Dave Clarke
Andres Löh
Institute of Information and Computing Sciences
Utrecht University, P.O. Box 80.089, 3508 TB Utrecht
The Netherlands
Generic H SKELL exploits the promising new incarnation of generic
programming due to Hinze. Apart from extending the programming
language Haskell, Hinze-style polytypism offers a simple approach to
defining generic functions which are applicable to types of all kinds.
Here we explore a number of simple but significant extensions to Hinze’s
ideas which make generic programming both more expressive and easier
to use. We illustrate our ideas with examples.
Keywords: generic programming, polytypism, program transformations, large bananas, Generic Haskell
Generic or polytypic programming languages provide a medium for
implementing functions whose behaviour is defined inductively on the
structure of types. Many algorithms can be defined generically. Commonly cited examples include mapping, folding, pretty printing, parsing,
and data compression. Once defined, a generic function can be re-used
for any type, even types not yet imagined. The dream of having a
complete programming language with adequate support for generic programming has come closer to fruition over recent years. Artifacts such as
PolyP (Jansson and Jeuring, 1997), G’Caml (Furuse, 2001a), FISh (Jay,
1999), the generic extension for Clean (Alimarine and Plasmeijer, 2001),
and of course, Generic H SKELL (Clarke et al., 2001), which is based
on the work of Hinze (Hinze, 2000c), among others, provide evidence of
this progress. Each is limited to some degree, but we believe that the
proposal which offers the best returns is Generic H SKELL.
Central to Hinze’s proposal is a constraint on the form of generic function’s type, namely that polytypic values possess polykinded types, which
ensures that the cases of a generic function are sufficiently polymorphic
and of a compatible form to be combined together when specialising a
generic function for any Haskell 98 type, including mutually recursive
datatypes and nested datatypes. Each Haskell type is first converted to a
structure type which is defined in terms of a fixed collection of constructors, over which generic functions are defined. Instances of generic functions are then specialised inductively over the target’s structure type,
with type abstraction, type application, and type-level fixed-point being
always interpreted as their value-level counterparts.
A significant advantage of Hinze’s approach, apart from being applicable to all Haskell 98 types, is its simplicity. A programmer need not
possess a strong theoretical background as programming generic functions is relatively straightforward.
For this paper we don a pragmatist’s hat. As we gained experience
with our Generic H SKELL compiler, we have come to realise that the
basic mechanisms do not support everything we wish to do in practice.
Thus we have designed and implemented a number of extensions which
allow not only more reuse of generic functions but also a larger class of
generic functions to be implemented. These extensions, which constitute
the contribution of this paper, are
default cases, with which one generic function can copy the cases
of an existing one, updating or adding cases as required;
constructor cases which allow generic function cases not only for
named types, but also for specific constructors; and
generic abstractions which allow generic functions to be defined by
abstracting a type variable from an expression which may involve
generic functions.
We describe these extensions and their implementation, and give examples to indicate their possible use.
Before explaining the extensions, we first give an overview of generic
programming in Generic H SKELL, including a description of how generic
functions are compiled into plain Haskell.
Classic Generic H SKELL
Generic H SKELL is an extension to the Haskell programming language (Peyton Jones et al., 1999) which offers a means for writing generic
functions. Generic functions, that are also known as polytypic functions (Jansson and Jeuring, 1997), are functions defined over the structure of types. Here we give a short introduction to the style of generic
Generic Haskell, Specifically
programming employed in Generic H SKELL, before giving an overview
of the implementation.
Generic functions
The most general way of declaring datatypes uses the data keyword
as in the following example:
data Tree a = Leaf
| IntLeaf Int
| Node{ref :: a, left :: Tree a, right :: Tree a } .
The left-hand side of a datatype specifies a new type name, along with
the type’s parameters. Thus this definition defines the parametric type
Tree a with parameter a. The right-hand side of a datatype may have
multiple alternatives, separated by a vertical bar. Each consists of a
constructor and a number of fields that may be labelled. The fields may
have any type.
