
Pointer may alternatively be a parser bu↵er, in which case it is equivalent to having
specified the bu↵er’s internal pointer.
parser-bu↵er-pointer-index pointer
parser-bu↵er-pointer-line pointer
procedure
procedure
Returns the character or line index, respectively, of pointer. Both indexes are zerobased.
14.11 Parser Language
Although it is possible to write parsers using the parser-bu↵er abstraction (see Section 14.10 [Parser Bu↵ers], page 204), it is tedious. The problem is that the abstraction
isn’t closely matched to the way that people think about syntactic structures. In this section, we introduce a higher-level mechanism that greatly simplifies the implementation of a
parser.
The parser language described here allows the programmer to write bnf-like specifications that are translated into efficient Scheme code at compile time. The language is
declarative, but it can be freely mixed with Scheme code; this allows the parsing of grammars that aren’t conveniently described in the language.
The language also provides backtracking. For example, this expression matches any
sequence of alphanumeric characters followed by a single alphabetic character:
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(*matcher
(seq (* (char-set char-set:alphanumeric))
(char-set char-set:alphabetic)))
The way that this works is that the matcher matches alphanumeric characters in the input
stream until it finds a non-alphanumeric character. It then tries to match an alphabetic
character, which of course fails. At this point, if it matched at least one alphanumeric character, it backtracks: the last matched alphanumeric is “unmatched”, and it again attempts
to match an alphabetic character. The backtracking can be arbitrarily deep; the matcher
will continue to back up until it finds a way to match the remainder of the expression.
So far, this sounds a lot like regular-expression matching (see Section 6.8 [Regular Expressions], page 96). However, there are some important di↵erences.
• The parser language uses a Scheme-like syntax that is easier to read and write than
regular-expression notation.
• The language provides macros so that common syntactic constructs can be abstracted.
• The language mixes easily with Scheme code, allowing the full power of Scheme to be
applied to program around limitations in the parser language.
• The language provides expressive facilities for converting syntax into parsed structure.
It also makes it easy to convert parsed strings into meaningful objects (e.g. numbers).
• The language is compiled into machine language; regular expressions are usually interpreted.
Here is an example that shows o↵ several of the features of the parser language. The
example is a parser for xml start tags:
(*parser
(with-pointer p
(seq "<"
parse-name
parse-attribute-list
(alt (match ">")
(match "/>")
(sexp
(lambda (b)
(error
(string-append
"Unterminated start tag at "
(parser-buffer-position-string p)))))))))
This shows that the basic description of a start tag is very similar to its bnf. Non-terminal
symbols parse-name and parse-attribute-list do most of the work, and the noise strings
"<" and ">" are the syntactic markers delimiting the form. There are two alternate endings
for start tags, and if the parser doesn’t find either of the endings, the Scheme code (wrapped
in sexp) is run to signal an error. The error procedure perror takes a pointer p, which it
uses to indicate the position in the input stream at which the error occurred. In this case,
that is the beginning of the start tag, i.e. the position of the leading "<" marker.
This example still looks pretty complicated, mostly due to the error-signalling code. In
practice, this is abstracted into a macro, after which the expression is quite succinct:
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209
(*parser
(bracket "start tag"
(seq (noise (string "<")) parse-name)
(match (alt (string ">") (string "/>")))
parse-attribute-list))
The bracket macro captures the pattern of a bracketed item, and hides much of the detail.
The parser language actually consists of two languages: one for defining matchers, and
one for defining parsers. The languages are intentionally very similar, and are meant to be
used together. Each sub-language is described below in its own section.
The parser language is a run-time-loadable option; to use it, execute
(load-option ’*parser)
once before compiling any code that uses the language.
14.11.1 *Matcher
The matcher language is a declarative language for specifying a matcher procedure. A
matcher procedure is a procedure that accepts a single parser-bu↵er argument and returns
a boolean value indicating whether the match it performs was successful. If the match
succeeds, the internal pointer of the parser bu↵er is moved forward over the matched text.
If the match fails, the internal pointer is unchanged.
For example, here is a matcher procedure that matches the character ‘a’:
(lambda (b) (match-parser-buffer-char b #\a))
Here is another example that matches two given characters, c1 and c2, in sequence:
(lambda (b)
(let ((p (get-parser-buffer-pointer b)))
(if (match-parser-buffer-char b c1)
(if (match-parser-buffer-char b c2)
#t
(begin
(set-parser-buffer-pointer! b p)
#f))
#f)))
This is code is clear, but has lots of details that get in the way of understanding what it is
doing. Here is the same example in the matcher language:
(*matcher (seq (char c1) (char c2)))
This is much simpler and more intuitive. And it generates virtually the same code:
(pp (*matcher (seq (char c1) (char c2))))
a (lambda (#[b1])
(let ((#[p1] (get-parser-buffer-pointer #[b1])))
a
(and (match-parser-buffer-char #[b1] c1)
a
(if (match-parser-buffer-char #[b1] c2)
a
#t
a
(begin
a
(set-parser-buffer-pointer! #[b1] #[p1])
a
#f)))))
a
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Now that we have seen an example of the language, it’s time to look at the detail. The
*matcher special form is the interface between the matcher language and Scheme.
*matcher mexp
special form
The operand mexp is an expression in the matcher language. The *matcher expression expands into Scheme code that implements a matcher procedure.
Here are the predefined matcher expressions. New matcher expressions can be defined
using the macro facility (see Section 14.11.3 [Parser-language Macros], page 215). We will
start with the primitive expressions.
char expression
char-ci expression
not-char expression
not-char-ci expression
matcher expression
matcher expression
matcher expression
matcher expression
These expressions match a given character. In each case, the expression operand
is a Scheme expression that must evaluate to a character at run time. The ‘-ci’
expressions do case-insensitive matching. The ‘not-’ expressions match any character
other than the given one.
string expression
string-ci expression
matcher expression
matcher expression
These expressions match a given string. The expression operand is a Scheme expression that must evaluate to a string at run time. The string-ci expression does
case-insensitive matching.
char-set expression
matcher expression
These expressions match a single character that is a member of a given character set.
The expression operand is a Scheme expression that must evaluate to a character set
at run time.
alphabet expression
matcher expression
These expressions match a single character that is a member of a given Unicode
alphabet (see Section 5.7 [Unicode], page 84). The expression operand is a Scheme
expression that must evaluate to an alphabet at run time.
end-of-input
matcher expression
The end-of-input expression is successful only when there are no more characters
available to be matched.
discard-matched
matcher expression
The discard-matched expression always successfully matches the null string. However, it isn’t meant to be used as a matching expression; it is used for its e↵ect.
discard-matched causes all of the bu↵ered text prior to this point to be discarded
(i.e. it calls discard-parser-buffer-head! on the parser bu↵er).
Note that discard-matched may not be used in certain places in a matcher expression. The reason for this is that it deliberately discards information needed for
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211
backtracking, so it may not be used in a place where subsequent backtracking will
need to back over it. As a rule of thumb, use discard-matched only in the last
operand of a seq or alt expression (including any seq or alt expressions in which it
is indirectly contained).
In addition to the above primitive expressions, there are two convenient abbreviations.
A character literal (e.g. ‘#\A’) is a legal primitive expression, and is equivalent to a char
expression with that literal as its operand (e.g. ‘(char #\A)’). Likewise, a string literal is
equivalent to a string expression (e.g. ‘(string "abc")’).
Next there are several combinator expressions. These closely correspond to similar combinators in regular expressions. Parameters named mexp are arbitrary expressions in the
matcher language.
seq mexp . . .
matcher expression
This matches each mexp operand in sequence. For example,
(seq (char-set char-set:alphabetic)
(char-set char-set:numeric))
matches an alphabetic character followed by a numeric character, such as ‘H4’.
Note that if there are no mexp operands, the seq expression successfully matches the
null string.
alt mexp . . .
matcher expression
This attempts to match each mexp operand in order from left to right. The first one
that successfully matches becomes the match for the entire alt expression.
The alt expression participates in backtracking. If one of the mexp operands matches,
but the overall match in which this expression is embedded fails, the backtracking
mechanism will cause the alt expression to try the remaining mexp operands. For
example, if the expression
(seq (alt "ab" "a") "b")
is matched against the text ‘abc’, the alt expression will initially match its first
operand. But it will then fail to match the second operand of the seq expression.
This will cause the alt to be restarted, at which time it will match ‘a’, and the overall
match will succeed.
Note that if there are no mexp operands, the alt match will always fail.
* mexp
matcher expression
This matches zero or more occurrences of the mexp operand. (Consequently this
match always succeeds.)
The * expression participates in backtracking; if it matches N occurrences of mexp,
but the overall match fails, it will backtrack to N-1 occurrences and continue. If the
overall match continues to fail, the * expression will continue to backtrack until there
are no occurrences left.
matcher expression
This matches one or more occurrences of the mexp operand. It is equivalent to
(seq mexp (* mexp))
+ mexp
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? mexp
matcher expression
This matches zero or one occurrences of the mexp operand. It is equivalent to
(alt mexp (seq))
sexp expression
matcher expression
The sexp expression allows arbitrary Scheme code to be embedded inside a matcher.
The expression operand must evaluate to a matcher procedure at run time; the procedure is called to match the parser bu↵er. For example,
(*matcher
(seq "a"
(sexp parse-foo)
"b"))
expands to
(lambda (#[b1])
(let ((#[p1] (get-parser-buffer-pointer #[b1])))
(and (match-parser-buffer-char #[b1] #\a)
(if (parse-foo #[b1])
(if (match-parser-buffer-char #[b1] #\b)
#t
(begin
(set-parser-buffer-pointer! #[b1] #[p1])
#f))
(begin
(set-parser-buffer-pointer! #[b1] #[p1])
#f)))))
The case in which expression is a symbol is so common that it has an abbreviation:
‘(sexp symbol)’ may be abbreviated as just symbol.
with-pointer identifier mexp
matcher expression
The with-pointer expression fetches the parser bu↵er’s internal pointer (using getparser-buffer-pointer), binds it to identifier, and then matches the pattern specified by mexp. Identifier must be a symbol.
This is meant to be used on conjunction with sexp, as a way to capture a pointer to
a part of the input stream that is outside the sexp expression. An example of the
use of with-pointer appears above (see [with-pointer example], page 208).
14.11.2 *Parser
The parser language is a declarative language for specifying a parser procedure. A parser
procedure is a procedure that accepts a single parser-bu↵er argument and parses some of
the input from the bu↵er. If the parse is successful, the procedure returns a vector of objects
that are the result of the parse, and the internal pointer of the parser bu↵er is advanced
past the input that was parsed. If the parse fails, the procedure returns #f and the internal
pointer is unchanged. This interface is much like that of a matcher procedure, except that
on success the parser procedure returns a vector of values rather than #t.
The *parser special form is the interface between the parser language and Scheme.
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*parser pexp
special form
The operand pexp is an expression in the parser language. The *parser expression
expands into Scheme code that implements a parser procedure.
There are several primitive expressions in the parser language. The first two provide a
bridge to the matcher language (see Section 14.11.1 [*Matcher], page 209):
match mexp
parser expression
The match expression performs a match on the parser bu↵er. The match to be
performed is specified by mexp, which is an expression in the matcher language. If
the match is successful, the result of the match expression is a vector of one element:
a string containing that text.
noise mexp
parser expression
The noise expression performs a match on the parser bu↵er. The match to be
performed is specified by mexp, which is an expression in the matcher language. If
the match is successful, the result of the noise expression is a vector of zero elements.
(In other words, the text is matched and then thrown away.)
The mexp operand is often a known character or string, so in the case that mexp is
a character or string literal, the noise expression can be abbreviated as the literal.
In other words, ‘(noise "foo")’ can be abbreviated just ‘"foo"’.
values expression . . .
parser expression
Sometimes it is useful to be able to insert arbitrary values into the parser result. The
values expression supports this. The expression arguments are arbitrary Scheme
expressions that are evaluated at run time and returned in a vector. The values
expression always succeeds and never modifies the internal pointer of the parser bu↵er.
discard-matched
parser expression
The discard-matched expression always succeeds, returning a vector of zero elements. In all other respects it is identical to the discard-matched expression in the
matcher language.
Next there are several combinator expressions. Parameters named pexp are arbitrary
expressions in the parser language. The first few combinators are direct equivalents of those
in the matcher language.
seq pexp . . .
parser expression
The seq expression parses each of the pexp operands in order. If all of the pexp
operands successfully match, the result is the concatenation of their values (by
vector-append).
alt pexp . . .
parser expression
The alt expression attempts to parse each pexp operand in order from left to right.
The first one that successfully parses produces the result for the entire alt expression.
Like the alt expression in the matcher language, this expression participates in backtracking.
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* pexp
parser expression
The * expression parses zero or more occurrences of pexp. The results of the parsed
occurrences are concatenated together (by vector-append) to produce the expression’s result.
Like the * expression in the matcher language, this expression participates in backtracking.
parser expression
The * expression parses one or more occurrences of pexp. It is equivalent to
(seq pexp (* pexp))
+ pexp
? pexp
parser expression
The * expression parses zero or one occurrences of pexp. It is equivalent to
(alt pexp (seq))
The next three expressions do not have equivalents in the matcher language. Each
accepts a single pexp argument, which is parsed in the usual way. These expressions perform
transformations on the returned values of a successful match.
transform expression pexp
parser expression
The transform expression performs an arbitrary transformation of the values returned by parsing pexp. Expression is a Scheme expression that must evaluate to a
procedure at run time. If pexp is successfully parsed, the procedure is called with
the vector of values as its argument, and must return a vector or #f. If it returns
a vector, the parse is successful, and those are the resulting values. If it returns #f,
the parse fails and the internal pointer of the parser bu↵er is returned to what it was
before pexp was parsed.
For example:
(transform (lambda (v) (if (= 0 (vector-length v)) #f v)) ...)
encapsulate expression pexp
parser expression
The encapsulate expression transforms the values returned by parsing pexp into a
single value. Expression is a Scheme expression that must evaluate to a procedure
at run time. If pexp is successfully parsed, the procedure is called with the vector
of values as its argument, and may return any Scheme object. The result of the
encapsulate expression is a vector of length one containing that object. (And consequently encapsulate doesn’t change the success or failure of pexp, only its value.)
For example:
(encapsulate vector->list ...)
map expression pexp
parser expression
The map expression performs a per-element transform on the values returned by parsing pexp. Expression is a Scheme expression that must evaluate to a procedure at run
time. If pexp is successfully parsed, the procedure is mapped (by vector-map) over
the values returned from the parse. The mapped values are returned as the result of
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the map expression. (And consequently map doesn’t change the success or failure of
pexp, nor the number of values returned.)
For example:
(map string->symbol ...)
Finally, as in the matcher language, we have sexp and with-pointer to support embedding Scheme code in the parser.
sexp expression
parser expression
The sexp expression allows arbitrary Scheme code to be embedded inside a parser.
The expression operand must evaluate to a parser procedure at run time; the procedure is called to parse the parser bu↵er. This is the parser-language equivalent of the
sexp expression in the matcher language.
The case in which expression is a symbol is so common that it has an abbreviation:
‘(sexp symbol)’ may be abbreviated as just symbol.
with-pointer identifier pexp
parser expression
The with-pointer expression fetches the parser bu↵er’s internal pointer (using getparser-buffer-pointer), binds it to identifier, and then parses the pattern specified
by pexp. Identifier must be a symbol. This is the parser-language equivalent of the
with-pointer expression in the matcher language.
14.11.3 Parser-language Macros
The parser and matcher languages provide a macro facility so that common patterns can
be abstracted. The macro facility allows new expression types to be independently defined
in the two languages. The macros are defined in heirarchically organized tables, so that
di↵erent applications can have private macro bindings.
define-*matcher-macro formals expression
define-*parser-macro formals expression
special form
special form
These special forms are used to define macros in the matcher and parser language,
respectively. Formals is like the formals list of a define special form, and expression
is a Scheme expression.
If formals is a list (or improper list) of symbols, the first symbol in the list is the name
of the macro, and the remaining symbols are interpreted as the formals of a lambda
expression. A lambda expression is formed by combining the latter formals with
the expression, and this lambda expression, when evaluated, becomes the expander.
The defined macro accepts the same number of operands as the expander. A macro
instance is expanded by applying the expander to the list of operands; the result of
the application is interpreted as a replacement expression for the macro instance.
If formals is a symbol, it is the name of the macro. In this case, the expander is
a procedure of no arguments whose body is expression. When the formals symbol
appears by itself as an expression in the language, the expander is called with no
arguments, and the result is interpreted as a replacement expression for the symbol.
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define-*matcher-expander identifier expander
define-*parser-expander identifier expander
procedure
procedure
These procedures provide a procedural interface to the macro-definition mechanism.
Identifier must be a symbol, and expander must be an expander procedure, as defined
above. Instances of the define-*matcher-macro and define-*parser-macro special
forms expand into calls to these procedures.
The remaining procedures define the interface to the parser-macros table abstraction.
Each parser-macro table has a separate binding space for macros in the matcher and parser
languages. However, the table inherits bindings from one specified table; it’s not possible
to inherit matcher-language bindings from one table and parser-language bindings from
another.
make-parser-macros parent-table
procedure
Create and return a new parser-macro table that inherits from parent-table. Parenttable must be either a parser-macro table, or #f; usually it is specified as the value
of global-parser-macros.
parser-macros? object
This is a predicate for parser-macro tables.
procedure
global-parser-macros
procedure
Return the global parser-macro table. This table is predefined and contains all of the
bindings documented here.
There is a “current” table at all times, and macro definitions are always placed in this
table. By default, the current table is the global macro table, but the following procedures
allow this to be changed.
current-parser-macros
Return the current parser-macro table.
procedure
set-current-parser-macros! table
procedure
Change the current parser-macro table to table, which must satisfy parser-macros?.
with-current-parser-macros table thunk
procedure
Bind the current parser-macro table to table, call thunk with no arguments, then
restore the original table binding. The value returned by thunk is the returned as the
value of this procedure. Table must satisfy parser-macros?, and thunk must be a
procedure of no arguments.
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14.12 XML Parser
MIT Scheme provides a simple non-validating xml parser. This parser is mostly conformant, with the exception that it doesn’t support utf-16. The parser also does not support
external document type declarations (dtds). The output of the parser is a record tree that
closely reflects the structure of the xml document.
There is also an output mechanism that writes an xml record tree to a port. There is no
guarantee that parsing an xml document and writing it back out will make a verbatim copy
of the document. The output will be semantically identical but may have small syntactic
di↵erences. For example, comments are discarded by the parser, and entities are substituted
during the parsing process.
The purpose of the xml support is to provide a mechanism for reading and writing
simple xml documents. In the future this support may be further developed to support a
standard interface such as dom or sax.
The xml support is a run-time-loadable option; to use it, execute
(load-option ’xml)
once before compiling any code that uses it.
The xml interface consists of an input procedure, an output procedure, and a set of
record types.
parse-xml-document bu↵er
procedure
This procedure parses an xml input stream and returns a newly-allocated xml record
tree. The bu↵er argument must be a parser bu↵er (see Section 14.10 [Parser Bu↵ers],
page 204). Most errors in the input stream are detected and signalled, with information identifying the location of the error where possible. Note that the input stream
is assumed to be utf-8.
write-xml xml-document port
procedure
This procedure writes an xml record tree to port. The xml-document argument must
be a record of type xml-document, which is the root record of an xml record tree.
The output is encoded in utf-8.
xml names are represented in memory as symbols. All symbols appearing within xml
records are xml names. Because xml names are case sensitive, there is a procedure to
intern these symbols:
xml-intern string
procedure
Returns the xml name called string. xml names are represented as symbols, but
unlike ordinary Scheme symbols, they are case sensitive. The following is true for any
two strings string1 and string2:
(let ((name1 (xml-intern string1))
(name2 (xml-intern string2)))
(if (string=? string1 string2)
(eq? name1 name2)
(not (eq? name1 name2))))
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The output from the xml parser and the input to the xml output procedure is a complex
data structure composed of a heirarchy of typed components. Each component is a record
whose fields correspond to parts of the xml structure that the record represents. There
are no special operations on these records; each is a tuple with named subparts. The root
record type is xml-document, which represents a complete xml document.
Each record type type has the following associated bindings:
type-rtd
is a variable bound to the record-type descriptor for type. The record-type
descriptor may be used as a specializer in sos method definitions, which greatly
simplifies code to dispatch on these types.
type?
is a predicate for records of type type. It accepts one argument, which can be
any object, and returns #t if the object is a record of this type, or #f otherwise.
make-type is a constructor for records of type type. It accepts one argument for each field
of type, in the same order that they are written in the type description, and
returns a newly-allocated record of that type.
type-field
is an accessor procedure for the field field in records of type type. It accepts
one argument, which must be a record of that type, and returns the contents
of the corresponding field in the record.
set-type-field!
is a modifier procedure for the field field in records of type type. It accepts two
arguments: the first must be a record of that type, and the second is a new
value for the corresponding field. The record’s field is modified to have the new
value.
xml-document declaration misc-1 dtd misc-2 root misc-3
record type
The xml-document record is the top-level record representing a complete xml document. Declaration is either an xml-declaration object or #f. Dtd is either an
xml-dtd object or #f. Root is an xml-element object. Misc-1, misc-2, and misc-3
are lists of miscellaneous items; a miscellaneous item is either an xml-processinginstructions object or a string of whitespace.
xml-declaration version encoding standalone
record type
The xml-declaration record represents the ‘<?xml ... ?>’ declaration that optionally appears at the beginning of an xml document. Version is a version string,
typically "1.0". Encoding is either an encoding string or #f. Standalone is either
"yes", "no", or #f.
xml-element name attributes contents
record type
The xml-element record represents general xml elements; the bulk of a typical xml
document consists of these elements. Name is the element name (a symbol). Attributes is a list of attributes; each attribute is a pair whose car is the attribute name
(a symbol), and whose cdr is the attribute value (a string). Contents is a list of the
contents of the element. Each element of this list is either a string, an xml-element
record, an xml-processing-instructions record, or an xml-uninterpreted record.
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xml-processing-instructions name text
record type
The xml-processing-instructions record represents processing instructions, which
have the form ‘<?name ... ?>’. These instructions are intended to contain non-xml
data that will be processed by another interpreter; for example they might contain
php programs. The name field is the processor name (a symbol), and the text field
is the body of the instructions (a string).
xml-uninterpreted text
record type
Some documents contain entity references that can’t be expanded by the parser,
perhaps because the document requires an external dtd. Such references are left uninterpreted in the output by wrapping them in xml-uninterpreted records. In some
situations, for example when they are embedded in attribute values, the surrounding
text is also included in the xml-uninterpreted record. The text field contains the
uninterpreted xml text (a string).
xml-dtd root external internal
record type
The xml-dtd record represents a document type declaration. The root field is an xml
name for the root element of the document. External is either an xml-external-id
record or #f. Internal is a list of dtd element records (e.g. xml-!element, xml!attlist, etc.).
The remaining record types are valid only within a dtd.
xml-!element name content-type
record type
The xml-!element record represents an element-type declaration. Name is the xml
name of the type being declared (a symbol). Content-type describes the type and
can have several di↵erent values, as follows:
• The xml names ‘EMPTY’ and ‘ANY’ correspond to the xml keywords of the same
name.
• A list ‘(MIX type ...)’ corresponds to the ‘(#PCDATA | type | ...)’ syntax.
xml-!attlist name definitions
record type
The xml-!attlist record represents an attribute-list declaration. Name is the xml
name of the type for which attributes are being declared (a symbol). Definitions is
a list of attribute definitions, each of which is a list of three elements (name type
default). Name is an xml name for the name of the attribute (a symbol). Type
describes the attribute type, and can have one of the following values:
• The xml names ‘CDATA’, ‘IDREFS’, ‘IDREF’, ‘ID’, ‘ENTITY’, ‘ENTITIES’,
‘NMTOKENS’, and ‘NMTOKEN’ correspond to the xml keywords of the same names.
• A list ‘(NOTATION name1 name2 ...)’ corresponds to the ‘NOTATION (name1 |
name2 ...)’ syntax.
• A list ‘(ENUMERATED name1 name2 ...)’ corresponds to the ‘(name1 | name2
...)’ syntax.
Default describes the default value for the attribute, and can have one of the following
values:
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• The xml names ‘#REQUIRED’ and ‘#IMPLIED’ correspond to the xml keywords of
the same names.
• A list ‘(#FIXED value)’ corresponds to the ‘#FIXED "value"’ syntax. Value is
represented as a string, but might also be an xml-uninterpreted record.
• A list ‘(DEFAULT value)’ corresponds to the ‘"value"’ syntax. Value is represented as a string, but might also be an xml-uninterpreted record.
xml-!entity name value
record type
The xml-!entity record represents a general entity declaration. Name is an xml
name for the entity. Value is the entity’s value, either a string, an xml-uninterpreted
record, or an xml-external-id record.
xml-parameter-!entity name value
record type
The xml-parameter-!entity record represents a parameter entity declaration. Name
is an xml name for the entity. Value is the entity’s value, either a string, an xmluninterpreted record, or an xml-external-id record.
xml-unparsed-!entity name id notation
record type
The xml-unparsed-!entity record represents an unparsed entity declaration. Name
is an xml name for the entity. Id is an xml-external-id record. Notation is an xml
name for the notation.
xml-!notation name id
record type
The xml-!notation record represents a notation declaration. Name is an xml name
for the notation. Id is an xml-external-id record.
xml-external-id id uri
record type
The xml-external-id record is a reference to an external dtd. This reference consists of two parts: id is a public id literal, corresponding to the ‘PUBLIC’ keyword,
while uri is a system literal, corresponding to the ‘SYSTEM’ keyword. Either or both
may be present, depending on the context. Each is represented as a string.
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15 Operating-System Interface
The Scheme standard provides a simple mechanism for reading and writing files: file
ports. MIT Scheme provides additional tools for dealing with other aspects of the operating
system:
• Pathnames are a reasonably operating-system independent tool for manipulating the
component parts of file names. This can be useful for implementing defaulting of file
name components.
• Control over the current working directory: the place in the file system from which
relative file names are interpreted.
• Procedures that rename, copy, delete, and test for the existence of files. Also, procedures that return detailed information about a particular file, such as its type (directory,
link, etc.) or length.
• Procedures for reading the contents of a directory.
• Procedures for obtaining times in various formats, converting between the formats, and
generating human-readable time strings.
• Procedures to run other programs as subprocesses of Scheme, to read their output, and
write input to them.
• A means to determine the operating system Scheme is running under.
15.1 Pathnames
MIT Scheme programs need to use names to designate files. The main difficulty in
dealing with names of files is that di↵erent file systems have di↵erent naming formats for
files. For example, here is a table of several file systems (actually, operating systems that
provide file systems) and what equivalent file names might look like for each one:
System
File Name
-------------TOPS-20
<LISPIO>FORMAT.FASL.13
TOPS-10
FORMAT.FAS[1,4]
ITS
LISPIO;FORMAT FASL
MULTICS
>udd>LispIO>format.fasl
TENEX
<LISPIO>FORMAT.FASL;13
VAX/VMS
[LISPIO]FORMAT.FAS;13
UNIX
/usr/lispio/format.fasl
DOS
C:\USR\LISPIO\FORMAT.FAS
It would be impossible for each program that deals with file names to know about
each di↵erent file name format that exists; a new operating system to which Scheme was
ported might use a format di↵erent from any of its predecessors. Therefore, MIT Scheme
provides two ways to represent file names: filenames (also called namestrings), which are
strings in the implementation-dependent form customary for the file system, and pathnames,
which are special abstract data objects that represent file names in an implementationindependent way. Procedures are provided to convert between these two representations,
and all manipulations of files can be expressed in machine-independent terms by using
pathnames.
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In order to allow MIT Scheme programs to operate in a network environment that may
have more than one kind of file system, the pathname facility allows a file name to specify
which file system is to be used. In this context, each file system is called a host, in keeping
with the usual networking terminology.1
Note that the examples given in this section are specific to unix pathnames. Pathnames
for other operating systems have di↵erent external representations.
15.1.1 Filenames and Pathnames
Pathname objects are usually created by parsing filenames (character strings) into component parts. MIT Scheme provides operations that convert filenames into pathnames and
vice versa.
->pathname object
procedure
Returns a pathname that is the equivalent of object. Object must be a pathname or
a string. If object is a pathname, it is returned. If object is a string, this procedure
returns the pathname that corresponds to the string; in this case it is equivalent to
(parse-namestring object #f #f).
) #[pathname 65 "foo"]
(->pathname "foo")
(->pathname "/usr/morris") ) #[pathname 66 "/usr/morris"]
parse-namestring thing [host [defaults]]
procedure
This turns thing into a pathname. Thing must be a pathname or a string. If thing is
a pathname, it is returned. If thing is a string, this procedure returns the pathname
that corresponds to the string, parsed according to the syntax of the file system
specified by host.
This procedure does not do defaulting of pathname components.
The optional arguments are used to determine what syntax should be used for parsing
the string. In general this is only really useful if your implementation of MIT Scheme
supports more than one file system, otherwise you would use ->pathname. If given,
host must be a host object or #f, and defaults must be a pathname. Host specifies the
syntax used to parse the string. If host is not given or #f, the host component from
defaults is used instead; if defaults is not given, the host component from *defaultpathname-defaults* is used.
->namestring pathname
procedure
->namestring returns a newly allocated string that is the filename corresponding to
pathname.
(->namestring (->pathname "/usr/morris/minor.van"))
) "/usr/morris/minor.van"
pathname-simplify pathname
procedure
Returns a pathname that locates the same file or directory as pathname, but is in some
sense simpler. Note that pathname-simplify might not always be able to simplify the
1
This introduction is adapted from Common Lisp, The Language, second edition, section 23.1.
Chapter 15: Operating-System Interface
223
pathname, e.g. on unix with symbolic links the directory ‘/usr/morris/../’ need not
be the same as ‘/usr/’. In cases of uncertainty the behavior is conservative, returning
the original or a partly simplified pathname.
(pathname-simplify "/usr/morris/../morris/dance")
) #[pathname "/usr/morris/dance"]
15.1.2 Components of Pathnames
A pathname object always has six components, described below. These components are
the common interface that allows programs to work the same way with di↵erent file systems;
the mapping of the pathname components into the concepts peculiar to each file system is
taken care of by the Scheme implementation.
host
The name of the file system on which the file resides. In the current implementation, this component is always a host object that is filled in automatically by
the runtime system. When specifying the host component, use either #f or the
value of the variable local-host.
device
Corresponds to the “device” or “file structure” concept in many host file systems: the name of a (logical or physical) device containing files. This component
is the drive letter for PC file systems, and is unused for unix file systems.
directory
Corresponds to the “directory” concept in many host file systems: the name of
a group of related files (typically those belonging to a single user or project).
This component is always used for all file systems.
name
The name of a group of files that can be thought of as conceptually the “same”
file. This component is always used for all file systems.
type
Corresponds to the “filetype” or “extension” concept in many host file systems.
This says what kind of file this is. Files with the same name but di↵erent type
are usually related in some specific way, such as one being a source file, another
the compiled form of that source, and a third the listing of error messages
from the compiler. This component is currently used for all file systems, and is
formed by taking the characters that follow the last dot in the namestring.
version
Corresponds to the “version number” concept in many host file systems. Typically this is a number that is incremented every time the file is modified. This
component is currently unused for all file systems.
Note that a pathname is not necessarily the name of a specific file. Rather, it is a
specification (possibly only a partial specification) of how to access a file. A pathname need
not correspond to any file that actually exists, and more than one pathname can refer to the
same file. For example, the pathname with a version of newest may refer to the same file
as a pathname with the same components except a certain number as the version. Indeed,
a pathname with version newest may refer to di↵erent files as time passes, because the
meaning of such a pathname depends on the state of the file system. In file systems with
such facilities as “links”, multiple file names, logical devices, and so on, two pathnames
that look quite di↵erent may turn out to address the same file. To access a file given a
pathname, one must do a file-system operation such as open-input-file.
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Two important operations involving pathnames are parsing and merging. Parsing is the
conversion of a filename (which might be something supplied interactively by the users
when asked to supply the name of a file) into a pathname object. This operation is
implementation-dependent, because the format of filenames is implementation-dependent.
Merging takes a pathname with missing components and supplies values for those components from a source of default values.
Not all of the components of a pathname need to be specified. If a component of a
pathname is missing, its value is #f. Before the file system interface can do anything
interesting with a file, such as opening the file, all the missing components of a pathname
must be filled in. Pathnames with missing components are used internally for various
purposes; in particular, parsing a namestring that does not specify certain components will
result in a pathname with missing components.
Any component of a pathname may be the symbol unspecific, meaning that the component simply does not exist, for file systems in which such a value makes no sense. For
example, unix, Windows, and OS/2 file systems usually do not support version numbers,
so the version component for such a host might be unspecific.2
In addition to #f and unspecific, the components of a pathname may take on the
following meaningful values:
host
An implementation-defined type which may be tested for using the host? predicate.
device
On systems that support this component (Windows and OS/2), it may be
specified as a string containing a single alphabetic character, for which the
alphabetic case is ignored.
directory
A non-empty list, which represents a directory path: a sequence of directories,
each of which has a name in the previous directory, the last of which is the
directory specified by the entire path. Each element in such a path specifies
the name of the directory relative to the directory specified by the elements
to its left. The first element of the list is either the symbol absolute or the
symbol relative. If the first element in the list is the symbol absolute, then
the directory component (and subsequently the pathname) is absolute; the first
component in the sequence is to be found at the “root” of the file system. If
the directory is relative then the first component is to be found in some as yet
unspecified directory; typically this is later specified to be the current working
directory.
Aside from absolute and relative, which may only appear as the first element
of the list, each subsequent element in the list is either: a string, which is a
literal component; the symbol wild, meaningful only when used in conjunction
with the directory reader; or the symbol up, meaning the next directory is the
“parent” of the previous one. up corresponds to the file ‘..’ in unix and PC
file systems.
(The following note does not refer to any file system currently supported by
MIT Scheme, but is included for completeness.) In file systems that do not
have “hierarchical” structure, a specified directory component will always be a
2
This description is adapted from Common Lisp, The Language, second edition, section 23.1.1.
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225
list whose first element is absolute. If the system does not support directories
other than a single global directory, the list will have no other elements. If
the system supports “flat” directories, i.e. a global set of directories with no
subdirectories, then the list will contain a second element, which is either a
string or wild. In other words, a non-hierarchical file system is treated as if it
were hierarchical, but the hierarchical features are unused. This representation
is somewhat inconvenient for such file systems, but it discourages programmers
from making code depend on the lack of a file hierarchy.
name
A string, which is a literal component; or the symbol wild, meaningful only
when used in conjunction with the directory reader.
type
A string, which is a literal component; or the symbol wild, meaningful only
when used in conjunction with the directory reader.
version
An exact positive integer, which is a literal component; the symbol newest,
which means to choose the largest available version number for that file; the
symbol oldest, which means to choose the smallest version number; or the symbol wild, meaningful only when used in conjunction with the directory reader.
In the future some other possible values may be added, e.g. installed. Note
that currently no file systems support version numbers; thus this component is
not used and should be specified as #f.
make-pathname host device directory name type version
procedure
Returns a pathname object whose components are the respective arguments. Each
argument must satisfy the restrictions for the corresponding component, which were
outlined above.
(make-pathname #f
#f
’(absolute "usr" "morris")
"foo"
"scm"
#f)
) #[pathname 67 "/usr/morris/foo.scm"]
pathname-host pathname
pathname-device pathname
pathname-directory pathname
pathname-name pathname
pathname-type pathname
pathname-version pathname
Returns a particular component of pathname.
(define x (->pathname "/usr/morris/foo.scm"))
) #[host 1]
(pathname-host x)
) unspecific
(pathname-device x)
(pathname-directory x) ) (absolute "usr" "morris")
) "foo"
(pathname-name x)
) "scm"
(pathname-type x)
) unspecific
(pathname-version x)
procedure
procedure
procedure
procedure
procedure
procedure
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pathname-new-device pathname device
pathname-new-directory pathname directory
pathname-new-name pathname name
pathname-new-type pathname type
pathname-new-version pathname version
procedure
procedure
procedure
procedure
procedure
Returns a new copy of pathname with the respective component replaced by the
second argument. Pathname is unchanged. Portable programs should not explicitly
replace a component with unspecific because this might not be permitted in some
situations.
(define p (->pathname "/usr/blisp/rel15"))
p
) #[pathname 71 "/usr/blisp/rel15"]
(pathname-new-name p "rel100")
) #[pathname 72 "/usr/blisp/rel100"]
(pathname-new-directory p ’(relative "test" "morris"))
) #[pathname 73 "test/morris/rel15"]
p
) #[pathname 71 "/usr/blisp/rel15"]
pathname-default-device pathname device
pathname-default-directory pathname directory
pathname-default-name pathname name
pathname-default-type pathname type
pathname-default-version pathname version
procedure
procedure
procedure
procedure
procedure
These operations are similar to the pathname-new-component operations, except that
they only change the specified component if it has the value #f in pathname.
15.1.3 Operations on Pathnames
pathname? object
Returns #t if object is a pathname; otherwise returns #f.
procedure
pathname=? pathname1 pathname2
procedure
Returns #t if pathname1 is equivalent to pathname2; otherwise returns #f. Pathnames are equivalent if all of their components are equivalent, hence two pathnames
that are equivalent must identify the same file or equivalent partial pathnames. However, the converse is not true: non-equivalent pathnames may specify the same file
(e.g. via absolute and relative directory components), and pathnames that specify no
file at all (e.g. name and directory components unspecified) may be equivalent.
pathname-absolute? pathname
procedure
Returns #t if pathname is an absolute rather than relative pathname object; otherwise
returns #f. Specifically, this procedure returns #t when the directory component of
pathname is a list starting with the symbol absolute, and returns #f in all other
cases. All pathnames are either absolute or relative, so if this procedure returns #f,
the argument is a relative pathname.
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227
directory-pathname? pathname
procedure
Returns #t if pathname has only directory components and no file components. This
is roughly equivalent to
(define (directory-pathname? pathname)
(string-null? (file-namestring pathname)))
except that it is faster.
pathname-wild? pathname
procedure
Returns #t if pathname contains any wildcard components; otherwise returns #f.
merge-pathnames pathname [defaults [default-version]]
procedure
Returns a pathname whose components are obtained by combining those of pathname
and defaults. Defaults defaults to the value of *default-pathname-defaults* and
default-version defaults to newest.
The pathnames are combined by components: if pathname has a non-missing component, that is the resulting component, otherwise the component from defaults is
used. The default version can be #f to preserve the information that the component
was missing from pathname. The directory component is handled specially: if both
pathnames have directory components that are lists, and the directory component
from pathname is relative (i.e. starts with relative), then the resulting directory
component is formed by appending pathname’s component to defaults’s component.
For example:
(define path1 (->pathname "scheme/foo.scm"))
(define path2 (->pathname "/usr/morris"))
path1
) #[pathname 74 "scheme/foo.scm"]
path2
) #[pathname 75 "/usr/morris"]
(merge-pathnames path1 path2)
) #[pathname 76 "/usr/scheme/foo.scm"]
(merge-pathnames path2 path1)
) #[pathname 77 "/usr/morris.scm"]
The merging rules for the version are more complex and depend on whether pathname
specifies a name. If pathname does not specify a name, then the version, if not
provided, will come from defaults. However, if pathname does specify a name then
the version is not a↵ected by defaults. The reason is that the version “belongs to”
some other file name and is unlikely to have anything to do with the new one. Finally,
if this process leaves the version missing, then default-version is used.
The net e↵ect is that if the user supplies just a name, then the host, device, directory
and type will come from defaults, but the version will come from default-version. If
the user supplies nothing, or just a directory, the name, type and version will come
over from defaults together.
*default-pathname-defaults*
variable
This is the default pathname-defaults pathname; if any pathname primitive that
needs a set of defaults is not given one, it uses this one. set-working-directory-
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pathname! sets this variable to a new value, computed by merging the new working
directory with the variable’s old value.
pathname-default pathname device directory name type version
procedure
This procedure defaults all of the components of pathname simultaneously. It could
have been defined by:
(define (pathname-default pathname
device directory name type version)
(make-pathname (pathname-host pathname)
(or (pathname-device pathname) device)
(or (pathname-directory pathname) directory)
(or (pathname-name pathname) name)
(or (pathname-type pathname) type)
(or (pathname-version pathname) version)))
file-namestring pathname
directory-namestring pathname
host-namestring pathname
enough-namestring pathname [defaults]
procedure
procedure
procedure
procedure
These procedures return a string corresponding to a subset of the pathname information. file-namestring returns a string representing just the name, type and version
components of pathname; the result of directory-namestring represents just the
host, device, and directory components; and host-namestring returns a string for
just the host portion.
enough-namestring takes another argument, defaults. It returns an abbreviated
namestring that is just sufficient to identify the file named by pathname when considered relative to the defaults (which defaults to *default-pathname-defaults*).
(file-namestring "/usr/morris/minor.van")
) "minor.van"
(directory-namestring "/usr/morris/minor.van")
) "/usr/morris/"
(enough-namestring "/usr/morris/men")
) "men"
;perhaps
file-pathname pathname
directory-pathname pathname
enough-pathname pathname [defaults]
procedure
procedure
procedure
These procedures return a pathname corresponding to a subset of the pathname information. file-pathname returns a pathname with just the name, type and version
components of pathname. The result of directory-pathname is a pathname containing the host, device and directory components of pathname.
enough-pathname takes another argument, defaults. It returns an abbreviated pathname that is just sufficient to identify the file named by pathname when considered
relative to the defaults (which defaults to *default-pathname-defaults*).
These procedures are similar to file-namestring, directory-namestring and
enough-namestring, but they return pathnames instead of strings.
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229
directory-pathname-as-file pathname
procedure
Returns a pathname that is equivalent to pathname, but in which the directory component is represented as a file. The last directory is removed from the directory
component and converted into name and type components. This is the inverse operation to pathname-as-directory.
(directory-pathname-as-file (->pathname "/usr/blisp/"))
) #[pathname "/usr/blisp"]
pathname-as-directory pathname
procedure
Returns a pathname that is equivalent to pathname, but in which any file components
have been converted to a directory component. If pathname does not have name,
type, or version components, it is returned without modification. Otherwise, these
file components are converted into a string, and the string is added to the end of the
list of directory components. This is the inverse operation to directory-pathnameas-file.
(pathname-as-directory (->pathname "/usr/blisp/rel5"))
) #[pathname "/usr/blisp/rel5/"]
15.1.4 Miscellaneous Pathname Procedures
This section gives some standard operations on host objects, and some procedures that
return some useful pathnames.
local-host
variable
This variable has as its value the host object that describes the local host’s file system.
host? object
Returns #t if object is a pathname host; otherwise returns #f.
procedure
host=? host1 host2
procedure
Returns #t if host1 and host2 denote the same pathname host; otherwise returns #f.
init-file-pathname [host]
procedure
Returns a pathname for the user’s initialization file on host. The host argument
defaults to the value of local-host. If the initialization file does not exist this
procedure returns #f.
Under unix, the init file is called ‘.scheme.init’; under Windows and OS/2, the init
file is called ‘scheme.ini’. In either case, it is located in the user’s home directory,
which is computed by user-homedir-pathname.
user-homedir-pathname [host]
procedure
Returns a pathname for the user’s “home directory” on host. The host argument
defaults to the value of local-host. The concept of a “home directory” is itself
somewhat implementation-dependent, but it should be the place where the user keeps
personal files, such as initialization files and mail.
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Under unix, the user’s home directory is specified by the HOME environment variable.
If this variable is undefined, the user name is computed using the getlogin system
call, or if that fails, the getuid system call. The resulting user name is passed to the
getpwnam system call to obtain the home directory.
Under OS/2, several heuristics are tried to find the user’s home directory. First,
if the environment variable HOME is defined, that is the home directory. If HOME
is undefined, but the USERDIR and USER environment variables are defined and the
directory ‘%USERDIR%\%USER%’ exists, then it is used. Failing that, if the directory
‘%USER%’ exists on the OS/2 system drive, then it is used. As a last resort, the OS/2
system drive is the home directory.
Like OS/2, the Windows implementation uses heuristics based on environment variables. The user’s home directory is computed by examining several environment
variables, in the following order:
• HOMEDRIVE and HOMEPATH are both defined and ‘%HOMEDRIVE%%HOMEPATH%’ is an
existing directory. (These variables are automatically defined by Windows NT.)
• HOME is defined and ‘%HOME%’ is an existing directory.
• USERDIR and USERNAME are defined and ‘%USERDIR%\%USERNAME%’ is an existing
directory.
• USERDIR and USER are defined and ‘%USERDIR%\%USER%’ is an existing directory.
• USERNAME is defined and ‘%USERNAME%’ is an existing directory on the Windows
system drive.
• USER is defined and ‘%USER%’ is an existing directory on the Windows system
drive.
• Finally, if all else fails, the Windows system drive is used as the home directory.
system-library-pathname pathname
procedure
Locates pathname in MIT Scheme’s system library directory. An error of type
condition-type:file-operation-error is signalled if pathname cannot be located
on the library search path.
(system-library-pathname "compiler.com")
) #[pathname 45 "/usr/local/lib/mit-scheme/compiler.com"]
system-library-directory-pathname pathname
procedure
Locates the pathname of an MIT Scheme system library directory. An error of type
condition-type:file-operation-error is signalled if pathname cannot be located
on the library search path.
(system-library-directory-pathname "options")
) #[pathname 44 "/usr/local/lib/mit-scheme/options/"]
15.2 Working Directory
When MIT Scheme is started, the current working directory (or simply, working directory) is initialized in an operating-system dependent manner; usually, it is the directory in
which Scheme was invoked. The working directory can be determined from within Scheme
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231
by calling the pwd procedure, and changed by calling the cd procedure. Each rep loop has
its own working directory, and inferior rep loops initialize their working directory from the
value in e↵ect in their superior at the time they are created.
working-directory-pathname
pwd
procedure
procedure
Returns the current working directory as a pathname that has no name, type, or
version components, just host, device, and directory components. pwd is an alias for
working-directory-pathname; the long name is intended for programs and the short
name for interactive use.
set-working-directory-pathname! filename
cd filename
procedure
procedure
Makes filename the current working directory and returns the new current working
directory as a pathname. Filename is coerced to a pathname using pathname-asdirectory. cd is an alias for set-working-directory-pathname!; the long name is
intended for programs and the short name for interactive use.
Additionally, set-working-directory-pathname! modifies the value of
*default-pathname-defaults* by merging the new working directory into it.
When this procedure is executed in the top-level rep loop, it changes the working
directory of the running Scheme executable.
(set-working-directory-pathname! "/usr/morris/blisp")
) #[pathname "/usr/morris/blisp/"]
(set-working-directory-pathname! "~")
) #[pathname "/usr/morris/"]
This procedure signals an error if filename does not refer to an existing directory.
If filename describes a relative rather than absolute pathname, this procedure interprets it as relative to the current working directory, before changing the working
directory.
(working-directory-pathname)
) #[pathname "/usr/morris/"]
(set-working-directory-pathname! "foo")
) #[pathname "/usr/morris/foo/"]
with-working-directory-pathname filename thunk
procedure
This procedure temporarily rebinds the current working directory to filename, invokes thunk (a procedure of no arguments), then restores the previous working directory and returns the value yielded by thunk. Filename is coerced to a pathname using pathname-as-directory. In addition to binding the working directory,
with-working-directory-pathname also binds the variable *default-pathnamedefaults*, merging the old value of that variable with the new working directory
pathname. Both bindings are performed in exactly the same way as dynamic binding
of a variable (see Section 2.3 [Dynamic Binding], page 18).
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15.3 File Manipulation
This section describes procedures that manipulate files and directories. Any of these
procedures can signal a number of errors for many reasons. The specifics of these errors
are much too operating-system dependent to document here. However, if such an error is
signalled by one of these procedures, it will be of type condition-type:file-operationerror.
file-exists? filename
file-exists-direct? filename
file-exists-indirect? filename
procedure
procedure
procedure
These procedures return #t if filename is an existing file or directory; otherwise they
return #f. In operating systems that support symbolic links, if the file is a symbolic
link, file-exists-direct? tests for the existence of the link, while file-existsindirect? and file-exists? test for the existence of the file pointed to by the
link.
copy-file source-filename target-filename
procedure
Makes a copy of the file named by source-filename. The copy is performed by creating
a new file called target-filename, and filling it with the same data as source-filename.
rename-file source-filename target-filename
procedure
Changes the name of source-filename to be target-filename. In the unix implementation, this will not rename across file systems.
delete-file filename
Deletes the file named filename.
procedure
delete-file-no-errors filename
procedure
Like delete-file, but returns a boolean value indicating whether an error occurred
during the deletion. If no errors occurred, #t is returned. If an error of type
condition-type:file-error or condition-type:port-error is signalled, #f is returned.
hard-link-file source-filename target-filename
procedure
Makes a hard link from source-filename to target-filename. This operation gives the
file specified by source-filename a new name, in addition to the old name.
This currently works only on unix systems. It is further restricted to work only when
source-filename and target-filename refer to names in the same file system.
soft-link-file source-filename target-filename
procedure
Creates a new soft link called target-filename that points at the file source-filename.
(Soft links are also sometimes called symbolic links.) Note that source-filename will
be interpreted as a string (although you may specify it as a pathname object, if
you wish). The contents of this string will be stored in the file system as the soft
link. When a file operation attempts to open the link, the contents of the link are
interpreted relative to the link’s location at that time.
This currently works only on unix systems.
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make-directory filename
procedure
Creates a new directory named filename. Signals an error if filename already exists,
or if the directory cannot be created.
delete-directory filename
procedure
Deletes the directory named filename. Signals an error if the directory does not exist,
is not a directory, or contains any files or subdirectories.
->truename filename
procedure
This procedure attempts to discover and return the “true name” of the file associated with filename within the file system. An error of type condition-type:fileoperation-error is signalled if the appropriate file cannot be located within the file
system.
call-with-temporary-file-pathname procedure
procedure
Calls temporary-file-pathname to create a temporary file, then calls procedure
with one argument, the pathname referring to that file. When procedure returns, if
the temporary file still exists, it is deleted; then, the value yielded by procedure is
returned. If procedure escapes from its continuation, and the file still exists, it is
deleted.
temporary-file-pathname [directory]
procedure
Creates a new empty temporary file and returns a pathname referring to it. The
temporary file is created with Scheme’s default permissions, so barring unusual circumstances it can be opened for input and/or output without error. The temporary
file will remain in existence until explicitly deleted. If the file still exists when the
Scheme process terminates, it will be deleted.
If directory is specified, the temporary file will be stored there. If it is not specified,
or if it is #f, the temporary file will be stored in the directory returned by temporarydirectory-pathname.
temporary-directory-pathname
procedure
Returns the pathname of an existing directory that can be used to store temporary
files. These directory names are tried, in order, until a writeable directory is found:
• The directories specified by the environment variables TMPDIR, TEMP, or TMP.
• Under unix, the directories ‘/var/tmp’, ‘/usr/tmp’, or ‘/tmp’.
• Under OS/2 or Windows, the following directories on the system drive: ‘\temp’,
‘\tmp’, or ‘\’.
• Under OS/2 or Windows, the current directory, as specified by *defaultpathname-defaults*.
file-directory? filename
procedure
Returns #t if the file named filename exists and is a directory. Otherwise returns #f.
In operating systems that support symbolic links, if filename names a symbolic link,
this examines the file linked to, not the link itself.
This is equivalent to
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MIT Scheme Reference
(eq? ’directory (file-type-indirect filename))
file-regular? filename
procedure
Returns #t if the file named filename exists and is a regular file (i.e. not a directory,
symbolic link, device file, etc.). Otherwise returns #f. In operating systems that
support symbolic links, if filename names a symbolic link, this examines the file
linked to, not the link itself.
This is equivalent to
(eq? ’regular (file-type-indirect filename))
file-symbolic-link? filename
procedure
In operating systems that support symbolic links, if the file named filename exists
and is a symbolic link, this procedure returns the contents of the symbolic link as a
newly allocated string. The returned value is the name of the file that the symbolic
link points to and must be interpreted relative to the directory of filename. If filename
either does not exist or is not a symbolic link, or if the operating system does not
support symbolic links, this procedure returns #f.
file-type-direct filename
file-type-indirect filename
procedure
procedure
If the file named filename exists, file-type-direct returns a symbol specifying what
type of file it is. For example, if filename refers to a directory, the symbol directory
is returned. If filename doesn’t refer to an existing file, #f is returned.
If filename refers to a symbolic link, file-type-direct returns the type of the link
itself, while file-type-indirect returns the type of the file linked to.
At this time, the symbols that can be returned are the following. The names are intended to be self-explanatory. Most of these names can only be returned on particular
operating systems, and so the operating-system name is prefixed to the name.
regular
directory
unix-symbolic-link
unix-character-device
unix-block-device
unix-named-pipe
unix-socket
os2-named-pipe
win32-named-pipe
file-readable? filename
procedure
Returns #t if filename names a file that can be opened for input; i.e. a readable file.
Otherwise returns #f.
file-writeable? filename
procedure
Returns #t if filename names a file that can be opened for output; i.e. a writeable
file. Otherwise returns #f.
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file-executable? filename
procedure
Returns #t if filename names a file that can be executed. Otherwise returns #f. Under
unix, an executable file is identified by its mode bits. Under OS/2, an executable file
has one of the file extensions ‘.exe’, ‘.com’, ‘.cmd’, or ‘.bat’. Under Windows, an
executable file has one of the file extensions ‘.exe’, ‘.com’, or ‘.bat’.
file-access filename mode
procedure
Mode must be an exact integer between 0 and 7 inclusive; it is a bitwise-encoded
predicate selector with 1 meaning “executable”, 2 meaning “writeable”, and 4 meaning “readable”. file-access returns #t if filename exists and satisfies the predicates
selected by mode. For example, if mode is 5, then filename must be both readable and
executable. If filename doesn’t exist, or if it does not satisfy the selected predicates,
#f is returned.
file-eq? filename1 filename2
procedure
Determines whether filename1 and filename2 refer to the same file. Under unix, this is
done by comparing the inodes and devices of the two files. Under OS/2 and Windows,
this is done by comparing the filename strings.
file-modes filename
procedure
If filename names an existing file, file-modes returns an exact non-negative integer
encoding the file’s permissions. The encoding of this integer is operating-system
dependent. Under unix, it is the least-significant 12 bits of the st_mode element of
the struct stat structure. Under OS/2 and Windows, it is the file attribute bits,
which are described below. If filename does not name an existing file, #f is returned.
set-file-modes! filename modes
procedure
Filename must name an existing file. Modes must be an exact non-negative integer
that could have been returned by a call to file-modes. set-file-modes! modifies
the file’s permissions to be those encoded by modes.
os2-file-mode/read-only
os2-file-mode/hidden
os2-file-mode/system
os2-file-mode/directory
os2-file-mode/archived
variable
variable
variable
variable
variable
The values of these variables are the “mode bits” that comprise the value returned by
file-modes under OS/2. These bits are small integers that are combined by adding
to form a complete set of modes. The integer zero represents a set of modes in which
none of these bits are set.
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nt-file-mode/read-only
nt-file-mode/hidden
nt-file-mode/system
nt-file-mode/directory
nt-file-mode/archive
nt-file-mode/normal
nt-file-mode/temporary
nt-file-mode/compressed
variable
variable
variable
variable
variable
variable
variable
variable
The values of these variables are the “mode bits” that comprise the value returned
by file-modes under Windows. These bits are small integers that are combined by
adding to form a complete set of modes. The integer zero represents a set of modes
in which none of these bits are set.
file-modification-time filename
procedure
Returns the modification time of filename as an exact non-negative integer. The result
may be compared to other file times using ordinary integer arithmetic. If filename
names a file that does not exist, file-modification-time returns #f.
In operating systems that support symbolic links, if filename names a symbolic
link, file-modification-time returns the modification time of the file linked to.
An alternate procedure, file-modification-time-direct, returns the modification
time of the link itself; in all other respects it is identical to file-modificationtime. For symmetry, file-modification-time-indirect is a synonym of filemodification-time.
file-access-time filename
procedure
Returns the access time of filename as an exact non-negative integer. The result may
be compared to other file times using ordinary integer arithmetic. If filename names
a file that does not exist, file-access-time returns #f.
In operating systems that support symbolic links, if filename names a symbolic link,
file-access-time returns the access time of the file linked to. An alternate procedure, file-access-time-direct, returns the access time of the link itself; in all
other respects it is identical to file-access-time. For symmetry, file-accesstime-indirect is a synonym of file-access-time.
set-file-times! filename access-time modification-time
procedure
Filename must name an existing file, while access-time and modification-time must
be valid file times that might have been returned by file-access-time and filemodification-time, respectively. set-file-times! alters the access and modification times of the file specified by filename to the values given by access-time and
modification-time, respectively. For convenience, either of the time arguments may be
specified as #f; in this case the corresponding time is not changed. set-file-times!
returns an unspecified value.
current-file-time
procedure
Returns the current time as an exact non-negative integer, in the same format used
by the above file-time procedures. This number can be compared to other file times
using ordinary arithmetic operations.
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237
file-touch filename
procedure
Touches the file named filename. If the file already exists, its modification time is set
to the current file time and #f is returned. Otherwise, the file is created and #t is
returned. This is an atomic test-and-set operation, so it is useful as a synchronization
mechanism.
file-length filename
procedure
Returns the length, in bytes, of the file named filename as an exact non-negative
integer.
file-attributes filename
procedure
This procedure determines if the file named filename exists, and returns information
about it if so; if the file does not exist, it returns #f.
In operating systems that support symbolic links, if filename names a symbolic
link, file-attributes returns the attributes of the link itself. An alternate procedure, file-attributes-indirect, returns the attributes of the file linked to; in all
other respects it is identical to file-attributes. For symmetry, file-attributesdirect is a synonym of file-attributes.
The information returned by file-attributes is decoded by accessor procedures. The
following accessors are defined in all operating systems:
file-attributes/type attributes
procedure
The file type: #t if the file is a directory, a character string (the name linked to) if a
symbolic link, or #f for all other types of file.
file-attributes/access-time attributes
procedure
file-attributes/modification-time attributes
procedure
file-attributes/change-time attributes
procedure
file-attributes/length attributes
procedure
The last access time of the file, an exact non-negative integer.
The last modification time of the file, an exact non-negative integer.
The last change time of the file, an exact non-negative integer.
The length of the file in bytes.
file-attributes/mode-string attributes
procedure
The mode string of the file, a newly allocated string showing the file’s mode bits.
Under unix, this string is in unix format. Under OS/2 and Windows, this string
shows the standard “DOS” attributes in their usual format.
file-attributes/n-links attributes
procedure
The number of links to the file, an exact positive integer. Under Windows and OS/2,
this is always 1.
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The following additional accessors are defined under unix:
file-attributes/uid attributes
procedure
file-attributes/gid attributes
procedure
file-attributes/inode-number attributes
procedure
The user id of the file’s owner, an exact non-negative integer.
The group id of the file’s group, an exact non-negative integer.
The inode number of the file, an exact non-negative integer.
The following additional accessor is defined under OS/2 and Windows:
file-attributes/modes attributes
procedure
The attribute bits of the file. This is an exact non-negative integer containing the
file’s attribute bits, exactly as specified by the operating system’s API.
The following additional accessor is defined under OS/2:
file-attributes/allocated-length attributes
procedure
The allocated length of the file, which can be larger than the length of the file due to
fixed-length allocation units.
15.4 Directory Reader
directory-read directory [sort?]
procedure
Directory must be an object that can be converted into a pathname by
->pathname. The directory specified by directory is read, and the contents of the
directory is returned as a newly allocated list of absolute pathnames. The result
is sorted according to the usual sorting conventions for directories, unless sort? is
specified as #f. If directory has name, type, or version components, the returned
list contains only those pathnames whose name, type, and version components match
those of directory; wild or #f as one of these components means “match anything”.
The OS/2 and Windows implementations support “globbing”, in which the characters * and ? are interpreted to mean “match anything” and “match any character”,
respectively. This “globbing” is supported only in the file part of directory.
15.5 Date and Time
MIT Scheme provides a simple set of procedures for manipulating date and time information. There are four time representations, each of which serves a di↵erent purpose. Each
representation may be converted to any of the others.
The primary time representation, universal time, is an exact non-negative integer counting the number of seconds that have elapsed since midnight January 1, 1900 UTC. (UTC
stands for Coordinated Universal Time, and is the modern name for Greenwich Mean Time.)
This format is produced by get-universal-time and decoded-time->universal-time.
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The second representation, decoded time, is a record structure in which the time is
broken down into components, such as month, minute, etc. Decoded time is always relative
to a particular time zone, which is a component of the structure. This format is produced
by global-decoded-time and local-decoded-time.
The third representation, file time, is an exact non-negative integer that is larger for
increasing time. Unlike universal time, this representation is operating-system dependent. This format is produced by all of the file-attribute procedures, for example filemodification-time and file-attributes.
The fourth representation, the time string, is an external representation for time. This
format is defined by RFC-822, Standard for the format of ARPA Internet text messages,
with the modification that years are represented as four-digit numbers rather than twodigit numbers. This format is the standard format for Internet email and numerous other
network protocols.
Within this section, argument variables named universal-time, decoded-time, file-time,
and time-string are respectively required to be of the corresponding format.
15.5.1 Universal Time
get-universal-time
Return the current time in universal format.
(get-universal-time) ) 3131453078
procedure
epoch
variable
epoch is the representation of midnight January 1, 1970 UTC in universal-time format.
epoch ) 2208988800
15.5.2 Decoded Time
Objects representing standard time components, such as seconds and minutes, are required to be exact non-negative integers. Seconds and minutes must be inclusively between
0 and 59; hours between 0 and 23; days between 1 and 31; months between 1 and 12; years
are represented in “four-digit” form, in which 1999 is represented as 1999 — not 99.
local-decoded-time
procedure
Return the current time in decoded format. The decoded time is represented in the
local time zone.
(pp (local-decoded-time))
a #[decoded-time 76]
a (second 2)
a (minute 12)
a (hour 11)
a (day 27)
a (month 4)
a (year 1999)
a (day-of-week 1)
a (daylight-savings-time 1)
a (zone 5)
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global-decoded-time
procedure
Return the current time in decoded format. The decoded time is represented in UTC.
(pp (global-decoded-time))
a #[decoded-time 77]
a (second 8)
a (minute 12)
a (hour 15)
a (day 27)
a (month 4)
a (year 1999)
a (day-of-week 1)
a (daylight-savings-time 0)
a (zone 0)
make-decoded-time second minute hour day month year [zone]
procedure
Return a new decoded-time object representing the given time. The arguments must
be valid components according to the above rules, and must form a valid date.
If zone is not supplied or is #f, the resulting decoded time will be represented in the
local time zone. Otherwise, zone must be a valid time zone, and the result will be
represented in that zone.
Warning: because this procedure depends on the operating system’s runtime library,
it is not capable of representing all dates. In particular, on most unix systems, it
is not possible to encode dates that occur prior to midnight, January 1, 1970 UTC.
Attempting to do this will signal an error.
(pp (make-decoded-time 0 9 11 26 3 1999))
a #[decoded-time 19]
a (second 0)
a (minute 9)
a (hour 11)
a (day 26)
a (month 3)
a (year 1999)
a (day-of-week 4)
a (daylight-savings-time 0)
a (zone 5)
(pp (make-decoded-time 0 9 11 26 3 1999 3))
a #[decoded-time 80]
a (second 0)
a (minute 9)
a (hour 11)
a (day 26)
a (month 3)
a (year 1999)
a (day-of-week 4)
a (daylight-savings-time 0)
a (zone 3)
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decoded-time/second decoded-time
decoded-time/minute decoded-time
decoded-time/hour decoded-time
decoded-time/day decoded-time
decoded-time/month decoded-time
decoded-time/year decoded-time
procedure
procedure
procedure
procedure
procedure
procedure
Return the corresponding component of decoded-time.
(decoded-time/second (local-decoded-time)) ) 17
(decoded-time/year (local-decoded-time)) ) 1999
(decoded-time/day (local-decoded-time)) ) 26
decoded-time/day-of-week decoded-time
procedure
Return the day of the week on which decoded-time falls, encoded as an exact integer
between 0 (Monday) and 6 (Sunday), inclusive.
(decoded-time/day-of-week (local-decoded-time)) ) 4
decoded-time/daylight-savings-time? decoded-time
procedure
Return #t if decoded-time is represented using daylight savings time. Otherwise
return #f.
(decoded-time/daylight-savings-time? (local-decoded-time))
) #f
decoded-time/zone decoded-time
procedure
Return the time zone in which decoded-time is represented. This is an exact rational
number between -24 and +24 inclusive, that when multiplied by 3600 is an integer.
The value is the number of hours west of UTC.
(decoded-time/zone (local-decoded-time)) ) 5
time-zone? object
procedure
Returns #t if object is an exact number between -24 and +24 inclusive, that when
multiplied by 3600 is an integer.
) #t
(time-zone? -5)
(time-zone? 11/2) ) #t
(time-zone? 11/7) ) #f
month/max-days month
procedure
Returns the maximum number of days possible in month. Month must be an exact
integer between 1 and 12 inclusive.
(month/max-days 2) ) 29
(month/max-days 3) ) 31
(month/max-days 4) ) 30
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15.5.3 File Time
As stated above, file time is operating-system dependent. As of this writing, two formats
are used. For unix and Windows systems, file time is the number of seconds since midnight
January 1, 1970 UTC (the standard unix time convention).
OS/2 represents file time as a 32-bit unsigned integer, in which the time components are
broken down into unsigned bit fields. The components are always stated in local time. The
fields, from MSB to LSB, are:
• 7 bits representing the year, relative to 1900.
• 4 bits representing the month, numbered 1 to 12.
• 5 bits representing the day of the month, numbered 1 to 31.
• 5 bits representing the hour of the day, numbered 0 to 23.
• 6 bits representing the minute, numbered 0 to 59.
• 5 bits representing the second. This field is unusual in that it counts units of two
seconds, so it is a number between 0 and 29, representing second counts corresponding
to 0 through 58.
The following procedures generate their results in file-time format:
file-access-time
file-access-time-direct
file-access-time-indirect
file-modification-time
file-modification-time-direct
file-modification-time-indirect
file-attributes/access-time
file-attributes/modification-time
file-attributes/change-time
Additionally, set-file-times! accepts its time arguments in file-time format.
15.5.4 Time-Format Conversion
The procedures described in this section convert times from one format to another.
universal-time->local-decoded-time universal-time
universal-time->global-decoded-time universal-time
procedure
procedure
Converts an argument in universal-time format to decoded-time format. The result
is in the local time zone or UTC, respectively.
(pp (universal-time->local-decoded-time (get-universal-time)))
a #[decoded-time 21]
a (second 23)
a (minute 57)
a (hour 17)
a (day 29)
a (month 4)
a (year 1999)
a (day-of-week 3)
a (daylight-savings-time 1)
a (zone 5)
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(pp (universal-time->global-decoded-time
(get-universal-time)))
a #[decoded-time 22]
a (second 27)
a (minute 57)
a (hour 21)
a (day 29)
a (month 4)
a (year 1999)
a (day-of-week 3)
a (daylight-savings-time 0)
a (zone 0)
universal-time->file-time universal-time
Converts an argument in universal-time format to file-time format.
(universal-time->file-time (get-universal-time))
) 925422988
procedure
universal-time->local-time-string universal-time
universal-time->global-time-string universal-time
procedure
procedure
Converts an argument in universal-time format to a time string. The result is in the
local time zone or UTC, respectively.
(universal-time->local-time-string (get-universal-time))
) "Thu, 29 Apr 1999 17:55:31 -0400"
(universal-time->global-time-string (get-universal-time))
) "Thu, 29 Apr 1999 21:55:51 +0000"
decoded-time->universal-time decoded-time
procedure
decoded-time->file-time decoded-time
procedure
decoded-time->string decoded-time
procedure
Converts an argument in decoded-time format to universal-time format.
(decoded-time->universal-time (local-decoded-time))
) 3134411942
(decoded-time->universal-time (global-decoded-time))
) 3134411947
Converts an argument in decoded-time format to file-time format.
(decoded-time->file-time (local-decoded-time))
) 925423191
(decoded-time->file-time (global-decoded-time))
) 925423195
Convert an argument in decoded-time format to a time string.
(decoded-time->string (local-decoded-time))
) "Thu, 29 Apr 1999 18:00:43 -0400"
(decoded-time->string (global-decoded-time))
) "Thu, 29 Apr 1999 22:00:46 +0000"
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file-time->universal-time file-time
Converts an argument in universal-time format to file-time format.
procedure
(file-time->universal-time (file-modification-time "/"))
) 3133891907
file-time->local-decoded-time file-time
file-time->global-decoded-time file-time
procedure
procedure
Converts an argument in file-time format to decoded-time format. The result is in
the local time zone or UTC, respectively.
(pp (file-time->local-decoded-time
(file-modification-time "/")))
a #[decoded-time 26]
a (second 47)
a (minute 31)
a (hour 17)
a (day 23)
a (month 4)
a (year 1999)
a (day-of-week 4)
a (daylight-savings-time 1)
a (zone 5)
(pp (file-time->global-decoded-time
(file-modification-time "/")))
a #[decoded-time 27]
a (second 47)
a (minute 31)
a (hour 21)
a (day 23)
a (month 4)
a (year 1999)
a (day-of-week 4)
a (daylight-savings-time 0)
a (zone 0)
file-time->local-time-string file-time
file-time->global-time-string file-time
procedure
procedure
Converts an argument in file-time format to a time string. The result is in the local
time zone or UTC, respectively.
(file-time->local-time-string (file-modification-time "/"))
) "Fri, 23 Apr 1999 17:31:47 -0400"
(file-time->global-time-string (file-modification-time "/"))
) "Fri, 23 Apr 1999 21:31:47 +0000"
string->universal-time time-string
Converts a time-string argument to universal-time format.
procedure
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245
(string->universal-time "Fri, 23 Apr 1999 21:31:47 +0000")
) 3133888307
(string->universal-time "Fri, 23 Apr 1999 17:31:47 -0400")
) 3133888307
string->decoded-time time-string
procedure
string->file-time time-string
procedure
Converts a time-string argument to decoded-time format.
(pp (string->decoded-time "Fri, 23 Apr 1999 17:31:47 -0400"))
a #[decoded-time 30]
a (second 47)
a (minute 31)
a (hour 17)
a (day 23)
a (month 4)
a (year 1999)
a (day-of-week 4)
a (daylight-savings-time 0)
a (zone 4)
Converts a time-string argument to file-time format.
(string->file-time "Fri, 23 Apr 1999 17:31:47 -0400")
) 924899507
15.5.5 External Representation of Time
The normal external representation for time is the time string, as described above. The
procedures in this section generate alternate external representations of time which are more
verbose and may be more suitable for presentation to human readers.
decoded-time/date-string decoded-time
decoded-time/time-string decoded-time
procedure
procedure
These procedures return strings containing external representations of the date and
time, respectively, represented by decoded-time. The results are implicitly in local
time.
(decoded-time/date-string (local-decoded-time))
) "Tuesday March 30, 1999"
(decoded-time/time-string (local-decoded-time))
) "11:22:38 AM"
day-of-week/long-string day-of-week
day-of-week/short-string day-of-week
procedure
procedure
Returns a string representing the given day-of-week. The argument must be an exact
non-negative integer between 0 and 6 inclusive. day-of-week/long-string returns
a long string that fully spells out the name of the day. day-of-week/short-string
returns a shortened string that abbreviates the day to three letters.
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(day-of-week/long-string 0) ) "Monday"
(day-of-week/short-string 0) ) "Mon"
(day-of-week/short-string 3) ) "Thu"
month/long-string month
month/short-string month
procedure
procedure
Returns a string representing the given month. The argument must be an exact
non-negative integer between 1 and 12 inclusive. month/long-string returns a long
string that fully spells out the name of the month. month/short-string returns a
shortened string that abbreviates the month to three letters.
) "January"
(month/long-string 1)
(month/short-string 1) ) "Jan"
(month/short-string 10) ) "Oct"
time-zone->string
procedure
Returns a string corresponding to the given time zone. This string is the same string
that is used to generate RFC-822 time strings.
) "-0500"
(time-zone->string 5)
) "+0400"
(time-zone->string -4)
(time-zone->string 11/2) ) "-0530"
15.6 Machine Time
The previous section dealt with procedures that manipulate clock time. This section
describes procedures that deal with computer time: elapsed CPU time, elapsed real time,
and so forth. These procedures are useful for measuring the amount of time it takes to
execute code.
Some of the procedures in this section manipulate a time representation called ticks. A
tick is a unit of time that is unspecified here but can be converted to and from seconds by
supplied procedures. A count in ticks is represented as an exact integer. At present each
tick is one millisecond, but this may change in the future.
process-time-clock
procedure
Returns the amount of process time, in ticks, that has elapsed since Scheme was
started. Process time is measured by the operating system and is time during which
the Scheme process is computing. It does not include time in system calls, but
depending on the operating system it may include time used by subprocesses.
(process-time-clock) ) 21290
real-time-clock
procedure
Returns the amount of real time, in ticks, that has elapsed since Scheme was started.
Real time is the time measured by an ordinary clock.
(real-time-clock) ) 33474836
internal-time/ticks->seconds ticks
procedure
Returns the number of seconds corresponding to ticks. The result is always a real
number.
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(internal-time/ticks->seconds 21290) ) 21.29
(internal-time/ticks->seconds 33474836) ) 33474.836
internal-time/seconds->ticks seconds
procedure
Returns the number of ticks corresponding to seconds. Seconds must be a real number.
(internal-time/seconds->ticks 20.88) ) 20880
(internal-time/seconds->ticks 20.83) ) 20830
system-clock
procedure
Returns the amount of process time, in seconds, that has elapsed since Scheme was
started. Roughly equivalent to:
(internal-time/ticks->seconds (process-time-clock))
Example:
(system-clock) ) 20.88
runtime
procedure
Returns the amount of process time, in seconds, that has elapsed since Scheme was
started. However, it does not include time spent in garbage collection.
(runtime) ) 20.83
with-timings thunk receiver
procedure
Calls thunk with no arguments. After thunk returns, receiver is called with three
arguments describing the time spent while computing thunk: the elapsed run time,
the amount of time spent in the garbage collector, and the elapsed real time. All
three times are in ticks.
This procedure is most useful for doing performance measurements, and is designed
to have relatively low overhead.
(with-timings
(lambda () ...hairy computation...)
(lambda (run-time gc-time real-time)
(write (internal-time/ticks->seconds run-time))
(write-char #\space)
(write (internal-time/ticks->seconds gc-time))
(write-char #\space)
(write (internal-time/ticks->seconds real-time))
(newline)))
measure-interval runtime? procedure
procedure
Calls procedure, passing it the current process time, in seconds, as an argument. The
result of this call must be another procedure. When procedure returns, the resulting
procedure is tail-recursively called with the ending time, in seconds, as an argument.
If runtime? is #f, the elapsed time is deducted from the elapsed system time returned
by runtime.
While this procedure can be used for time measurement, its interface is somewhat
clumsy for that purpose. We recommend that you use with-timings instead, because
it is more convenient and has lower overhead.
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(measure-interval #t
(lambda (start-time)
(let ((v ...hairy computation...))
(lambda (end-time)
(write (- end-time start-time))
(newline)
v))))
15.7 Subprocesses
MIT Scheme provides the ability to run and control subprocesses. This support is divided
into two parts: a low-level set of primitives that maps onto the underlying operating system’s
process-control primitives, and a high-level set of procedures for starting a subprocess and
running it to completion in a single call. Subprocesses that are run in the latter fashion
are referred to as synchronous, because they are started and stopped in synchrony with a
Scheme procedure call.
This chapter documents Scheme’s high-level synchronous-subprocess support. The lowlevel support is not documented but is available for those who are willing to read the source
code.
Synchronous-subprocess support is a run-time-loadable option. To use it, execute
(load-option ’synchronous-subprocess)
once before calling it.
15.7.1 Subprocess Procedures
There are two commands for running synchronous subprocesses under Scheme. runshell-command is very simple to use, provides access to all shell features, and is to be
preferred in most situations. run-synchronous-subprocess allows direct execution of a
program and precise control of the command-line arguments passed to the program, but
does not provide file globbing, I/O redirection, or other shell features.
run-shell-command command option . . .
procedure
Runs command, which must be a string. Command is passed to a command shell for
interpretation; how the shell is chosen is detailed below.
The options are a sequence of keyword/value pairs that specify optional behavior.
See below for more information about options.
run-shell-command waits until the subprocess completes its execution and returns
the exit code from the subprocess. If the subprocess is killed or stopped, an error is
signalled and the procedure does not return.
run-synchronous-subprocess program arguments option . . .
procedure
Runs program, passing it the given command-line arguments. Program must be
either the name of a program on the path, or else a pathname to a specific program.
Arguments must be a list of strings; each string is a single command-line argument
to the program.
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249
The options are a sequence of keyword/value pairs that specify optional behavior.
See below for more information about options.
run-synchronous-subprocess waits until the subprocess completes its execution and
returns the exit code from the subprocess. If the subprocess is killed or stopped, an
error is signalled and the procedure does not return.
15.7.2 Subprocess Conditions
If a subprocess spawned by one of the above procedures is killed or suspended, then one
of the following errors will be signalled.
condition-type:subprocess-signalled subprocess reason
condition type
This condition type is a subtype of condition-type:subprocess-abnormaltermination. It is signalled when the subprocess is killed.
Subprocess is an object that represents the subprocess involved. The internals of this
object can be accessed but the interface is not documented at this time; see the source
code for details.
Reason is interesting only on unix systems, where it is the signal that killed the
process. On other systems it has a fixed value that conveys no useful information.
condition-type:subprocess-stopped subprocess reason
condition type
This condition type is a subtype of condition-type:subprocess-abnormaltermination. It is signalled when the subprocess is stopped or suspended.
Subprocess is an object that represents the subprocess involved. The internals of this
object can be accessed but the interface is not documented at this time; see the source
code for details.
Reason is interesting only on unix systems, where it is the signal that stopped the
process. On other systems it has a fixed value that conveys no useful information.
condition-type:subprocess-abnormal-termination
condition type
subprocess reason
This condition type is a subtype of condition-type:error. This is an abstract type
that is never signalled. It is provided so that condition handlers can be bound to it.
15.7.3 Subprocess Options
The following subprocess options may be passed to run-shell-command or
run-synchronous-subprocess. These options are passed as alternating keyword/value
pairs, for example:
(run-shell-command "ls /"
’output my-output-port
’output-buffer-size 8192)
The example shows a shell command being run with two options specified: output and
output-buffer-size.
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input port
subprocess option
Specifies the standard input of the subprocess. Port may be an input port, in which
case characters are read from port and fed to the subprocess until port reaches endof-file. Alternatively, port may be #f, indicating that the subprocess has no standard
input.
The default value of this option is #f.
(call-with-input-file "foo.in"
(lambda (port)
(run-shell-command "cat > /dev/null" ’input port)))
input-line-translation line-ending
subprocess option
Specifies how line-endings should be translated when writing characters to the subprocess. Ignored if the input option is #f. Line-ending must be either a string specifying
the line ending, or the symbol default, meaning to use the operating system’s standard line ending. In either case, newline characters to be written to the input port
are translated to the specified line ending before being written.
The default value of this option is default.
(call-with-input-file "foo.in"
(lambda (port)
(run-shell-command "cat > /dev/null"
’input port
’input-line-translation "\r\n")))
input-bu↵er-size n
subprocess option
Specifies the size of the input bu↵er for the standard input of the subprocess. (This
is the bu↵er on the Scheme side, and has nothing to do with any bu↵ering done on
the subprocess side.) Ignored if the input option is #f. N must be an exact positive
integer specifying the number of characters the bu↵er can hold.
The default value of this option is 512.
(call-with-input-file "foo.in"
(lambda (port)
(run-shell-command "cat > /dev/null"
’input port
’input-buffer-size 4096)))
output port
subprocess option
Specifies the standard output and standard error of the subprocess. Port may be
an output port, in which case characters are read from the subprocess and fed to
port until the subprocess finishes. Alternatively, port may be #f, indicating that the
subprocess has no standard output or standard error.
The default value of this option is the value of (current-output-port).
(call-with-output-file "foo.out"
(lambda (port)
(run-shell-command "ls -la /etc" ’output port)))
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251
output-line-translation line-ending
subprocess option
Specifies how line-endings should be translated when reading characters from the
standard output of the subprocess. Ignored if the output option is #f. Line-ending
must be either a string specifying the line ending, or the symbol default, meaning
to use the operating system’s standard line ending. In either case, newline characters
read from the subprocess port are translated to the specified line ending.
The default value of this option is default.
(call-with-output-file "foo.out"
(lambda (port)
(run-shell-command "ls -la /etc"
’output port
’output-line-translation "\r\n")))
output-bu↵er-size n
subprocess option
Specifies the size of the output bu↵er for the standard output of the subprocess. (This
is the bu↵er on the Scheme side, and has nothing to do with any bu↵ering done on
the subprocess side.) Ignored if the output option is #f. N must be an exact positive
integer specifying the number of characters the bu↵er can hold.
The default value of this option is 512.
(call-with-output-file "foo.out"
(lambda (port)
(run-shell-command "ls -la /etc"
’output port
’output-buffer-size 4096)))
redisplay-hook thunk
subprocess option
Specifies that thunk is to be run periodically when output from the subprocess is
available. Thunk must be a procedure of no arguments, or #f indicating that no hook
is supplied. This option is mostly useful for interactive systems. For example, the
Edwin text editor uses this to update output bu↵ers when running some subprocesses.
The default value of this option is #f.
(run-shell-command "ls -la /etc"
’redisplay-hook
(lambda ()
(update-buffer-contents buffer)))
environment environment
subprocess option
Specifies the environment variables that are to be used for the subprocess. Environment must be either a vector of strings or #f indicating the default environment. If
it is a vector of strings, each string must be a name/value pair where the name and
value are separated by an equal sign, for example, "foo=bar". To define a variable
with no value, just omit the value, as in "foo=".
Note that the variable scheme-subprocess-environment is bound to the default
subprocess environment.
The default value of this option is #f.
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(run-shell-command "ls -la /etc"
’environment
(let* ((v scheme-subprocess-environment)
(n (vector-length v))
(v (vector-grow v (+ n 1))))
(vector-set! v n "TERM=none")
v))
working-directory pathname
Specifies the working directory in which the subprocess will run.
subprocess option
The default value of this option is (working-directory-pathname).
(run-shell-command "ls -la" ’working-directory "/etc/")
use-pty? boolean
subprocess option
This option is meaningful only on unix systems; on other systems it is ignored. Specifies whether to communicate with the subprocess using pty devices; if true, ptys
will be used, otherwise pipes will be used.
The default value of this option is #f.
(run-shell-command "ls -la /etc" ’use-pty? #t)
shell-file-name pathname
Specifies the shell program to use for run-shell-command.
subprocess option
The default value of this option is (os/shell-file-name). This is the value of the
environment variable SHELL, or if SHELL is not set, the value is operating-system
dependent as follows:
• On unix systems, ‘/bin/sh’ is used.
• On OS/2 systems, the value of the environment variable COMSPEC is used, or if
that is not set, ‘cmd.exe’ on the current path.
• On Windows systems, the value of the environment variable COMSPEC is used. If
that is not set, ‘cmd.exe’ is used for Windows NT, or ‘command.com’ is used for
Windows 9x; in each case the shell is found by searching the path.
(run-shell-command "ls -la /etc"
’shell-file-name "/usr/local/bin/bash")
15.8 TCP Sockets
MIT Scheme provides access to sockets, which are a mechanism for inter-process communication. tcp stream sockets are supported, which communicate between computers over a
tcp/ip network. tcp sockets are supported on all operating systems.
tcp sockets have two distinct interfaces: one interface to implement a client and another
to implement a server. The basic protocol is that servers set up a listening port and wait
for connections from clients. Implementation of clients is simpler and will be treated first.
The socket procedures accept two special arguments, called host-name and service. Hostname is a string which must be the name of an internet host. It is looked up using the
Chapter 15: Operating-System Interface
253
ordinary lookup rules for your computer. For example, if your host is foo.mit.edu and
host-name is "bar", then it specifies bar.mit.edu.
Service specifies the service to which you will connect. A networked computer normally
provides several di↵erent services, such as telnet or ftp. Each service is associated with a
unique port number; for example, the "www" service is associated with port 80. The service
argument specifies the port number, either as a string, or directly as an exact non-negative
integer. Port strings are decoded by the operating system using a table; for example, on
unix the table is in ‘/etc/services’. Usually you will use a port string rather than a
number.
open-tcp-stream-socket host-name service [bu↵er-size
procedure
[line-translation]]
open-tcp-stream-socket opens a connection to the host specified by host-name.
Host-name is looked up using the ordinary lookup rules for your computer. The
connection is established to the service specified by service. The returned value is
an i/o port, to which you can read and write characters using ordinary Scheme i/o
procedures such as read-char and write-char.