Haskell datatypes are thus sums of products, where the arity of the
sum or product is unbounded. This makes the datatypes difficult to process generically, as witnessed in the approaches of Jay, Bellè and Moggi
(Jay et al., 1998). Hinze, however, adopts a simpler model, dealing only
with binary sums and products (Hinze, 2000b). Each Haskell datatype
can be realised in this form by firstly replacing the alternatives with the
binary sum operator :+:. In a similar way the list of fields for a constructor is replaced by a nested tuple constructed using the binary product
operator :*:, with the special type Unit being used when there are no
fields. The constructors and field names are then replaced by special
type constructors which mark the positions where they occurred in the
original type. Explicitly, the following set of type constructors is used
to construct an isomorphic type that exhibits the top-level structure of
a type:1
data a :+: b = Inl a | Inr b
data a :*: b = a :*: b
data Unit
= Unit
data Con a = Con a
data Lab a
= Lab a
1 Haskell
98 does not allow infix type constructors. To simplify the notation, we use them
here anyway, and further assume that both :+: and :*: are right associative. In the actual
implementation, the following types are used instead:
data Sum a b
data Prod a b
Inl a | Inr b
a :*: b
This type is called the structure type for the Haskell datatype.
For example, the type Tree has structure type:
type Tree ◦ a
= Con Unit
:+: Con Int
:+: Con (Lab a :*: Lab (Tree a) :*: Lab (Tree a)) .
Notice that the recursive instances of Tree a are left intact. This is
because the specialisation of generic functions follows only the top-level
structure of a type, relying on recursion when processing further instances of Tree a. Base types, such as Int in our example, are unaffected
by the translation: they appear in the structure type in the same way
as in the original type.
We can write generic functions which can be specialised to all types
by giving cases for the structure type constructors (and primitive types)
only. An example generic definition is
gmaph:+:i gmapA gmapB
gmaph:+:i gmapA gmapB
gmaph:*:i gmapA gmapB
gmaphCon ci gmapA
gmaphLab l i gmapA
(Inl a)
(Inr b)
(a :*: b)
(Con a)
(Lab a)
Inl (gmapA a)
Inr (gmapB b)
(gmapA a) :*:(gmapB b)
Con (gmapA a)
Lab (gmapA a) .
This reimplements the function map, which is predefined for lists in the
Haskell prelude, generically for all datatypes. The cases for Con and
Lab take an extra argument (c and l in the example) that is bound to
a descriptor of the constructor or label in question. These descriptors
can be used to query information about constructors and labels, such as
their names.
If a generic function is to be applicable to types containing primitive
types such as Int or Char or abstract types such as IO, then cases for
these types must be added to the function definition. The case for Int,
which is needed for gmap to work on our example Tree datatype, is given
gmaphInti i = i .
Whenever a generic function is used on a type, that type is implicitly
viewed as the corresponding structure type. Thus, in the call
gmaphTreei (+1) :: (a → b) → Tree a → Tree b ,
which could be used to increase all values in an integer tree by one, the
instance gmaphTreei is really defined over Tree ◦ . The precise details
Generic Haskell, Specifically
of the underlying specialisation process are deferred until Section 2.3.3.
Note that above call will only increase integers in the ref positions of
the nodes of a tree. The integers appearing in the leaves will be treated
according to the case for Int, thus left unaltered.
Different cases in the definition of gmap have different types. All but
the case for Unit require additional arguments, equal in number to the
number of arguments of the type constructor. This is due to the fact
that Generic H SKELL implements so-called MPC-style generic functions
(Hinze, 2000c) which allows generic functions to be used with types of
arbitrary kind. A consequence is that generic definitions possess kindindexed types. So the complete type information for gmap is given by
the following recurrence:
type GMaphh?ii s t
type GMaphhκ → νii s t
gmapht :: κi
= s→t
= ∀a b . GMaphhκii a b
→ GMaphhνii (s a) (t b)
:: GMaphhκii t t .
Here a kind-indexed type GMap is defined inductively over the structure
of kinds. The first line is for type arguments of kind ?. The second line
extends the type to arguments of higher kind. Its form is a consequence
of the theory explained in Hinze’s paper. The kind-indexed type GMap
is then assigned to the generic function gmap.