Bu↵er-size specifies the size of the read and write bu↵ers used by the port; if this is
unspecified or #f, the bu↵ers will hold 4096 bytes.
Line-translation specifies how end-of-line characters will be translated when reading
or writing to the socket. If this is unspecified or #f, then lines will be terminated by
cr-lf, which is the standard for most internet protocols. Otherwise, it must be a
string, which specifies the line-ending character sequence to use.
When you wish to close the connection, just use close-port.
As an example, here is how you can open a connection to a web server:
(open-tcp-stream-socket "web.mit.edu" "www")
Next we will treat setting up a tcp server, which is slightly more complicated. Creating
a server is a two-part process. First, you must open a server socket, which causes the
operating system to listen to the network on a port that you specify. Once the server socket
is opened, the operating system will allow clients to connect to your computer on that port.
In the second step of the process, you accept the connection, which completes the connection initiated by the client, and allows you to communicate with the client. Accepting
a connection does not a↵ect the server socket; it continues to listen for additional client
connections. You can have multiple client connections to the same server socket open simultaneously.
open-tcp-server-socket service [address]
procedure
This procedure opens a server socket that listens for connections to service; the socket
will continue to listen until you close it. The returned value is a server socket object.
An error is signalled if another process is already listening on the service. Additionally,
ports whose number is less than 1024 are privileged on many operating systems, and
cannot be used by non-privileged processes; if service specifies such a port and you
do not have administrative privileges, an error may be signalled.
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The optional argument address specifies the ip address on which the socket will listen.
If this argument is not supplied or is given as #f, then the socket listens on all ip
addresses for this machine. (This is equivalent to passing the result of calling hostaddress-any.)
tcp-server-connection-accept server-socket block? peer-address
procedure
Checks to see if a client has connected to server-socket. If so, an i/o port is returned.
The returned port can be read and written using ordinary Scheme i/o procedures
such as read-char and write-char.
The argument block? says what to do if no client has connected at the time of the
call. If #f, it says to return immediately with two values of #f. Otherwise, the call
waits until a client connects.
The argument peer-address is either #f or an ip address as allocated by allocatehost-address. If it is an ip address, the address is modified to be the address of the
client making the connection.
Note that closing the port returned by this procedure does not a↵ect server-socket;
it just closes the particular client connection that was opened by the call. To close
server-socket, use close-tcp-server-socket.
close-tcp-server-socket server-socket
procedure
Closes the server socket server-socket. The operating system will cease listening for
network connections to that service. Client connections to server-socket that have
already been accepted will not be a↵ected.
15.9 Miscellaneous OS Facilities
This section contains assorted operating-system facilities that don’t fit into other categories.
microcode-id/operating-system
microcode-id/operating-system-name
variable
variable
microcode-id/operating-system is bound to a symbol that specifies the type of
operating system that Scheme is running under. There are three possible values:
unix, os/2, or nt.
microcode-id/operating-system-name is a string containing the same name as
microcode-id/operating-system; the latter is created by interning the former as a
symbol.
microcode-id/operating-system-variant
variable
This variable is a string that identifies the particular variant of the operating system
that Scheme is running under. Here are some of the possible values:
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255
"GNU/Linux"
"FreeBSD"
"HP-UX"
"SunOS"
"OS/2 2.1"
"OS/2 4.0"
"Microsoft Windows NT 4.0 (Build 1381; Service Pack 3)"
"Microsoft Windows 98 (Build 410)"
For Windows systems, it is recommended that you match on the prefix of this string
and ignore the "Build" suffix. This is because the suffix may contain information
about service packs or fixes, while the prefix will be constant for a particular version
of Windows.
The next few procedures provide access to the domain name service (dns), which maintains associations between internet host names such as "www.swiss.ai.mit.edu" and ip
addresses, such as 18.23.0.16. In MIT Scheme, we represent an internet host name as
a string, and an ip address as a byte vector of length 4 (byte vectors are just character
strings that are accessed using vector-8b-ref rather than string-ref). The bytes in an
ip address read in the same order as they do when written out:
(get-host-by-name "www.swiss") ) #("\022\027\000\020")
get-host-by-name host-name
procedure
Looks up the internet host name host-name using the dns, returning a vector of
ip addresses for the corresponding host, or #f if there is no such host. Usually
the returned vector has only one element, but if a host has more than one network
interface, the vector might have more than one element.
(get-host-by-name "www.swiss") ) #("\022\027\000\020")
get-host-by-address ip-address
procedure
Does a reverse dns lookup on ip-address, returning the internet host name corresponding to that address, or #f if there is no such host.
(get-host-by-address "\022\027\000\020") ) "swissnet.ai.mit.edu"
canonical-host-name host-name
procedure
Finds the “canonical” internet host name for host-name. For example:
) "zurich.ai.mit.edu"
(canonical-host-name "zurich")
(canonical-host-name "www.swiss") ) "swissnet.ai.mit.edu"
In both examples, the default internet domain ‘ai.mit.edu’ is added to host-name.
In the second example, "www.swiss" is an alias for another computer named
"swissnet".
get-host-name
procedure
Returns the string that identifies the computer that MIT Scheme is running on.
Usually this is an unqualified internet host name, i.e. the host name without the
domain suffix:
(get-host-name) ) "aarau"
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os/hostname
procedure
Returns the canonical internet host name of the computer that MIT Scheme is running
on. So, in contrast to the example for get-host-name:
(os/hostname) ) "aarau.ai.mit.edu"
allocate-host-address
procedure
Allocates and returns an ip address object. This is just a string of a fixed length
(current 4 bytes) into which an ip address may be stored. This procedure is used to
generate an appropriate argument to be passed to tcp-server-connection-accept.
(allocate-host-address) ) "Xe\034\241"
host-address-any
procedure
Return an ip address object that specifies “any host”. This object is useful only when
passed as the address argument to open-tcp-server-socket.
(host-address-any) ) "\000\000\000\000"
host-address-loopback
procedure
Return an ip address object that specifies the local loopback network interface. The
loopback interface is a software network interface that can be used only for communicating between processes on the same computer. This address object is useful only
when passed as the address argument to open-tcp-server-socket.
(host-address-loopback) ) "\177\000\000\001"
Chapter 16: Error System
257
16 Error System
The MIT Scheme error system provides a uniform mechanism for the signalling of errors
and other exceptional conditions. The simplest and most generally useful procedures in the
error system are:
error
is used to signal simple errors, specifying a message and some irritant objects
(see Section 16.1 [Condition Signalling], page 258). Errors are usually handled
by stopping the computation and putting the user in an error repl.
warn
is used to signal warnings (see Section 16.1 [Condition Signalling], page 258).
Warnings are usually handled by printing a message on the console and continuing the computation normally.
ignore-errors
is used to suppress the normal handling of errors within a given dynamic extent
(see Section 16.3 [Condition Handling], page 261). Any error that occurs within
the extent is trapped, returning immediately to the caller of ignore-errors.
More demanding applications require more powerful facilities. To give a concrete example, suppose you want floating-point division to return a very large number whenever the
denominator is zero. This behavior can be implemented using the error system.
The Scheme arithmetic system can signal many di↵erent kinds of errors, including
floating-point divide by zero. In our example, we would like to handle this particular
condition specially, allowing the system to handle other arithmetic errors in its usual way.
The error system supports this kind of application by providing mechanisms for distinguishing di↵erent types of error conditions and for specifying where control should be
transferred should a given condition arise. In this example, there is a specific object that
represents the “floating-point divide by zero” condition type, and it is possible to dynamically specify an arbitrary Scheme procedure to be executed when a condition of that type
is signalled. This procedure then finds the stack frame containing the call to the division
operator, and returns the appropriate value from that frame.
Another useful kind of behavior is the ability to specify uniform handling for related
classes of conditions. For example, it might be desirable, when opening a file for input,
to gracefully handle a variety of di↵erent conditions associated with the file system. One
such condition might be that the file does not exist, in which case the program will try
some other action, perhaps opening a di↵erent file instead. Another related condition is
that the file exists, but is read protected, so it cannot be opened for input. If these or any
other related conditions occur, the program would like to skip this operation and move on
to something else.
At the same time, errors unrelated to the file system should be treated in their usual
way. For example, calling car on the argument 3 should signal an error. Or perhaps the
name given for the file is syntactically incorrect, a condition that probably wants to be
handled di↵erently from the case of the file not existing.
To facilitate the handling of classes of conditions, the error system taxonomically organizes all condition types. The types are related to one another by taxonomical links, which
specify that one type is a “kind of” another type. If two types are linked this way, one is
considered to be a specialization of the other; or vice-versa, the second is a generalization
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of the first. In our example, all of the errors associated with opening an input file would be
specializations of the condition type “cannot open input file”.
The taxonomy of condition types permits any condition type to have no more than one
immediate generalization. Thus, the condition types form a forest (set of trees). While users
can create new trees, the standard taxonomy (see Section 16.7 [Taxonomy], page 272) is
rooted at condition-type:serious-condition, condition-type:warning, conditiontype:simple-condition, and condition-type:breakpoint; users are encouraged to add
new subtypes to these condition types rather than create new trees in the forest.
To summarize, the error system provides facilities for the following tasks. The sections
that follow will describe these facilities in more detail.
Signalling a condition
A condition may be signalled in a number of di↵erent ways. Simple errors may
be signalled, without explicitly defining a condition type, using error. The
signal-condition procedure provides the most general signalling mechanism.
Handling a condition
The programmer can dynamically specify handlers for particular condition
types or for classes of condition types, by means of the bind-conditionhandler procedure. Individual handlers have complete control over the handling of a condition, and additionally may decide not to handle a particular
condition, passing it on to previously bound handlers.
Restarting from a handler
The with-restart procedure provides a means for condition-signalling code to
communicate to condition-handling code what must be done to proceed past
the condition. Handlers can examine the restarts in e↵ect when a condition was
signalled, allowing a structured way to continue an interrupted computation.
Packaging condition state
Each condition is represented by an explicit object. Condition objects contain
information about the nature of the condition, information that describes the
state of the computation from which the condition arose, and information about
the ways the computation can be restarted.
Classification of conditions
Each condition has a type, represented by a condition type object. Each condition type may be a specialization of some other condition types. A group of
types that share a common generalization can be handled uniformly by specifying a handler for the generalization.
16.1 Condition Signalling
Once a condition instance has been created using make-condition (or any condition
constructor), it can be signalled. The act of signalling a condition is separated from the
act of creating the condition to allow more flexibility in how conditions are handled. For
example, a condition instance could be returned as the value of a procedure, indicating that
something unusual has happened, to allow the caller to clean up some state. The caller
could then signal the condition once it is ready.
Chapter 16: Error System
259
A more important reason for having a separate condition-signalling mechanism is that
it allows resignalling. When a signalled condition has been caught by a particular handler,
and the handler decides that it doesn’t want to process that particular condition, it can
signal the condition again. This is one way to allow other handlers to get a chance to see
the condition.
error reason argument...
procedure
This is the simplest and most common way to signal a condition that requires intervention before a computation can proceed (when intervention is not required, warn
is more appropriate). error signals a condition (using signal-condition), and if
no handler for that condition alters the flow of control (by invoking a restart, for
example) it calls the procedure standard-error-handler, which normally prints an
error message and stops the computation, entering an error repl. Under normal
circumstances error will not return a value (although an interactive debugger can be
used to force this to occur).
Precisely what condition is signalled depends on the first argument to error. If
reason is a condition, then that condition is signalled and the arguments are ignored.
If reason is a condition type, then a new instance of this type is generated and
signalled; the arguments are used to generate the values of the fields for this condition
type (they are passed as the field-plist argument to make-condition). In the most
common case, however, reason is neither a condition nor a condition type, but rather
a string or symbol. In this case a condition of type condition-type:simple-error is
created with the message field containing the reason and the irritants field containing
the arguments.
warn reason argument...
procedure
When a condition is not severe enough to warrant intervention, it is appropriate to
signal the condition with warn rather than error. As with error, warn first calls
signal-condition; the condition that is signalled is chosen exactly as in error except
that a condition of type condition-type:simple-warning is signalled if reason is
neither a condition nor a condition type. If the condition is not handled, warn calls
the procedure standard-warning-handler, which normally prints a warning message
and continues the computation by returning from warn.
warn establishes a restart named muffle-warning before calling signal-condition.
This allows a signal handler to prevent the generation of the warning message by
calling muffle-warning. The value of a call to warn is unspecified.
signal-condition condition
procedure
This is the fundamental operation for signalling a condition. The precise operation of
signal-condition depends on the condition type of which condition is an instance,
the condition types set by break-on-signals, and the handlers established by bindcondition-handler and bind-default-condition-handler.
If the condition is an instance of a type that is a specialization of any of the types
specified by break-on-signals, then a breakpoint repl is initiated. Otherwise (or
when that repl returns), the handlers established by bind-condition-handler are
checked, most recent first. Each applicable handler is invoked, and the search for a
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handler continues if the handler returns normally. If all applicable handlers return,
then the applicable handlers established by bind-default-condition-handler are
checked, again most recent first. Finally, if no handlers apply (or all return in a
normal manner), signal-condition returns an unspecified value.
Note: unlike many other systems, the MIT Scheme runtime library does not establish handlers of any kind. (However, the Edwin text editor uses condition handlers
extensively.) Thus, calls to signal-condition will return to the caller unless there
are user supplied condition handlers, as the following example shows:
(signal-condition
(make-condition
condition-type:error
(call-with-current-continuation (lambda (x) x))
’()
; no restarts
’())) ; no fields
) unspecified
16.2 Error Messages
By convention, error messages (and in general, the reports generated by writecondition-report) should consist of one or more complete sentences. The usual rules for
sentences should be followed: the first word of the sentence should be capitalized, and the
sentence should be terminated by a period. The message should not contain extraneous
whitespace such as line breaks or indentation.
The error system provides a simple formatting language that allows the programmer
to have some control over the printing of error messages. This formatting language will
probably be redesigned in a future release.
Error messages typically consist of a string describing the error, followed by some irritant
objects. The string is printed using display, and the irritants are printed using write, typically with a space between each irritant. To allow simple formatting, we introduce a noise
object, printed using display. The irritant list may contain ordinary objects interspersed
with noise objects. Each noise object is printed using display, with no extra whitespace,
while each normal object is printed using write, prefixed by a single space character.
Here is an example:
(define (error-within-procedure message irritant procedure)
(error message
irritant
(error-irritant/noise "within procedure")
procedure
(error-irritant/noise ".")))
This would format as follows:
(error-within-procedure "Bad widget" ’widget-32 ’invert-widget)
Bad widget widget-32 within procedure invert-widget.
Here are the operations supporting error messages:
error
Chapter 16: Error System
261
format-error-message message irritants port
procedure
Message is typically a string (although this is not required), irritants a list of irritant
objects, and port an output port. Formats message and irritants to port in the
standard way. Note that, during the formatting process, the depth and breadth to
which lists are printed are each limited to small numbers, to guarantee that the output
from each irritant is not arbitrarily large.
error-irritant/noise value
Creates and returns a noise object whose value is value.
procedure
16.3 Condition Handling
The occurrence of a condition is signalled using signal-condition. signal-condition
attempts to locate and invoke a condition handler that is prepared to deal with the type
of condition that has occurred. A condition handler is a procedure of one parameter, the
condition that is being signalled. A procedure is installed as a condition handler by calling
bind-condition-handler (to establish a handler that is in e↵ect only while a particular thunk is executing) or bind-default-condition-handler (to establish a handler that
is in e↵ect permanently). As implied by the name, handlers created by bind-defaultcondition-handler are invoked only after all other applicable handlers have been invoked.
A handler may process a signal in any way it deems appropriate, but the common
patterns are:
Ignore the condition.
By returning from the handler in the usual manner.
Handle the condition.
By doing some processing and then invoking a restart (or, less preferably, a
continuation) that was established at some point prior to the call to signalcondition.
Resignal a condition.
By doing some processing and calling signal-condition with either the same
condition or a newly created one. In order to support this, signal-condition
runs handler in such a way that a subsequent call to signal-condition sees
only the handlers that were established prior to this one.
As an aid to debugging condition handlers, Scheme maintains a set of condition types
that will cause an interactive breakpoint to occur prior to normal condition signalling. That
is, signal-condition creates a new repl prior to its normal operation when its argument is
a condition that is a specialization of any of these types. The procedure break-on-signals
establishes this set of condition types.
ignore-errors thunk
procedure
Executes thunk with a condition handler that intercepts the signalling of any specialization of condition-type:error (including those produced by calls to error) and
immediately terminates the execution of thunk and returns from the call to ignoreerrors with the signalled condition as its value. If thunk returns normally, its value
is returned from ignore-errors.
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Notice that ignore-errors does not “turn o↵ signalling” or condition handling. Condition handling takes place in the normal manner but conditions specialized from
condition-type:error are trapped rather than propogated as they would be by
default.
bind-condition-handler condition-types handler thunk
procedure
Invokes thunk after adding handler as a condition handler for the conditions specified
by condition-types. Condition-types must be a list of condition types; signalling a
condition whose type is a specialization of any of these types will cause the handler
to be invoked. See signal-condition for a description of the mechanism used to
invoke handlers.
By special extension, if condition-types is the empty list then the handler is called
for all conditions.
bind-default-condition-handler condition-types handler
procedure
Installs handler as a (permanent) condition handler for the conditions specified by
condition-types. Condition-types must be a list of condition types; signalling a condition whose type is a specialization of any of these types will cause the handler to be
invoked. See signal-condition for a description of the mechanism used to invoke
handlers.
By special extension, if condition-types is the empty list then the handler is called
for all conditions.
break-on-signals condition-types
procedure
Arranges for signal-condition to create an interactive repl before it signals a
condition that is a specialization of any of the types in the list of condition-types.
This can be extremely helpful when trying to debug code that uses custom condition
handlers. In order to create a repl when any condition type is signalled it is best to
actually put a breakpoint on entry to signal-condition.
standard-error-handler condition
procedure
Called internally by error after it calls signal-condition. Normally creates creates
a new repl with the prompt "error>" (but see standard-error-hook). In order to
simulate the e↵ect of calling error, code may call signal-condition directly and
then call standard-error-handler if signal-condition returns.
standard-error-hook
variable
This variable controls the behavior of the procedure standard-error-handler, and
hence error. It is intended to be bound with fluid-let and is normally #f. It may
be changed to a procedure of one argument and will then be invoked (with standarderror-hook rebound to #f) by standard-error-handler just prior to starting the
error repl. It is passed one argument, the condition being signalled.
standard-warning-handler condition
procedure
This is the procedure called internally by warn after it calls signal-condition.
The normal behavior of standard-warning-handler is to print a message (but see
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263
standard-warning-hook). More precisely, the message is printed to the port returned
by notification-output-port. The message is formed by first printing the string
"Warning: " to this port, and then calling write-condition-report on condition
and the port.
In order to simulate the e↵ect of calling warn, code may call signal-condition directly and then call standard-warning-handler if signal-condition returns. (This
is not sufficient to implement the muffle-warning protocol, however. For that purpose an explicit restart must be provided.)
standard-warning-hook
variable
This variable controls the behavior of the procedure standard-warning-handler,
and hence warn. It is intended to be bound with fluid-let and is normally #f.
It may be changed to a procedure of one argument and will then be invoked (with
standard-warning-hook rebound to #f) by standard-warning-handler in lieu of
writing the warning message. It is passed one argument, the condition being signalled.
16.4 Restarts
The Scheme error system provides a mechanism, known as restarts, that helps coordinate
condition-signalling code with condition-handling code. A module of code that detects and
signals conditions can provide procedures (using with-simple-restart or with-restart)
to be invoked by handlers that wish to continue, abort, or restart the computation. These
procedures, called restart e↵ectors, are encapsulated in restart objects.
When a condition object is created, it contains a set of restart objects, each of which
contains a restart e↵ector. Condition handlers can inspect the condition they are handling (using find-restart to find restarts by name, or condition/restarts to see the
entire set), and they can invoke the associated e↵ectors (using invoke-restart or invokerestart-interactively). E↵ectors can take arguments, and these may be computed
directly by the condition-handling code or by gathering them interactively from the user.
The names of restarts can be chosen arbitrarily, but the choice of name is significant.
These names are used to coordinate between the signalling code (which supplies names for
restarts) and the handling code (which typically chooses a restart e↵ector by the name of
its restart). Thus, the names specify the restart protocol implemented by the signalling
code and invoked by the handling code. The protocol indicates the number of arguments
required by the e↵ector code as well as the semantics of the arguments.
Scheme provides a conventional set of names (hence, protocols) for common use. By
choosing the names of restarts from this set, signalling code can indicate that it is able to
perform a small set of fairly common actions (abort, continue, muffle-warning, retry,
store-value, use-value). In turn, simple condition-handling code can look for the kind
of action it wishes to perform and simply invoke it by name. All of Scheme’s conventional
names are symbols, although in general restart names are not restricted to any particular
data type. In addition, the object #f is reserved to indicate the “not for automated use”
protocol: these restarts should be activated only under human control.
Restarts themselves are first-class objects. They encapsulate their name, a procedure
(known as the e↵ector) to be executed if they are invoked, and a thunk (known as the
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reporter) that can be invoked to display a description of the restart (used, for example,
by the interactive debugger). Invoking a restart is an indication that a handler has chosen
to accept control for a condition; as a consequence, the e↵ector of the restart should not
return, since this would indicate that the handler declined to handle the condition. Thus, the
e↵ector should call a continuation captured before the condition-signalling process began.
The most common pattern of usage by signalling code is encapsulated in with-simplerestart.
Within this chapter, a parameter named restarts will accept any of the following values:
• A list of restart objects.
• A condition. The procedure condition/restarts is called on the condition, and the
resulting list of restarts is used in place of the condition.
• The symbol bound-restarts. The procedure bound-restarts is called (with no arguments), and the resulting list of restarts is used in place of the symbol.
• If the restarts parameter is optional and is not supplied, it is equivalent to having
specified the symbol bound-restarts.
16.4.1 Establishing Restart Code
with-simple-restart name reporter thunk
procedure
Invokes thunk in a dynamic environment created by adding a restart named name to
the existing named restarts. Reporter may be used during the execution of thunk to
produce a description of the newly created restart; it must either be a procedure of one
argument (a port) or a string. By convention, the description generated by reporter
should be a short complete sentence, with first word capitalized and terminated by
a period. The sentence should fit on one line with a little room to spare (see the
examples below); usually this means that the sentence should be 70 characters or less
in length.
If the restart created by with-simple-restart is invoked it simply aborts the computation in progress by returning an unspecified value from the call to with-simplerestart. Otherwise with-simple-restart returns the value computed by thunk.
(with-simple-restart ’george "This restart is named george."
(lambda () 3)) ) 3
(with-simple-restart ’george "This restart is named george."
(lambda ()
(invoke-restart (find-restart ’george)))) ) unspecific
(with-simple-restart ’george "This restart is named george."
(lambda () (car 3)))
;The object 3, passed as the first argument to car,
; is not the correct type.
;To continue, call RESTART with an option number:
; (RESTART 3) => Specify an argument to use in its place.
; (RESTART 2) => This restart is named george.
; (RESTART 1) => Return to read-eval-print level 1.
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with-restart name reporter e↵ector interactor thunk
procedure
Invokes thunk in a dynamic environment created by adding a restart named name
to the existing named restarts. Reporter may be used during the execution of thunk
to produce a description of the newly created restart; it must either be a procedure
of one argument (a port) or a string. E↵ector is a procedure which will be called
when the restart is invoked by invoke-restart. Interactor specifies the arguments
that are to be passed to e↵ector when it is invoked interactively; it may be either a
procedure of no arguments, or #f. If interactor is #f, this restart is not meant to be
invoked interactively.
The value returned by with-restart is the value returned by thunk. Should the
restart be invoked by a condition handler, however, the e↵ector will not return back
to the handler that invoked it. Instead, the e↵ector should call a continuation created
before the condition-signalling process began, and with-restart will therefore not
return in the normal manner.
(define (by-george! thunk)
; This code handles conditions that arise while executing thunk
; by invoking the GEORGE restart, passing 1 and 2 to the restart’s
; e↵ector code.
(bind-condition-handler ’() ; All conditions
(lambda (condition)
(invoke-restart (find-restart ’george) 1 2))
thunk))
(define (can-george! thunk)
; This code provides a way of handling errors: the GEORGE restart.
; In order to GEORGE you must supply two values.
(lambda ()
(call-with-current-continuation
(lambda (kappa)
(with-restart
’george
; Name
"This restart is named george." ; Reporter
(lambda (a b)
; E↵ector
(kappa (list ’george a b)))
values
; Interactor
thunk)))))
; Thunk
) -3
(by-george! (can-george! (lambda () -3))
(by-george! (can-george! (lambda () (car ’x)))) ) (george 1 2)
16.4.2 Invoking Standard Restart Code
Scheme supports six standard protocols for restarting from a condition, each encapsulated using a named restart (for use by condition-signalling code) and a simple procedure
(for use by condition-handling code). Unless otherwise specified, if one of these procedures
is unable to find its corresponding restart, it returns immediately with an unspecified value.
Each of these procedures accepts an optional argument restarts, which is described above
in Section 16.4 [Restarts], page 263.
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abort [restarts]
procedure
Abort the computation, using the restart named abort. The corresponding e↵ector
takes no arguments and abandons the current line of computation. This is the restart
provided by Scheme’s repl.
If there is no restart named abort, this procedure signals an error of type conditiontype:no-such-restart.
continue [restarts]
procedure
Continue the current computation, using the restart named continue. The corresponding e↵ector takes no arguments and continues the computation beyond the
point at which the condition was signalled.
mu✏e-warning [restarts]
procedure
Continue the current computation, using the restart named muffle-warning. The
corresponding e↵ector takes no arguments and continues the computation beyond
the point at which any warning message resulting from the condition would be presented to the user. The procedure warn establishes a muffle-warning restart for this
purpose.
If there is no restart named muffle-warning, this procedure signals an error of type
condition-type:no-such-restart.
retry [restarts]
procedure
Retry the current computation, using the restart named retry. The corresponding
e↵ector takes no arguments and simply retries the same computation that triggered
the condition. The condition may reoccur, of course, if the root cause has not been
eliminated. The code that signals a “file does not exist” error can be expected to
supply a retry restart. The restart would be invoked after first creating the missing
file, since the computation is then likely to succeed if it is simply retried.
store-value new-value [restarts]
procedure
Retry the current computation, using the restart named store-value, after first
storing new-value. The corresponding e↵ector takes one argument, new-value, and
stores it away in a restart-dependent location, then retries the same computation that
triggered the condition. The condition may reoccur, of course, if the root cause has
not been eliminated. The code that signals an “unassigned variable” error can be
expected to supply a store-value restart; this would store the value in the variable
and continue the computation.
use-value new-value [restarts]
procedure
Retry the current computation, using the restart named use-value, but substituting
new-value for a value that previously caused a failure. The corresponding e↵ector
takes one argument, new-value, and retries the same computation that triggered the
condition with the new value substituted for the failing value. The condition may
reoccur, of course, if the new value also induces the condition.
The code that signals an “unassigned variable” error can be expected to supply a usevalue restart; this would simply continue the computation with new-value instead of
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267
the value of the variable. Contrast this with the retry and store-value restarts.
If the retry restart is used it will fail because the variable still has no value. The
store-value restart could be used, but it would alter the value of the variable, so
that future references to the variable would not be detected.
16.4.3 Finding and Invoking General Restart Code
Restarts are a general mechanism for establishing a protocol between condition-signalling
and condition-handling code. The Scheme error system provides “packaging” for a number
of common protocols. It also provides lower-level hooks that are intended for implementing
customized protocols. The mechanism used by signalling code (with-restart and withsimple-restart) is used for both purposes.
Four additional operations are provided for the use of condition-handling code. Two
operations (bound-restarts and find-restart) allow condition-handling code to locate
active restarts. The other two operations (invoke-restart and invoke-restartinteractively) allow restart e↵ectors to be invoked once the restart object has been
located.
In addition, there is a data abstraction that provides access to the information encapsulated in restart objects.
bound-restarts
procedure
Returns a list of all currently active restart objects, most recently installed first.
bound-restarts should be used with caution by condition-handling code, since it
reveals all restarts that are active at the time it is called, rather than at the time
the condition was signalled. It is useful, however, for collecting the list of restarts for
inclusion in newly generated condition objects or for inspecting the current state of
the system.
find-restart name [restarts]
procedure
Returns the first restart object named name in the list of restarts (permissible values
for restarts are described above in Section 16.4 [Restarts], page 263). When used in
a condition handler, find-restart is usually passed the name of a particular restart
and the condition object that has been signalled. In this way the handler finds only
restarts that were available when the condition was created (usually the same as when
it was signalled). If restarts is omitted, the currently active restarts would be used,
and these often include restarts added after the condition ocurred.
invoke-restart restart argument...
procedure
Calls the restart e↵ector encapsulated in restart, passing the specified arguments to it.
invoke-restart is intended for use by condition-handling code that understands the
protocol implemented by restart, and can therefore calculate and pass an appropriate
set of arguments.
If a condition handler needs to interact with a user to gather the arguments for an
e↵ector (e.g. if it does not understand the protocol implemented by restart) invokerestart-interactively should be used instead of invoke-restart.
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invoke-restart-interactively restart
procedure
First calls the interactor encapsulated in restart to interactively gather the arguments
needed for restart’s e↵ector. It then calls the e↵ector, passing these arguments to it.
invoke-restart-interactively is intended for calling interactive restarts (those
for which restart/interactor is not #f). For convenience, invoke-restartinteractively will call the restart’s e↵ector with no arguments if the restart has
no interactor; this behavior may change in the future.
16.4.4 The Named Restart Abstraction
A restart object is very simple, since it encapsulates only a name, e↵ector, interactor,
and description.
restart? object
Returns #f if and only if object is not a restart.
procedure
restart/name restart
procedure
Returns the name of restart. While the Scheme error system uses only symbols and
the object #f for its predefined names, programs may use arbitrary objects (name
equivalence is tested using eq?).
restart/e↵ector restart
procedure
Returns the e↵ector encapsulated in restart. Normally this procedure is not used since
invoke-restart and invoke-restart-interactively capture the most common
invocation patterns.
restart/interactor restart
procedure
Returns the interactor encapsulated in restart. This is either a procedure of no arguments or the object #f. Normally this procedure is not used since invoke-restartinteractively captures the most common usage. Thus restart/interactor is
most useful as a predicate to determine if restart is intended to be invoked interactively.
write-restart-report restart port
procedure
Writes a description of restart to port. This works by either displaying (if it is a
string) or calling (if it is a procedure) the reporter that was supplied when the restart
was created.
16.5 Condition Instances
A condition, in addition to the information associated with its type, usually contains
other information that is not shared with other conditions of the same type. For example,
the condition type associated with “unbound variable” errors does not specify the name
of the variable that was unbound. The additional information is captured in a condition
object, also called a condition instance.
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In addition to information that is specific to a given type of condition (such as the
variable name for “unbound variable” conditions), every condition instance also contains a
continuation that encapsulates the state of the computation in which the condition occurred.
This continuation is used for analyzing the computation to learn more about the context
in which the condition occurred. It is not intended to provide a mechanism for continuing
the computation; that mechanism is provided by restarts.
16.5.1 Generating Operations on Conditions
Scheme provides four procedures that take a condition type as input and produce operations on the corresponding condition object. These are reminiscent of the operations on
record types that produce record operators (see Section 10.4 [Records], page 132). Given
a condition type it is possible to generate: a constructor for instances of the type (using
condition-constructor); an accessor to extract the contents of a field in instances of
the type (using condition-accessor); a predicate to test for instances of the type (using
condition-predicate); and a procedure to create and signal an instance of the type (using
condition-signaller).
Notice that the creation of a condition object is distinct from signalling an occurrence
of the condition. Condition objects are first-class; they may be created and never signalled,
or they may be signalled more than once. Further notice that there are no procedures for
modifying conditions; once created, a condition cannot be altered.
condition-constructor condition-type field-names
procedure
Returns a constructor procedure that takes as arguments values for the fields specified
in field-names and creates a condition of type condition-type. Field-names must be a
list of symbols that is a subset of the field-names in condition-type. The constructor
procedure returned by condition-constructor has signature
(lambda (continuation restarts . field-values) ...)
where the field-names correspond to the field-values. The constructor argument
restarts is described in Section 16.4 [Restarts], page 263. Conditions created by the
constructor procedure have #f for the values of all fields other than those specified
by field-names.
For example, the following procedure make-simple-warning constructs a condition
of type condition-type:simple-warning given a continuation (where the condition
occurred), a description of the restarts to be made available, a warning message, and
a list of irritants that caused the warning:
(define make-simple-warning
(condition-constructor condition-type:simple-warning
’(message irritants)))
condition-accessor condition-type field-name
procedure
Returns a procedure that takes as input a condition object of type condition-type
and extracts the contents of the specified field-name. condition-accessor signals error:bad-range-argument if the field-name isn’t one of the named fields of
condition-type; the returned procedure will signal error:wrong-type-argument if
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passed an object other than a condition of type condition-type or one of its specializations.
If it is known in advance that a particular field of a condition will be accessed repeatedly it is worth constructing an accessor for the field using condition-accessor
rather than using the (possibly more convenient, but slower) access-condition procedure.
condition-predicate condition-type
procedure
Returns a predicate procedure for testing whether an object is a condition of type
condition-type or one of its specializations (there is no predefined way to test for a
condition of a given type but not a specialization of that type).
condition-signaller condition-type field-names default-handler
procedure
Returns a signalling procedure with parameters field-names. When the signalling
procedure is called it creates and signals a condition of type condition-type. If the
condition isn’t handled (i.e. if no handler is invoked that causes an escape from the
current continuation) the signalling procedure reduces to a call to default-handler
with the condition as its argument.
There are several standard procedures that are conventionally used for
default-handler. If condition-type is a specialization of condition-type:error,
default-handler should be the procedure
standard-error-handler. If condition-type is a specialization of conditiontype:warning, default-handler should be the procedure standard-warninghandler. If condition-type is a specialization of condition-type:breakpoint,
default-handler should be the procedure standard-breakpoint-handler.
16.5.2 Condition Abstraction
The condition data type is abstracted through a predicate condition? and a set of
accessor procedures.
condition? object
procedure
condition/type condition
procedure
Returns #f if and only if object is not a condition.
Returns the condition type of which condition is an instance.
condition/error? condition
procedure
Returns #t if the condition is an instance of condition type condition-type:error
or a specialization of it, #f otherwise.
condition/restarts condition
Returns the list of restarts specified when condition was created.
condition/continuation condition
procedure
procedure
Returns the continuation specified when condition was created. This is provided for
inspecting the state of the system when the condition occurred, not for continuing or
restarting the computation.
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write-condition-report condition port
procedure
Writes a description of condition to port, using the reporter function from the condition type associated with condition. See also condition/report-string.
16.5.3 Simple Operations on Condition Instances
The simple procedures described in this section are built on top of the more detailed
abstraction of condition objects described above. While these procedures are sometimes
easier to use, they are often less efficient.
make-condition condition-type continuation restarts field-plist
procedure
Create a new condition object as an instance of condition-type, associated with continuation. The continuation is provided for inspection purposes only, not for restarting the computation. The restarts argument is described in Section 16.4 [Restarts],
page 263. The field-plist is an alternating list of field names and values for those fields,
where the field names are those that would be returned by (condition-type/fieldnames condition-type). It is used to provide values for fields in the condition object;
fields with no value specified are set to #f. Once a condition object has been created
there is no way to alter the values of these fields.
access-condition condition field-name
procedure
Returns the value stored in the field field-name within condition. Field-name must
be one of the names returned by (condition-type/field-names (condition/type
condition)). access-condition looks up the field-name at runtime, so it is more
efficient to use condition-accessor to create an access function if the same field is
to be extracted from several instances of the same condition type.
condition/report-string condition
procedure
Returns a string containing a report of the condition. This is generated by calling
write-condition-report on condition and a string output port, and returning the
output collected by the port as a string.
16.6 Condition Types
Each condition has a condition type object associated with it. These objects are used
as a means of focusing on related classes of conditions, first by concentrating all of the
information about a specific class of condition in a single place, and second by specifying an
inheritance relationship between types. This inheritance relationship forms the taxonomic
structure of the condition hierarchy (see Section 16.7 [Taxonomy], page 272).
The following procedures consititute the abstraction for condition types.
make-condition-type name generalization field-names reporter
procedure
Creates and returns a (new) condition type that is a specialization of generalization (if
it is a condition type) or is the root of a new tree of condition types (if generalization
is #f). For debugging purposes, the condition type has a name, and instances of
this type contain storage for the fields specified by field-names (a list of symbols) in
addition to the fields common to all conditions (type, continuation and restarts).
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Reporter is used to produce a description of a particular condition of this type. It
may be a string describing the condition, a procedure of arity two (the first argument
will be a condition of this type and the second a port) that will write the message to
the given port, or #f to specify that the reporter should be taken from the condition
type generalization (or produce an “undocumented condition of type ...” message if
generalization is #f). The conventions used to form descriptions are spelled out in
Section 16.2 [Error Messages], page 260.
condition-type/error? condition-type
procedure
Returns #t if the condition-type is condition-type:error or a specialization of it,
#f otherwise.
condition-type/field-names condition-type
procedure
Returns a list of all of the field names for a condition of type condition-type. This
is the set union of the fields specified when this condition-type was created with the
condition-type/field-names of the generalization of this condition-type.
condition-type/generalizations condition-type
procedure
Returns a list of all of the generalizations of condition-type. Notice that every condition type is considered a generalization of itself.
condition-type? object
Returns #f if and only if object is not a condition type.
procedure
16.7 Condition-Type Taxonomy
The MIT Scheme error system provides a rich set of predefined condition types. These are
organized into a forest through taxonomic links providing the relationships for “specializes”
and “generalizes”. The chart appearing below shows these relationships by indenting all the
specializations of a given type relative to the type. Note that the variables that are bound to
these condition types are prefixed by ‘condition-type:’; for example, the type appearing in
the following table as ‘simple-error’ is stored in the variable condition-type:simpleerror. Users are encouraged to add new condition types by creating specializations of
existing ones.
Following the chart are detailed descriptions of the predefined condition types. Some
of these types are marked as abstract types. Abstract types are not intended to be used
directly as the type of a condition; they are to be used as generalizations of other types,
and for binding condition handlers. Types that are not marked as abstract are concrete;
they are intended to be explicitly used as a condition’s type.
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serious-condition
error
simple-error
illegal-datum
wrong-type-datum
wrong-type-argument
wrong-number-of-arguments
datum-out-of-range
bad-range-argument
inapplicable-object
file-error
file-operation-error
derived-file-error
port-error
derived-port-error
variable-error
unbound-variable
unassigned-variable
arithmetic-error
divide-by-zero
floating-point-overflow
floating-point-underflow
control-error
no-such-restart
not-loading
primitive-procedure-error
system-call-error
warning
simple-warning
simple-condition
breakpoint
condition-type:serious-condition
condition type
This is an abstract type. All serious conditions that require some form of intervention
should inherit from this type. In particular, all errors inherit from this type.
condition-type:error
This is an abstract type. All errors should inherit from this type.
condition type
condition-type:simple-error message irritants
condition type
This is the condition generated by the error procedure when its first argument is not
a condition or condition type. The fields message and irritants are taken directly from
the arguments to error; message contains an object (usually a string) and irritants
contains a list of objects. The reporter for this type uses format-error-message to
generate its output from message and irritants.
condition-type:illegal-datum datum
condition type
This is an abstract type. This type indicates the class of errors in which a program
discovers an object that lacks specific required properties. Most commonly, the object
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is of the wrong type or is outside a specific range. The datum field contains the
o↵ending object.
condition-type:wrong-type-datum datum type
condition type
This type indicates the class of errors in which a program discovers an object that
is of the wrong type. The type field contains a string describing the type that was
expected, and the datum field contains the object that is of the wrong type.
(error:wrong-type-datum 3.4 "integer")
error
;The object 3.4 is not an integer.
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.
error:wrong-type-datum datum type
procedure
This procedure signals a condition of type condition-type:wrong-type-datum. The
datum and type fields of the condition are filled in from the corresponding arguments
to the procedure.
condition-type:wrong-type-argument datum type operator
condition type
operand
This type indicates that a procedure was passed an argument of the wrong type.
The operator field contains the procedure (or a symbol naming the procedure), the
operand field indicates the argument position that was involved (this field contains
either a symbol, a non-negative integer, or #f), the type field contains a string describing the type that was expected, and the datum field contains the o↵ending argument.
(+ ’a 3)
error
;The object a, passed as the first argument to integer-add,
; is not the correct type.
;To continue, call RESTART with an option number:
; (RESTART 2) => Specify an argument to use in its place.
; (RESTART 1) => Return to read-eval-print level 1.
(list-copy 3)
;The object 3, passed as an argument to list-copy, is not a list.
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.
error:wrong-type-argument datum type operator
procedure
This procedure signals a condition of type condition-type:wrong-type-argument.
The datum, type and operator fields of the condition are filled in from the corresponding arguments to the procedure; the operand field of the condition is set to
#f.
condition-type:wrong-number-of-arguments datum type
condition type
operands
This type indicates that a procedure was called with the wrong number of arguments.
The datum field contains the procedure being called, the type field contains the
number of arguments that the procedure accepts, and the operands field contains a
list of the arguments that were passed to the procedure.
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(car 3 4)
error
;The procedure car has been called with 2 arguments;
; it requires exactly 1 argument.
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.
error:wrong-number-of-arguments datum type operands
procedure
This procedure signals a condition of type condition-type:wrong-number-ofarguments. The datum, type and operands fields of the condition are filled in from
the corresponding arguments to the procedure.
condition-type:datum-out-of-range datum
condition type
This type indicates the class of errors in which a program discovers an object that is
of the correct type but is otherwise out of range. Most often, this type indicates that
an index to some data structure is outside of the range of indices for that structure.
The datum field contains the o↵ending object.
(error:datum-out-of-range 3)
error
;The object 3 is not in the correct range.
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.
error:datum-out-of-range datum
procedure
This procedure signals a condition of type condition-type:datum-out-of-range.
The datum field of the condition is filled in from the corresponding argument to the
procedure.
condition-type:bad-range-argument datum operator
condition type
operand
This type indicates that a procedure was passed an argument that is of the correct
type but is otherwise out of range. Most often, this type indicates that an index to
some data structure is outside of the range of indices for that structure. The operator
field contains the procedure (or a symbol naming the procedure), the operand field
indicates the argument position that was involved (this field contains either a symbol,
a non-negative integer, or #f), and the datum field is the o↵ending argument.
(string-ref "abc" 3)
error
;The object 3, passed as the second argument to string-ref,
; is not in the correct range.
;To continue, call RESTART with an option number:
; (RESTART 2) => Specify an argument to use in its place.
; (RESTART 1) => Return to read-eval-print level 1.
error:bad-range-argument datum operator
procedure
This procedure signals a condition of type condition-type:bad-range-argument.
The datum and operator fields of the condition are filled in from the corresponding
arguments to the procedure; the operand field of the condition is set to #f.
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condition-type:inapplicable-object datum operands
condition type
This type indicates an error in which a program attempted to apply an object that
is not a procedure. The object being applied is saved in the datum field, and the
arguments being passed to the object are saved as a list in the operands field.
(3 4)
;The object 3
;To continue,
; (RESTART 2)
; (RESTART 1)
error
is not applicable.
call RESTART with an option number:
=> Specify a procedure to use in its place.
=> Return to read-eval-print level 1.
condition-type:file-error filename
condition type
This is an abstract type. It indicates that an error associated with a file has occurred.
For example, attempting to delete a nonexistent file will signal an error. The filename
field contains a filename or pathname associated with the operation that failed.
condition-type:file-operation-error filename verb noun
condition type
reason operator operands
This is the most common condition type for file system errors. The filename field
contains the filename or pathname that was being operated on. The verb field contains
a string which is the verb or verb phrase describing the operation being performed,
and the noun field contains a string which is a noun or noun phrase describing the
object being operated on. The reason field contains a string describing the error
that occurred. The operator field contains the procedure performing the operation
(or a symbol naming that procedure), and the operands field contains a list of the
arguments that were passed to that procedure. For example, an attempt to delete a
nonexistent file would have the following field values:
filename
"/zu/cph/tmp/no-such-file"
verb
"delete"
noun
"file"
reason
"no such file or directory"
operator
file-remove
operands
("/zu/cph/tmp/no-such-file")
and would generate a message like this:
(delete-file "/zu/cph/tmp/no-such-file") error
;Unable to delete file "/zu/cph/tmp/no-such-file" because:
; No such file or directory.
;To continue, call RESTART with an option number:
; (RESTART 3) => Try to delete the same file again.
; (RESTART 2) => Try to delete a different file.
; (RESTART 1) => Return to read-eval-print level 1.
error:file-operation-error filename verb noun reason operator
procedure
operands
This procedure signals a condition of type condition-type:file-operation-error.
The fields of the condition are filled in from the corresponding arguments to the
procedure.
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condition-type:derived-file-error filename condition
condition type
This is another kind of file error, which is generated by obscure file-system errors that
do not fit into the standard categories. The filename field contains the filename or
pathname that was being operated on, and the condition field contains a condition
describing the error in more detail. Usually the condition field contains a condition
of type condition-type:system-call-error.
error:derived-file filename condition
procedure
This procedure signals a condition of type condition-type:derived-file-error.
The filename and condition fields of the condition are filled in from the corresponding
arguments to the procedure.
condition-type:port-error port
condition type
This is an abstract type. It indicates that an error associated with a I/O port has
occurred. For example, writing output to a file port can signal an error if the disk
containing the file is full; that error would be signalled as a port error. The port field
contains the associated port.
condition-type:derived-port-error port condition
condition type
This is a concrete type that is signalled when port errors occur. The port field contains
the port associated with the error, and the condition field contains a condition object
that describes the error in more detail. Usually the condition field contains a condition
of type condition-type:system-call-error.
error:derived-port port condition
procedure
This procedure signals a condition of type condition-type:derived-port-error.
The port and condition fields of the condition are filled in from the corresponding
arguments to the procedure.
condition-type:variable-error location environment
condition type
This is an abstract type. It indicates that an error associated with a variable has
occurred. The location field contains the name of the variable, and the environment
field contains the environment in which the variable was referenced.
condition-type:unbound-variable location environment
condition type
This type is generated when a program attempts to access or modify a variable that is
not bound. The location field contains the name of the variable, and the environment
field contains the environment in which the reference occurred.
foo
error
;Unbound variable: foo
;To continue, call RESTART with an option number:
; (RESTART 3) => Specify a value to use instead of foo.
; (RESTART 2) => Define foo to a given value.
; (RESTART 1) => Return to read-eval-print level 1.
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condition-type:unassigned-variable location environment
condition type
This type is generated when a program attempts to access a variable that is not
assigned. The location field contains the name of the variable, and the environment
field contains the environment in which the reference occurred.
foo
error
;Unassigned variable: foo
;To continue, call RESTART with an option number:
; (RESTART 3) => Specify a value to use instead of foo.
; (RESTART 2) => Set foo to a given value.
; (RESTART 1) => Return to read-eval-print level 1.
condition-type:arithmetic-error operator operands
condition type
This is an abstract type. It indicates that a numerical operation was unable to complete because of an arithmetic error. (For example, division by zero.) The operator
field contains the procedure that implements the operation (or a symbol naming the
procedure), and the operands field contains a list of the arguments that were passed
to the procedure.
condition-type:divide-by-zero operator operands
condition type
This type is generated when a program attempts to divide by zero. The operator field
contains the procedure that implements the failing operation (or a symbol naming the
procedure), and the operands field contains a list of the arguments that were passed
to the procedure.
(/ 1 0)
;Division by zero signalled by /.
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.
error:divide-by-zero operator operands
procedure
This procedure signals a condition of type condition-type:divide-by-zero. The
operator and operands fields of the condition are filled in from the corresponding
arguments to the procedure.
condition-type:floating-point-overflow operator operands
condition type
This type is generated when a program performs an arithmetic operation that results
in a floating-point overflow. The operator field contains the procedure that implements the operation (or a symbol naming the procedure), and the operands field
contains a list of the arguments that were passed to the procedure.
condition-type:floating-point-underflow operator operands
condition type
This type is generated when a program performs an arithmetic operation that results
in a floating-point underflow. The operator field contains the procedure that implements the operation (or a symbol naming the procedure), and the operands field
contains a list of the arguments that were passed to the procedure.
Chapter 16: Error System
condition-type:primitive-procedure-error operator
279
condition type
operands
This is an abstract type. It indicates that an error was generated by a primitive
procedure call. Primitive procedures are distinguished from ordinary procedures in
that they are not written in Scheme but instead in the underlying language of the
Scheme implementation. The operator field contains the procedure that implements
the operation (or a symbol naming the procedure), and the operands field contains a
list of the arguments that were passed to the procedure.
condition-type:system-call-error operator operands
condition type
system-call error-type
This is the most common condition type generated by primitive procedures. A condition of this type indicates that the primitive made a system call to the operating
system, and that the system call signalled an error. The system-call error is reflected
back to Scheme as a condition of this type, except that many common system-call
errors are automatically translated by the Scheme implementation into more useful
forms; for example, a system-call error that occurs while trying to delete a file will be
translated into a condition of type condition-type:file-operation-error. The
operator field contains the procedure that implements the operation (or a symbol
naming the procedure), and the operands field contains a list of the arguments that
were passed to the procedure. The system-call and error-type fields contain symbols
that describe the specific system call that was being made and the error that occurred,
respectively; these symbols are completely operating-system dependent.
condition-type:control-error
condition type
This is an abstract type. It describes a class of errors relating to program control
flow.
condition-type:no-such-restart name
condition type
This type indicates that a named restart was not active when it was expected to be.
Conditions of this type are signalled by several procedures that look for particular
named restarts, for example muffle-warning. The name field contains the name
that was being searched for.
(muffle-warning)
error
;The restart named muffle-warning is not bound.
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.
error:no-such-restart name
procedure
This procedure signals a condition of type condition-type:no-such-restart. The
name field of the condition is filled in from the corresponding argument to the procedure.
condition-type:not-loading
condition type
A condition of this type is generated when the procedure current-load-pathname is
called from somewhere other than inside a file being loaded.
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(current-load-pathname)
error
;No file being loaded.
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.
condition-type:warning
condition type
This is an abstract type. All warnings should inherit from this type. Warnings are
a class of conditions that are usually handled by informing the user of the condition
and proceeding the computation normally.
condition-type:simple-warning message irritants
condition type
This is the condition generated by the warn procedure. The fields message and
irritants are taken directly from the arguments to warn; message contains an object
(usually a string) and irritants contains a list of objects. The reporter for this type
uses format-error-message to generate its output from message and irritants.
condition-type:simple-condition message irritants
condition type
This is an unspecialized condition that does not fall into any of the standard condition
classes. The message field contains an object (usually a string) and irritants contains
a list of objects. The reporter for this type uses format-error-message to generate
its output from message and irritants.
condition-type:breakpoint environment message prompt
condition type
A condition of this type is generated by the breakpoint mechanism. The contents of
its fields are beyond the scope of this document.
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17 Graphics
MIT Scheme has a simple two-dimensional line-graphics interface that is suitable for
many graphics applications. In particular it is often used for plotting data points from
experiments. The interface is generic in that it can support di↵erent types of graphics
devices in a uniform manner. At the present time only one type of graphics device is
implemented on each operating system.
Procedures are available for drawing points, lines, and text; defining the coordinate
system; clipping graphics output; controlling some of the drawing characteristics; and controlling the output bu↵er (for devices that perform bu↵ering). Additionally, devices may
support custom operations, such as control of colors.
There are some constraints on the arguments to the procedures described in this chapter.
Any argument named graphics-device must be a graphics device object that was returned
from a call to make-graphics-device. Any argument that is a coordinate must be either
an exact integer or an inexact real.
17.1 Opening and Closing of Graphics Devices
graphics-type-available? graphics-device-type
procedure
This predicate returns #t if the graphics system named by the symbol graphics-devicetype is implemented by the Scheme system. Otherwise it returns #f, in which case it
is an error to attempt to make a graphics device using graphics-device-type.
enumerate-graphics-types
procedure
This procedure returns a list of symbols which are the names of all the graphics device
types that are supported by the Scheme system. The result is useful in deciding
what additional arguments to supply to make-graphics-device, as each device type
typically has a unique way of specifying the initial size, shape and other attributes.
make-graphics-device graphics-device-type object . . .
procedure
This operation creates and returns a graphics device object. Graphics-device-type is
a symbol naming a graphics device type, and both the number and the meaning of
the remaining arguments is determined by that type (see the description of each device type for details); graphics-device-type must satisfy graphics-type-available?.
Graphics-device-type may also be #f, in which case the graphics device type is chosen by the system from what is available. This allows completely portable graphics
programs to be written provided no custom graphics operations are used. When
graphics-device-type is #f no further arguments may be given; each graphics device
type will use some “sensible” defaults. If more control is required then the program
should use one of the two procedures above to dispatch on the available types.
This procedure opens and initializes the device, which remains valid until explicitly
closed by the procedure graphics-close. Depending on the implementation of the
graphics device, if this object is reclaimed by the garbage collector, the graphics device
may remain open or it may be automatically closed. While a graphics device remains
open the resources associated with it are not released.
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graphics-close graphics-device
procedure
Closes graphics-device, releasing its resources. Subsequently it is an error to use
graphics-device.
17.2 Coordinates for Graphics
Each graphics device has two di↵erent coordinate systems associated with it: device
coordinates and virtual coordinates. Device coordinates are generally defined by low-level
characteristics of the device itself, and often cannot be changed. Most device coordinate
systems are defined in terms of pixels, and usually the upper-left-hand corner is the origin
of the coordinate system, with x coordinates increasing to the right and y coordinates
increasing downwards.
In contrast, virtual coordinates are more flexible in the units employed, the position of
the origin, and even the direction in which the coordinates increase. A virtual coordinate
system is defined by assigning coordinates to the edges of a device. Because these edge
coordinates are arbitrary real numbers, any Cartesian coordinate system can be defined.
All graphics procedures that use coordinates are defined on virtual coordinates. For
example, to draw a line at a particular place on a device, the virtual coordinates for the
endpoints of that line are given.
When a graphics device is initialized, its virtual coordinate system is reset so that the
left edge corresponds to an x-coordinate of -1, the right edge to x-coordinate 1, the bottom
edge to y-coordinate -1, and the top edge to y-coordinate 1.
graphics-device-coordinate-limits graphics-device
procedure
Returns (as multiple values) the device coordinate limits for graphics-device. The
values, which are exact non-negative integers, are: x-left, y-bottom, x-right, and ytop.
graphics-coordinate-limits graphics-device
procedure
Returns (as multiple values) the virtual coordinate limits for graphics-device. The
values, which are real numbers, are: x-left, y-bottom, x-right, and y-top.
graphics-set-coordinate-limits graphics-device x-left y-bottom
procedure
x-right y-top
Changes the virtual coordinate limits of graphics-device to the given arguments.
X-left, y-bottom, x-right, and y-top must be real numbers. Subsequent calls to
graphics-coordinate-limits will return the new limits. This operation has no
e↵ect on the device’s displayed contents.
Note: This operation usually resets the clip rectangle, although it is not guaranteed
to do so. If a clip rectangle is in e↵ect when this procedure is called, it is necessary
to redefine the clip rectangle afterwards.
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17.3 Drawing Graphics
The procedures in this section provide the basic drawing capabilities of Scheme’s graphics
system.
graphics-clear graphics-device
procedure
Clears the display of graphics-device. Una↵ected by the current drawing mode.
graphics-draw-point graphics-device x y
procedure
Draws a single point on graphics-device at the virtual coordinates given by x and y,
using the current drawing mode.
graphics-erase-point graphics-device x y
procedure
Erases a single point on graphics-device at the virtual coordinates given by x and y.
This procedure is una↵ected by the current drawing mode.
This is equivalent to
(lambda (device x y)
(graphics-bind-drawing-mode device 0
(lambda ()
(graphics-draw-point device x y))))
graphics-draw-line graphics-device x-start y-start x-end y-end
procedure
X-start, y-start, x-end, and y-end must be real numbers. Draws a line on graphicsdevice that connects the points (x-start, y-start) and (x-end, y-end). The line is
drawn using the current drawing mode and line style.
graphics-draw-text graphics-device x y string
procedure
Draws the characters of string at the point (x, y) on graphics-device, using the current
drawing mode. The characteristics of the characters drawn are device-dependent, but
all devices are initialized so that the characters are drawn upright, from left to right,
with the leftmost edge of the leftmost character at x, and the baseline of the characters
at y.
The following two procedures provide an alternate mechanism for drawing lines, which is
more akin to using a plotter. They maintain a cursor, which can be positioned to a particular
point and then dragged to another point, producing a line. Sequences of connected line
segments can be drawn by dragging the cursor from point to point.
Many graphics operations have an unspecified e↵ect on the cursor. The following exceptions are guaranteed to leave the cursor una↵ected:
graphics-device-coordinate-limits
graphics-coordinate-limits
graphics-enable-buffering
graphics-disable-buffering
graphics-flush
graphics-bind-drawing-mode
graphics-set-drawing-mode
graphics-bind-line-style
graphics-set-line-style
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The initial state of the cursor is unspecified.
graphics-move-cursor graphics-device x y
procedure
Moves the cursor for graphics-device to the point (x, y). The contents of the device’s
display are unchanged.
graphics-drag-cursor graphics-device x y
procedure
Draws a line from graphics-device’s cursor to the point (x, y), simultaneously moving
the cursor to that point. The line is drawn using the current drawing mode and line
style.
17.4 Characteristics of Graphics Output
Two characteristics of graphics output are so useful that they are supported uniformly
by all graphics devices: drawing mode and line style. A third characteristic, color, is equally
useful (if not more so), but implementation restrictions prohibit a uniform interface.
The drawing mode, an exact integer in the range 0 to 15 inclusive, determines how the
figure being drawn is combined with the background over which it is drawn to generate the
final result. Initially the drawing mode is set to “source”, so that the new output overwrites
whatever appears in that place. Useful alternative drawing modes can, for example, erase
what was already there, or invert it.
Altogether 16 boolean operations are available for combining the source (what is being
drawn) and the destination (what is being drawn over). The source and destination are
combined by the device on a pixel-by-pixel basis as follows:
Mode
Meaning
---------0
ZERO [erase; use background color]
1
source AND destination
2
source AND (NOT destination)
3
source
4
(NOT source) AND destination
5
destination
6
source XOR destination
7
source OR destination
8
NOT (source OR destination)
9
NOT (source XOR destination)
10
NOT destination
11
source OR (NOT destination)
12
NOT source
13
(NOT source) OR destination
14
(NOT source) OR (NOT destination)
15
ONE [use foreground color]
The line style, an exact integer in the range 0 to 7 inclusive, determines which parts of
a line are drawn in the foreground color, and which in the background color. The default
line style, “solid”, draws the entire line in the foreground color. Alternatively, the “dash”
style alternates between foreground and background colors to generate a dashed line. This
capability is useful for plotting several things on the same graph.
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Here is a table showing the name and approximate pattern of the di↵erent styles. A
‘1’ in the pattern represents a foreground pixel, while a ‘-’ represents a background pixel.
Note that the precise output for each style will vary from device to device. The only style
that is guaranteed to be the same for every device is “solid”.
Style
Name
Pattern
----------------0
solid
1111111111111111
1
dash
11111111-------2
dot
1-1-1-1-1-1-1-13
dash dot
1111111111111-14
dash dot dot
11111111111-1-15
long dash
11111111111----6
center dash
111111111111-117
center dash dash
111111111-11-11-
graphics-bind-drawing-mode graphics-device drawing-mode thunk
graphics-bind-line-style graphics-device line-style thunk
procedure
procedure
These procedures bind the drawing mode or line style, respectively, of graphics-device,
invoke the procedure thunk with no arguments, then undo the binding when thunk
returns. The value of each procedure is the value returned by thunk. Graphics
operations performed during thunk’s dynamic extent will see the newly bound mode
or style as current.
graphics-set-drawing-mode graphics-device drawing-mode
graphics-set-line-style graphics-device line-style
procedure
procedure
These procedures change the drawing mode or line style, respectively, of graphicsdevice. The mode or style will remain in e↵ect until subsequent changes or bindings.
17.5 Bu↵ering of Graphics Output
To improve performance of graphics output, most graphics devices provide some form
of bu↵ering. By default, Scheme’s graphics procedures flush this bu↵er after every drawing
operation. The procedures in this section allow the user to control the flushing of the output
bu↵er.
graphics-enable-bu↵ering graphics-device
procedure
Enables bu↵ering for graphics-device. In other words, after this procedure is called,
graphics operations are permitted to bu↵er their drawing requests. This usually
means that the drawing is delayed until the bu↵er is flushed explicitly by the user, or
until it fills up and is flushed by the system.
graphics-disable-bu↵ering graphics-device
procedure
Disables bu↵ering for graphics-device. By default, all graphics devices are initialized
with bu↵ering disabled. After this procedure is called, all drawing operations perform
their output immediately, before returning.
Note: graphics-disable-buffering flushes the output bu↵er if necessary.
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graphics-flush graphics-device
procedure
Flushes the graphics output bu↵er for graphics-device. This operation has no e↵ect
for devices that do not support bu↵ering, or if bu↵ering is disabled for the device.
17.6 Clipping of Graphics Output
Scheme provides a rudimentary mechanism for restricting graphics output to a given
rectangular subsection of a graphics device. By default, graphics output that is drawn
anywhere within the device’s virtual coordinate limits will appear on the device. When
a clip rectangle is specified, however, output that would have appeared outside the clip
rectangle is not drawn.
Note that changing the virtual coordinate limits for a device will usually reset the clip
rectangle for that device, as will any operation that a↵ects the size of the device (such as a
window resizing operation). However, programs should not depend on this.
graphics-set-clip-rectangle graphics-device x-left y-bottom x-right
procedure
y-top
Specifies the clip rectangle for graphics-device in virtual coordinates. X-left, ybottom, x-right, and y-top must be real numbers. Subsequent graphics output is
clipped to the intersection of this rectangle and the device’s virtual coordinate limits.
graphics-reset-clip-rectangle graphics-device
procedure
Eliminates the clip rectangle for graphics-device. Subsequent graphics output is
clipped to the virtual coordinate limits of the device.
17.7 Custom Graphics Operations
In addition to the standard operations, a graphics device may support custom operations.
For example, most devices have custom operations to control color. graphics-operation
is used to invoke custom operations.
graphics-operation graphics-device name object . . .
procedure
Invokes the graphics operation on graphics-device whose name is the symbol name,
passing it the remaining arguments. This procedure can be used to invoke the standard operations, as well as custom operations that are specific to a particular graphics
device type. The names of the standard graphics operations are formed by removing
the graphics- prefix from the corresponding procedure. For example, the following
are equivalent:
(graphics-draw-point device x y)
(graphics-operation device ’draw-point x y)
For information on the custom operations for a particular device, see the documentation for its type.
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17.8 Images
Some graphics device types support images, which are rectangular pieces of picture that
may be drawn into a graphics device. Images are often called something else in the host
graphics system, such as bitmaps or pixmaps. The operations supported vary between
devices, so look under the di↵erent device types to see what operations are available. All
devices that support images support the following operations.
create-image width height
operation on graphics-device
Images are created using the create-image graphics operation, specifying the width
and height of the image in device coordinates (pixels).
(graphics-operation device ’create-image 200 100)
The initial contents of an image are unspecified.
create-image is a graphics operation rather than a procedure because the kind of
image returned depends on the kind of graphics device used and the options specified
in its creation. The image may be used freely with other graphics devices created
with the same attributes, but the e↵ects of using an image with a graphics device
with di↵erent attributes (for example, di↵erent colors) is undefined. Under X, the
image is display dependent.
draw-image x y image
operation on graphics-device
The image is copied into the graphics device at the specified position.
draw-subimage x y image im-x im-y w h
operation on graphics-device
Part of the image is copied into the graphics device at the specified (x, y) position.
The part of the image that is copied is the rectangular region at im-x and im-y and
of width w and height h. These four numbers are given in device coordinates (pixels).
image? object
Returns #t if object is an image, otherwise returns #f.
procedure
image/destroy image
procedure
This procedure destroys image, returning storage to the system. Programs should
destroy images after they have been used because even modest images may use large
amounts of memory. Images are reclaimed by the garbage collector, but they may be
implemented using memory outside of Scheme’s heap. If an image is reclaimed before
being destroyed, the implementation might not deallocate that non-heap memory,
which can cause a subsequent call to create-image to fail because it is unable to
allocate enough memory.
image/height image
procedure
image/width image
procedure
Returns the height of the image in device coordinates.
Returns the width of the image in device coordinates.
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image/fill-from-byte-vector image bytes
procedure
The contents of image are set in a device-dependent way, using one byte per pixel
from bytes (a string). Pixels are filled row by row from the top of the image to the
bottom, with each row being filled from left to right. There must be at least (*
(image/height image) (image/width image)) bytes in bytes.
17.9 X Graphics
MIT Scheme supports graphics in the X window system (version 11). Arbitrary numbers
of displays may be opened, and arbitrary numbers of graphics windows may be created for
each display. A variety of operations is available to manipulate various aspects of the
windows, to control their size, position, colors, and mapping. The X graphics device type
supports images, which are implemented as Xlib XImage objects. X display, window, and
image objects are automatically closed if they are reclaimed by the garbage collector.
17.9.1 X Graphics Type
A graphics device for X windows is created by passing the symbol x as the graphics
device type name to make-graphics-device:
(make-graphics-device ’x #!optional display geometry suppress-map?)
where display is either a display object, #f, or a string; geometry is either #f or a string;
and suppress-map? is a boolean or a vector (see below). A new window is created on the
appropriate display, and a graphics device representing that window is returned.
Display specifies which X display the window is to be opened on; if it is #f or a string,
it is passed as an argument to x-open-display, and the value returned by that procedure
is used in place of the original argument. Geometry is an X geometry string, or #f which
means to use the default geometry (which is specified as a resource).
Suppress-map?, if given, may take two forms. First, it may be a boolean: if #f (the
default), the window is automatically mapped after it is created; otherwise, #t means to
suppress this automatic mapping. The second form is a vector of three elements. The
first element is a boolean with the same meaning as the boolean form of suppress-map?.
The second element is a string, which specifies an alternative resource name to be used for
looking up the window’s resources. The third element is also a string, which specifies a class
name for looking up the window’s resources. The default value for suppress-map? is #f.
The default resource and class names are "schemeGraphics" and "SchemeGraphics"
respectively.
The window is initialized using the resource and class names specified by suppress-map?,
and is sensitive to the following resource properties:
Chapter 17: Graphics
Property
-------geometry
font
borderWidth
internalBorder
background
foreground
borderColor
cursorColor
pointerColor
289
Class
----Geometry
Font
BorderWidth
BorderWidth
Background
Foreground
BorderColor
Foreground
Foreground
Default
------512x384+0+0
fixed
2
[border width]
white
black
[foreground color]
[foreground color]
[foreground color]
The window is created with a backing_store attribute of Always. The window’s name
and icon name are initialized to "scheme-graphics".
17.9.2 Utilities for X Graphics
x-graphics/open-display display-name
procedure
Opens a connection to the display whose name is display-name, returning a display
object. If unable to open a connection, #f is returned. Display-name is normally a
string, which is an X display name in the usual form; however, #f is also allowed,
meaning to use the value of the unix environment variable DISPLAY.
x-graphics/close-display display
procedure
Closes display; after calling this procedure, it is an error to use display for any
purpose. Any windows that were previously opened on display are destroyed and
their resources returned to the operating system.
x-close-all-displays
procedure
Closes all open connections to X displays. Equivalent to calling x-close-display on
all open displays.
x-geometry-string x y width height
procedure
This procedure creates and returns a standard X geometry string from the given
arguments. X and y must be either exact integers or #f, while width and height
must be either exact non-negative integers or #f. Usually either x and y are both
specified or both #f; similarly for width and height. If only one of the elements of
such a pair is specified, it is ignored.
Examples:
(x-geometry-string #f #f 100 200) ) "100x200"
(x-geometry-string 2 -3 100 200) ) "100x200+2-3"
(x-geometry-string 2 -3 #f #f) ) "+2-3"
Note that the x and y arguments cannot distinguish between +0 and -0, even though
these have di↵erent meanings in X. If either of those arguments is 0, it means +0 in
X terminology. If you need to distinguish these two cases you must create your own
geometry string using Scheme’s string and number primitives.
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17.9.3 Custom Operations on X Graphics Devices
Custom operations are invoked using the procedure graphics-operation. For example,
(graphics-operation device ’set-foreground-color "blue")
set-background-color color-name
set-foreground-color color-name
set-border-color color-name
set-mouse-color color-name
operation on x-graphics-device
operation on x-graphics-device
operation on x-graphics-device
operation on x-graphics-device
These operations change the colors associated with a window. Color-name must be
a string, which is the X server’s name for the desired color. set-border-color and
set-mouse-color immediately change the border and mouse-cursor colors. setbackground-color and set-foreground-color change the colors to be used when
drawing, but have no e↵ect on anything drawn prior to their invocation. Because
changing the background color a↵ects the entire window, we recommend calling
graphics-clear on the window’s device afterwards. Color names include both
mnemonic names, like "red", and intensity names specified in the "#rrggbb" notation.
draw-arc x y radius-x radius-y angle-start
operation on x-graphics-device
angle-sweep fill?
Operation draw-arc draws or fills an arc. An arc is a segment of a circle, which may
have been stretched along the x- or y- axis to form an ellipse.
The parameters x, y, radius-x and radius-y describe the circle and angle-start and
angle-sweep choose which part of the circle is drawn. The arc is drawn on the graphics
device with the center of the circle at the virtual coordinates given by x and y. radiusx and radius-y determine the size of the circle in virtual coordinate units.
The parameter angle-start determines where the arc starts. It is measured in degrees
in an anti-clockwise direction, starting at 3 o’clock. angle-sweep determines how much
of the circle is drawn. It too is measured anti-clockwise in degrees. A negative value
means the measurement is in a clockwise direction.
Note that the angles are determined on a unit circle before it is stretched into an
ellipse, so the actual angles that you will see on the computer screen depends on all
of: radius-x and radius-y, the window size, and the virtual coordinates.
If fill? is #f then just the segment of the circle is drawn, otherwise the arc is filled in
a pie-slice fashion.
This draws a quarter circle pie slice, standing on its point, with point at virtual
coordinates (3,5):
(graphics-opereration g ’draw-arc 3 5 .5 .5 45 90 #t)
draw-circle x y radius
fill-circle x y radius
operation on x-graphics-device
operation on x-graphics-device
These operations draw a circle (outline) or a filled circle (solid) at on the graphics device at the virtual coordinates given by x and y. These operations could be
implemented trivially interms of the draw-arc operation.
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set-border-width width
set-internal-border-width width
operation on x-graphics-device
operation on x-graphics-device
These operations change the external and internal border widths of a window. Width
must be an exact non-negative integer, specified in pixels. The change takes place immediately. Note that changing the internal border width can cause displayed graphics
to be garbled; we recommend calling graphics-clear on the window’s device after
doing so.
set-font font-name
operation on x-graphics-device
Changes the font used when drawing text in a window. Font-name must be a string
that is a font name known to the X server. This operation does not a↵ect text drawn
prior to its invocation.
set-mouse-shape shape-number
operation on x-graphics-device
Changes the shape of the mouse cursor. Shape-number is an exact non-negative
integer that is used as an index into the mouse-shape font; when multiplied by 2 this
number corresponds to an index in the file
‘/usr/include/X11/cursorfont.h’.
map-window
withdraw-window
operation on x-graphics-device
operation on x-graphics-device
These operations control the mapping of windows. They correspond directly to Xlib’s
XMapWindow and XWithdrawWindow.
resize-window width height
operation on x-graphics-device
Changes the size of a window. Width and height must be exact non-negative integers.
The operation corresponds directly to Xlib’s XResizeWindow.
This operation resets the virtual coordinate system and the clip rectangle.
move-window x y
operation on x-graphics-device
Changes the position of a window on the display. X and y must be exact integers.
The operation corresponds directly to Xlib’s XMoveWindow. Note that the coordinates
x and y do not take the external border into account, and therefore will not position
the window as you might like. The only reliable way to position a window is to ask
a window manager to do it for you.
get-default resource property
operation on x-graphics-device
This operation corresponds directly to Xlib’s XGetDefault. Resource and property
must be strings. The operation returns the character string corresponding to the
association of resource and property; if no such association exists, #f is returned.
copy-area source-x-left source-y-top width height
operation on x-graphics-device
destination-x-left destination-y-top
This operation copies the contents of the rectangle specified by source-x-left, sourcey-top, width, and height to the rectangle of the same dimensions at destination-x-left
and destination-y-top.
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font-structure font-name
operation on x-graphics-device
Returns a Scheme equivalent of the X font structure for the font named font-name. If
the string font-name does not name a font known to the X server, or names a 16-bit
font, #f is returned.
x-font-structure/name font-structure
x-font-structure/direction font-structure
x-font-structure/all-chars-exist font-structure
x-font-structure/default-char font-structure
x-font-structure/min-bounds font-structure
x-font-structure/max-bounds font-structure
x-font-structure/start-index font-structure
x-font-structure/character-bounds font-structure
x-font-structure/max-ascent font-structure
x-font-structure/max-descent font-structure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
procedure
These procedures extract the components of the font description structure returned by
the X graphics operation font-structure. A more complete description of these components appears in documentation of the XLoadQueryFont Xlib call. start-index is
the index of the first character available in the font. The min-bounds and max-bounds
components are structures of type x-character-bounds, and the character-bounds
component is a vector of the same type.
x-character-bounds/lbearing character-bounds
x-character-bounds/rbearing character-bounds
x-character-bounds/width character-bounds
x-character-bounds/ascent character-bounds
x-character-bounds/descent character-bounds
procedure
procedure
procedure
procedure
procedure
These procedures extract components of objects of type x-character-bounds. A
more complete description of them appears in documentation of the
XLoadQueryFont Xlib call.
17.10 Win32 Graphics
MIT Scheme supports graphics on Microsoft Windows 95, Windows 98, and Windows
NT. In addition to the usual operations, there are operations to control the size, position
and colors of a graphics window. Win32 devices support images, which are implemented as
device independent bitmaps (dibs).
The Win32 graphics device type is implemented as a top level window. graphicsenable-buffering is implemented and gives a 2x to 4x speedup on many graphics operations. As a convenience, when bu↵ering is enabled clicking on the graphics window’s
title bar e↵ects a graphics-flush operation. The user has the benefit of the increased
performance and the ability to view the progress in drawing at the click of a mouse button.
17.10.1 Win32 Graphics Type
Win32 graphics devices are created by specifying the symbol win32 as the graphicsdevice-type argument to make-graphics-device. The Win32 graphics device type is im-
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293
plemented as a top-level window and supports color drawing in addition to the standard
Scheme graphics operations.
Graphics devices are opened as follows:
(make-graphics-device ’win32 #!optional width height palette)
where width and height specify the size, in pixels, of the drawing area in the graphics
window (i.e. excluding the frame). Palette determines the colors available for drawing in
the window.
When a color is specified for drawing, the nearest color available in the palette is used.
Permitted values for palette are
’grayscale
The window allocates colors from a grayscale palette of approximately 236
shades of gray.
’grayscale-128
The window allocates colors from a grayscale palette of 128 shades of gray.
’standard
The standard palette has good selection of colors and grays.
#f or ’system
The colors available are those in the system palette. There are usually 16
to 20 colors in the system palette and these are usually sufficent for simple
applications like line drawings and x-vs-y graphs of mathematical functions.
Drawing with the system palette can be more efficient.
If palette is not specified then the standard palette is used.
17.10.2 Custom Operations for Win32 Graphics
Custom operations are invoked using the procedure graphics-operation. For example,
(graphics-operation device ’set-foreground-color "blue")
set-background-color color-name
set-foreground-color color-name
operation on win32-graphics-device
operation on win32-graphics-device
These operations change the colors associated with a window. Color-name must be
of one of the valid color specification forms listed below. set-background-color and
set-foreground-color change the colors to be used when drawing, but have no e↵ect
on anything drawn prior to their invocation. Because changing the background color
a↵ects the entire window, we recommend calling graphics-clear on the window’s
device afterwards.
The foreground color a↵ects the drawing of text, points, lines, ellipses and filled
polygons.
Colors are specified in one of three ways:
An integer This is the Win32 internal RGB value.
By name
A limited number of names are understood by the system. Names are
strings, e.g. "red", "blue", "black". More names can be registered with
the define-color operation.
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RGB (Red-Green-Blue) triples
A triple is either a vector or list of three integers in the range 0–255
inclusive which specify the intensity of the red, green and blue components
of the color. Thus #(0 0 0) is black, (0 0 128) is dark blue and #(255
255 255) is white.
If the color is not available in the graphics device then the nearest available color is
used instead.
define-color name spec
operation on win32-graphics-device
Define the string name to be the color specified by spec. Spec may be any acceptable
color specification. Note that the color names defined this way are available to any
Win32 graphics device, and the names do not have to be defined for each device.
Color names defined by this interface may also be used when setting the colors of the
Scheme console window, or the colors of Edwin editor windows.
find-color name
operation on win32-graphics-device
Looks up a color previously defined by define-color. This returns the color in
its most efficient form for operations set-foreground-color or set-backgroundcolor.
draw-ellipse left top right bottom
operation on win32-graphics-device
Draw an ellipse. Left, top, right and bottom indicate the coordinates of the bounding
rectangle of the ellipse. Circles are merely ellipses with equal width and height. Note
that the bounding rectangle has horizontal and vertical sides. Ellipses with rotated
axes cannot be drawn. The rectangle applies to the center of the line used to draw
the ellipse; if the line width has been set to greater than 1 then the ellipse will spill
outside the bounding rectange by half of the line width.
fill-polygon points
operation on win32-graphics-device
Draws a filled polygon using the current foreground color. Points is a vector of real
numbers. The numbers are in the order x1 y1 x2 y2 ... xn yn. For example,
(graphics-operation device ’fill-polygon #(0 0 0 1 1 0))
draws a solid triangular region between the points (0, 0), (0, 1) and (1, 0).
load-bitmap pathname
operation on win32-graphics-device
The graphics device contents and size are initialized from the windows bitmap file
specified by pathname. If no file type is supplied then a ".BMP" extension is added.
If a clip rectangle is in e↵ect when this procedure is called, it is necessary to redefine
the clip rectangle afterwards.
save-bitmap pathname
operation on win32-graphics-device
The graphics device contents are saved as a bitmap to the file specified by pathname.
If no file type is supplied then a ".BMP" extension is added. The saved bitmap may
be incorporated into documents or printed.
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move-window x y
operation on win32-graphics-device
The graphics device window is moved to the screen position specified by x and y.
resize-window width height
operation on win32-graphics-device
The graphics device window is resized to the specified width and height in device
coordinates (pixels). If a clip rectangle is in e↵ect when this procedure is called, it is
necessary to redefine the clip rectangle afterwards.
set-line-width width
operation on win32-graphics-device
This operation sets the line width for future drawing of lines, points and ellipses. It
does not a↵ect existing lines and has no e↵ect on filled polygons. The line width is
specified in device units. The default and initial value of this parameter is 1 pixel.
set-window-name name
operation on win32-graphics-device
This sets the window title to the string name. The window is given the name "Scheme
Graphics" at creation.
set-font handle
operation on win32-graphics-device
Sets the font for drawing text. Currently not well supported. If you can get a Win32
font handle it can be used here.
copy-area source-x-left source-y-top width
operation on win32-graphics-device
height destination-x-left destination-y-top
This operation copies the contents of the rectangle specified by source-x-left, sourcey-top, width, and height to the rectangle of the same dimensions at destination-x-left
and destination-y-top.
17.11 OS/2 Graphics
MIT Scheme supports graphics under the OS/2 Presentation Manager in OS/2 version
2.1 and later. The OS/2 graphics device type is implemented as a top level window. In
addition to the usual operations, there are operations to control the size, position, and
colors of a graphics window. OS/2 graphics devices support images, which are implemented
as memory presentation spaces.
The custom graphics operations defined in this section are invoked using the procedure
graphics-operation. For example,
(graphics-operation device ’set-foreground-color "blue")
17.11.1 OS/2 Graphics Type
OS/2 graphics devices are created by specifying the symbol os/2 as the graphics-devicetype argument to make-graphics-device. The OS/2 graphics device type is implemented
as a top-level window and supports color drawing in addition to the standard Scheme
graphics operations.
Graphics devices are opened as follows:
(make-graphics-device ’os/2 #!optional width height)
where width and height specify the size, in pixels, of the drawing area in the graphics
window (i.e. excluding the frame).
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17.11.2 Color Operations for OS/2 Graphics
These operations control the colors used when drawing on an OS/2 graphics device.
color?
operation on os2-graphics-device
This operation returns #t if the display supports color.
set-background-color color-name
set-foreground-color color-name
operation on os2-graphics-device
operation on os2-graphics-device
These operations change the colors associated with a window. Color-name must be
one of the valid color specification forms listed below. set-background-color and
set-foreground-color change the colors to be used when drawing, but have no e↵ect
on anything drawn prior to their invocation. Because changing the background color
a↵ects the entire window, we recommend calling graphics-clear on the window’s
device afterwards.
The foreground color a↵ects the drawing of text, points, and lines. Colors are specified
in one of these ways:
An integer between 0 and #xffffff inclusive
This is the OS/2 internal RGB value.
By name
A limited number of names are understood by the system. Names are
strings, e.g. "red", "blue", "black". More names can be registered with
the define-color operation.
RGB (Red-Green-Blue) triples
A triple is a list of three integers between 0 and #xff inclusive which
specify the intensity of the red, green and blue components of the color.
Thus (0 0 0) is black, (0 0 128) is dark blue and (255 255 255) is white.
If the color is not available in the graphics device then the nearest available color is
used instead.
define-color name spec
operation on os2-graphics-device
Define the string name to be the color specified by spec. Spec may be any acceptable
color specification. Note that the color names defined this way are available to any
OS/2 graphics device, and the names do not have to be defined for each device.
Color names defined by this interface may also be used when setting the colors of the
Scheme console window, or the colors of Edwin editor windows.
find-color name
operation on os2-graphics-device
Looks up a color previously defined by define-color. This returns the color in
its most efficient form for operations set-foreground-color or set-backgroundcolor.
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17.11.3 Window Operations for OS/2 Graphics
These operations control the window that contains the OS/2 graphics device. They
provide facilities to change the window’s size and position; to raise and lower the window
relative to other windows on the desktop; to hide or minimize the window, and to restore it
from the hidden or minimized state; to activate or deactivate the window (that is, control
the keyboard focus); and to control the text that appears in the window’s title bar.
window-position
operation on os2-graphics-device
This operation returns the position of the graphics-device window on the desktop.
The position is returned as two values (see Section 12.3 [Continuations], page 167),
which are the x and y coordinates of the position. These coordinates are in units
of pels (pixels), and measure the distance between the lower left hand corner of the
desktop and the lower left hand corner of the graphics device window’s frame.
set-window-position x y
operation on os2-graphics-device
The graphics-device window is moved to the screen position specified by x and y. The
coordinates x and y are in units of pels (pixels), and measure the distance between
the lower left hand corner of the desktop and the lower left hand corner of the graphics
device window’s frame.
window-size
operation on os2-graphics-device
This operation returns the size of the client area of the graphics-device window. The
client area is the part of the window that you draw on; it does not include the
window frame, title bar, etc. The size is returned as two values (see Section 12.3
[Continuations], page 167), which are the width and height of the client area in units
of pels (pixels).
set-window-size width height
operation on os2-graphics-device
This operation sets the size of the client area of the graphics-device window to the
specified width and height, which are in units of pels (pixels). The client area is the
part of the window that you draw on; it does not include the window frame, title bar,
etc.
window-frame-size
operation on os2-graphics-device
This operation returns the size of the graphics-device window’s frame. This includes
the client area, as well as the border, title bar, etc. The size is returned as two values
(see Section 12.3 [Continuations], page 167), which are the width and height of the
frame in units of pels (pixels).
The frame size is useful in conjunction with the window position and the desktop size
to determine relative placement of the window or to guarantee that the entire window
is visible on the desktop.
desktop-size
operation on os2-graphics-device
This operation returns the size of the OS/2 desktop. The size is returned as two
values (see Section 12.3 [Continuations], page 167), which are the width and height
of the frame in units of pels (pixels).