The type required for each of the cases in the definition of gmap
can be determined by applying GMaphhκii t t to the type and kind of
the particular case. Observing that Unit :: ?, Con, Lab :: ? → ?, and
:+:, :*: :: ? → ? → ?, we obtain:
Unit → Unit
(a → c) → (b → d ) → a :+: b → c :+: d
(a → c) → (b → d ) → a :*: b → c :*: d
(a → b) → Con a → Con b
(a → b) → Lab a → Lab b .
The same kind-indexed type, GMaphhκii t t, determines the types of
instances of the generic functions. For our example type, Tree :: ? → ?,
we expect
gmaphTreei :: (a → b) → Tree a → Tree b .
Notice that for a functor of one argument, such as Tree, the instance
of gmap requires one function argument for transforming the values of
type a into b. For a two argument functor, such as :+: above, two
function arguments are required, one for each type parameter.
A generic function can be written using cases for arbitrary named
types, i.e., for simple types or type constructors of any kind. Generic
definitions therefore need not have cases for all the type constructors
which form the structure types, nor need they be limited to just those.
If a case for a different type is present in a generic definition, then this
overrides the default behaviour. Apart from giving a distinct behaviour
for a specific type, such lines can also be employed to provide a more efficient implementation for a particular type constructor. For instance, the
gmap function could be made to use the (probably optimised) predefined
map function on lists by adding the line
gmaph[ ]i gmapA xs = map gmapA xs .
Generic types
In addition to generic functions, Generic H SKELL also supports types
that are defined over the structure of datatypes. Similar in form to
generic functions, a generic type definition consists of multiple cases
for different named types, each resembling a Haskell type or newtype
declaration. Generic types, also called type-indexed types, are described
in depth in (Hinze et al., 2002), with several examples of their use, and
are therefore not the focus of this paper.
Inside the Generic H SKELL compiler
Generic H SKELL compiles modules written in an enriched Haskell
syntax. A Generic H SKELL module may contain, in addition to regular
Haskell code, definitions of generic functions, kind-indexed types, and
type-indexed types, as well as applications of these to types or kinds.
The compiler translates a Generic H SKELL module into an ordinary
Haskell module by performing a number of tasks:
translating generic definitions into Haskell code;
translating calls to generic functions into an appropriate Haskell
expression; and
specialising generic entities to the types at which they are applied
(Consequently no type information is passed around at run-time).
In addition, the compiler generates structure types for all datatypes,
together with functions that allow conversion between a datatype and
its structure type.
Generic H SKELL programs may consist of multiple modules. Generic
functions defined in one module may be imported into and reused in
Generic Haskell, Specifically
other modules. Generic H SKELL comes with a library that provides a
collection of common generic functions, among which are those usually
generated by means of Haskell’s deriving mechanism.
Translation of Generic Functions. In general, a generic
function definition consists of a type signature referring to a kind-indexed
type, and a collection of cases indexed by named types.
Each of the cases is translated into one ordinary Haskell function definition. The generated function is given a name which depends upon the
generic function’s name and the type name for which it is defined.2 The
special argument for a Con or Lab case is turned into an extra argument
to the generated Haskell function, of type ConDescr or LabDescr , respectively. Both ConDescr and LabDescr are abstract types, for which
Generic H SKELL provides a collection of functions to query information
such as the name of the constructor or label. As an example, the :+:
and the Con cases of the gmap function are translated to
Sum :: (a → c) → (b → d ) → a :+: b → c :+: d
Sum gmapA gmapB (Inl a) = Inl (gmapA a)
Sum gmapA gmapB (Inr b) = Inr (gmapB b)
Con :: ConDescr → (a → b) → Con a → Con b
Con c gmapA
(Con a) = Con (gmapA a) .
The type signatures are generated internally by expanding the function’s
kind-indexed type.
Calls to Generic Functions.
poly in the source code takes the form
A call to a generic function
polyhT i
where T is a type expression that can be either a named type or an
application of one type expression to another.