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raise-window
operation on os2-graphics-device
This operation raises the graphics-device window so that it is on top of any other
windows on the desktop.
lower-window
operation on os2-graphics-device
This operation lowers the graphics-device window so that it is below all other windows
on the desktop.
hide-window
operation on os2-graphics-device
This operation hides the graphics-device window. The window disappears from the
desktop, but still appears in the window list.
minimize-window
operation on os2-graphics-device
This operation minimizes the graphics-device window. The window disappears from
the desktop, but still appears in the window list. Depending on how you have configured your desktop, the window may appear as an icon, either on the desktop or in
the minimized window viewer.
maximize-window
operation on os2-graphics-device
This operation maximizes the graphics-device window. This causes the window to fill
the entire desktop.
restore-window
operation on os2-graphics-device
This operation restores the graphics-device window to its normal state. If the window
is hidden or minimized, it is shown again, at its former position on the desktop. If
the window is maximized, it is returned to its normal size.
activate-window
operation on os2-graphics-device
This operation makes the graphics-device window be the active window. This causes
the window to be put in front of all other windows on the desktop, highlights its
frame, and gives it the keyboard focus.
deactivate-window
operation on os2-graphics-device
This operation deactivates the graphics-device window if it was active (otherwise it
has no e↵ect). This causes some other window to be chosen to be active in its place.
set-window-title title
operation on os2-graphics-device
This operation changes the text that appears in the graphics device window’s title
bar. The new text is given by title, which must be a string.
17.11.4 Event Operations for OS/2 Graphics
These operations allow you to read some of the events that are generated by the Presentation Manager and put in the message queue of a graphics-device window.
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read-button
operation on os2-graphics-device
This operation waits for the user to push a mouse button inside the client area of the
graphics-device window. It then returns four values (see Section 12.3 [Continuations],
page 167) which are: the button number; the x and y coordinates of the mouse pointer
at the time the button was pressed, in pels (pixels) relative to the lower left hand
corner of the client area; and the graphics device that the mouse pointer was over at
the time the button was pressed.
Note that this operation only works when button events are selected (which is the
default).
select-user-events mask
operation on os2-graphics-device
This operation sets the event-selection mask for the graphics device to mask. The
event-selection mask is an exact non-negative integer that specifies which types of
incoming events are to be saved in the user-event queue for later retrieval by the
read-user-event operation. The mask is specified by setting the bits corresponding
to the event types that you are interested in, as follows:
Number Mask
Description
------ --------------0
#x001
Button press/release
1
#x002
Close (close the window) [WM_CLOSE]
2
#x004
Focus change [WM_SETFOCUS]
3
#x008
Key press/release [WM_CHAR]
4
#x010
Paint [WM_PAINT]
5
#x020
Size change [WM_SIZE]
6
#x040
Visibility change [WM_SHOW]
7
#x080
Command [WM_COMMAND]
8
#x100
Help [WM_HELP]
9
#x200
Mouse-move [WM_MOUSEMOVE]
Note that this operation does not a↵ect any events that are already in the user-event
queue. Changing the mask only a↵ects what events will be added to the queue in the
future.
read-user-event
operation on os2-graphics-device
This operation returns the next user event available from the user-event queue. If
there are no events in the queue, the operation waits for an event to arrive before
returning.
An event is a vector whose first element is the event-type number, whose second element is the graphics device that the event refers to, and whose remaining elements provide
information about the event. Here is a table of the possible event types and their vector
layout:
#(0 device number type x y flags)
A button event. Number is the button number, for example button number
0 is usually the left mouse button, 1 is usually the right button, etc. Type
specifies what occurred: 0 means the button was pressed, 1 means the button
was released, 2 means the button was clicked, and 3 means the button was
double clicked. X and y are the position of the mouse pointer at the time of
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the event, in units of pels (pixels) measured from the lower left corner of the
client area of the associated window. Finally, flags specifies what shift keys were
pressed at the time of the button event; it is a mask word created by combining
zero or more of the following flags: #x08 means the shift key was pressed, #x10
means the control key was pressed, and #x20 means the alt key was pressed.
#(1 device)
A close event. The user has selected the close button from the system menu,
or typed hAlt-f4i.
#(2 device gained?)
A focus event. If gained? is #t, the keyboard focus is being gained, and if
gained? is #f, it is being lost.
#(3 device code flags repeat)
A keyboard event. This is much too complicated to describe here. See the OS/2
toolkit documentation for details.
#(4 device xl xh yl yh)
A paint event. Part of the graphics-device window that was obscured has been
revealed and the Presentation Manager is informing the window that it must
repaint that area. Scheme will take care of the painting for you, so this event
isn’t very useful.
#(5 device width height)
A size-change event. The size of the graphics-device window has changed, and
width and height specify the new size in pels (pixels).
#(6 device shown?)
A visibility event. Indicates that the graphics-device window has been hidden
or revealed. If shown? is #f, the window is hidden, and if it is #t, the window
is shown.
#(7 device source mouse?)
#(8 device source mouse?)
A menu command. Source specifies which menu item was selected to cause this
event, and mouse? is a boolean indicating whether the item was selected with
the mouse or the keyboard. The event-type number 7 indicates a command
from a ‘WM_COMMAND’ message, while 8 is a command from a ‘WM_HELP’ message.
#(9 device x y hit-test flags)
The mouse was moved. X and y specify the position of the mouse, hit-test
contains the hit-test information, and flags specifies the modifier keys that
were pressed at the time.
discard-events
operation on os2-graphics-device
This operation discards any events that are in the user-event queue. This is sometimes
useful when you want to prompt the user for some input and don’t want to consider
any previous input.
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17.11.5 Miscellaneous Operations for OS/2 Graphics
These operations allow you to: change the font used for drawing text in a graphics-device
window; take a snapshot of a graphics-device window and return it as an image object; and
draw multiple lines efficiently.
set-font font-name
operation on os2-graphics-device
This operation sets the font used for drawing text in the graphics-device
window. Font-name is a string describing the font; this string is in the form
"<point-size>.<family-name>", for example, "10.Courier". You may specify any
fixed-pitch font family, in any point size that is supported for that font family. This
includes both image fonts and outline fonts.
capture-image x-left y-bottom x-right y-top
operation on os2-graphics-device
This operation creates and returns an image that contains part of the client area of
the graphics-device window. The portion of the client area that is selected is specified
by the four coordinate arguments, which are given in the current virtual coordinates
for the device. See Section 17.8 [Images], page 287, for more information about
manipulating images.
draw-lines xv yv
operation on os2-graphics-device
This operation draws multiple disjoint lines; it is like multiple calls to graphicsdraw-line but much faster. The arguments xv and yv are vectors of coordinates;
these vectors must be the same length, and the length must be a multiple of two. The
contents of the vectors are alternating start/end pairs. For example, the following are
equivalent:
(graphics-draw-line device xs ys xe ye)
(graphics-operation device ’draw-lines
(vector xs xe)
(vector ys ye))
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Chapter 18: Win32 Package Reference
303
18 Win32 Package Reference
18.1 Overview
The Win32 implementation is still in a state of development. It is expected that changes
will be necessary when MIT Scheme is ported to Windows NT on the DEC Alpha architecture. In particular, the current system is not arranged in a way that adequately
distinguishes between issues that are a consequence of the NT operating system and those
which are a consequence of the Intel x86 architecture.
Thus this documentation is not definitive, it merely outlines how the current system
works. Parts of the system will change and any project implemented using the win32
system must plan for a re-implementation stage.
The Win32 implementation has several components:
• Special microcode primitives.
• A foreign function interface (FFI) for calling procedures in dynamically linked libraries
(DLLs).
• An interface for Edwin.
• The Win32 package provides support for using the features of the Windows 3.1 and
Windows NT 3.1 environments.
• Device Independent Bitmap utilities. These are used by the win32 Scheme Graphics
implementation. (The Scheme Graphics implementation is described in the Reference
Manual).
Note that all the names in the Win32 support are part of the win32 package. The names
are bound in the (win32) environment, and do not appear as bindings in the user or root
environments. An e↵ect of this is that it is far easier to develop Win32 software in the
(win32) package environment or a child environment.
18.2 Foreign Function Interface
The Win32 foreign function interface (FFI) is a primitive and fairly simple system for
calling procedures written in C in a dynamically linked library (DLL). Both user’s procedures from a custom DLL and system procedures (e.g. MessageBox) are called using the
same mechanism.
Warning: The FFI as it stands has several flaws which make it difficult to use reliably. It
is expected that both the interface to and the mechanisms used by the FFI will be changed
in the future. We provide it, and this documentation, only to give people an early start in
accessing some of the features of Win32 from Scheme. Should you use it in an experiment
we welcome any feedback.
The FFI is designed for calling C procedures that use C data types rather than Scheme
data objects. Thus it is not possible to write and call a C procedure that returns, for
example, a Scheme list. The object returned will always be an integer (which may represent
the address of a C data structure).
Warning: It is extremely dangerous to try to pass Scheme callback procedures to C
procedures. It is only possible by passing integer ‘handles’ rather than the actual procedures,
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and even so, if a garbage collection occurs during the execution of the callback procedure
objects in Scheme’s heap will have moved. Thus in a foreign procedure that has a callback
and a string, after calling the callback the string value may no longer be valid. Playing this
game requires a profound knowledge of the implementation.
The interface to the FFI has two main components: a language for declaring the types of
values passed to and returned from the foreign procedures and a form for declaring foreign
procedures.
18.2.1 Windows Types
Foreign types are designed to represent a correspondence between a Scheme data type
that is used to represent an object within the Scheme world and a C data type that represents
the data object in the C world. Thus we cannot manipulate true C objects in Scheme, nor
can we manipulate Scheme objects in C.
Each foreign type has four aspects that together ensure that the correspondence between
the Scheme and C objects is maintained. These aspects are all encoded as procedures that
either check for validity or convert between representations. Thus a foreign type is not a
declarative type so much as a procedural description of how to pass the type. The underlying
foreign procedure call mechanism can pass integers and vector-like Scheme objects, and
returns integer values. All other objects must be translated into integers or some other
basic type, and must be recovered from integers.
The aspects are:
check
A predicate that returns #t if the argument is of an acceptable Scheme type,
otherwise returns #f. The check procedure is used for type-checking.
convert
A procedure of one argument which returns a Scheme object of one of the basic
types. It is used to convert an object into a ‘simpler’ object that will eventually
be converted into a C object. The legal simpler objects are integers and strings.
return-convert
A procedure of one argument that, given an integer, returns a Scheme object
of a type satisfying check. Its purpose is to convert the result returned by the
foreign procedure into a Scheme value.
revert
Some C procedures modify one or more of their arguments. These arguments
are passed by reference, i.e. as a pointer to their address. Since a Scheme object
might have a di↵erent memory layout and storage conventions, it must be passed
by copy-in and copy-out rather than by reference. Revert is a procedure of two
parameters, the original object passed and the result of convert on that object.
Revert may then inspect the converted object and copy back the changes to
the original.
define-windows-type name check convert return revert
define-similar-windows-type name model [check [convert [return
special form
special form
[revert]]]]
Both forms define a windows type. The first form defines a type in terms of its aspects
as described above. The second defines the type as being like another type, except
for certain aspects, which are redefined. Name is the name of the type. Model is the
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305
name of a type. Check, convert, return and revert are procedures or the value #f. A
#f means use the default value, which in the second form means use the definition
provided for model. The defaults are
check
(lambda (x) #t), i.e. unchecked.
convert
(lambda (x) x), i.e. no translation performed.
return
(lambda (x) x), i.e. no translation performed.
revert
(lambda (x y) unspecific), i.e. no update performed
The unchecked windows type (see below) is defined as:
(define-windows-type unchecked #f #f #f #f)
Windows types are not first class values, so they cannot be stored in variables or
defined using define:
(define my-type unchecked)
Unbound variable
error
(define-similar-windows-type my-type unchecked)
;; the correct way
Scheme characters must be converted to integers. This is accomplished as follows:
(define-windows-type char
char?
; check
char->integer ; convert
integer->char ; convert return value
#f
; cannot be passed by reference
)
unchecked
windows type
The type which is not checked and undergoes only the basic conversion from a Scheme
integer to a C integer or from a Scheme string to a C pointer to the first byte of the
string. Returned unchecked values are returned as integers.
bool
windows type
Scheme booleans are analogous to C integers 0 and 1. Windows type bool have been
defined as:
(define-windows-type bool
boolean?
(lambda (x) (if x 1 0))
(lambda (x) (if (eq? x 0) #f #t))
#f)
char
windows type
Scheme characters are converted into C objects of type char, which are indistinguishable from small integers.
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int
uint
long
ulong
short
ushort
word
byte
Various integer types that are passed without conversion.
windows
windows
windows
windows
windows
windows
windows
windows
type
type
type
type
type
type
type
type
string
windows type
A string that is passed as a C pointer of type char* to the first character in the string.
char*
windows type
A string or #f. The string is passed as a pointer to characters. The string is correctly
null-terminated. #f is passed as the null pointer. This is an example where there is
a more complex mapping between C objects and Scheme objects. C’s char* type is
represented as one of two Scheme types depending on its value. This allows us us
to distinguish between the C string (pointer) that points to the empty sequence of
characters and the null pointer (which doesnt point anywhere).
handle
hbitmap
hbrush
hcursor
hdc
hicon
hinstance
hmenu
hpalette
hpen
hrgn
hwnd
windows type
windows type
windows type
windows type
windows type
windows type
windows type
windows type
windows type
windows type
windows type
windows type
Various kinds of Win32 handle. These names correspond to the same, but all uppercase, names in the Windows C language header files. Win32 API calls are the
source of values of this type and the values are meaningless except as arguments to
other Win32 API calls. Currently these values are represented as integers but we
expect that Win32 handles will in future be represented by allocated Scheme objects
(e.g. records) that will allow predicates (e.g. hmenu?) and sensible interlocking with
the garbage collector to free the programmer of the current tedious allocation and
deallocation of handles.
resource-id
windows type
A Windows resource identifier is either a small integer or a string. In C, this distinction is possible because pointers look like larger integers, so a machine word representing a small integer can be distinguished from a machine word that is a pointer to
the text of the name of the resource.
Chapter 18: Win32 Package Reference
307
18.2.2 Windows Foreign Procedures
Foreign procedures are declared as callable entry-points in a module, usually a dynamically linked library (DLL).
find-module name
procedure
Returns a module suitable for use in creating procedures with windows-procedure.
Name is a string which is the name of a DLL file. Internally, find-module uses the
LoadLibrary Win32 API, so name should conform to the specifications for this call.
Name should be either a full path name of a DLL, or the name of a DLL that resides
in the same directory as the Scheme binary ‘SCHEME.EXE’ or in the system directory.
The module returned is a description for the DLL, and the DLL need not necessarily
be linked at or immediately after this call. DLL modules are linked on need and
unlinked before Scheme exits and when there are no remaining references to entry
points after a garbage-collection. This behavior ensures that the Scheme system can
run when a DLL is absent, provided the DLL is not actually used (i.e. no attempt is
made to call a procedure in the DLL).
gdi32.dll
variable
This variable is bound to the module describing the ‘GDI32.DLL’ library, which contains the Win32 API graphics calls, e.g. LineTo.
kernel32.dll
This variable is bound to the module describing the ‘KERNEL32.DLL’ library.
variable
user32.dll
variable
This variable is bound to the module describing the ‘USER32.DLL’ library. This
module contains many useful Win32 API procedures, like MessageBox and
SetWindowText.
windows-procedure (name (parameter type) ...) return-type
special form
module entry-name [options]
This form creates a procedure, and could be thought of as “foreign-named-lambda”.
The form creates a Scheme procedure that calls the C procedure identified by the
exported entry point entry-name in the module identified by the value of module.
Both entry-name and module are evaluated at procedure creation time, so either
may be expression. Entry-name must evaluate to a string and module must evaluate
to a module as returned by find-module. These are the only parts of the form that
are evaluated at procedure creation time.
Name is the name of the procedure and is for documentation purposes only. This
form does not define a procedure called name. It is more like lambda. The name
might be used for debugging and pretty-printing.
A windows procedure has a fixed number of parameters (i.e. no ‘rest’ parameters or
‘varargs’), each of which is named and associated with a windows type type. Both the
name parameter and the windows type type must be symbols and are not evaluated.
The procedure returns a value of the windows type return-type.
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The following example creates a procedure that takes a window handle (hwnd) and
a string and returns a boolean (bool) result. The procedure does this by calling the
SetWindowText entry in the module that is the value of the variable user32.dll.
The variable set-window-title is defined to have this procedure as it’s value.
(define set-window-title
(windows-procedure
(set-window-text (window hwnd) (text string))
bool user32.dll "SetWindowText"))
(set-window-title my-win "Hi")
) #t
;; Changes window’s title/text
set-window-title
set-window-text
) #[compiled-procedure
Unbound variable
error
...]
When there are no options the created procedure will (a) check its arguments against
the types, (b) convert the arguments, (c) call the C procedure and (d) convert the
returned value. No reversion is performed, even if one of the types has a reversion
defined. (Reverted types are rare [I have never used one], so paying a cost for this
unless it is used seems silly).
The following options are allowed:
with-reversions
The reversions are included in the type conversions.
expand
A synonym for with-reversions.
Scheme code
The Scheme code is placed between steps (a) and (b) in the default process. The Scheme code can enforce constraints on the arguments, including constraints between arguments such as checking that an index refers
to a valid position in a string.
If both options (i.e. with-reversions and Scheme code) are used, with-reversions
must appear first. There can be arbitrarily many Scheme expression.
18.2.3 Win32 API names and procedures
This section is a moving target.
The #define values from ‘wingdi.h’ and ‘winuser.h’ are available as bindings in the
(win32) package environment. The #define symbols are all uppercase; these have been
translated to all lowercase Scheme identifiers, thus WM_LBUTTONUP is the scheme variable wm_
lbuttonup. As Scheme is case insensitive, the upper-case version may be used and probably
should to make the code look more like conventional Windows code. The Scheme bindings
have been produced automagically. Most of the #define-symbols contain an underscore so
there are not many name clashes. There is one very notable name clash, however: ERROR is
#defined to 0, which shadows the scheme procedure error in the root package environment.
To signal an error, use access to get error from the system global environment:
Chapter 18: Win32 Package Reference
309
(declare (usual-integrations))
...
((access error system-global-environment) "Complain" ...)
The set of procedures is incomplete because procedures have been added on a by-need
basis for the implementation of other parts of the system, e.g. Scheme Graphics. Look in
the implementation for further details.
Win32 API procedure names have been uniformly converted into Scheme identifiers as
follows:
• A leading uppercase letter is translated into a lowercase letter.
• Subsequent sequences of uppercase letters are translated into lowercase letters preceeded by a hyphen (minus symbol), i.e. hyphens are inserted at a lowercase to uppercase transition.
• Predicates beginning with Is finally have a question-mark appended.
Example: applying these rules to IsWindow yields is-window?, and GetDC is translated into
get-dc.
18.3 Device Independent Bitmap Utilities
The Device Independent Bitmap (DIB) utilities library ‘DIBUTILS.DLL’ and the associated procedures in ‘dib.scm’ in the Win32 system source is an example of how to use the
foreign function interface to access and manipulate non-Scheme objects.
dib
windows type
In the C world a DIB is a handle to a piece of memory containing the bits that
represent information about the image and the pixels of the image. The handle is a
machine-word sized piece of data which may be thought of as a 32 bit integer. The
handle may be null (i.e. zero), indicating that there is no block of memory describing
the DIB. The null value is usually returned by C functions that are supposed to create
a DIB but failed, for some reason like the memory could not be allocated or a file
could not be opened.
In the Scheme world a DIB is a structure containing information about the bitmap
(specifically the integer that represents the handle). We also include #f in the dib
windows type to mirror the null handle error value.
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MIT Scheme Reference
(define dib-result
(lambda (handle)
(if (= handle 0)
#f
(make-dib handle))))
(define dib-arg
(lambda (dib)
(if dib
(cell-contents (dib-handle dib))
0)))
(define-windows-type dib
(lambda (thing) (or (dib? thing) (eq? thing #f)))
dib-arg
dib-result)
18.3.1 DIB procedures
The following procedures have typed parameters, using the same convention as windowsprocedure.
open-dib (filename string)
procedure
Return type: dib. Calls the OpenDIB entry of ‘DIBUTILS.DLL’. If the return value is
not #f then the file filename was found, successfully opened, and the contents were
suitable for loading into memory as a device independent bitmap.
write-dib (filename string) (dib dib)
procedure
Return type: bool. Calls the WriteDIB entry of ‘DIBUTILS.DLL’. Returns #t if the
file filename could be opened and written to. After this operation the file contains
the bitmap data in a standard format that is understood by open-dib and various
system utilities like the bitmap editor. Any problems resulting in failure are signalled
by a #f return value.
bitmap-from-dib (dib dib) (palette hpalette)
procedure
Return type: hbitmap. Calls the BitmapFromDib entry of ‘DIBUTILS.DLL’. The
returned value is a device dependent bitmap. The colours from the DIB are matched
against colors in palette.
dib-from-bitmap (bitmap hbitmap) (style dword) (bits word) (palette
procedure
hpalette)
Return type: dib. Returns a DIB containing the same image as the device dependent
bitmap bitmap. Style determines the kind of DIB, e.g. compression style. Calls the
DibFromBitmap entry of ‘DIBUTILS.DLL’.
dib-blt (dest hdc) (x int) (y int) (w int) (h int) (src dib) (src-x int)
procedure
(src-y int) (raster-op long)
Return type: bool. Calls the DibBlt entry of ‘DIBUTILS.DLL’. Similar to the Win32
API BitBlt call, but draws a DIB rather than a piece of another device context.
Chapter 18: Win32 Package Reference
311
Draws the dib on device context hdc at position (x,y). A rectangle of width w and
height h is copied from position (src-x,src-y) of dib. Raster-op is supposed to allow
the source and destination to be combined but I don’t think I got this right so stick
to SRCCOPY.
%delete-dib (dib-handle handle)
procedure
Return type: bool. Calls the DeleteDIB entry of ‘DIBUTILS.DLL’. Note that the
parameter is a handle, and not a dib. This allows us to destroy a DIB and reclaim
its memory by knowing only the handle value, and not needing the dib record. The
importance of this is that if the dib record is GC-ed then a GC hook can reclaim the
storage knowing only the handle.
delete-dib (dib dib)
procedure
Return type: bool. This procedure calls %delete-dib to reclaim the storage occupied
by a DIB. After being deleted, the DIB should not be used. This procedure allows
the programmer to reclaim external heap storage rather than risking it running out
before the next garbage collection.
dib-height (dib dib)
procedure
Return type: int. Calls the DibHeight expand entry of ‘DIBUTILS.DLL’, which returns
the height of the bitmap in pixels.
dib-width (dib dib)
procedure
Return type: int. Calls the DibWidth entry of ‘DIBUTILS.DLL’, which returns the
width of the bitmap in pixels.
copy-bitmap (bm hbitmap)
procedure
Return type: hbitmap. Calls the CopyBitmap of ‘DIBUTILS.DLL’, which creates a new
bitmap with the same size and contents as the original.
create-dib (width int) (height int) (style int) (depth int) (palette
procedure
hpalette)
Return type: dib. Calls the CreateDIB entry of ‘DIBUTILS.DLL’. Creates a DIB of
width by height pixels and depth bits of colour information. The style parameter
determines how the bitmap is stored. I have only ever used BI_RGB. If depth<=8
then the palette determines the DIB’s colour table.
crop-bitmap (bm hbitmap) (left int) (top int) (right int) (bottom int)
procedure
Return type: hbitmap. Calls the CropBitmap entry of ‘DIBUTILS.DLL’. Returns a
new bitmap containing the image from a region of the original.
dib-set-pixels-unaligned dib (pixels string)
procedure
Return type: bool. Calls the DIBSetPixelsUnaligned entry of
‘DIBUTILS.DLL’. Stu↵s bytes from pixels into the bitmap. There are no alignment
constraints on pixels (the usual way of doing this is to use the SetDIBits function
which requires that every scan line of the bitmap is 32-bit word aligned, even if the
scan lines are not a multiple of 4 bytes long). doing this
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18.3.2 Other parts of the DIB Utilities implementation
The ‘DIBUTILS.DLL’ library is an ordinary DLL. See the standard Microsoft Windows
documentation on how to create DLLs. Look at the code in the ‘WIN32/DIBUTILS’ directory
of the Scheme source.
Please note:
• For the foreign function interface to find the procedures they must be declared as
exports in the ‘.DEF’ definition file.
• To load the ‘.DLL’ file use the find-module Scheme function. Look at ‘WIN32/DIB.SCM’
to see how this is done.
• The current system works with C procedures with the __stdcall and __cdecl calling
conventions but not the __fastcall calling convention.
GNU Free Documentation License
313
GNU Free Documentation License
Version 1.1, March 2000
Copyright c 2000 Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other written document
“free” in the sense of freedom: to assure everyone the e↵ective freedom to copy and
redistribute it, with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way to get credit for
their work, while not being considered responsible for modifications made by others.
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We have designed this License in order to use it for manuals for free software, because
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The “Invariant Sections” are certain Secondary Sections whose titles are designated, as
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The “Cover Texts” are certain short passages of text that are listed, as Front-Cover
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as they preserve the title of the Document and satisfy these conditions, can be treated
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4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditions
of sections 2 and 3 above, provided that you release the Modified Version under precisely
this License, with the Modified Version filling the role of the Document, thus licensing
distribution and modification of the Modified Version to whoever possesses a copy of
it. In addition, you must do these things in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title distinct from that of the
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a previous version if the original publisher of that version gives permission.
B. List on the Title Page, as authors, one or more persons or entities responsible for
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D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications adjacent to the other
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permission to use the Modified Version under the terms of this License, in the form
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Texts given in the Document’s license notice.
H. Include an unaltered copy of this License.
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include translations of some or all Invariant Sections in addition to the original versions
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Permission is granted to copy, distribute and/or modify this document
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Free Documentation License’’.
If you have no Invariant Sections, write “with no Invariant Sections” instead of saying
which ones are invariant. If you have no Front-Cover Texts, write “no Front-Cover Texts”
instead of “Front-Cover Texts being list”; likewise for Back-Cover Texts.
If your document contains nontrivial examples of program code, we recommend releasing
these examples in parallel under your choice of free software license, such as the GNU
General Public License, to permit their use in free software.
Binding Index
319
Binding Index
"
*
" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 62, 211, 214
*default-pathname-defaults* . . . . . . . . . . . . . . 227
*matcher . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
*parser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
*parser-canonicalize-symbols?* . . . . . . . . . . . 188
*parser-radix* . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
*random-state* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
*unparse-with-maximum-readability?* . . . . . 191
*unparser-list-breadth-limit* . . . . . . . . . . . . 190
*unparser-list-depth-limit* . . . . . . . . . . . . . . 190
*unparser-radix* . . . . . . . . . . . . . . . . . . . . . . . . . . 190
*unparser-string-length-limit* . . . . . . . . . . . 190
#
# . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
#!optional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 16
#!rest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 16
#( . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
#\ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
#\altmode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
#\backnext . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
#\backspace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
#\call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
#\linefeed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78, 83
#\newline . . . . . . . . . . . . . . . . . . . . . . . . 77, 78, 83, 87
#\page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78, 83, 87
#\return . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78, 83
#\rubout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
#\space . . . . . . . . . . . . . . . . . . . . . . . . . . 77, 78, 83, 92
#\tab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78, 83, 87
#b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
#d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
#e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
#f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 24, 127
#i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
#o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
#t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 24, 127
#x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
+
+ . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 58, 62, 211, 214
+inf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
,
, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 105
,@. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 105
- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
->namestring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
->pathname . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222, 231
->truename . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
-1+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
-inf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
%
.
%delete-dib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
’
’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 105
/
/ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59, 62
(
<
( . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
< . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
<= . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61, 81
)
) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
=
= . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51, 60, 61, 90
=> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
320
MIT Scheme Reference
>
> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
>= . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
?
? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212, 214
‘
‘ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 105
\
\ ...........................................
\f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
\n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
\t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
87
87
87
1
1+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
1d-table/alist . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
1d-table/get . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
1d-table/lookup . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
1d-table/put! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
1d-table/remove! . . . . . . . . . . . . . . . . . . . . . . . . . . 142
1d-table? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
2
2d-get . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2d-get-alist-x . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2d-get-alist-y . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2d-put! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2d-remove! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
143
143
143
142
143
8
8-bit-alphabet? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
A
abort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263, 266
abs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
access-condition . . . . . . . . . . . . . . . . . . . . . 270, 271
acos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
activate-windowon os2-graphics-device . . . . . . 298
alist->rb-tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
alist->wt-tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
alist-copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
alist? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
allocate-host-address . . . . . . . . . . . . . . . . . . . . 256
alphabet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
alphabet+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
alphabet- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
alphabet->char-set . . . . . . . . . . . . . . . . . . . . . . . . . 86
alphabet->code-points . . . . . . . . . . . . . . . . . . . . . 85
alphabet->string . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
alphabet? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
alt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211, 213
and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 127
angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66, 67
append . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111, 115
append! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111, 115
append-map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
append-map! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
append-map* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
append-map*! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
apply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
apply-hook-extra . . . . . . . . . . . . . . . . . . . . . . . . . . 170
apply-hook-procedure. . . . . . . . . . . . . . . . . . . . . . 170
apply-hook? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
ascii->char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
ascii-range->char-set . . . . . . . . . . . . . . . . . . . . . 84
asin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
assoc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
association-procedure . . . . . . . . . . . . . . . . . . . . 140
assq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
assv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
atan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66, 74
B
beep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
begin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
bind-cell-contents! . . . . . . . . . . . . . . . . . . . . . . . 131
bind-condition-handler . . . . . . 258, 259, 261, 262
bind-default-condition-handler . . 259, 261, 262
bit-string->signed-integer . . . . . . . . . . . . . . . 126
bit-string->unsigned-integer . . . . . . . . . . . . . 126
bit-string-allocate . . . . . . . . . . . . . . . . . . . . . . . 123
bit-string-and . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-and! . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-andc . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-andc! . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-append . . . . . . . . . . . . . . . . . . . . . . . . . 124
bit-string-clear! . . . . . . . . . . . . . . . . . . . . . . . . . 124
bit-string-copy . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
bit-string-fill! . . . . . . . . . . . . . . . . . . . . . . . . . . 126
bit-string-length . . . . . . . . . . . . . . . . . . . . . . . . . 124
bit-string-move! . . . . . . . . . . . . . . . . . . . . . . . . . . 126
bit-string-movec! . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-not . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-or . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-or! . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-ref . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
bit-string-set! . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
bit-string-xor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-xor! . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string-zero? . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
bit-string? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
bit-substring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Binding Index
bit-substring-find-next-set-bit . . . . . . . . . .
bit-substring-move-right! . . . . . . . . . . . . . . . .
bitmap-from-dib . . . . . . . . . . . . . . . . . . . . . . . . . . .
bool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
boolean/and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
boolean/or . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
boolean=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
boolean? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
bound-restart . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
bound-restarts . . . . . . . . . . . . . . . . . . . . . . . . . . . .
break-on-signals . . . . . . . . . . . . . . . . . 259, 261,
buffered-input-charson input port . . . . . . . . .
buffered-output-charson output port . . . . . . .
byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
321
124
126
310
305
128
128
127
127
267
267
262
200
201
306
C
caaaar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
caaadr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
caaar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
caadar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
caaddr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
caadr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
caar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cadaar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cadadr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cadar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
caddar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cadddr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
caddr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cadr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
call-with-binary-input-file . . . . . . . . . . . . . . 183
call-with-binary-output-file . . . . . . . . . . . . . 183
call-with-current-continuation . . . . . . . . . . . 167
call-with-input-file. . . . . . . . . . . . . . . . . . . . . . 182
call-with-output-file . . . . . . . . . . . . . . . . . . . . 182
call-with-temporary-file-pathname . . . . . . . 233
call-with-values . . . . . . . . . . . . . . . . . . . . . . . . . . 169
canonical-host-name . . . . . . . . . . . . . . . . . . . . . . . 255
capture-imageon os2-graphics-device . . . . . . . . 301
capture-syntactic-environment . . . . . . . . . . . . . 43
car . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 106, 136
case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25, 27
cd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
cdaaar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cdaadr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cdaar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cdadar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cdaddr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cdadr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cdar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cddaar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cddadr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cddar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cdddar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cddddr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cdddr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cddr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
cdr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 106, 136
ceiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
ceiling->exact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
cell-contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
cell? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210, 305
char* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
char->ascii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81, 82
char->digit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char->integer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
char->name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
char-alphabetic? . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-alphanumeric? . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-ascii? . . . . . . . . . . . . . . . . . . . . . . . . . 81, 82, 87
char-bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
char-bits-limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
char-ci . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
char-ci<=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char-ci<? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char-ci=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char-ci>=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char-ci>? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char-code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
char-code-limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
char-downcase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char-graphic? . . . . . . . . . . . . . . . . . . . . . . . . . . 83, 195
char-in-alphabet? . . . . . . . . . . . . . . . . . . . . . . . . . . 86
char-integer-limit . . . . . . . . . . . . . . . . . . . . . . . . . 81
char-lower-case? . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-numeric? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-ready? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
char-ready?on input port . . . . . . . . . . . . . . . . . . . 199
char-set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83, 210
char-set->alphabet . . . . . . . . . . . . . . . . . . . . . . . . . 86
char-set-difference . . . . . . . . . . . . . . . . . . . . . . . . 84
char-set-intersection . . . . . . . . . . . . . . . . . . . . . 84
char-set-invert . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
char-set-member? . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-set-members . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-set-union . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
char-set:alphabetic . . . . . . . . . . . . . . . . . . . . . . . . 83
char-set:alphanumeric . . . . . . . . . . . . . . . . . . . . . 83
char-set:graphic . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-set:lower-case . . . . . . . . . . . . . . . . . . . . . . . . 83
char-set:not-graphic . . . . . . . . . . . . . . . . . . . . . . . 83
char-set:not-whitespace . . . . . . . . . . . . . . . . . . . 83
char-set:numeric . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-set:standard . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-set:upper-case . . . . . . . . . . . . . . . . . . . . . . . . 82
char-set:whitespace . . . . . . . . . . . . . . . . . . . . 83, 93
char-set? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
char-standard? . . . . . . . . . . . . . . . . . . . . . . . . . . 79, 83
char-upcase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char-upper-case? . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char-whitespace? . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
char<=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79, 81
char<? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51, 79
322
char>=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char>? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
char? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
chars->char-set . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
chars-remainingon input port . . . . . . . . . . . . . . 200
circular-list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
clear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
close-all-open-files. . . . . . . . . . . . . . . . . . . . . . 182
close-input-port . . . . . . . . . . . . . . . . . . . . . . . . . . 181
close-output-port . . . . . . . . . . . . . . . . . . . . . . . . . 181
close-port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
close-syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
close-tcp-server-socket . . . . . . . . . . . . . . . . . . 254
code-point->utf8-string . . . . . . . . . . . . . . . . . . . 85
code-point-in-alphabet? . . . . . . . . . . . . . . . . . . . 86
code-points->alphabet . . . . . . . . . . . . . . . . . . . . . 85
color? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296
color?on os2-graphics-device . . . . . . . . . . . . . . . . 296
compiled-procedure? . . . . . . . . . . . . . . . . . . . . . . . 166
complex?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
compound-procedure? . . . . . . . . . . . . . . . . . . . . . . . 166
conc-name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
cond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 24, 27, 127
condition-accessor . . . . . . . . . . . . . . . . . . . 269, 271
condition-constructor . . . . . . . . . . . . . . . . . . . . 269
condition-predicate . . . . . . . . . . . . . . . . . . 269, 270
condition-signaller . . . . . . . . . . . . . . . . . . 269, 270
condition-type/error? . . . . . . . . . . . . . . . . . . . . 272
condition-type/field-names . . . . . . . . . . . . . . . 272
condition-type/generalizations . . . . . . . . . . . 272
condition-type:arithmetic-error . . . . . . . . . . 278
condition-type:bad-range-argument. . . . . 62, 67,
82, 126, 152, 275
condition-type:breakpoint . . . . . . . . . . . . 258, 280
condition-type:control-error . . . . . . . . . . . . . 279
condition-type:datum-out-of-range . . . . . . . 275
condition-type:derived-file-error . . . . . . . 277
condition-type:derived-port-error . . . . . . . 277
condition-type:divide-by-zero . . . . . . . . . . . . 278
condition-type:error . . . . . . . . 261, 270, 272, 273
condition-type:file-error . . . . . . . . . . . . . . . . 276
condition-type:file-operation-error . . . . . 182,
230, 232, 233, 276
condition-type:floating-point-overflow . . 278
condition-type:floating-point-underflow
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
condition-type:illegal-datum . . . . . . . . . . . . . 273
condition-type:inapplicable-object . . . . . . 276
condition-type:macro-binding . . . . . . . . . . . . . 173
condition-type:no-such-restart . . . . . . 266, 279
condition-type:not-loading . . . . . . . . . . . . . . . 279
condition-type:port-error . . . . . . . . . . . . . . . . 277
condition-type:primitive-procedure-error
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
condition-type:serious-condition . . . . 258, 273
condition-type:simple-condition . . . . . 258, 280
condition-type:simple-error . . . . . . . . . 259, 273
condition-type:simple-warning . . . . . . . 259, 280
MIT Scheme Reference
condition-type:subprocess-abnormaltermination . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
condition-type:subprocess-signalled . . . . . 249
condition-type:subprocess-stopped . . . . . . . 249
condition-type:system-call-error . . . . . . . . . 279
condition-type:unassigned-variable . . . 6, 173,
278
condition-type:unbound-variable . . . . 6, 19, 277
condition-type:variable-error . . . . . . . . . . . . 277
condition-type:warning . . . . . . . . . . . . . . . 258, 280
condition-type:wrong-number-of-arguments
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15, 166, 274
condition-type:wrong-type-argument . . . 6, 180,
274
condition-type:wrong-type-datum . . . . . . . . . . 274
condition-type? . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
condition/continuation . . . . . . . . . . . . . . . . . . . 270
condition/error? . . . . . . . . . . . . . . . . . . . . . . . . . . 270
condition/report-string . . . . . . . . . . . . . . . . . . 271
condition/restarts . . . . . . . . . . . . . . . 269, 270, 271
condition/type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
condition? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
conjugate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
cons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
cons* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
cons-stream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
console-i/o-port . . . . . . . . . . . . . . . . . . . . . 179, 181
console-input-port . . . . . . . . . . . . . . . . . . . . . . . . 180
console-output-port . . . . . . . . . . . . . . . . . . . . . . . 180
constructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
continuation? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
continue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263, 266
copier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
copy-areaon win32-graphics-device . . . . . . . . . . 295
copy-areaon x-graphics-device . . . . . . . . . . . . . . . 291
copy-bitmap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
copy-file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
cos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
create-dib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
create-imageon graphics-device . . . . . . . . . . . . . 287
crop-bitmap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
current-file-time . . . . . . . . . . . . . . . . . . . . . . . . . 236
current-input-port . . . . . . . . . . . . . . . . . . . 180, 183
current-output-port . . . . . . . . . . . . . . 180, 183, 184
current-parser-macros . . . . . . . . . . . . . . . . . . . . 216
D
day-of-week/long-string . . . . . . . . . . . . . . . . . .
day-of-week/short-string . . . . . . . . . . . . . . . . .
deactivate-windowon os2-graphics-device . . . .
debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
decoded-time->file-time . . . . . . . . . . . . . . . . . .
decoded-time->string. . . . . . . . . . . . . . . . . . . . . .
decoded-time->universal-time . . . . . . . . . . . . .
decoded-time/date-string . . . . . . . . . . . . . . . . .
decoded-time/day . . . . . . . . . . . . . . . . . . . . . . . . . .
decoded-time/day-of-week . . . . . . . . . . . . . . . . .
245
245
298
195
243
243
243
245
241
241
Binding Index
decoded-time/daylight-savings-time? . . . . . 241
decoded-time/hour . . . . . . . . . . . . . . . . . . . . . . . . . 241
decoded-time/minute . . . . . . . . . . . . . . . . . . . . . . . 241
decoded-time/month . . . . . . . . . . . . . . . . . . . . . . . . 241
decoded-time/second . . . . . . . . . . . . . . . . . . . . . . . 240
decoded-time/time-string . . . . . . . . . . . . . . . . . 245
decoded-time/year . . . . . . . . . . . . . . . . . . . . . . . . . 241
decoded-time/zone . . . . . . . . . . . . . . . . . . . . . . . . . 241
default-object? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
define . . . . . . . . . . . . . . . . . . . . . . . . 7, 20, 21, 27, 175
define-*matcher-expander . . . . . . . . . . . . . . . . . 216
define-*matcher-macro . . . . . . . . . . . . . . . . . . . . 215
define-*parser-expander . . . . . . . . . . . . . . . . . . 216
define-*parser-macro. . . . . . . . . . . . . . . . . . . . . . 215
define-coloron os2-graphics-device. . . . . . . . . . 296
define-coloron win32-graphics-device . . . . . . . 294
define-similar-windows-type . . . . . . . . . . . . . . 304
define-structure . . . . . . . . . . . . . . . . . . . . . . 29, 132
define-syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
define-windows-type . . . . . . . . . . . . . . . . . . . . . . . 304
del-assoc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
del-assoc! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
del-assq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
del-assq! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
del-assv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
del-assv! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 133
delete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
delete! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112, 113
delete-association-procedure . . . . . . . . . . . . . 141
delete-dib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
delete-directory . . . . . . . . . . . . . . . . . . . . . . . . . . 233
delete-file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
delete-file-no-errors . . . . . . . . . . . . . . . . . . . . 232
delete-matching-items . . . . . . . . . . . . . . . . . . . . 112
delete-matching-items! . . . . . . . . . . . . . . . . . . . 112
delete-member-procedure . . . . . . . . . . . . . . . . . . 113
delq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
delq! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
delv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112, 113
delv! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
denominator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
desktop-sizeon os2-graphics-device. . . . . . . . . . 297
dib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309
dib-blt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
dib-from-bitmap . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
dib-height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
dib-set-pixels-unaligned . . . . . . . . . . . . . . . . . 311
dib-width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
digit->char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
directory-namestring. . . . . . . . . . . . . . . . . . . . . . 228
directory-pathname . . . . . . . . . . . . . . . . . . . . . . . . 228
directory-pathname-as-file . . . . . . . . . . . . . . . 229
directory-pathname? . . . . . . . . . . . . . . . . . . . . . . . 227
directory-read . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
discard-charon input port . . . . . . . . . . . . . . . . . . 199
discard-charson input port . . . . . . . . . . . . . . . . . 199
discard-eventson os2-graphics-device . . . . . . . 300
323
discard-matched . . . . . . . . . . . . . . . . . . . . . . 210, 213
discard-parser-buffer-head! . . . . . . . . . . . . . . 206
discretionary-flush-output . . . . . . . . . . . . . . . 188
discretionary-flush-outputon output port
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
display . . . . . . . . . . . . . . . . . . . . . . . . . . . 179, 189, 260
do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 27, 28
draw-arc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
draw-arcon x-graphics-device . . . . . . . . . . . . . . . . 290
draw-circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
draw-circleon x-graphics-device . . . . . . . . . . . . 290
draw-ellipseon win32-graphics-device . . . . . . . 294
draw-imageon graphics-device . . . . . . . . . . . . . . . 287
draw-lineson os2-graphics-device . . . . . . . . . . . . 301
draw-subimageon graphics-device . . . . . . . . . . . . 287
dynamic-wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
E
eighth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
else. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 25
encapsulate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
end-of-input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
enough-namestring . . . . . . . . . . . . . . . . . . . . . . . . . 228
enough-pathname . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
entity-extra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
entity-procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 171
entity? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
enumerate-graphics-types . . . . . . . . . . . . . . . . . 281
environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
environment-assign! . . . . . . . . . . . . . . . . . . . . . . . 174
environment-assignable? . . . . . . . . . . . . . . . . . . 174
environment-assigned? . . . . . . . . . . . . . . . . . . . . 174
environment-bindings. . . . . . . . . . . . . . . . . . . . . . 173
environment-bound-names . . . . . . . . . . . . . . . . . . 173
environment-bound? . . . . . . . . . . . . . . . . . . . . . . . . 174
environment-definable? . . . . . . . . . . . . . . . . . . . 174
environment-define . . . . . . . . . . . . . . . . . . . . . . . . 175
environment-define-macro . . . . . . . . . . . . . . . . . 175
environment-has-parent? . . . . . . . . . . . . . . . . . . 173
environment-lookup . . . . . . . . . . . . . . . . . . . . . . . . 174
environment-lookup-macro . . . . . . . . . . . . . . . . . 174
environment-macro-names . . . . . . . . . . . . . . . . . . 173
environment-parent . . . . . . . . . . . . . . . . . . . . . . . . 173
environment-reference-type . . . . . . . . . . . . . . . 174
environment? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
eof-object? . . . . . . . . . . . . . . . . . . . . . . . . . . . 186, 198
eof?on input port . . . . . . . . . . . . . . . . . . . . . . . . . . 200
epoch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
eq-hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
eq-hash-mod . . . . . . . . . . . . . . . . . . . . . . . . . . . 145, 149
eq? . . . . . . 51, 54, 112, 114, 128, 130, 140, 141, 142,
144, 145, 151, 152, 268
equal-hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
equal-hash-mod. . . . . . . . . . . . . . . . . . . . . . . . 145, 149
equal? . . . . 23, 51, 55, 112, 114, 140, 141, 144, 145
eqv-hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
eqv-hash-mod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
324
MIT Scheme Reference
eqv? . . . 9, 26, 51, 101, 106, 112, 114, 140, 141, 144
er-macro-transformer . . . . . . . . . . . . . . . . . . . . . . . 47
error . . . . . . . . . . . . . . . . . . . . . . 4, 257, 259, 261, 262
error-irritant/noise. . . . . . . . . . . . . . . . . . . . . . 261
error:bad-range-argument . . . . . . . . . . . . . . . . . 275
error:datum-out-of-range . . . . . . . . . . . . . . . . . 275
error:derived-file . . . . . . . . . . . . . . . . . . . . . . . . 277
error:derived-port . . . . . . . . . . . . . . . . . . . . . . . . 277
error:divide-by-zero. . . . . . . . . . . . . . . . . . . . . . 278
error:file-operation-error . . . . . . . . . . . . . . . 276
error:no-such-restart . . . . . . . . . . . . . . . . . . . . 279
error:wrong-number-of-arguments . . . . . . . . . . 275
error:wrong-type-argument . . . . . . . . . . . . . . . . 274
error:wrong-type-datum . . . . . . . . . . . . . . . . . . . 274
eval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
even? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
exact->inexact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
exact-integer? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
exact-nonnegative-integer? . . . . . . . . . . . . . . . . 61
exact-rational? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
exact? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
except-last-pair . . . . . . . . . . . . . . . . . . . . . . . . . . 112
except-last-pair! . . . . . . . . . . . . . . . . . . . . . . . . . 112
exp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
expt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
extend-top-level-environment . . . . . . . . . . . . . 176
F
false . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
false? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
fifth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
file-access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
file-access-time . . . . . . . . . . . . . . . . . . . . . . . . . .
file-access-time-direct . . . . . . . . . . . . . . . . . .
file-access-time-indirect . . . . . . . . . . . . . . . .
file-attributes . . . . . . . . . . . . . . . . . . . . . . . . . . .
file-attributes-direct . . . . . . . . . . . . . . . . . . .
file-attributes-indirect . . . . . . . . . . . . . . . . .
file-attributes/access-time . . . . . . . . . . . . . .
file-attributes/allocated-length . . . . . . . . .
file-attributes/change-time . . . . . . . . . . . . . .
file-attributes/gid . . . . . . . . . . . . . . . . . . . . . . .
file-attributes/inode-number . . . . . . . . . . . . .
file-attributes/length . . . . . . . . . . . . . . . . . . .
file-attributes/mode-string . . . . . . . . . . . . . .
file-attributes/modes . . . . . . . . . . . . . . . . . . . .
file-attributes/modification-time . . . . . . .
file-attributes/n-links . . . . . . . . . . . . . . . . . .
file-attributes/type. . . . . . . . . . . . . . . . . . . . . .
file-attributes/uid . . . . . . . . . . . . . . . . . . . . . . .
file-directory? . . . . . . . . . . . . . . . . . . . . . . . . . . .
file-eq? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
file-executable? . . . . . . . . . . . . . . . . . . . . . . . . . .
file-exists-direct? . . . . . . . . . . . . . . . . . . . . . . .
file-exists-indirect? . . . . . . . . . . . . . . . . . . . .
file-exists? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
file-length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
127
127
110
235
236
236
236
237
237
237
237
238
237
238
238
237
237
238
237
237
237
238
233
235
235
232
232
232
237
file-modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
file-modification-time . . . . . . . . . . . . . . . . . . . 236
file-modification-time-direct . . . . . . . . . . . . 236
file-modification-time-indirect . . . . . . . . . . 236
file-namestring . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
file-pathname . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
file-readable? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
file-regular? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
file-symbolic-link? . . . . . . . . . . . . . . . . . . . . . . . 234
file-time->global-decoded-time . . . . . . . . . . . 244
file-time->global-time-string . . . . . . . . . . . . 244
file-time->local-decoded-time . . . . . . . . . . . . 244
file-time->local-time-string . . . . . . . . . . . . . 244
file-time->universal-time . . . . . . . . . . . . . . . . 244
file-touch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
file-type-direct . . . . . . . . . . . . . . . . . . . . . . . . . . 234
file-type-indirect . . . . . . . . . . . . . . . . . . . . . . . . 234
file-writeable? . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
fill-circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
fill-circleon x-graphics-device . . . . . . . . . . . . 290
fill-polygon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
fill-polygonon win32-graphics-device . . . . . . . 294
find-coloron os2-graphics-device . . . . . . . . . . . . 296
find-coloron win32-graphics-device . . . . . . . . . 294
find-matching-item . . . . . . . . . . . . . . . . . . . . . . . . 113
find-module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
find-restart . . . . . . . . . . . . . . . . . . . . . . . . . . 263, 267
first . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
fix:* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:-1+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:< . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:<= . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:= . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:>= . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:1+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
fix:andc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
fix:divide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:fixnum? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:gcd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:lsh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
fix:negative? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:not . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
fix:or . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
fix:positive? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:quotient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:remainder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
fix:xor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
fix:zero? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
flo:* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:/ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:< . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:= . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Binding Index
flo:> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:abs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:acos. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:asin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:atan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:atan2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:ceiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:ceiling->exact . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:cos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:exp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:expt. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:finite? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:flonum? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:floor->exact . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:negate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:negative? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:positive? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flo:random-unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
flo:round . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:round->exact . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:sin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:sqrt. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:tan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:truncate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:truncate->exact . . . . . . . . . . . . . . . . . . . . . . . . 74
flo:zero? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
flonum-parser-fast? . . . . . . . . . . . . . . . . . . . . . . . . 68
flonum-unparser-cutoff . . . . . . . . . . . . . . . . . . . . 68
floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
floor->exact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
fluid-let . . . . . . . . . . . . . . . . . . . . . . . . 18, 20, 21, 27
flush-output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
flush-outputon output port . . . . . . . . . . . . . . . . 200
fold-left . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
fold-right . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
font-structureon x-graphics-device . . . . . . . . . 292
for-all? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
for-each . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133, 136
format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
format-error-message. . . . . . . . . . . . . . . . . . . . . . 261
fourth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
fresh-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
fresh-lineon output port. . . . . . . . . . . . . . . . . . . 200
G
gcd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
gdi32.dll . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
ge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 176
general-car-cdr . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
generate-uninterned-symbol . . . . . . . . . . . . . . . 130
get-defaulton x-graphics-device . . . . . . . . . . . . 291
get-host-by-address . . . . . . . . . . . . . . . . . . . . . . . 255
get-host-by-name . . . . . . . . . . . . . . . . . . . . . . . . . . 255
get-host-name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255
325
get-parser-buffer-pointer . . . . . . . . . . . . . . . .
get-parser-buffer-tail . . . . . . . . . . . . . . . . . . .
get-universal-time . . . . . . . . . . . . . . . . . . . . . . . .
global-decoded-time . . . . . . . . . . . . . . . . . . . . . . .
global-parser-macros. . . . . . . . . . . . . . . . . . . . . .
graphics-bind-drawing-mode . . . . . . . . . . . . . . .
graphics-bind-line-style . . . . . . . . . . . . . . . . .
graphics-clear . . . . . . . . . . . . . . . . . . . 283, 290,
graphics-close . . . . . . . . . . . . . . . . . . . . . . . . . . . .
graphics-coordinate-limits . . . . . . . . . . . . . . .
graphics-device-coordinate-limits . . . . . . .
graphics-disable-buffering . . . . . . . . . . . . . . .
graphics-drag-cursor. . . . . . . . . . . . . . . . . . . . . .
graphics-draw-line . . . . . . . . . . . . . . . . . . . . . . . .
graphics-draw-point . . . . . . . . . . . . . . . . . . . . . . .
graphics-draw-text . . . . . . . . . . . . . . . . . . . . . . . .
graphics-enable-buffering . . . . . . . . . . . . . . . .
graphics-erase-point. . . . . . . . . . . . . . . . . . . . . .
graphics-flush . . . . . . . . . . . . . . . . . . . . . . . . . . . .
graphics-move-cursor. . . . . . . . . . . . . . . . . . . . . .
graphics-operation . . . . . . . . . . . . . . . . . . . . . . . .
graphics-reset-clip-rectangle . . . . . . . . . . . .
graphics-set-clip-rectangle . . . . . . . . . . . . . .
graphics-set-coordinate-limits . . . . . . . . . . .
graphics-set-drawing-mode . . . . . . . . . . . . . . . .
graphics-set-line-style . . . . . . . . . . . . . . . . . .
graphics-type-available? . . . . . . . . . . . . . . . . .
guarantee-i/o-port . . . . . . . . . . . . . . . . . . . . . . . .
guarantee-input-port. . . . . . . . . . . . . . . . . . . . . .
guarantee-output-port . . . . . . . . . . . . . . . . . . . .
guarantee-port . . . . . . . . . . . . . . . . . . . . . . . . . . . .
205
206
239
240
216
285
285
291
282
282
282
285
284
283
283
283
285
283
286
284
286
286
286
282
285
285
281
180
180
180
180
H
handle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hard-link-file . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151,
hash-table->alist . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/clean! . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/clear! . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/constructor . . . . . . . . . . . . . . . . . . .
hash-table/count . . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/datum-list . . . . . . . . . . . . . . . . . . . .
hash-table/entries-list . . . . . . . . . . . . . . . . . .
hash-table/entries-vector . . . . . . . . . . . . . . . .
hash-table/entry-datum . . . . . . . . . . . . . . . . . . .
hash-table/entry-key. . . . . . . . . . . . . . . . . . . . . .
hash-table/entry-valid? . . . . . . . . . . . . . . . . . .
hash-table/for-each . . . . . . . . . . . . . . . . . . . . . . .
hash-table/get . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/key-hash . . . . . . . . . . . . . . . . . . . . . . .
hash-table/key-list . . . . . . . . . . . . . . . . . . . . . . .
hash-table/key=? . . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/lookup . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/make . . . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/make-entry . . . . . . . . . . . . . . . . . . . .
hash-table/put! . . . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/rehash-size . . . . . . . . . . . . . . . . . . .
306
232
194
146
145
145
150
146
146
151
151
151
151
151
146
145
151
146
151
146
153
151
145
148
326
hash-table/rehash-threshold . . . . . . . . . . . . . .
hash-table/remove! . . . . . . . . . . . . . . . . . . . . . . . .
hash-table/set-entry-datum! . . . . . . . . . . . . . .
hash-table/size . . . . . . . . . . . . . . . . . . . . . . . . . . .
hash-table? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hbitmap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hbrush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hcursor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hdc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hide-windowon os2-graphics-device . . . . . . . . . . .
hinstance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hmenu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
host-address-any . . . . . . . . . . . . . . . . . . . . . . . . . .
host-address-loopback . . . . . . . . . . . . . . . . . . . .
host-namestring . . . . . . . . . . . . . . . . . . . . . . . . . . .
host=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
host? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hpalette . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hpen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hrgn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
hwnd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MIT Scheme Reference
148
145
151
148
145
306
306
306
306
306
298
306
306
256
256
228
229
229
306
306
306
306
I
i/o-port-type? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
i/o-port? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
identifier=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
identifier? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
if. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 127
ignore-error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
ignore-errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
imag-part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
image/destroy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
image/fill-from-byte-vector . . . . . . . . . . . . . . 288
image/height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
image/width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
image? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
implemented-primitive-procedure? . . . . . . . . . 167
inexact->exact . . . . . . . . . . . . . . . . . . . . . . . . . . 58, 67
inexact?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
init-file-pathname . . . . . . . . . . . . . . . . . . . . . . . . 229
initial-offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
input-buffer-size . . . . . . . . . . . . . . . . . . . . . . . . . 250
input-buffer-sizeon input port . . . . . . . . . . . . 200
input-line-translation . . . . . . . . . . . . . . . . . . . 250
input-port->parser-buffer . . . . . . . . . . . . . . . . 204
input-port-type? . . . . . . . . . . . . . . . . . . . . . . . . . . 197
input-port/char-ready? . . . . . . . . . . . . . . . . . . . 199
input-port/discard-char . . . . . . . . . . . . . . . . . . 199
input-port/discard-chars . . . . . . . . . . . . . . . . . 199
input-port/peek-char. . . . . . . . . . . . . . . . . . . . . . 199
input-port/read-char. . . . . . . . . . . . . . . . . . . . . . 199
input-port/read-string . . . . . . . . . . . . . . . . . . . 199
input-port/read-substring . . . . . . . . . . . . . . . . 199
input-port? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
int . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
integer->char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
integer-ceiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
integer-divide . . . . . . . . . . . . . . . . . . . . . . . . . . 64, 71
integer-divide-quotient . . . . . . . . . . . . . . . . 64, 71
integer-divide-remainder . . . . . . . . . . . . . . . 64, 71
integer-floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
integer-round . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
integer-truncate . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
integer?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
interaction-i/o-port . . . . . . . . . . . . . . . . . 180, 194
intern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
intern-soft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
internal-time/seconds->ticks . . . . . . . . . . . . . 247
internal-time/ticks->seconds . . . . . . . . . . . . . 246
interpreter-environment? . . . . . . . . . . . . . . . . . 176
invoke-restart . . . . . . . . . . . . . . . 263, 265, 267, 268
invoke-restart-interactively . . . . 263, 267, 268
K
keep-matching-items . . . . . . . . . . . . . . . . . . . . . . . 112
keep-matching-items!. . . . . . . . . . . . . . . . . . . . . . 112
kernel32.dll . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
keyword-constructor . . . . . . . . . . . . . . . . . . . . . . . . 31
L
lambda . . . . . . . . 5, 8, 12, 14, 15, 18, 20, 21, 27, 165
last-pair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
lcm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58, 110
let . . . . . . . . . . . . . . . . . . . . . . . . . 7, 17, 18, 20, 21, 27
let* . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 17, 20, 21, 27
let*-syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
let-syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
letrec . . . . . . . . . . . . . . . . . . . . . . . . . 7, 18, 20, 21, 27
letrec-syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
link-variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
list. . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 108, 117, 119
list->stream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
list->string . . . . . . . . . . . . . . . . . . . . . . . . . . . 88, 109
list->vector . . . . . . . . . . . . . . . . . . . . . . . . . . 109, 119
list-copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109, 141
list-deletor . . . . . . . . . . . . . . . . . . . . . . . . . . 113, 141
list-deletor! . . . . . . . . . . . . . . . . . . . . . . . . . 113, 141
list-head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
list-ref . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
list-search-negative. . . . . . . . . . . . . . . . . . . . . . 114
list-search-positive. . . . . . . . . . . . . . . . . . . . . . 114
list-tail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110, 111
list-transform-negative . . . . . . . . . . . . . . . . . . 112
list-transform-positive . . . . . . . . . . . . . . . . . . 112
list? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109, 140, 142
load-bitmapon win32-graphics-device . . . . . . . . 294
load-option . . . . . . . . . . . . . 153, 157, 191, 209, 217
local-decoded-time . . . . . . . . . . . . . . . . . . . . . . . . 239
local-host . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Binding Index
long . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
lower-windowon os2-graphics-device. . . . . . . . . . 298
M
magnitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
make-1d-table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
make-apply-hook . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
make-bit-string . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
make-cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
make-char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
make-circular-list . . . . . . . . . . . . . . . . . . . . . . . . 117
make-condition . . . . . . . . . . . . . . . . . . . 258, 259, 271
make-condition-type . . . . . . . . . . . . . . . . . . . . . . . 271
make-decoded-time . . . . . . . . . . . . . . . . . . . . . . . . . 240
make-directory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
make-entity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
make-eof-object . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
make-eq-hash-table . . . . . . . . . . . . . . . . . . . . . . . . 144
make-equal-hash-table . . . . . . . . . . . . . . . . . . . . 144
make-eqv-hash-table . . . . . . . . . . . . . . . . . . . . . . . 144
make-graphics-device. . . . . . . . . . . . . . . . . . . . . . 281
make-initialized-vector . . . . . . . . . . . . . . . . . . 120
make-list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108, 117
make-parser-macros . . . . . . . . . . . . . . . . . . . . . . . . 216
make-pathname . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
make-polar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
make-port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
make-port-type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
make-primitive-procedure . . . . . . . . . . . . . . . . . 166
make-random-state . . . . . . . . . . . . . . . . . . . . . . . . . . 75
make-rb-tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
make-record-type . . . . . . . . . . . . . . . . . . . . . . . . . . 132
make-rectangular . . . . . . . . . . . . . . . . . . . . . . . 66, 67
make-root-top-level-environment . . . . . . . . . . 176
make-string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
make-string-hash-table . . . . . . . . . . . . . . . . . . . 144
make-syntactic-closure . . . . . . . . . . . . . . . . . . . . 42
make-synthetic-identifier . . . . . . . . . . . . . . . . . 47
make-vector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
make-wt-tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
make-wt-tree-type . . . . . . . . . . . . . . . . . . . . . . . . . 157
make-xml-!attlist . . . . . . . . . . . . . . . . . . . . . . . . . 219
make-xml-!element . . . . . . . . . . . . . . . . . . . . . . . . . 219
make-xml-!entity . . . . . . . . . . . . . . . . . . . . . . . . . . 220
make-xml-!notation . . . . . . . . . . . . . . . . . . . . . . . . 220
make-xml-declaration. . . . . . . . . . . . . . . . . . . . . . 218
make-xml-document . . . . . . . . . . . . . . . . . . . . . . . . . 218
make-xml-dtd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
make-xml-element . . . . . . . . . . . . . . . . . . . . . . . . . . 218
make-xml-external-id. . . . . . . . . . . . . . . . . . . . . . 220
make-xml-parameter-!entity . . . . . . . . . . . . . . . 220
make-xml-processing-instructions . . . . . . . . . 219
make-xml-uninterpreted . . . . . . . . . . . . . . . . . . . 219
make-xml-unparsed-!entity . . . . . . . . . . . . . . . . 220
map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114, 214
map* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
map-windowon x-graphics-device. . . . . . . . . . . . . . 291
327
match . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
match-parser-buffer-char . . . . . . . . . . . . . . . . . 206
match-parser-buffer-char-ci . . . . . . . . . . . . . . 206
match-parser-buffer-char-ci-no-advance . . 206
match-parser-buffer-char-in-set . . . . . . . . . . 206
match-parser-buffer-char-in-set-no-advance
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
match-parser-buffer-char-no-advance . . . . . 206
match-parser-buffer-not-char . . . . . . . . . . . . . 206
match-parser-buffer-not-char-ci . . . . . . . . . . 206
match-parser-buffer-not-char-ci-no-advance
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
match-parser-buffer-not-char-no-advance
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
match-parser-buffer-string . . . . . . . . . . . . . . . 206
match-parser-buffer-string-ci . . . . . . . . . . . . 206
match-parser-buffer-string-ci-no-advance
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
match-parser-buffer-string-no-advance . . . 206
match-parser-buffer-substring . . . . . . . . . . . . 207
match-parser-buffer-substring-ci . . . . . . . . . 207
match-parser-buffer-substring-ci-no-advance
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
match-parser-buffer-substring-no-advance
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
match-utf8-char-in-alphabet . . . . . . . . . . . . . . 207
max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
maximize-windowon os2-graphics-device . . . . . . 298
measure-interval . . . . . . . . . . . . . . . . . . . . . . . . . . 247
member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
member-procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 114
memq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
memv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
merge-pathnames . . . . . . . . . . . . . . . . . . . . . . 181, 227
merge-sort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
merge-sort! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
microcode-id/operating-system . . . . . . . . . . . . 254
microcode-id/operating-system-name . . . . . . 254
microcode-id/operating-system-variant . . . 254
min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
minimize-windowon os2-graphics-device . . . . . . 298
modulo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
month/long-string . . . . . . . . . . . . . . . . . . . . . . . . . 246
month/max-days . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
month/short-string . . . . . . . . . . . . . . . . . . . . . . . . 246
move-windowon win32-graphics-device . . . . . . . . 295
move-windowon x-graphics-device . . . . . . . . . . . . 291
muffle-warning . . . . . . . . . . . . . . . . . . . 259, 263, 266
328
N
name->char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
named . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
named-lambda . . . . . . . . . . . . . . . . . . . . . . . . 16, 20, 27
NaN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
nearest-repl/environment . . . . . . . . . . . . . . . . . 176
negative? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
newline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
nil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
ninth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
not . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
not-char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
not-char-ci . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
notification-output-port . . . . . . . . . . . . . 180, 262
nt-file-mode/archive. . . . . . . . . . . . . . . . . . . . . . 236
nt-file-mode/compressed . . . . . . . . . . . . . . . . . . 236
nt-file-mode/directory . . . . . . . . . . . . . . . . . . . 236
nt-file-mode/hidden . . . . . . . . . . . . . . . . . . . . . . . 236
nt-file-mode/normal . . . . . . . . . . . . . . . . . . . . . . . 236
nt-file-mode/read-only . . . . . . . . . . . . . . . . . . . 235
nt-file-mode/system . . . . . . . . . . . . . . . . . . . . . . . 236
nt-file-mode/temporary . . . . . . . . . . . . . . . . . . . 236
null? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109, 110, 136
number->string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
number-wt-type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
number? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
numerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
O
object-hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
object-hashed? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
object-unhash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
odd? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
open-binary-i/o-file. . . . . . . . . . . . . . . . . . . . . . 182
open-binary-input-file . . . . . . . . . . . . . . . . . . . 182
open-binary-output-file . . . . . . . . . . . . . . . . . . 182
open-dib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
open-i/o-file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
open-input-file . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
open-output-file . . . . . . . . . . . . . . . . . . . . . . . . . . 182
open-tcp-server-socket . . . . . . . . . . . . . . . . . . . 253
open-tcp-stream-socket . . . . . . . . . . . . . . . . . . . 253
or. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 127
os/hostname . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
os2-file-mode/archived . . . . . . . . . . . . . . . . . . . 235
os2-file-mode/directory . . . . . . . . . . . . . . . . . . 235
os2-file-mode/hidden. . . . . . . . . . . . . . . . . . . . . . 235
os2-file-mode/read-only . . . . . . . . . . . . . . . . . . 235
os2-file-mode/system. . . . . . . . . . . . . . . . . . . . . . 235
output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
output-buffer-size . . . . . . . . . . . . . . . . . . . . . . . . 251
output-buffer-sizeon output port . . . . . . . . . . 201
output-line-translation . . . . . . . . . . . . . . . . . . 251
output-port-type? . . . . . . . . . . . . . . . . . . . . . . . . . 197
output-port/discretionary-flush-output . . 201
output-port/flush-output . . . . . . . . . . . . . . . . . 201
MIT Scheme Reference
output-port/fresh-line . . . . . . . . . . . . . . . . . . .
output-port/write-char . . . . . . . . . . . . . . . . . . .
output-port/write-string . . . . . . . . . . . . . . . . .
output-port/write-substring . . . . . . . . . . . . . .
output-port/x-size . . . . . . . . . . . . . . . . . . . . . . . .
output-port/y-size . . . . . . . . . . . . . . . . . . . . . . . .
output-port? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
201
201
201
201
201
202
179
P
pair? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106, 109,
parse-namestring . . . . . . . . . . . . . . . . . . . . . . . . . .
parse-xml-document . . . . . . . . . . . . . . . . . . . . . . . .
parser-buffer-pointer-index . . . . . . . . . . . . . .
parser-buffer-pointer-line . . . . . . . . . . . . . . .
parser-buffer-pointer? . . . . . . . . . . . . . . . . . . .
parser-buffer-position-string . . . . . . . . . . . .
parser-buffer-ref . . . . . . . . . . . . . . . . . . . . . . . . .
parser-buffer? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
parser-macros? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-absolute? . . . . . . . . . . . . . . . . . . . . . . . .
pathname-as-directory . . . . . . . . . . . . . . . . 229,
pathname-default . . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-default-device . . . . . . . . . . . . . . . . . .
pathname-default-directory . . . . . . . . . . . . . . .
pathname-default-name . . . . . . . . . . . . . . . . . . . .
pathname-default-type . . . . . . . . . . . . . . . . . . . .
pathname-default-version . . . . . . . . . . . . . . . . .
pathname-device . . . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-directory . . . . . . . . . . . . . . . . . . . . . . . .
pathname-host . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-name . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-new-device . . . . . . . . . . . . . . . . . . . . . . .
pathname-new-directory . . . . . . . . . . . . . . . . . . .
pathname-new-name . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-new-type . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-new-version. . . . . . . . . . . . . . . . . . . . . .
pathname-simplify . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-type . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-version . . . . . . . . . . . . . . . . . . . . . . . . . .
pathname-wild? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pathname=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pathname? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
peek-char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
peek-charon input port . . . . . . . . . . . . . . . . . . . . .
peek-parser-buffer-char . . . . . . . . . . . . . . . . . .
port-type/operation . . . . . . . . . . . . . . . . . . . . . . .
port-type/operation-names . . . . . . . . . . . . . . . .
port-type/operations. . . . . . . . . . . . . . . . . . . . . .
port-type? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
port/input-blocking-mode . . . . . . . . . . . . . . . . .
port/input-terminal-mode . . . . . . . . . . . . . . . . .
port/operation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
port/operation-names. . . . . . . . . . . . . . . . . . . . . .
port/output-blocking-mode . . . . . . . . . . . . . . . .
port/output-terminal-mode . . . . . . . . . . . . . . . .
port/set-input-blocking-mode . . . . . . . . . . . . .
port/set-input-terminal-mode . . . . . . . . . . . . .
137
222
217
207
207
205
207
205
205
216
226
231
228
226
226
226
226
226
225
225
225
225
226
226
226
226
226
222
225
225
227
226
226
185
198
205
197
197
197
197
202
203
198
198
202
204
202
203
Binding Index
port/set-output-blocking-mode . . . . . . . . . . . . 202
port/set-output-terminal-mode . . . . . . . . . . . . 204
port/state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
port/type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
port/with-input-blocking-mode . . . . . . . . . . . . 202
port/with-input-terminal-mode . . . . . . . . . . . . 203
port/with-output-blocking-mode . . . . . . . . . . . 203
port/with-output-terminal-mode . . . . . . . . . . . 204
port? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
positive? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
pp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
predicate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
predicate->char-set . . . . . . . . . . . . . . . . . . . . . . . . 84
primitive-procedure-name . . . . . . . . . . . . . . . . . 167
primitive-procedure?. . . . . . . . . . . . . . . . . . . . . . 166
print-procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
procedure-arity . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
procedure-arity-valid? . . . . . . . . . . . . . . . . . . . 166
procedure-environment . . . . . . . . . . . . . . . . . . . . 166
procedure? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
process-time-clock . . . . . . . . . . . . . . . . . . . . . . . . 246
promise-forced? . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
promise-value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
promise? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
prompt-for-command-char . . . . . . . . . . . . . . . . . . 195
prompt-for-command-expression . . . . . . . . . . . . 195
prompt-for-confirmation . . . . . . . . . . . . . . . . . . 196
prompt-for-evaluated-expression . . . . . . . . . . 196
prompt-for-expression . . . . . . . . . . . . . . . . . . . . 195
pwd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
Q
quasiquote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23, 105
quick-sort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
quick-sort! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
quote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 105
quotient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63, 64
R
raise-windowon os2-graphics-device. . . . . . . . . . 298
random . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
random-state? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
rational? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
rationalize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
rationalize->exact . . . . . . . . . . . . . . . . . . . . . . . . . 65
rb-tree->alist . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
rb-tree/copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
rb-tree/datum-list . . . . . . . . . . . . . . . . . . . . . . . . 154
rb-tree/delete! . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
rb-tree/delete-max! . . . . . . . . . . . . . . . . . . . . . . . 156
rb-tree/delete-max-datum! . . . . . . . . . . . . . . . . 156
rb-tree/delete-max-pair! . . . . . . . . . . . . . . . . . 156
rb-tree/delete-min! . . . . . . . . . . . . . . . . . . . . . . . 156
rb-tree/delete-min-datum! . . . . . . . . . . . . . . . . 156
rb-tree/delete-min-pair! . . . . . . . . . . . . . . . . . 156
rb-tree/empty? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
329
rb-tree/equal? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
rb-tree/height . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
rb-tree/insert! . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
rb-tree/key-list . . . . . . . . . . . . . . . . . . . . . . . . . . 154
rb-tree/lookup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
rb-tree/max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
rb-tree/max-datum . . . . . . . . . . . . . . . . . . . . . . . . . 156
rb-tree/max-pair . . . . . . . . . . . . . . . . . . . . . . . . . . 156
rb-tree/min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
rb-tree/min-datum . . . . . . . . . . . . . . . . . . . . . . . . . 155
rb-tree/min-pair . . . . . . . . . . . . . . . . . . . . . . . . . . 155
rb-tree/size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
rb-tree? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
re-match-end-index . . . . . . . . . . . . . . . . . . . . . . . . . 98
re-match-extract . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
re-match-start-index . . . . . . . . . . . . . . . . . . . . . . . 98
re-string-match . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
re-string-search-backward . . . . . . . . . . . . . . . . . 97
re-string-search-forward . . . . . . . . . . . . . . . . . . 97
re-substring-match . . . . . . . . . . . . . . . . . . . . . . . . . 97
re-substring-search-backward . . . . . . . . . . . . . . 97
re-substring-search-forward . . . . . . . . . . . . . . . 97
read . . . . . . . . . . . . . . . . . 4, 8, 78, 105, 128, 179, 186
read-buttonon os2-graphics-device . . . . . . . . . . . 299
read-char . . . . . . . . . . . . . . . . . . . . . . . . 179, 185, 186
read-charon input port . . . . . . . . . . . . . . . . . . . . . 198
read-char-no-hang . . . . . . . . . . . . . . . . . . . . . . . . . 186
read-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
read-only . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
read-parser-buffer-char . . . . . . . . . . . . . . . . . . 205
read-string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
read-stringon input port . . . . . . . . . . . . . . . . . . . 199
read-string! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
read-substringon input port . . . . . . . . . . . . . . . 199
read-substring! . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
read-user-eventon os2-graphics-device . . . . . . 299
read-utf8-code-point . . . . . . . . . . . . . . . . . . . . . . . 85
real-part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
real-time-clock . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
real? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
record-accessor . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
record-constructor . . . . . . . . . . . . . . . . . . . . . . . . 132
record-modifier . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
record-predicate . . . . . . . . . . . . . . . . . . . . . . . . . . 132
record-type-descriptor . . . . . . . . . . . . . . . . . . . 133
record-type-field-names . . . . . . . . . . . . . . . . . . 133
record-type-name . . . . . . . . . . . . . . . . . . . . . . . . . . 133
record-type? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
record? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
redisplay-hook . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
reduce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
reduce-right . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
regexp-group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
remainder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63, 64
rename-file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
resize-windowon win32-graphics-device . . . . . . 295
resize-windowon x-graphics-device . . . . . . . . . . 291
resource-id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
330
restart/effector . . . . . . . . . . . . . . . . . . . . . . . . . . 268
restart/interactor . . . . . . . . . . . . . . . . . . . . . . . . 268
restart/name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
restart? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
restore-windowon os2-graphics-device . . . . . . . 298
retry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263, 266
reverse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
reverse! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
reverse-string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
reverse-string! . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
reverse-substring . . . . . . . . . . . . . . . . . . . . . . . . . 102
reverse-substring! . . . . . . . . . . . . . . . . . . . . . . . . 102
rexp* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
rexp+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
rexp->regexp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-alternatives . . . . . . . . . . . . . . . . . . . . . . . . . 100
rexp-any-char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-case-fold . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
rexp-compile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
rexp-line-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-line-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-not-syntax-char. . . . . . . . . . . . . . . . . . . . . . 100
rexp-not-word-char . . . . . . . . . . . . . . . . . . . . . . . . 100
rexp-not-word-edge . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-optional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
rexp-sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
rexp-string-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-string-start . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-syntax-char . . . . . . . . . . . . . . . . . . . . . . . . . . 100
rexp-word-char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-word-edge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-word-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp-word-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
rexp? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
round . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
round->exact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
rsc-macro-transformer . . . . . . . . . . . . . . . . . . . . . 41
run-shell-command . . . . . . . . . . . . . . . . . . . . . . . . . 248
run-synchronous-subprocess . . . . . . . . . . . . . . . 248
runtime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
S
safe-accessors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
save-bitmapon win32-graphics-device . . . . . . . . 294
sc-macro-transformer . . . . . . . . . . . . . . . . . . . . . . . 40
scheme-subprocess-environment . . . . . . . . . . . . 251
second . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
select-user-eventson os2-graphics-device . . . 299
seq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211, 213
sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
set! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21, 22
set-apply-hook-extra! . . . . . . . . . . . . . . . . . . . . 170
set-apply-hook-procedure! . . . . . . . . . . . . . . . . 170
set-background-color . . . . . . . . . . . . . . . . . 293, 296
set-background-coloron os2-graphics-device
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296
MIT Scheme Reference
set-background-coloron win32-graphics-device
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
set-background-coloron x-graphics-device . . . 290
set-border-coloron x-graphics-device . . . . . . . 290
set-border-widthon x-graphics-device . . . . . . . 291
set-car! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
set-cdr! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105, 107
set-cell-contents! . . . . . . . . . . . . . . . . . . . . . . . . 131
set-current-input-port! . . . . . . . . . . . . . . . . . . 181
set-current-output-port! . . . . . . . . . . . . . . . . . 181
set-current-parser-macros! . . . . . . . . . . . . . . . 216
set-entity-extra! . . . . . . . . . . . . . . . . . . . . . . . . . 171
set-entity-procedure! . . . . . . . . . . . . . . . . . . . . 171
set-file-modes! . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
set-file-times! . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
set-fonton os2-graphics-device . . . . . . . . . . . . . . 301
set-fonton win32-graphics-device . . . . . . . . . . . 295
set-fonton x-graphics-device . . . . . . . . . . . . . . . . 291
set-foreground-color . . . . . . . . . . . . . . . . . 293, 296
set-foreground-coloron os2-graphics-device
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296
set-foreground-coloron win32-graphics-device
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
set-foreground-coloron x-graphics-device . . . 290
set-hash-table/rehash-size! . . . . . . . . . . . . . . 148
set-hash-table/rehash-threshold! . . . . . . . . . 149
set-input-buffer-sizeon input port . . . . . . . . 200
set-interaction-i/o-port! . . . . . . . . . . . . . . . . 181
set-internal-border-widthon x-graphics-device
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
set-line-widthon win32-graphics-device . . . . . 295
set-mouse-coloron x-graphics-device . . . . . . . . 290
set-mouse-shapeon x-graphics-device . . . . . . . . 291
set-notification-output-port! . . . . . . . . . . . . 181
set-output-buffer-sizeon output port. . . . . . 201
set-parser-buffer-pointer! . . . . . . . . . . . . . . . 205
set-port/state! . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
set-record-type-unparser-method! . . . . . 31, 193
set-string-length! . . . . . . . . . . . . . . . . . . . . . . . . 103
set-trace-output-port! . . . . . . . . . . . . . . . . . . . 181
set-window-nameon win32-graphics-device . . . . 295
set-window-positionon os2-graphics-device . . 297
set-window-sizeon os2-graphics-device . . . . . . 297
set-window-titleon os2-graphics-device . . . . . 298
set-working-directory-pathname! . . . . . . . . . . 231
set-xml-!attlist-definitions! . . . . . . . . . . . . 219
set-xml-!attlist-name! . . . . . . . . . . . . . . . . . . . 219
set-xml-!element-content-type! . . . . . . . . . . . 219
set-xml-!element-name! . . . . . . . . . . . . . . . . . . . 219
set-xml-!entity-name! . . . . . . . . . . . . . . . . . . . . 220
set-xml-!entity-value! . . . . . . . . . . . . . . . . . . . 220
set-xml-!notation-id! . . . . . . . . . . . . . . . . . . . . 220
set-xml-!notation-name! . . . . . . . . . . . . . . . . . . 220
set-xml-declaration-encoding! . . . . . . . . . . . . 218
set-xml-declaration-standalone! . . . . . . . . . . 218
set-xml-declaration-version! . . . . . . . . . . . . . 218
set-xml-document-declaration! . . . . . . . . . . . . 218
set-xml-document-dtd! . . . . . . . . . . . . . . . . . . . . 218
Binding Index
set-xml-document-misc-1! . . . . . . . . . . . . . . . . . 218
set-xml-document-misc-2! . . . . . . . . . . . . . . . . . 218
set-xml-document-misc-3! . . . . . . . . . . . . . . . . . 218
set-xml-document-root! . . . . . . . . . . . . . . . . . . . 218
set-xml-dtd-external! . . . . . . . . . . . . . . . . . . . . 219
set-xml-dtd-internal! . . . . . . . . . . . . . . . . . . . . 219
set-xml-dtd-root! . . . . . . . . . . . . . . . . . . . . . . . . . 219
set-xml-element-attributes! . . . . . . . . . . . . . . 218
set-xml-element-contents! . . . . . . . . . . . . . . . . 218
set-xml-element-name! . . . . . . . . . . . . . . . . . . . . 218
set-xml-external-id-id! . . . . . . . . . . . . . . . . . . 220
set-xml-external-id-uri! . . . . . . . . . . . . . . . . . 220
set-xml-parameter-!entity-name! . . . . . . . . . . 220
set-xml-parameter-!entity-value! . . . . . . . . . 220
set-xml-processing-instructions-name! . . . 219
set-xml-processing-instructions-text! . . . 219
set-xml-uninterpreted-text! . . . . . . . . . . . . . . 219
set-xml-unparsed-!entity-id! . . . . . . . . . . . . . 220
set-xml-unparsed-!entity-name! . . . . . . . . . . . 220
set-xml-unparsed-!entity-notation! . . . . . . 220
seventh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
sexp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212, 215
shell-file-name . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
short . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
signal-condition . . . . . . . . . . . . . . . . . 258, 259, 262
signed-integer->bit-string . . . . . . . . . . . . . . . 126
simplest-exact-rational . . . . . . . . . . . . . . . . . . . 66
simplest-rational . . . . . . . . . . . . . . . . . . . . . . . . . . 66
sin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
singleton-wt-tree . . . . . . . . . . . . . . . . . . . . . . . . . 158
sixth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
soft-link-file . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
sort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
sort! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
source->parser-buffer . . . . . . . . . . . . . . . . . . . . 205
sqrt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59, 67
standard-error-handler . . . . . . . . . . . . . . . . . . . 262
standard-error-hook . . . . . . . . . . . . . . . . . . . . . . . 262
standard-unparser-method . . . . . . . . . . . . . . . . . 194
standard-warning-handler . . . . . . . . . . . . . 262, 263
standard-warning-hook . . . . . . . . . . . . . . . . 262, 263
store-value . . . . . . . . . . . . . . . . . . . . . . . . . . . 263, 266
stream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
stream->list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
stream-car . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
stream-cdr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
stream-first . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
stream-head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
stream-length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
stream-map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
stream-null? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
stream-pair? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
stream-ref . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
stream-rest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
stream-tail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88, 210, 306
string->alphabet . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
string->char-set . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
331
string->decoded-time. . . . . . . . . . . . . . . . . . . . . . 245
string->file-time . . . . . . . . . . . . . . . . . . . . . . . . . 245
string->input-port . . . . . . . . . . . . . . . . . . . . . . . . 183
string->list . . . . . . . . . . . . . . . . . . . . . . . . . . . 88, 109
string->number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
string->parser-buffer . . . . . . . . . . . . . . . . . . . . 204
string->symbol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
string->uninterned-symbol . . . . . . . . . . . . . . . . 130
string->universal-time . . . . . . . . . . . . . . . . . . . 244
string-append . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
string-capitalize . . . . . . . . . . . . . . . . . . . . . . . . . . 91
string-capitalize! . . . . . . . . . . . . . . . . . . . . . . . . . 91
string-capitalized? . . . . . . . . . . . . . . . . . . . . . . . . 91
string-ci . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
string-ci<=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
string-ci<? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
string-ci=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
string-ci>=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
string-ci>? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
string-compare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
string-compare-ci . . . . . . . . . . . . . . . . . . . . . . . . . . 90
string-copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
string-downcase . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
string-downcase! . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
string-fill! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
string-find-next-char . . . . . . . . . . . . . . . . . . . . . 94
string-find-next-char-ci . . . . . . . . . . . . . . . . . . 94
string-find-next-char-in-set . . . . . . . . . . . . . . 95
string-find-previous-char . . . . . . . . . . . . . . . . . 95
string-find-previous-char-ci . . . . . . . . . . . . . . 95
string-find-previous-char-in-set . . . . . . . . . . 95
string-hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90, 131
string-hash-mod. . . . . . . . . . . . . . . . . . . . . . . . 90, 145
string-head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
string-length . . . . . . . . . . . . . . . . . . . . . . 58, 89, 102
string-lower-case? . . . . . . . . . . . . . . . . . . . . . . . . . 91
string-match-backward . . . . . . . . . . . . . . . . . . . . . 96
string-match-backward-ci . . . . . . . . . . . . . . . . . . 96
string-match-forward . . . . . . . . . . . . . . . . . . . . . . . 95
string-match-forward-ci . . . . . . . . . . . . . . . . . . . 95
string-maximum-length . . . . . . . . . . . . . . . . . . . . 102
string-null? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
string-pad-left . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
string-pad-right . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
string-prefix-ci? . . . . . . . . . . . . . . . . . . . . . . . . . . 96
string-prefix? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
string-ref . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 89, 103
string-replace . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
string-replace! . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
string-search-all . . . . . . . . . . . . . . . . . . . . . . . . . . 94
string-search-backward . . . . . . . . . . . . . . . . . . . . 93
string-search-forward . . . . . . . . . . . . . . . . . . . . . 93
string-set! . . . . . . . . . . . . . . . . . . . . . . . . . 9, 89, 129
string-suffix-ci? . . . . . . . . . . . . . . . . . . . . . . . . . . 96
string-suffix? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
string-tail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
string-trim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
string-trim-left . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
332
string-trim-right . . . . . . . . . . . . . . . . . . . . . . . . . . 93
string-upcase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
string-upcase! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
string-upper-case? . . . . . . . . . . . . . . . . . . . . . . . . . 91
string-wt-type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
string<=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
string<?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
string=? . . . . . . . . . . . 51, 89, 90, 128, 129, 144, 145
string>=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
string>?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
string? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
strong-hash-table/constructor . . . . . . . . . . . . 144
sublist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
substring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
substring->list . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
substring->parser-buffer . . . . . . . . . . . . . . . . . 204
substring-capitalize! . . . . . . . . . . . . . . . . . . . . . 91
substring-capitalized? . . . . . . . . . . . . . . . . . . . . 91
substring-ci<? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
substring-ci=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
substring-downcase! . . . . . . . . . . . . . . . . . . . . . . . . 91
substring-fill! . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
substring-find-next-char . . . . . . . . . . . . . . . . . . 94
substring-find-next-char-ci . . . . . . . . . . . . . . . 94
substring-find-next-char-in-set . . . . . . . . . . . 95
substring-find-previous-char . . . . . . . . . . . . . . 95
substring-find-previous-char-ci . . . . . . . . . . . 95
substring-find-previous-char-in-set . . . . . . 95
substring-lower-case? . . . . . . . . . . . . . . . . . . . . . 91
substring-match-backward . . . . . . . . . . . . . . . . . . 96
substring-match-backward-ci . . . . . . . . . . . . . . . 96
substring-match-forward . . . . . . . . . . . . . . . . . . . 95
substring-match-forward-ci . . . . . . . . . . . . . . . . 95
substring-move-left!. . . . . . . . . . . . . . . . . . . . . . 101
substring-move-right! . . . . . . . . . . . . . . . . . . . . 101
substring-prefix-ci? . . . . . . . . . . . . . . . . . . . . . . . 96
substring-prefix? . . . . . . . . . . . . . . . . . . . . . . . . . . 96
substring-replace . . . . . . . . . . . . . . . . . . . . . . . . . 101
substring-replace! . . . . . . . . . . . . . . . . . . . . . . . . 101
substring-search-all . . . . . . . . . . . . . . . . . . . . . . . 94
substring-search-backward . . . . . . . . . . . . . . . . . 94
substring-search-forward . . . . . . . . . . . . . . . . . . 93
substring-suffix-ci? . . . . . . . . . . . . . . . . . . . . . . . 96
substring-suffix? . . . . . . . . . . . . . . . . . . . . . . . . . . 96
substring-upcase! . . . . . . . . . . . . . . . . . . . . . . . . . . 91
substring-upper-case? . . . . . . . . . . . . . . . . . . . . . 91
substring<? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
substring=? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
substring? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
subvector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
subvector->list . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
subvector-fill! . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
subvector-move-left!. . . . . . . . . . . . . . . . . . . . . . 122
subvector-move-right! . . . . . . . . . . . . . . . . . . . . 122
symbol->string . . . . . . . . . . . . . . . . . . . . . . 9, 51, 129
symbol-append . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
symbol-hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
symbol-hash-mod . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
MIT Scheme Reference
symbol<? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
symbol? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
syntax-rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
system-clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
system-global-environment . . . . . . . . . . . . . . . . 175
system-library-directory-pathname . . . . . . . 230
system-library-pathname . . . . . . . . . . . . . . . . . . 230
T
t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
tan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
tcp-server-connection-accept . . . . . . . . . . . . . 254
template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
temporary-directory-pathname . . . . . . . . . . . . . 233
temporary-file-pathname . . . . . . . . . . . . . . . . . . 233
tenth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
the-environment . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
there-exists? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
third . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
time-zone->string . . . . . . . . . . . . . . . . . . . . . . . . . 246
time-zone? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
top-level-environment? . . . . . . . . . . . . . . . . . . . 176
trace-output-port . . . . . . . . . . . . . . . . . . . . . . . . . 180
transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
tree-copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
true . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
truncate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
truncate->exact . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 32
type-descriptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
U
uint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
ulong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
unbind-variable . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
unchecked . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
unhash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
unicode-code-point? . . . . . . . . . . . . . . . . . . . . . . . . 85
universal-time->file-time . . . . . . . . . . . . . . . . 243
universal-time->global-decoded-time . . . . . 242
universal-time->global-time-string . . . . . . 243
universal-time->local-decoded-time . . . . . . 242
universal-time->local-time-string . . . . . . . 243
unparser/set-tagged-pair-method! . . . . . . . . . 193
unparser/set-tagged-vector-method! . . . . . . 193
unquote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 105
unquote-splicing . . . . . . . . . . . . . . . . . . . . . . 24, 105
unsigned-integer->bit-string . . . . . . . . . . . . . 126
use-pty? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
use-value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263, 266
user-homedir-pathname . . . . . . . . . . . . . . . . . . . . 229
user-initial-environment . . . . . . . . . . 7, 175, 176
user32.dll . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
ushort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
utf8-string->code-point . . . . . . . . . . . . . . . . . . . 85
Binding Index
333
V
valid-hash-number? . . . . . . . . . . . . . . . . . . . . . . . .
values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170,
vector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector->list . . . . . . . . . . . . . . . . . . . . . . . . . . 109,
vector-8b-fill! . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-8b-find-next-char . . . . . . . . . . . . . . . . .
vector-8b-find-next-char-ci . . . . . . . . . . . . . .
vector-8b-find-previous-char . . . . . . . . . . . . .
vector-8b-find-previous-char-ci . . . . . . . . . .
vector-8b-ref . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-8b-set! . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-binary-search. . . . . . . . . . . . . . . . . . . . . .
vector-copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-eighth . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-fifth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-fill! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-first . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-fourth . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-grow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-length . . . . . . . . . . . . . . . . . . . . . . . . . . 58,
vector-map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-ref . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9,
vector-second . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-set! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-seventh . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-sixth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-tail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector-third . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vector? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
152
213
119
119
103
103
103
103
103
103
103
121
119
121
121
121
120
121
120
121
120
120
120
121
120
121
121
121
121
120
W
warn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259, 266
weak-car . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
weak-cdr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
weak-cons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
weak-hash-table/constructor . . . . . . . . . . . . . . 144
weak-pair/car?. . . . . . . . . . . . . . . . . . . . . . . . 137, 150
weak-pair? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
weak-set-car! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
weak-set-cdr! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
well-formed-code-points-list? . . . . . . . . . . . . . 85
where . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
window-frame-sizeon os2-graphics-device . . . . 297
window-positionon os2-graphics-device . . . . . . 297
window-sizeon os2-graphics-device . . . . . . . . . . . 297
windows-procedure . . . . . . . . . . . . . . . . . . . . . . . . . 307
with-current-parser-macros . . . . . . . . . . . . . . . 216
with-current-unparser-state . . . . . . . . . . . . . . 194
with-input-from-binary-file . . . . . . . . . . . . . . 183
with-input-from-file. . . . . . . . . . . . . . . . . . . . . . 183
with-input-from-port. . . . . . . . . . . . . . . . . . . . . . 180
with-input-from-string . . . . . . . . . . . . . . . . . . . 183
with-interaction-i/o-port . . . . . . . . . . . . . . . . 180
with-notification-output-port . . . . . . . . . . . . 180
with-output-to-binary-file . . . . . . . . . . . . . . . 183
with-output-to-file . . . . . . . . . . . . . . . . . . . . . . . 183
with-output-to-port . . . . . . . . . . . . . . . . . . . . . . . 180
with-output-to-string . . . . . . . . . . . . . . . . . . . . 184
with-output-to-truncated-string . . . . . . . . . . 184
with-pointer . . . . . . . . . . . . . . . . . . . . . . . . . . 212, 215
with-restart . . . . . . . . . . . . . . . . . 258, 263, 265, 267
with-simple-restart . . . . . . . . . . . . . . 263, 264, 267
with-string-output-port . . . . . . . . . . . . . . . . . . 184
with-timings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
with-trace-output-port . . . . . . . . . . . . . . . . . . . 180
with-working-directory-pathname . . . . . . . . . . 231
withdraw-windowon x-graphics-device . . . . . . . . 291
within-continuation . . . . . . . . . . . . . . . . . . . . . . . 168
word . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
working-directory . . . . . . . . . . . . . . . . . . . . . . . . . 252
working-directory-pathname . . . . . . . . . . . . . . . 231
write . . . . . . . . . . . . . . . . . . . . . . 8, 128, 189, 194, 260
write-char. . . . . . . . . . . . . . . . . . . . . . . . . . 6, 179, 188
write-charon output port. . . . . . . . . . . . . . . . . . . 200
write-condition-report . . . . . . . . . . . . . . . 262, 271
write-dib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
write-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
write-restart-report. . . . . . . . . . . . . . . . . . . . . . 268
write-string . . . . . . . . . . . . . . . . . . . . . 188, 191, 194
write-substring . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
write-substringon output port . . . . . . . . . . . . . 200
write-to-string . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
write-utf8-code-point . . . . . . . . . . . . . . . . . . . . . 85
write-xml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
wt-tree/add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
wt-tree/add! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
wt-tree/delete . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
wt-tree/delete! . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
wt-tree/delete-min . . . . . . . . . . . . . . . . . . . . . . . . 163
wt-tree/delete-min! . . . . . . . . . . . . . . . . . . . . . . . 163
wt-tree/difference . . . . . . . . . . . . . . . . . . . . . . . . 160
wt-tree/empty? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
wt-tree/fold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
wt-tree/for-each . . . . . . . . . . . . . . . . . . . . . . . . . . 161
wt-tree/index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
wt-tree/index-datum . . . . . . . . . . . . . . . . . . . . . . . 162
wt-tree/index-pair . . . . . . . . . . . . . . . . . . . . . . . . 162
wt-tree/intersection. . . . . . . . . . . . . . . . . . . . . . 160
wt-tree/lookup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
wt-tree/member? . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
wt-tree/min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
wt-tree/min-datum . . . . . . . . . . . . . . . . . . . . . . . . . 163
wt-tree/min-pair . . . . . . . . . . . . . . . . . . . . . . . . . . 163
wt-tree/rank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
wt-tree/set-equal? . . . . . . . . . . . . . . . . . . . . . . . . 161
wt-tree/size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
wt-tree/split< . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
wt-tree/split> . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
wt-tree/subset? . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
wt-tree/union . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
wt-tree/union-merge . . . . . . . . . . . . . . . . . . . . . . . 161
wt-tree? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
334
MIT Scheme Reference
X
x-character-bounds/ascent . . . . . . . . . . . . . . . .
x-character-bounds/descent . . . . . . . . . . . . . . .
x-character-bounds/lbearing . . . . . . . . . . . . . .
x-character-bounds/rbearing . . . . . . . . . . . . . .
x-character-bounds/width . . . . . . . . . . . . . . . . .
x-close-all-displays. . . . . . . . . . . . . . . . . . . . . .
x-font-structure/all-chars-exist . . . . . . . . .
x-font-structure/character-bounds . . . . . . .
x-font-structure/default-char . . . . . . . . . . . .
x-font-structure/direction . . . . . . . . . . . . . . .
x-font-structure/max-ascent . . . . . . . . . . . . . .
x-font-structure/max-bounds . . . . . . . . . . . . . .
x-font-structure/max-descent . . . . . . . . . . . . .
x-font-structure/min-bounds . . . . . . . . . . . . . .
x-font-structure/name . . . . . . . . . . . . . . . . . . . .
x-font-structure/start-index . . . . . . . . . . . . .
x-geometry-string . . . . . . . . . . . . . . . . . . . . . . . . .
x-graphics/close-display . . . . . . . . . . . . . . . . .
x-graphics/open-display . . . . . . . . . . . . . . . . . .
x-open-display . . . . . . . . . . . . . . . . . . . . . . . . . . . .
x-sizeon output port . . . . . . . . . . . . . . . . . . . . . . .
xml-!attlist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!attlist-definitions . . . . . . . . . . . . . . . . .
xml-!attlist-name . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!attlist-rtd . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!attlist? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!element-content-type . . . . . . . . . . . . . . . .
xml-!element-name . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!element-rtd . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!element? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!entity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!entity-name . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!entity-rtd . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!entity-value . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!entity? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!notation-id . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!notation-name . . . . . . . . . . . . . . . . . . . . . . . .
xml-!notation-rtd . . . . . . . . . . . . . . . . . . . . . . . . .
xml-!notation? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-declaration . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-declaration-encoding . . . . . . . . . . . . . . . . .
xml-declaration-rtd . . . . . . . . . . . . . . . . . . . . . . .
xml-declaration-standalone . . . . . . . . . . . . . . .
xml-declaration-version . . . . . . . . . . . . . . . . . .
xml-declaration? . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-document-declaration . . . . . . . . . . . . . . . . .
xml-document-dtd . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-document-misc-1 . . . . . . . . . . . . . . . . . . . . . . .
292
292
292
292
292
289
292
292
292
292
292
292
292
292
292
292
289
289
289
288
201
219
219
219
219
219
219
219
219
219
219
220
220
220
220
220
220
220
220
220
220
218
218
218
218
218
218
218
218
218
218
xml-document-misc-2 . . . . . . . . . . . . . . . . . . . . . . .
xml-document-misc-3 . . . . . . . . . . . . . . . . . . . . . . .
xml-document-root . . . . . . . . . . . . . . . . . . . . . . . . .
xml-document-rtd . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-document? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-dtd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-dtd-external . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-dtd-internal . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-dtd-root . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-dtd-rtd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-dtd? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-element-attributes . . . . . . . . . . . . . . . . . . .
xml-element-contents. . . . . . . . . . . . . . . . . . . . . .
xml-element-name . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-element-rtd . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-element? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-external-id . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-external-id-id . . . . . . . . . . . . . . . . . . . . . . . .
xml-external-id-rtd . . . . . . . . . . . . . . . . . . . . . . .
xml-external-id-uri . . . . . . . . . . . . . . . . . . . . . . .
xml-external-id? . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-intern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xml-parameter-!entity . . . . . . . . . . . . . . . . . . . .
xml-parameter-!entity-name . . . . . . . . . . . . . . .
xml-parameter-!entity-rtd . . . . . . . . . . . . . . . .
xml-parameter-!entity-value . . . . . . . . . . . . . .
xml-parameter-!entity? . . . . . . . . . . . . . . . . . . .
xml-processing-instructions . . . . . . . . . . . . . .
xml-processing-instructions-name . . . . . . . . .
xml-processing-instructions-rtd . . . . . . . . . .
xml-processing-instructions-text . . . . . . . . .
xml-processing-instructions? . . . . . . . . . . . . .
xml-uninterpreted . . . . . . . . . . . . . . . . . . . . . . . . .
xml-uninterpreted-rtd . . . . . . . . . . . . . . . . . . . .
xml-uninterpreted-text . . . . . . . . . . . . . . . . . . .
xml-uninterpreted? . . . . . . . . . . . . . . . . . . . . . . . .
xml-unparsed-!entity. . . . . . . . . . . . . . . . . . . . . .
xml-unparsed-!entity-id . . . . . . . . . . . . . . . . . .
xml-unparsed-!entity-name . . . . . . . . . . . . . . . .
xml-unparsed-!entity-notation . . . . . . . . . . . .
xml-unparsed-!entity-rtd . . . . . . . . . . . . . . . . .
xml-unparsed-!entity? . . . . . . . . . . . . . . . . . . . .
218
218
218
218
218
219
219
219
219
219
219
218
218
218
218
218
218
220
220
220
220
220
217
220
220
220
220
220
219
219
219
219
219
219
219
219
219
220
220
220
220
220
220
Y
y-size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
y-sizeon output port . . . . . . . . . . . . . . . . . . . . . . . 201
Z
zero? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60, 61
Concept Index
335
Concept Index
!
;
! in mutation procedure names . . . . . . . . . . . . . . . 11
; as external representation . . . . . . . . . . . . . . . . . . . 11
"
=
" as external representation. . . . . . . . . . . . . . . . . . . 87
=> in cond clause . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
=> notational convention . . . . . . . . . . . . . . . . . . . . . . 4
#
?
# as format parameter . . . . . . . . . . . . . . . . . . . . . . 192
# in external representation of number . . . . . . . . 60
#( as external representation . . . . . . . . . . . . . . . . 119
#* as external representation . . . . . . . . . . . . . . . . 123
#[ as external representation . . . . . . . . . . . . . . . . 194
#\ as external representation . . . . . . . . . . . . . . . . . 77
#| as external representation . . . . . . . . . . . . . . . . . 11
#b as external representation . . . . . . . . . . . . . . . . . 60
#d as external representation . . . . . . . . . . . . . . . . . 60
#e as external representation . . . . . . . . . . . . . . . . . 60
#f as external representation . . . . . . . . . . . . . . . . 127
#i as external representation . . . . . . . . . . . . . . . . . 60
#o as external representation . . . . . . . . . . . . . . . . . 60
#t as external representation . . . . . . . . . . . . . . . . 127
#x as external representation . . . . . . . . . . . . . . . . . 60
? in predicate names . . . . . . . . . . . . . . . . . . . . . . . . . 11
[
[ in entries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
]
] in entries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
‘
‘ as external representation . . . . . . . . . . . . . . . . . . . 24
\
’
\ as escape character in string . . . . . . . . . . . . . . . . 87
’ as external representation . . . . . . . . . . . . . . . . . . . 22
1
(
( as external representation . . . . . . . . . . . . . . . . . . 105
)
) as external representation . . . . . . . . . . . . . . . . . . 105
,
, as external representation . . . . . . . . . . . . . . . . . . . 24
,@ as external representation . . . . . . . . . . . . . . . . . 24
-| notational convention . . . . . . . . . . . . . . . . . . . . . . . 4
-ci, in string procedure name . . . . . . . . . . . . . . . . . 87
.
. as external representation . . . . . . . . . . . . . . . . . . 105
... in entries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1D table (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
A
absolute pathname (defn) . . . . . . . . . . . . . . . . . . . 230
absolute value, of number . . . . . . . . . . . . . . . . . . . . 63
access time, of file . . . . . . . . . . . . . . . . . . . . . . . . . . 236
access, used with set!. . . . . . . . . . . . . . . . . . . . . . . . . 22
addition, of numbers . . . . . . . . . . . . . . . . . . . . . . . . . 62
address hashing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
alias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40, 45
alist (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Alphabet, Unicode . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
alphabetic case, of interned symbol. . . . . . . . . . . 128
alphabetic case, of string . . . . . . . . . . . . . . . . . . . . . 91
alphabetic case-insensitivity of programs (defn)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
alphabetic character (defn) . . . . . . . . . . . . . . . . . . . 83
alphanumeric character (defn) . . . . . . . . . . . . . . . . 83
anonymous syntactic keyword. . . . . . . . . . . . . . . . . 35
apostrophe, as external representation . . . . . . . . . 22
appending, of bit strings . . . . . . . . . . . . . . . . . . . . 124
appending, of lists . . . . . . . . . . . . . . . . . . . . . . . . . . 111
336
appending, of strings . . . . . . . . . . . . . . . . . . . . . . . . . 92
appending, of symbols . . . . . . . . . . . . . . . . . . . . . . 130
appending, to output file . . . . . . . . . . . . . . . . . . . . 182
application hook (defn) . . . . . . . . . . . . . . . . . 165, 170
application, of procedure . . . . . . . . . . . . . . . . . . . . 165
apply hook (defn). . . . . . . . . . . . . . . . . . . . . . . . . . . 170
argument evaluation order . . . . . . . . . . . . . . . . . . . . 14
ASCII character . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
association list (defn) . . . . . . . . . . . . . . . . . . . . . . . 139
association table (defn) . . . . . . . . . . . . . . . . . . . . . 142
asterisk, as external representation . . . . . . . . . . . 123
attribute, of file . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
B
backquote, as external representation . . . . . . . . . . 24
backslash, as escape character in string . . . . . . . . 87
Backtracking, in parser language . . . . . . . . . . . . . 207
balanced binary trees . . . . . . . . . . . . . . . . . . . 153, 156
bell, ringing on console . . . . . . . . . . . . . . . . . . . . . . 189
binary file ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
binary trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153, 156
binary trees, as discrete maps . . . . . . . . . . . . . . . . 157
binary trees, as sets . . . . . . . . . . . . . . . . . . . . . . . . . 157
binding expression (defn) . . . . . . . . . . . . . . . . . . . . . . 7
binding expression, dynamic (or fluid) . . . . . . . . . 18
binding expression, lexical . . . . . . . . . . . . . . . . . . . . 17
binding, of variable . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
binding, syntactic keyword . . . . . . . . . . . . . . . . . . 173
binding, unassigned . . . . . . . . . . . . . . . . . . . . . . . . . 173
binding, variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
bit string (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
bit string index (defn). . . . . . . . . . . . . . . . . . . . . . . 123
bit string length (defn) . . . . . . . . . . . . . . . . . . . . . . 123
bitmaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
bitmaps, graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
bitwise-logical operations, on fixnums . . . . . . . . . 72
block structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
blocking mode, of port . . . . . . . . . . . . . . . . . . . . . . 202
BOA constructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
BOA constructor (defn) . . . . . . . . . . . . . . . . . . . . . . 31
body, of special form (defn) . . . . . . . . . . . . . . . . . . . . 5
boolean object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
boolean object (defn) . . . . . . . . . . . . . . . . . . . . . . . 127
boolean object, equivalence predicate . . . . . . . . . 127
bound variable (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 6
bound-restarts . . . . . . . . . . . . . . . . . . . . . . . . . 269, 271
bracket, in entries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
bucky bit, of character (defn) . . . . . . . . . . . . . . . . . 80
bucky bit, prefix (defn) . . . . . . . . . . . . . . . . . . . . . . . 77
bu↵ering, of graphics output . . . . . . . . . . . . . . . . . 285
bu↵ering, of output . . . . . . . . . . . . . . . . . . . . . . . . . 188
built-in procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 165
byte vector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
MIT Scheme Reference
C
call by need evaluation (defn) . . . . . . . . . . . . . . . . 133
capitalization, of string . . . . . . . . . . . . . . . . . . . . . . . 91
car field, of pair (defn) . . . . . . . . . . . . . . . . . . . . . . 105
case clause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
case conversion, of character . . . . . . . . . . . . . . . . . . 79
case sensitivity, of string operations . . . . . . . . . . . 87
case, of interned symbol . . . . . . . . . . . . . . . . . . . . . 128
case, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
case-insensitivity of programs (defn) . . . . . . . . . . . 11
cdr field, of pair (defn) . . . . . . . . . . . . . . . . . . . . . . 105
cell (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
character (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
character bits (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 80
character code (defn) . . . . . . . . . . . . . . . . . . . . . . . . 80
character set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
character, alphabetic (defn). . . . . . . . . . . . . . . . . . . 83
character, alphanumeric (defn) . . . . . . . . . . . . . . . . 83
character, graphic (defn) . . . . . . . . . . . . . . . . . . . . . 83
character, input from port . . . . . . . . . . . . . . 185, 198
character, ISO-8859-1 (defn) . . . . . . . . . . . . . . . . . . 82
character, named (defn) . . . . . . . . . . . . . . . . . . . . . . 77
character, numeric (defn) . . . . . . . . . . . . . . . . . . . . . 83
character, output to port . . . . . . . . . . . . . . . 188, 200
character, searching string for . . . . . . . . . . . . . . . . . 93
character, standard . . . . . . . . . . . . . . . . . . . . . . . . . . 79
character, standard (defn) . . . . . . . . . . . . . . . . . . . . 83
character, whitespace (defn) . . . . . . . . . . . . . . . . . . 83
characters, special, in programs . . . . . . . . . . . . . . . 11
child, of environment (defn) . . . . . . . . . . . . . . . . . . . 6
circle, graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
circles, drawing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
circular list . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109, 117
circular structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
clause, of case expression . . . . . . . . . . . . . . . . . . . . . 25
clause, of cond expression . . . . . . . . . . . . . . . . . . . . 24
clearing the console screen . . . . . . . . . . . . . . . . . . . 189
client socket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
clip rectangle, graphics (defn). . . . . . . . . . . . . . . . 286
clipping, of graphics . . . . . . . . . . . . . . . . . . . . . . . . 286
closing environment, of procedure (defn) . . . . . . . 15
closing, of file port . . . . . . . . . . . . . . . . . . . . . . . . . . 182
closing, of port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
Code point, Unicode . . . . . . . . . . . . . . . . . . . . . . . . . 84
code, of character (defn) . . . . . . . . . . . . . . . . . . . . . 80
color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293, 296
combination (defn). . . . . . . . . . . . . . . . . . . . . . . . . . . 14
comma, as external representation . . . . . . . . . . . . 24
comment, extended, in programs (defn) . . . . . . . . 11
comment, in programs (defn) . . . . . . . . . . . . . . . . . 11
comparison predicate. . . . . . . . . . . . . . . . . . . . . . . . . 48
comparison, for equivalence . . . . . . . . . . . . . . . . . . . 51
comparison, of bit strings. . . . . . . . . . . . . . . . . . . . 125
comparison, of boolean objects. . . . . . . . . . . . . . . 127
comparison, of characters . . . . . . . . . . . . . . . . . . . . . 79
comparison, of numbers . . . . . . . . . . . . . . . . . . . . . . 61
comparison, of strings . . . . . . . . . . . . . . . . . . . . . . . . 89
compiled, procedure type . . . . . . . . . . . . . . . . . . . . 165
Concept Index
component selection, of bit string . . . . . . . . . . . . 124
component selection, of cell . . . . . . . . . . . . . . . . . . 131
component selection, of character . . . . . . . . . . . . . 80
component selection, of list . . . . . . . . . . . . . . . . . . 109
component selection, of pair . . . . . . . . . . . . . . . . . 106
component selection, of stream . . . . . . . . . . . . . . 136
component selection, of string . . . . . . . . . . . . . . . . 88
component selection, of vector . . . . . . . . . . . . . . . 120
component selection, of weak pair . . . . . . . . . . . . 137
components, of pathname . . . . . . . . . . . . . . . . . . . 223
compound procedure . . . . . . . . . . . . . . . . . . . . . . . . 165
cond clause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
condition (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
condition handler (defn) . . . . . . . . . . . . . . . . . . . . . 261
condition instance (defn) . . . . . . . . . . . . . . . . . . . . 268
condition signalling (defn) . . . . . . . . . . . . . . . . . . . 258
condition type . . . . . . . . . . . . . . . . . . . . . . . . . 258, 271
conditional expression (defn) . . . . . . . . . . . . . . . . . 24
console, clearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
console, port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
console, ringing the bell . . . . . . . . . . . . . . . . . . . . . 189
constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
constant expression (defn) . . . . . . . . . . . . . . . . . . . . 13
constant, and quasiquote . . . . . . . . . . . . . . . . . . . . . 23
constant, and quote . . . . . . . . . . . . . . . . . . . . . . . . . . 22
construction, of bit string . . . . . . . . . . . . . . . . . . . 123
construction, of cell . . . . . . . . . . . . . . . . . . . . . . . . . 131
construction, of character. . . . . . . . . . . . . . . . . . . . . 80
construction, of character set . . . . . . . . . . . . . . . . . 83
construction, of circular list. . . . . . . . . . . . . . . . . . 117
construction, of continuation. . . . . . . . . . . . . . . . . 167
construction, of EOF object . . . . . . . . . . . . . . . . . 198
construction, of file input port . . . . . . . . . . . . . . . 182
construction, of file output port . . . . . . . . . . . . . . 182
construction, of hash table . . . . . . . . . . . . . . . . . . 143
construction, of list . . . . . . . . . . . . . . . . . . . . . . . . . 108
construction, of pair . . . . . . . . . . . . . . . . . . . . . . . . 106
construction, of pathname . . . . . . . . . . . . . . 222, 225
construction, of port type . . . . . . . . . . . . . . . . . . . 197
construction, of procedure . . . . . . . . . . . . . . . . . . . . 15
construction, of promise . . . . . . . . . . . . . . . . . . . . . 133
construction, of stream . . . . . . . . . . . . . . . . . . . . . . 135
construction, of string . . . . . . . . . . . . . . . . . . . . . . . . 88
construction, of string input port . . . . . . . . . . . . 183
construction, of string output port . . . . . . . . . . . 184
construction, of symbols . . . . . . . . . . . . . . . . . . . . . 129
construction, of vector . . . . . . . . . . . . . . . . . . . . . . 119
construction, of weak pair . . . . . . . . . . . . . . . . . . . 137
continuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
continuation, alternate invocation . . . . . . . . . . . . 168
continuation, and dynamic binding . . . . . . . . . . . . 19
control, bucky bit prefix (defn). . . . . . . . . . . . . . . . 77
conventions for error messages . . . . . . . . . . . . . . . 260
conventions, lexical . . . . . . . . . . . . . . . . . . . . . . . . . . 10
conventions, naming . . . . . . . . . . . . . . . . . . . . . . . . . 11
conventions, notational . . . . . . . . . . . . . . . . . . . . . . . . 4
conversion, pathname to string . . . . . . . . . . 222, 228
cooked mode, of terminal port . . . . . . . . . . . . . . . 203
337
coordinates, graphics . . . . . . . . . . . . . . . . . . . . . . . . 282
copying, of alist . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
copying, of bit string . . . . . . . . . . . . . . . . . . . . . . . . 123
copying, of file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
copying, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
copying, of tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
copying, of vector . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
current environment . . . . . . . . . . . . . . . . . . . . . . . . 176
current environment (defn) . . . . . . . . . . . . . . . . . . . . 7
current input port (defn) . . . . . . . . . . . . . . . . . . . . 179
current input port, rebinding . . . . . . . . . . . . . . . . 183
current output port (defn) . . . . . . . . . . . . . . . . . . . 179
current output port, rebinding . . . . . . . . . . 183, 184
current working directory . . . . . . . . . . . . . . . . . . . 221
current working directory (defn) . . . . . . . . . . . . . 230
cursor, graphics (defn) . . . . . . . . . . . . . . . . . . . . . . 283
custom operations, on graphics device . . . . . . . . 286
custom operations, on port . . . . . . . . . . . . . . . . . . 197
cutting, of bit string . . . . . . . . . . . . . . . . . . . . . . . . 124
cutting, of list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
cutting, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
cutting, of vector . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
D
d, as exponent marker in number . . . . . . . . . . . . . 60
decoded time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
default object (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 15
defaulting, of pathname . . . . . . . . . . . . . . . . . . . . . 227
define, procedure (defn) . . . . . . . . . . . . . . . . . . . . . . 20
defining foreign procedures . . . . . . . . . . . . . . . . . . 307
defining foreign types . . . . . . . . . . . . . . . . . . . . . . . 304
definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
definition, internal . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
definition, internal (defn) . . . . . . . . . . . . . . . . . . . . . 20
definition, top-level . . . . . . . . . . . . . . . . . . . . . . . . . . 21
definition, top-level (defn) . . . . . . . . . . . . . . . . . . . . 20
deletion, of alist element . . . . . . . . . . . . . . . . . . . . 140
deletion, of file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
deletion, of list element . . . . . . . . . . . . . . . . . . . . . 112
delimiter, in programs (defn) . . . . . . . . . . . . . . . . . 10
device coordinates, graphics (defn) . . . . . . . . . . . 282
device, pathname component . . . . . . . . . . . . . . . . 223
di↵erence, of numbers . . . . . . . . . . . . . . . . . . . . . . . . 62
directive, format (defn) . . . . . . . . . . . . . . . . . . . . . 191
directory path (defn) . . . . . . . . . . . . . . . . . . . . . . . . 224
directory, converting pathname to . . . . . . . . . . . . 229
directory, current working (defn) . . . . . . . . . . . . . 230
directory, pathname component . . . . . . . . . . . . . . 223
directory, predicate for . . . . . . . . . . . . . . . . . . . . . . 233
directory, reading . . . . . . . . . . . . . . . . . . . . . . 229, 238
discrete maps, using binary trees . . . . . . . . . . . . . 157
discretionary flushing, of bu↵ered output . . . . . 188
disembodied property list . . . . . . . . . . . . . . . . . . . 128
display, clearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
display, X graphics . . . . . . . . . . . . . . . . . . . . . . . . . . 289
division, of integers . . . . . . . . . . . . . . . . . . . . . . . . . . 63
division, of numbers. . . . . . . . . . . . . . . . . . . . . . . . . . 62
338
DLL, DIBUTILS.DLL . . . . . . . . . . . . . . . . . . . . . . 309
DLL, exports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
DLL, GDI32.DLL . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
DLL, KERNEL32.DLL . . . . . . . . . . . . . . . . . . . . . . 307
DLL, loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
DLL, USER32.DLL . . . . . . . . . . . . . . . . . . . . . . . . . 307
dot, as external representation . . . . . . . . . . . . . . . 105
dotted notation, for pair (defn) . . . . . . . . . . . . . . 105
dotted pair (see pair) . . . . . . . . . . . . . . . . . . . . . . . 105
double precision, of inexact number . . . . . . . . . . . 60
double quote, as external representation . . . . . . . 87
drawing arcs and circles, graphics . . . . . . . . . . . . 290
drawing mode, graphics (defn) . . . . . . . . . . . . . . . 284
dynamic binding . . . . . . . . . . . . . . . . . . . . 18, 262, 263
dynamic binding, and continuations . . . . . . . . . . . 19
dynamic binding, versus static scoping. . . . . . . . . . 7
dynamic types (defn). . . . . . . . . . . . . . . . . . . . . . . . . . 3
E
e, as exponent marker in number . . . . . . . . . . . . . . 60
e↵ector, restart (defn) . . . . . . . . . . . . . . . . . . . . . . . 263
element, of list (defn) . . . . . . . . . . . . . . . . . . . . . . . 105
ellipse, graphics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
ellipsis, in entries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
else clause, of case expression (defn) . . . . . . . . . . . 25
else clause, of cond expression (defn) . . . . . . . . . . 25
empty list (defn). . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
empty list, external representation . . . . . . . . . . . 105
empty list, predicate for . . . . . . . . . . . . . . . . . . . . . 110
empty stream, predicate for . . . . . . . . . . . . . . . . . 136
empty string, predicate for . . . . . . . . . . . . . . . . . . . 89
end of file object (see EOF object) . . . . . . . . . . . 186
end, of substring (defn) . . . . . . . . . . . . . . . . . . . . . . 87
end, of subvector (defn) . . . . . . . . . . . . . . . . . . . . . 119
entity (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
entry format. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
environment (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
environment, current . . . . . . . . . . . . . . . . . . . . . . . . 176
environment, current (defn). . . . . . . . . . . . . . . . . . . . 7
environment, extension (defn). . . . . . . . . . . . . . . . . . 6
environment, initial (defn) . . . . . . . . . . . . . . . . . . . . . 7
environment, interpreter. . . . . . . . . . . . . . . . . . . . . 176
environment, of procedure . . . . . . . . . . . . . . . . . . . . 15
environment, procedure closing (defn) . . . . . . . . . 15
environment, procedure invocation (defn) . . . . . . 15
environment, top-level . . . . . . . . . . . . . . . . . . . . . . 176
EOF object, construction. . . . . . . . . . . . . . . . . . . . 198
EOF object, predicate for . . . . . . . . . . . . . . . . . . . 186
equivalence predicate (defn) . . . . . . . . . . . . . . . . . . 51
equivalence predicate, for bit strings. . . . . . . . . . 125
equivalence predicate, for boolean objects . . . . . 127
equivalence predicate, for characters . . . . . . . . . . . 79
equivalence predicate, for fixnums . . . . . . . . . . . . . 71
equivalence predicate, for flonums . . . . . . . . . . . . . 73
equivalence predicate, for numbers . . . . . . . . . . . . 61
equivalence predicate, for pathname host . . . . . 229
equivalence predicate, for pathnames . . . . . . . . . 226
MIT Scheme Reference
equivalence predicate, for strings . . . . . . . . . . . . . . 89
equivalence predicate, of hash table . . . . . . . . . . 143
equivalence predicates, for characters . . . . . . . . . . 79
error messages, conventions . . . . . . . . . . . . . . . . . . 260
error, in examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
error, unassigned variable. . . . . . . . . . . . . . . . . . . . . . 6
error, unbound variable (defn) . . . . . . . . . . . . . . . . . 6
error–> notational convention . . . . . . . . . . . . . . . . . . 4
errors, notational conventions . . . . . . . . . . . . . . . . . . 4
escape character, for string . . . . . . . . . . . . . . . . . . . 87
escape procedure (defn) . . . . . . . . . . . . . . . . . . . . . 167
escape procedure, alternate invocation . . . . . . . . 168
evaluation order, of arguments . . . . . . . . . . . . . . . . 14
evaluation, call by need (defn) . . . . . . . . . . . . . . . 133
evaluation, in examples . . . . . . . . . . . . . . . . . . . . . . . . 4
evaluation, lazy (defn) . . . . . . . . . . . . . . . . . . . . . . 133
evaluation, of s-expression . . . . . . . . . . . . . . . . . . . 175
even number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
exactness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
existence, testing of file . . . . . . . . . . . . . . . . . . . . . 232
exit, non-local . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
explicit renaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
exponent marker (defn) . . . . . . . . . . . . . . . . . . . . . . 60
expression (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
expression, binding (defn) . . . . . . . . . . . . . . . . . . . . . 7
expression, conditional (defn) . . . . . . . . . . . . . . . . . 24
expression, constant (defn) . . . . . . . . . . . . . . . . . . . 13
expression, input from port . . . . . . . . . . . . . . . . . . 186
expression, iteration (defn) . . . . . . . . . . . . . . . . . . . 27
expression, literal (defn) . . . . . . . . . . . . . . . . . . . . . . 13
expression, output to port . . . . . . . . . . . . . . . . . . . 189
expression, procedure call (defn) . . . . . . . . . . . . . . 14
expression, special form (defn) . . . . . . . . . . . . . . . . 13
extended comment, in programs (defn) . . . . . . . . 11
extension, of environment (defn) . . . . . . . . . . . . . . . 6
extent, of dynamic binding (defn) . . . . . . . . . . . . . 19
extent, of objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
external representation (defn) . . . . . . . . . . . . . . . . . . 8
external representation, and quasiquote . . . . . . . . 23
external representation, and quote . . . . . . . . . . . . 22
external representation, for bit string . . . . . . . . . 123
external representation, for character . . . . . . . . . . 77
external representation, for empty list . . . . . . . . 105
external representation, for list . . . . . . . . . . . . . . 105
external representation, for number . . . . . . . . . . . 60
external representation, for pair . . . . . . . . . . . . . . 105
external representation, for procedure . . . . . . . . 165
external representation, for string . . . . . . . . . . . . . 87
external representation, for symbol . . . . . . . . . . . 128
external representation, for vector . . . . . . . . . . . . 119
external representation, generating . . . . . . . . . . . 189
external representation, parsing . . . . . . . . . . . . . . 186
extra object, of application hook . . . . . . . . . . . . . 170
Concept Index
F
f, as exponent marker in number . . . . . . . . . . . . . . 60
false, boolean object . . . . . . . . . . . . . . . . . . . . . . . . . . 8
false, boolean object (defn) . . . . . . . . . . . . . . . . . . 127
false, in conditional expression (defn) . . . . . . . . . . 24
false, predicate for . . . . . . . . . . . . . . . . . . . . . . . . . . 127
file (regular), predicate for . . . . . . . . . . . . . . . . . . . 234
file name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
file time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
file type, procedure for . . . . . . . . . . . . . . . . . . . . . . 234
file, converting pathname directory to . . . . . . . . 229
file, end-of-file marker (see EOF object) . . . . . . 186
file, input and output ports . . . . . . . . . . . . . . . . . . 181
filename (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
filling, of bit string . . . . . . . . . . . . . . . . . . . . . . . . . . 126
filling, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
filling, of vector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
filtering, of list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
fixnum (defn). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
flonum (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
fluid binding. . . . . . . . . . . . . . . . . . . . . . . . 18, 262, 263
flushing, of bu↵ered output . . . . . . . . . . . . . . . . . . 188
forcing, of promise . . . . . . . . . . . . . . . . . . . . . . . . . . 133
foreign type declarations . . . . . . . . . . . . . . . . . . . . 304
form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
form, special (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . 13
formal parameter list, of lambda (defn) . . . . . . . . 15
format directive (defn) . . . . . . . . . . . . . . . . . . . . . . 191
format, entry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
G
generalization, of condition types . . . 258, 259, 271,
272
generalization, of condition types (defn) . . . . . . 257
generating, external representation . . . . . . . . . . . 189
gensym (see uninterned symbol) . . . . . . . . . . . . . 130
geometry string, X graphics . . . . . . . . . . . . . . . . . 289
graphic character (defn) . . . . . . . . . . . . . . . . . . . . . . 83
graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
graphics, bitmaps . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
graphics, bu↵ering of output . . . . . . . . . . . . . . . . . 285
graphics, circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
graphics, clipping . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
graphics, coordinate systems . . . . . . . . . . . . . . . . . 282
graphics, cursor (defn) . . . . . . . . . . . . . . . . . . . . . . 283
graphics, custom operations . . . . . . . . . . . . . . . . . 286
graphics, device coordinates (defn) . . . . . . . . . . . 282
graphics, drawing . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
graphics, drawing arcs and circles . . . . . . . . . . . . 290
graphics, drawing mode (defn) . . . . . . . . . . . . . . . 284
graphics, ellipse. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
graphics, images . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
graphics, line style (defn) . . . . . . . . . . . . . . . . . . . . 284
graphics, opening and closing devices . . . . . . . . . 281
graphics, output characteristics . . . . . . . . . . . . . . 284
graphics, virtual coordinates (defn) . . . . . . . . . . 282
greatest common divisor, of numbers . . . . . . . . . . 64
339
growing, of vector. . . . . . . . . . . . . . . . . . . . . . . . . . . 120
H
handler, condition (defn) . . . . . . . . . . . . . . . . . . . . 261
hard linking, of file . . . . . . . . . . . . . . . . . . . . . . . . . 232
hash table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
hashing, of key in hash table. . . . . . . . . . . . . . . . . 144
hashing, of object . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
hashing, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
hashing, of symbol . . . . . . . . . . . . . . . . . . . . . . . . . . 131
home directory, as pathname . . . . . . . . . . . . . . . . 229
hook, application (defn) . . . . . . . . . . . . . . . . . . . . . 165
host, in filename . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
host, pathname component . . . . . . . . . . . . . . . . . . 223
hostname, TCP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
hygienic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
hyper, bucky bit prefix (defn) . . . . . . . . . . . . . . . . . 77
I
I/O, to files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
I/O, to strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
identifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
identifier (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
identity, additive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
identity, multiplicative . . . . . . . . . . . . . . . . . . . . . . . 62
images, graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
immutable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
implementation restriction . . . . . . . . . . . . . . . . . . . . 58
implicit begin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
improper list (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 105
index, of bit string (defn). . . . . . . . . . . . . . . . . . . . 123
index, of list (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 110
index, of string (defn) . . . . . . . . . . . . . . . . . . . . . . . . 87
index, of subvector (defn) . . . . . . . . . . . . . . . . . . . 119
index, of vector (defn). . . . . . . . . . . . . . . . . . . . . . . 119
inheritance, of environment bindings (defn) . . . . . 6
initial environment (defn) . . . . . . . . . . . . . . . . . . . . . 7
initial size, of hash table . . . . . . . . . . . . . . . . . . . . 147
input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
input form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
input form, to macro . . . . . . . . . . . . . . . . . . . . . . . . . 47
input operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
input port operations . . . . . . . . . . . . . . . . . . . . . . . 198
input port, console . . . . . . . . . . . . . . . . . . . . . . . . . . 181
input port, current (defn) . . . . . . . . . . . . . . . . . . . 179
input port, file. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
input port, string . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
insensitivity, to case in programs (defn). . . . . . . . 11
installed, as pathname component . . . . . . . . . . . 225
instance, of condition (defn) . . . . . . . . . . . . . . . . . 268
integer division . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
integer, converting to bit string . . . . . . . . . . . . . . 126
interactive input ports (defn) . . . . . . . . . . . . . . . . 185
internal definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
internal definition (defn) . . . . . . . . . . . . . . . . . . . . . 20
internal representation, for character . . . . . . . . . . 80
340
internal representation, for inexact number . . . . 60
interned symbol (defn) . . . . . . . . . . . . . . . . . . . . . . 128
interning, of symbols . . . . . . . . . . . . . . . . . . . . . . . . 129
interpreted, procedure type . . . . . . . . . . . . . . . . . . 165
interpreter environment . . . . . . . . . . . . . . . . . . . . . 176
inverse, additive, of number . . . . . . . . . . . . . . . . . . 62
inverse, multiplicative, of number . . . . . . . . . . . . . 62
inverse, of bit string. . . . . . . . . . . . . . . . . . . . . . . . . 125
inverse, of boolean object. . . . . . . . . . . . . . . . . . . . 127
invocation environment, of procedure (defn) . . . 15
ISO-8859-1 character (defn) . . . . . . . . . . . . . . . . . . 82
iteration expression (defn) . . . . . . . . . . . . . . . . . . . . 27
K
key, of association list element (defn) . . . . . . . . . 139
keyword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
keyword binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
keyword constructor . . . . . . . . . . . . . . . . . . . . . . . . . 29
keyword constructor (defn) . . . . . . . . . . . . . . . . . . . 31
keyword, of special form (defn) . . . . . . . . . . . . . . . 13
L
l, as exponent marker in number . . . . . . . . . . . . . . 60
lambda expression (defn) . . . . . . . . . . . . . . . . . . . . . 15
lambda list (defn). . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
lambda, implicit in define . . . . . . . . . . . . . . . . . . . . 20
lambda, implicit in let. . . . . . . . . . . . . . . . . . . . . . . . 17
latent types (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
lazy evaluation (defn) . . . . . . . . . . . . . . . . . . . . . . . 133
least common multiple, of numbers . . . . . . . . . . . . 64
length, of bit string . . . . . . . . . . . . . . . . . . . . . . . . . 124
length, of bit string (defn) . . . . . . . . . . . . . . . . . . . 123
length, of list (defn). . . . . . . . . . . . . . . . . . . . . . . . . 105
length, of stream . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
length, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
length, of string (defn) . . . . . . . . . . . . . . . . . . . . . . . 87
length, of vector (defn) . . . . . . . . . . . . . . . . . . . . . . 119
letrec, implicit in define . . . . . . . . . . . . . . . . . . . . . . 21
lexical binding expression. . . . . . . . . . . . . . . . . . . . . 17
lexical conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
lexical scoping (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . 8
library, system pathname . . . . . . . . . . . . . . . . . . . . 230
limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
line style, graphics (defn) . . . . . . . . . . . . . . . . . . . . 284
linking (hard), of file . . . . . . . . . . . . . . . . . . . . . . . . 232
linking (soft), of file . . . . . . . . . . . . . . . . . . . . . . . . . 232
list (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
list index (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
list, association (defn) . . . . . . . . . . . . . . . . . . . . . . . 139
list, converting to stream . . . . . . . . . . . . . . . . . . . . 135
list, converting to string . . . . . . . . . . . . . . . . . . . . . . 88
list, converting to vector . . . . . . . . . . . . . . . . . . . . 119
list, external representation . . . . . . . . . . . . . . . . . . 105
list, improper (defn) . . . . . . . . . . . . . . . . . . . . . . . . 105
literal expression (defn) . . . . . . . . . . . . . . . . . . . . . . 13
literal, and quasiquote . . . . . . . . . . . . . . . . . . . . . . . 23
MIT Scheme Reference
literal, and quote . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
literal, identifier as . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
loading DLLs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
location, of variable . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
logical operations, on fixnums . . . . . . . . . . . . . . . . . 72
long precision, of inexact number . . . . . . . . . . . . . 60
loopback interface . . . . . . . . . . . . . . . . . . . . . . . . . . 256
looping (see iteration expressions) . . . . . . . . . . . . . 27
lowercase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
lowercase, character conversion . . . . . . . . . . . . . . . 79
lowercase, in string. . . . . . . . . . . . . . . . . . . . . . . . . . . 91
M
macro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
macro keyword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
macro transformer . . . . . . . . . . . . . . . . . . . . 34, 41, 47
macro use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
magnitude, of real number . . . . . . . . . . . . . . . . . . . . 63
manifest types (defn). . . . . . . . . . . . . . . . . . . . . . . . . . 3
mapping, of list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
mapping, of stream . . . . . . . . . . . . . . . . . . . . . . . . . 137
mapping, of vector . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Matcher language . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
Matcher procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 209
matching, of strings . . . . . . . . . . . . . . . . . . . . . . . . . . 95
maximum length, of string (defn) . . . . . . . . . . . . 102
maximum, of numbers . . . . . . . . . . . . . . . . . . . . . . . 62
memoization, of promise . . . . . . . . . . . . . . . . . . . . 133
merging, of pathnames . . . . . . . . . . . . . . . . . . . . . . 227
meta, bucky bit prefix (defn) . . . . . . . . . . . . . . . . . 77
method, unparser (defn). . . . . . . . . . . . . . . . . . . . . 193
minimum, of numbers . . . . . . . . . . . . . . . . . . . . . . . . 62
modification time, of file . . . . . . . . . . . . . . . . . . . . 236
modification, of bit string . . . . . . . . . . . . . . . . . . . 126
modification, of string . . . . . . . . . . . . . . . . . . . . . . . 101
modification, of vector . . . . . . . . . . . . . . . . . . . . . . 121
modulus, of hashing procedure . . . . . . . . . . . . . . . 144
modulus, of integers. . . . . . . . . . . . . . . . . . . . . . . . . . 63
moving, of bit string elements . . . . . . . . . . . . . . . 126
moving, of string elements . . . . . . . . . . . . . . . . . . . 101
moving, of vector elements . . . . . . . . . . . . . . . . . . 121
multiple values, from procedure . . . . . . . . . . . . . . 169
multiplication, of numbers . . . . . . . . . . . . . . . . . . . . 62
must be, notational convention . . . . . . . . . . . . . . . . . 4
mutable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
mutation procedure (defn) . . . . . . . . . . . . . . . . . . . . 11
Concept Index
341
N
P
name, of character . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
name, of file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
name, of symbol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
name, of value (defn). . . . . . . . . . . . . . . . . . . . . . . . . . 6
name, pathname component . . . . . . . . . . . . . . . . . 223
named lambda (defn) . . . . . . . . . . . . . . . . . . . . . . . . 16
named let (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
names, XML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
naming conventions . . . . . . . . . . . . . . . . . . . . . 11, 308
negative infinity (-inf) . . . . . . . . . . . . . . . . . . . . . . . 73
negative number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
nesting, of quasiquote expressions . . . . . . . . . . . . . 23
newest, as pathname component . . . . . . . . . . . . . 225
newline character (defn) . . . . . . . . . . . . . . . . . . . . . . 78
newline character, output to port . . . . . . . . . . . . 189
newline translation . . . . . . . . . . . . . . . . . . . . . . . . . 181
non-local exit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
not a number (NaN) . . . . . . . . . . . . . . . . . . . . . . . . . . 73
notation, dotted (defn) . . . . . . . . . . . . . . . . . . . . . . 105
notational conventions . . . . . . . . . . . . . . . . . . . . . . . . 4
null string, predicate for . . . . . . . . . . . . . . . . . . . . . . 89
number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
number, external representation . . . . . . . . . . . . . . . 60
number, pseudo-random . . . . . . . . . . . . . . . . . . . . . . 74
numeric character (defn) . . . . . . . . . . . . . . . . . . . . . 83
numeric precision, inexact . . . . . . . . . . . . . . . . . . . . 60
numerical input and output. . . . . . . . . . . . . . . . . . . 68
numerical operations . . . . . . . . . . . . . . . . . . . . . . . . . 60
numerical types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
padding, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
pair (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
pair, external representation . . . . . . . . . . . . . . . . . 105
pair, weak (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
parameter list, of lambda (defn) . . . . . . . . . . . . . . 15
parameter, optional (defn) . . . . . . . . . . . . . . . . . . . . 15
parameter, required (defn) . . . . . . . . . . . . . . . . . . . . 15
parameter, rest (defn) . . . . . . . . . . . . . . . . . . . . . . . . 15
parent, of directory . . . . . . . . . . . . . . . . . . . . . . . . . 224
parent, of environment (defn) . . . . . . . . . . . . . . . . . . 6
parenthesis, as external representation . . . 105, 119
Parser bu↵er . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
Parser language . . . . . . . . . . . . . . . . . . . . . . . . 207, 212
Parser procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
parser, XML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
Parser-bu↵er pointer . . . . . . . . . . . . . . . . . . . . . . . . 204
parsing, of external representation . . . . . . . . . . . 186
pasting, of bit strings . . . . . . . . . . . . . . . . . . . . . . . 124
pasting, of lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
pasting, of strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
pasting, of symbols . . . . . . . . . . . . . . . . . . . . . . . . . 130
path, directory (defn) . . . . . . . . . . . . . . . . . . . . . . . 224
pathname . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
pathname (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
pathname components . . . . . . . . . . . . . . . . . . . . . . 223
pathname, absolute (defn) . . . . . . . . . . . . . . . . . . . 230
pathname, relative (defn) . . . . . . . . . . . . . . . . . . . . 230
period, as external representation . . . . . . . . . . . . 105
physical size, of hash table (defn) . . . . . . . . . . . . 147
port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
port (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
port number, TCP . . . . . . . . . . . . . . . . . . . . . . . . . . 253
port primitives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
port type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
port, console . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
port, current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
port, file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
port, string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
positive infinity (+inf) . . . . . . . . . . . . . . . . . . . . . . . 73
positive number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
precision, of inexact number . . . . . . . . . . . . . . . . . . 60
predicate (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 51
predicate, equivalence (defn) . . . . . . . . . . . . . . . . . . 51
prefix, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
pretty printer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
primitive procedure (defn) . . . . . . . . . . . . . . . . . . . 165
primitive, procedure type . . . . . . . . . . . . . . . . . . . . 165
print name, of symbol . . . . . . . . . . . . . . . . . . . . . . . 129
printed output, in examples . . . . . . . . . . . . . . . . . . . 4
printing graphics output . . . . . . . . . . . . . . . . . . . . 294
procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
procedure call (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 14
procedure define (defn) . . . . . . . . . . . . . . . . . . . . . . . 20
procedure, closing environment (defn) . . . . . . . . . 15
procedure, compiled . . . . . . . . . . . . . . . . . . . . . . . . 165
procedure, compound . . . . . . . . . . . . . . . . . . . . . . . 165
procedure, construction . . . . . . . . . . . . . . . . . . . . . . 15
O
object hashing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
odd number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
oldest, as pathname component . . . . . . . . . . . . . . 225
one-dimensional table (defn) . . . . . . . . . . . . . . . . . 141
operand, of procedure call (defn) . . . . . . . . . . . . . . 14
Operating-System Interface . . . . . . . . . . . . . . . . . . 221
operator, of procedure call (defn) . . . . . . . . . . . . . 14
option, run-time-loadable . . 153, 157, 191, 209, 217
optional component, in entries . . . . . . . . . . . . . . . . . 5
optional parameter (defn) . . . . . . . . . . . . . . . . . . . . 15
order, of argument evaluation . . . . . . . . . . . . . . . . . 14
ordering, of characters . . . . . . . . . . . . . . . . . . . . . . . 79
ordering, of numbers . . . . . . . . . . . . . . . . . . . . . . . . . 61
ordering, of strings . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
OS/2 graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
output form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
output port operations . . . . . . . . . . . . . . . . . . . . . . 200
output port, console . . . . . . . . . . . . . . . . . . . . . . . . 181
output port, current (defn) . . . . . . . . . . . . . . . . . . 179
output port, file . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
output port, string . . . . . . . . . . . . . . . . . . . . . . . . . . 183
output procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 188
output, XML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
342
MIT Scheme Reference
procedure, entry format . . . . . . . . . . . . . . . . . . . . . . . 5
procedure, escape (defn) . . . . . . . . . . . . . . . . . . . . 167
procedure, interpreted. . . . . . . . . . . . . . . . . . . . . . . 165
procedure, invocation environment (defn) . . . . . . 15
procedure, of application hook . . . . . . . . . . . . . . . 170
procedure, primitive . . . . . . . . . . . . . . . . . . . . . . . . 165
procedure, type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
product, of numbers . . . . . . . . . . . . . . . . . . . . . . . . . 62
promise (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
promise, construction . . . . . . . . . . . . . . . . . . . . . . . 133
promise, forcing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
prompting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
proper tail recursion (defn) . . . . . . . . . . . . . . . . . . . . 3
property list . . . . . . . . . . . . . . . . . . . . . . . 139, 141, 142
property list, of symbol . . . . . . . . . . . . . . . . . . . . . 128
protocol, restart (defn) . . . . . . . . . . . . . . . . . . . . . . 263
pseudo-random number . . . . . . . . . . . . . . . . . . . . . . 74
resizing, of hash table . . . . . . . . . . . . . . . . . . . . . . . 146
resources, X graphics. . . . . . . . . . . . . . . . . . . . . . . . 288
rest parameter (defn) . . . . . . . . . . . . . . . . . . . . . . . . 15
restart (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
restart e↵ector (defn) . . . . . . . . . . . . . . . . . . . . . . . 263
restart protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
restarts, bound . . . . . . . . . . . . . . . . . . . . . . . . . 269, 271
result of evaluation, in examples . . . . . . . . . . . . . . . 4
result, unspecified (defn) . . . . . . . . . . . . . . . . . . . . . . 5
reversal, of list. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
REXP abstraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
ringing the console bell . . . . . . . . . . . . . . . . . . . . . . 189
root, as pathname component . . . . . . . . . . . . . . . 224
run-time-loadable option . . 153, 157, 191, 209, 217
runtime system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Q
s, as exponent marker in number . . . . . . . . . . . . . . 60
s-expression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
scheme concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Scheme standard. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
scope (see region) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
scoping, lexical (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 8
scoping, static . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
screen, clearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
searching, for regular expression . . . . . . . . . . . . . . 97
searching, of alist . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
searching, of bit string . . . . . . . . . . . . . . . . . . . . . . 124
searching, of list . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
searching, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
searching, of vector . . . . . . . . . . . . . . . . . . . . . . . . . 121
selecting, of stream component . . . . . . . . . . . . . . 136
selection, components of pathname . . . . . . . . . . . 228
selection, of bit string component . . . . . . . . . . . . 124
selection, of cell component . . . . . . . . . . . . . . . . . . 131
selection, of character component . . . . . . . . . . . . . 80
selection, of list component . . . . . . . . . . . . . . . . . . 109
selection, of pair component . . . . . . . . . . . . . . . . . 106
selection, of string component . . . . . . . . . . . . . . . . 88
selection, of vector component . . . . . . . . . . . . . . . 120
selection, of weak pair component . . . . . . . . . . . . 137
semicolon, as external representation . . . . . . . . . . 11
sensitivity, to case in programs (defn) . . . . . . . . . 11
sequencing expressions . . . . . . . . . . . . . . . . . . . . . . . 26
server socket . . . . . . . . . . . . . . . . . . . . . . . . . . . 252, 253
service, TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
set, of characters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
sets, using binary trees . . . . . . . . . . . . . . . . . . . . . . 157
shadowing, of variable binding (defn) . . . . . . . . . . . 6
short precision, of inexact number . . . . . . . . . . . . . 60
signal an error (defn). . . . . . . . . . . . . . . . . . . . . . . . . . 4
signalling, of condition (defn) . . . . . . . . . . . . . . . . 258
simplest rational (defn) . . . . . . . . . . . . . . . . . . . . . . 65
simplification, of pathname . . . . . . . . . . . . . . . . . . 222
single precision, of inexact number . . . . . . . . . . . . 60
size, of hash table (defn) . . . . . . . . . . . . . . . . . . . . 146
socket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
quote, as external representation . . . . . . . . . . . . . .
quotient, of integers . . . . . . . . . . . . . . . . . . . . . . . . . .
quotient, of numbers . . . . . . . . . . . . . . . . . . . . . . . . .
quoting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
63
62
22
R
R4RS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
random number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
rational, simplest (defn) . . . . . . . . . . . . . . . . . . . . . . 65
raw mode, of terminal port . . . . . . . . . . . . . . . . . . 203
record-type descriptor (defn) . . . . . . . . . . . . . . . . 132
recursion (see tail recursion) . . . . . . . . . . . . . . . . . . . 3
red-black binary trees . . . . . . . . . . . . . . . . . . . . . . . 153
reduction, of list . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
reference, variable (defn) . . . . . . . . . . . . . . . . . . . . . 13
referentially transparent . . . . . . . . . . . . . . . . . . . . . . 35
region of variable binding, do . . . . . . . . . . . . . . . . . 28
region of variable binding, internal definition . . . 21
region of variable binding, lambda . . . . . . . . . . . . 15
region of variable binding, let . . . . . . . . . . . . . . . . . 17
region of variable binding, let* . . . . . . . . . . . . . . . . 17
region of variable binding, letrec . . . . . . . . . . . . . . 18
region, of variable binding (defn) . . . . . . . . . . . . . . . 8
regular expression, searching string for . . . . . . . . 97
regular file, predicate for . . . . . . . . . . . . . . . . . . . . 234
rehash size, of hash table (defn) . . . . . . . . . . . . . . 148
rehash threshold, of hash table (defn) . . . . . . . . 148
relative pathname (defn) . . . . . . . . . . . . . . . . . . . . 230
remainder, of integers . . . . . . . . . . . . . . . . . . . . . . . . 63
renaming procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 47
renaming, of file . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
REP loop . . . . . . . . . . . . . . . . . . . . . 259, 261, 262, 266
REP loop (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
REP loop, environment of . . . . . . . . . . . . . . . . . . . . . 7
replacement, of string component . . . . . . . . . . . . 101
representation, external (defn) . . . . . . . . . . . . . . . . . 8
required parameter (defn) . . . . . . . . . . . . . . . . . . . . 15
S
Concept Index
soft linking, of file . . . . . . . . . . . . . . . . . . . . . . . . . . 232
special characters, in programs . . . . . . . . . . . . . . . . 11
special form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
special form (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
special form, entry category . . . . . . . . . . . . . . . . . . . 5
specialization, of condition types . . . 258, 259, 269,
270, 272
specialization, of condition types (defn) . . . . . . . 257
specified result, in examples . . . . . . . . . . . . . . . . . . . 4
standard character . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
standard character (defn). . . . . . . . . . . . . . . . . . . . . 83
standard operations, on port. . . . . . . . . . . . . . . . . 196
standard ports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
standard Scheme (defn) . . . . . . . . . . . . . . . . . . . . . . . 3
start, of substring (defn) . . . . . . . . . . . . . . . . . . . . . 87
start, of subvector (defn) . . . . . . . . . . . . . . . . . . . . 119
static scoping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
static scoping (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . 3
static types (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
stream (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
stream, converting to list . . . . . . . . . . . . . . . . . . . . 135
string index (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
string length (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . 87
string, character (defn) . . . . . . . . . . . . . . . . . . . . . . . 87
string, converting to input port . . . . . . . . . . . . . . 183
string, converting to list . . . . . . . . . . . . . . . . . . . . . 109
string, input and output ports . . . . . . . . . . . . . . . 183
string, input from port . . . . . . . . . . . . . . . . . 186, 199
string, interning as symbol . . . . . . . . . . . . . . . . . . 129
string, of bits (defn) . . . . . . . . . . . . . . . . . . . . . . . . 123
string, output to port . . . . . . . . . . . . . . . . . . . . . . . 188
strong types (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
strongly held keys, of hash table . . . . . . . . . . . . . 143
subprocess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
substring (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
substring, of bit string . . . . . . . . . . . . . . . . . . . . . . 124
substring, output to port . . . . . . . . . . . . . . . . . . . . 200
substring, searching string for . . . . . . . . . . . . . . . . . 93
subtraction, of numbers . . . . . . . . . . . . . . . . . . . . . . 62
subvector (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
suffix, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
sum, of numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
super, bucky bit prefix (defn) . . . . . . . . . . . . . . . . . 77
symbol (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
symbolic link, predicate for . . . . . . . . . . . . . . . . . . 234
symbolic linking, of file . . . . . . . . . . . . . . . . . . . . . . 232
synchronous subprocess . . . . . . . . . . . . . . . . . . . . . 248
syntactic closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
syntactic closures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
syntactic environment . . . . . . . . . . . . . . . . . . . . . . . . 40
syntactic keyword . . . . . . . . . . . . . . . . . . . . . . . . 14, 34
syntactic keyword (defn) . . . . . . . . . . . . . . . . . . . . . 13
syntactic keyword binding . . . . . . . . . . . . . . . . . . . 173
syntactic keyword, identifier as. . . . . . . . . . . . . . . . 10
synthetic identifier . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
343
T
table, association (defn) . . . . . . . . . . . . . . . . . . . . . 142
table, one-dimensional (defn) . . . . . . . . . . . . . . . . 141
tail recursion (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
tail recursion, vs. iteration expression . . . . . . . . . 27
taxonomical link, of condition type (defn) . . . . 257
terminal mode, of port . . . . . . . . . . . . . . . . . . . . . . 203
terminal screen, clearing. . . . . . . . . . . . . . . . . . . . . 189
tick . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
time, decoded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
time, file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
time, string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
time, universal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
token, in programs (defn) . . . . . . . . . . . . . . . . . . . . 10
top, bucky bit prefix (defn) . . . . . . . . . . . . . . . . . . . 77
top-level definition . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
top-level definition (defn) . . . . . . . . . . . . . . . . . . . . . 20
top-level environment . . . . . . . . . . . . . . . . . . . . . . . 176
total ordering (defn) . . . . . . . . . . . . . . . . . . . . . . . . 117
transformer environment . . . . . . . . . . . . . . . . . . . . . 41
tree, copying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
trees, balanced binary . . . . . . . . . . . . . . . . . . 153, 156
trimming, of string . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
true, boolean object . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
true, boolean object (defn) . . . . . . . . . . . . . . . . . . 127
true, in conditional expression (defn) . . . . . . . . . . 24
truename, of input file . . . . . . . . . . . . . . . . . . . . . . 233
type predicate, for 1D table . . . . . . . . . . . . . . . . . 142
type predicate, for alist . . . . . . . . . . . . . . . . . . . . . 140
type predicate, for apply hook . . . . . . . . . . . . . . . 170
type predicate, for bit string . . . . . . . . . . . . . . . . . 124
type predicate, for boolean . . . . . . . . . . . . . . . . . . 127
type predicate, for cell . . . . . . . . . . . . . . . . . . . . . . 131
type predicate, for character . . . . . . . . . . . . . . . . . . 79
type predicate, for character set . . . . . . . . . . . . . . . 82
type predicate, for compiled procedure . . . . . . . 166
type predicate, for compound procedure . . . . . . 166
type predicate, for continuation . . . . . . . . . . . . . . 168
type predicate, for empty list . . . . . . . . . . . . . . . . 110
type predicate, for entity . . . . . . . . . . . . . . . . . . . . 171
type predicate, for environment . . . . . . . . . . . . . . 173
type predicate, for EOF object. . . . . . . . . . . . . . . 186
type predicate, for fixnum . . . . . . . . . . . . . . . . . . . . 71
type predicate, for flonum . . . . . . . . . . . . . . . . . . . . 73
type predicate, for hash table . . . . . . . . . . . . . . . . 145
type predicate, for list. . . . . . . . . . . . . . . . . . . . . . . 109
type predicate, for number . . . . . . . . . . . . . . . . . . . 60
type predicate, for pair . . . . . . . . . . . . . . . . . . . . . . 106
type predicate, for pathname . . . . . . . . . . . . . . . . 226
type predicate, for pathname host . . . . . . . . . . . . 229
type predicate, for port . . . . . . . . . . . . . . . . . . . . . 179
type predicate, for primitive procedure . . . . . . . 166
type predicate, for procedure . . . . . . . . . . . . . . . . 165
type predicate, for promise . . . . . . . . . . . . . . . . . . 134
type predicate, for record . . . . . . . . . . . . . . . . . . . . 133
type predicate, for record type . . . . . . . . . . . . . . . 133
type predicate, for stream pair . . . . . . . . . . . . . . . 136
type predicate, for string . . . . . . . . . . . . . . . . . . . . . 88
344
type predicate, for symbol . . . . . . . . . . . . . . . . . . . 129
type predicate, for top-level environment . . . . . 176
type predicate, for vector . . . . . . . . . . . . . . . . . . . . 120
type predicate, for weak pair . . . . . . . . . . . . . . . . 137
type, condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
type, of condition . . . . . . . . . . . . . . . . . . . . . . . . . . . 271
type, of procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 165
type, pathname component . . . . . . . . . . . . . . . . . . 223
types, latent (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
types, manifest (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 3
types, Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
U
unassigned binding . . . . . . . . . . . . . . . . . . . . . . . . . 173
unassigned variable . . . . . . . . . . . . . . . . . . . . . . . . . . 13
unassigned variable (defn) . . . . . . . . . . . . . . . . . . . . . 6
unassigned variable, and assignment . . . . . . . . . . . 22
unassigned variable, and definition . . . . . . . . . . . . 21
unassigned variable, and dynamic bindings. . . . . 19
unassigned variable, and named let . . . . . . . . . . . . 27
unbound variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
unbound variable (defn) . . . . . . . . . . . . . . . . . . . . . . . 6
Unicode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
uninterned symbol (defn) . . . . . . . . . . . . . . . . . . . . 128
universal time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
unparser method (defn) . . . . . . . . . . . . . . . . . . . . . 193
unspecified result (defn) . . . . . . . . . . . . . . . . . . . . . . . 5
up, as pathname component . . . . . . . . . . . . . . . . . 224
uppercase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
uppercase, character conversion . . . . . . . . . . . . . . . 79
uppercase, in string . . . . . . . . . . . . . . . . . . . . . . . . . . 91
usable size, of hash table (defn) . . . . . . . . . . . . . . 147
usage environment . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
V
V as format parameter . . . . . . . . . . . . . . . . . . . . . . 192
valid index, of bit string (defn) . . . . . . . . . . . . . . 123
valid index, of list (defn) . . . . . . . . . . . . . . . . . . . . 110
valid index, of string (defn) . . . . . . . . . . . . . . . . . . . 87
valid index, of subvector (defn) . . . . . . . . . . . . . . 119
valid index, of vector (defn) . . . . . . . . . . . . . . . . . 119
value, of variable (defn) . . . . . . . . . . . . . . . . . . . . . . . 6
values, multiple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
variable binding . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 173
variable binding, do . . . . . . . . . . . . . . . . . . . . . . . . . . 28
variable binding, fluid-let . . . . . . . . . . . . . . . . . . . . . 18
variable binding, internal definition. . . . . . . . . . . . 21
variable binding, lambda . . . . . . . . . . . . . . . . . . . . . 15
MIT Scheme Reference
variable binding, let . . . . . . . . . . . . . . . . . . . . . . . . . . 17
variable binding, let* . . . . . . . . . . . . . . . . . . . . . . . . . 17
variable binding, letrec . . . . . . . . . . . . . . . . . . . . . . . 18
variable binding, top-level definition . . . . . . . . . . . 21
variable reference (defn) . . . . . . . . . . . . . . . . . . . . . . 13
variable, adding to environment . . . . . . . . . . . . . . . 20
variable, assigning values to . . . . . . . . . . . . . . . . . . 22
variable, binding region (defn) . . . . . . . . . . . . . . . . . 8
variable, entry category . . . . . . . . . . . . . . . . . . . . . . . 5
variable, identifier as . . . . . . . . . . . . . . . . . . . . . . . . . 10
vector (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
vector index (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 119
vector length (defn) . . . . . . . . . . . . . . . . . . . . . . . . . 119
vector, byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
vector, converting to list . . . . . . . . . . . . . . . . . . . . 109
version, pathname component . . . . . . . . . . . . . . . 223
virtual coordinates, graphics (defn) . . . . . . . . . . 282
W
warning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
weak pair (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
weak pair, and 1D table . . . . . . . . . . . . . . . . . . . . . 141
weak types (defn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
weakly held keys, of hash table . . . . . . . . . . . . . . 143
weight-balanced binary trees . . . . . . . . . . . . . . . . . 156
whitespace character (defn) . . . . . . . . . . . . . . . . . . . 83
whitespace, in programs (defn). . . . . . . . . . . . . . . . 10
Win32 API names . . . . . . . . . . . . . . . . . . . . . . . . . . 308
Win32 graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292
Windows types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
working directory (see current working directory)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230
X
X display, graphics . . . . . . . . . . . . . . . . . . . . . . . . . .
X geometry string, graphics . . . . . . . . . . . . . . . . .
X graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X resources, graphics. . . . . . . . . . . . . . . . . . . . . . . .
X window system . . . . . . . . . . . . . . . . . . . . . . . . . . .
XML name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XML names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XML output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XML parser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
289
289
288
288
288
217
217
217
217
Z
zero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
i
Short Contents
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Special Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3 Equivalence Predicates . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4 Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5 Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6 Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
7 Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
8 Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
9 Bit Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
10 Miscellaneous Datatypes . . . . . . . . . . . . . . . . . . . . . . . . . 127
11 Associations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
12 Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
13 Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
14 Input/Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
15 Operating-System Interface . . . . . . . . . . . . . . . . . . . . . . . 221
16 Error System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
17 Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
18 Win32 Package Reference . . . . . . . . . . . . . . . . . . . . . . . . 303
GNU Free Documentation License . . . . . . . . . . . . . . . . . . . . . 313
Binding Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
Concept Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
ii
MIT Scheme Reference
iii
Table of Contents
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
1.2
1.3
1.4
2
Notational Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.3 Entry Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Scheme Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1 Variable Bindings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 Environment Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.3 Initial and Current Environments . . . . . . . . . . . . . . . . 7
1.2.4 Static Scoping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.5 True and False . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2.6 External Representations . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2.7 Disjointness of Types . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.8 Storage Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Lexical Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.1 Whitespace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.2 Delimiters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.3 Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.4 Uppercase and Lowercase . . . . . . . . . . . . . . . . . . . . . . 11
1.3.5 Naming Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3.6 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3.7 Additional Notations. . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.1 Literal Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.2 Variable References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.3 Special Form Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.4 Procedure Call Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 14
Special Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
Lambda Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lexical Binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dynamic Binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.1 Top-Level Definitions . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.2 Internal Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Quoting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Iteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
17
18
20
21
21
22
22
24
26
27
iv
MIT Scheme Reference
2.10
2.11
Structure Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.11.1 Binding Constructs for Syntactic Keywords . . . . .
2.11.2 Pattern Language . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.11.3 Syntactic Closures . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.11.3.1 Syntax Terminology . . . . . . . . . . . . . . . . . .
2.11.3.2 Transformer Definition . . . . . . . . . . . . . . .
2.11.3.3 Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.11.4 Explicit Renaming . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
34
35
37
39
40
40
45
47
3
Equivalence Predicates . . . . . . . . . . . . . . . . . . . . 51
4
Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
5
57
58
58
60
60
68
70
70
73
74
Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5.1
5.2
5.3
5.4
5.5
5.6
5.7
6
Numerical types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exactness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Implementation restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Syntax of numerical constants . . . . . . . . . . . . . . . . . . . . . . . . . . .
Numerical operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Numerical input and output. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fixnum and Flonum Operations . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.1 Fixnum Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.2 Flonum Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Random Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
External Representation of Characters . . . . . . . . . . . . . . . . . . .
Comparison of Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Miscellaneous Character Operations . . . . . . . . . . . . . . . . . . . . .
Internal Representation of Characters . . . . . . . . . . . . . . . . . . . .
ISO-8859-1 Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Character Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Unicode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
79
79
80
82
82
84
Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
Construction of Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Selecting String Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Comparison of Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Alphabetic Case in Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Cutting and Pasting Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
Searching Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Matching Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.8.1 Regular-expression procedures . . . . . . . . . . . . . . . . . . 97
6.8.2 REXP abstraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.9 Modification of Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.10 Variable-Length Strings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.11 Byte Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
v
7
Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8
Construction of Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Selecting Vector Components . . . . . . . . . . . . . . . . . . . . . . . . . .
Cutting Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modifying Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
119
120
121
121
Bit Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
9.1
9.2
9.3
9.4
9.5
9.6
10
106
108
109
110
112
113
114
115
117
Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8.1
8.2
8.3
8.4
9
Pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Construction of Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Selecting List Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cutting and Pasting Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Filtering Lists. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Searching Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mapping of Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reduction of Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Miscellaneous List Operations . . . . . . . . . . . . . . . . . . . . . . . . . .
Construction of Bit Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Selecting Bit String Components . . . . . . . . . . . . . . . . . . . . . . .
Cutting and Pasting Bit Strings . . . . . . . . . . . . . . . . . . . . . . . .
Bitwise Operations on Bit Strings . . . . . . . . . . . . . . . . . . . . . .
Modification of Bit Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Integer Conversions of Bit Strings . . . . . . . . . . . . . . . . . . . . . .
123
124
124
125
126
126
Miscellaneous Datatypes . . . . . . . . . . . . . . . . 127
10.1
10.2
10.3
10.4
10.5
10.6
10.7
Booleans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Promises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Weak Pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
127
128
131
132
133
135
137
vi
11
MIT Scheme Reference
Associations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
11.1 Association Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
11.2 1D Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
11.3 The Association Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
11.4 Hash Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
11.4.1 Construction of Hash Tables. . . . . . . . . . . . . . . . . . 143
11.4.2 Basic Hash Table Operations . . . . . . . . . . . . . . . . . 145
11.4.3 Resizing of Hash Tables . . . . . . . . . . . . . . . . . . . . . . 146
11.4.4 Address Hashing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
11.4.5 Low-Level Hash Table Operations. . . . . . . . . . . . . 149
11.5 Object Hashing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
11.6 Red-Black Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
11.7 Weight-Balanced Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
11.7.1 Construction of Weight-Balanced Trees . . . . . . . . 157
11.7.2 Basic Operations on Weight-Balanced Trees . . . 158
11.7.3 Advanced Operations on Weight-Balanced Trees
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
11.7.4 Indexing Operations on Weight-Balanced Trees
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
12
Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
12.1 Procedure Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2 Primitive Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.3 Continuations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.4 Application Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
Environments . . . . . . . . . . . . . . . . . . . . . . . . . . 173
13.1
13.2
13.3
13.4
14
165
166
167
170
Environment Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Environment Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REPL Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Top-level Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
173
175
176
176
Input/Output . . . . . . . . . . . . . . . . . . . . . . . . . . 179
14.1
14.2
14.3
14.4
14.5
14.6
14.7
14.8
14.9
Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
File Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
String Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Input Procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Custom Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Prompting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Port Primitives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.9.1 Port Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.9.2 Constructors and Accessors for Ports . . . . . . . . . .
14.9.3 Input Port Operations . . . . . . . . . . . . . . . . . . . . . . .
14.9.4 Output Port Operations . . . . . . . . . . . . . . . . . . . . .
14.9.5 Blocking Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
179
181
183
185
188
191
193
194
196
197
198
198
200
202
vii
14.10
14.11
14.12
15
203
204
207
209
212
215
217
Operating-System Interface . . . . . . . . . . . . . 221
15.1
15.2
15.3
15.4
15.5
15.6
15.7
15.8
15.9
16
14.9.6 Terminal Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parser Bu↵ers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parser Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.11.1 *Matcher . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.11.2 *Parser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.11.3 Parser-language Macros . . . . . . . . . . . . . . . . . . . . .
XML Parser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pathnames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.1.1 Filenames and Pathnames . . . . . . . . . . . . . . . . . . . .
15.1.2 Components of Pathnames . . . . . . . . . . . . . . . . . . .
15.1.3 Operations on Pathnames . . . . . . . . . . . . . . . . . . . .
15.1.4 Miscellaneous Pathname Procedures . . . . . . . . . .
Working Directory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
File Manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Directory Reader . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Date and Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.5.1 Universal Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.5.2 Decoded Time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.5.3 File Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.5.4 Time-Format Conversion . . . . . . . . . . . . . . . . . . . . .
15.5.5 External Representation of Time. . . . . . . . . . . . . .
Machine Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Subprocesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.7.1 Subprocess Procedures . . . . . . . . . . . . . . . . . . . . . . .
15.7.2 Subprocess Conditions . . . . . . . . . . . . . . . . . . . . . . .
15.7.3 Subprocess Options . . . . . . . . . . . . . . . . . . . . . . . . . .
TCP Sockets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Miscellaneous OS Facilities. . . . . . . . . . . . . . . . . . . . . . . . . . . .
221
222
223
226
229
230
232
238
238
239
239
242
242
245
246
248
248
249
249
252
254
Error System . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
16.1
16.2
16.3
16.4
16.5
16.6
16.7
Condition Signalling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Error Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Condition Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Restarts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.4.1 Establishing Restart Code . . . . . . . . . . . . . . . . . . . .
16.4.2 Invoking Standard Restart Code . . . . . . . . . . . . . .
16.4.3 Finding and Invoking General Restart Code . . .
16.4.4 The Named Restart Abstraction . . . . . . . . . . . . . .
Condition Instances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.5.1 Generating Operations on Conditions . . . . . . . . .
16.5.2 Condition Abstraction . . . . . . . . . . . . . . . . . . . . . . .
16.5.3 Simple Operations on Condition Instances . . . . .
Condition Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Condition-Type Taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . .
258
260
261
263
264
265
267
268
268
269
270
271
271
272
viii
17
MIT Scheme Reference
Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
17.1
17.2
17.3
17.4
17.5
17.6
17.7
17.8
17.9
Opening and Closing of Graphics Devices . . . . . . . . . . . . . .
Coordinates for Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Drawing Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Characteristics of Graphics Output . . . . . . . . . . . . . . . . . . . .
Bu↵ering of Graphics Output . . . . . . . . . . . . . . . . . . . . . . . . .
Clipping of Graphics Output . . . . . . . . . . . . . . . . . . . . . . . . . .
Custom Graphics Operations. . . . . . . . . . . . . . . . . . . . . . . . . .
Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.9.1 X Graphics Type . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.9.2 Utilities for X Graphics . . . . . . . . . . . . . . . . . . . . . .
17.9.3 Custom Operations on X Graphics Devices . . . .
17.10 Win32 Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.10.1 Win32 Graphics Type . . . . . . . . . . . . . . . . . . . . . .
17.10.2 Custom Operations for Win32 Graphics . . . . . .
17.11 OS/2 Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.11.1 OS/2 Graphics Type. . . . . . . . . . . . . . . . . . . . . . . .
17.11.2 Color Operations for OS/2 Graphics . . . . . . . . .
17.11.3 Window Operations for OS/2 Graphics. . . . . . .
17.11.4 Event Operations for OS/2 Graphics . . . . . . . . .
17.11.5 Miscellaneous Operations for OS/2 Graphics . .
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283
284
285
286
286
287
288
288
289
290
292
292
293
295
295
296
297
298
301
Win32 Package Reference . . . . . . . . . . . . . . . 303
18.1
18.2
18.3
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
Foreign Function Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
18.2.1 Windows Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
18.2.2 Windows Foreign Procedures . . . . . . . . . . . . . . . . . 307
18.2.3 Win32 API names and procedures . . . . . . . . . . . . 308
Device Independent Bitmap Utilities . . . . . . . . . . . . . . . . . . . 309
18.3.1 DIB procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
18.3.2 Other parts of the DIB Utilities implementation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
GNU Free Documentation License . . . . . . . . . . . 313
ADDENDUM: How to use this License for your documents . . . . 318
Binding Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
Concept Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
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