One of the fundamental ideas of MPC-style generic definitions is to
interpret type application as value application. In other words, for type
expressions F :: κ → ν and A :: κ, the equation
polyhF Ai = polyhF i (polyhAi)
holds. Knowing this fact, calls to generic functions can always be rewritten to an application expression containing several calls to the same
2 The
names assigned by the current implementation are additionally prefixed to prevent
name clashes and differ from the names used in this paper.
generic function having only named types as arguments. For example,
the call
gmaphEither [String ]i
is rewritten as
gmaphEither i (gmaph[ ]i (gmaphStringi)) .
Calls that refer to named types are replaced by a call to the appropriate
specialised function, such as gmap Either .
Specialisation of Generic Functions.
When a generic
function is specialised to a named type, the compiler first checks whether
the function in question has a case defined for that particular named
type. If one is present, then it is used. The presence of a special case
for a user-defined datatype thus overrides the standard behaviour of
specialisation. In the absence of a special case, the compiler proceeds in
two steps:
the generic function is specialised to the named type’s structure
type; and
the resulting specialised function is converted using a generic wrapper to a specialisation for the original named type.
Recall that a structure type is defined as a type synonym. Now a call
to a generic function on a structure type can be viewed as a call on the
right-hand side of that synonym. Hence, the specialisation of the generic
function is again reduced to calls to named types as described in Section
For example, specialising gmap to Tree ◦ results in
gmap Tree ◦
:: (a → b) → Tree ◦ a → Tree ◦ b
gmap Tree a =
gmap Con a
‘gmap :+:‘
gmap Con (gmap Lab a ‘gmap :*:‘ gmap Lab (gmap Tree a)
‘gmap :*:‘ gmap Lab (gmap Tree a)) .
The type specialisation process is guaranteed to terminate, since structure types are only generated for the top-level of a type and the number
of named types in a program is finite.
These specialisations work for a structure type rather than the original
type. Fortunately, the isomorphisms mapping between a type and its
Generic Haskell, Specifically
structure type are straightforward to generate. For our example, the
bimap Tree :: (Tree ◦ a → Tree ◦ b) → (Tree a → Tree b)
is firstly generated as a lifted isomorphism, then gmap Tree, which
corresponds to gmaphTreei, becomes
gmap Tree
:: (a → b) → Tree a → Tree b
gmap Tree a = bimap Expr (gmap Tree ◦ a) .
These isomorphisms can be lifted to arbitrary types generically, and
then used to wrap the function generated for structure types to convert
it into a function for the original type. This technique is described in
detail in (de Wit, 2002) and (Hinze, 2000a).
Contemporary Generic H SKELL
In the previous section we described the basic features of Generic
H SKELL, which are to a high degree based on work published by
Hinze (Hinze, 2000c; Hinze, 2000a). Recently a number of extensions
for Generic H SKELL have been designed. These extensions, which have
been implemented in the current version of our compiler, constitute the
contribution of this paper and are described in this section.
The first, default cases, is a method to ease the programming of similar generic functions. A new generic function may be based on a previous
one, inheriting the definitions for all cases that are not explicitly overridden.
Secondly, constructor cases allow generic function cases to be defined
not only for named types, but also for particular constructors. This, at
the first glance, may seem not particularly generic, but it happens to
be useful in situations similar to those which require special cases to be
defined for particular types, for example, when defining a traversal over
a datatype with a large number of constructors which requires special
action only at certain constructors.
Furthermore, we introduce generic abstractions, which allow generic
functions to be defined by abstracting a type variable out of an expression which may involve generic functions.
All these extensions work well together, allowing some applications
that are not among the typical generic programming examples. Along
with the extensions, we provide demonstrations of their possible use,
and a larger example showing their interaction.
Default cases
Many generic functions follow some commonly occurring pattern for
most type constructors. Two such examples are map-like and crushlike functions (also called type-preserving and type-unifying traversals
(Lämmel et al., 2000)) which, respectively, perform a computation during the traversal of a data structure while preserving the type, and collect
information from the data structure into a value of a new type. Such
functions exhibit certain general behaviour which differs for just a few
types or constructors. Rather than duplicate the bulk of such definitions,
Generic H SKELL introduces default cases so that the mechanics of the
traversal can be written as one generic function, and then copied with
modification or extension into other generic functions. Default cases are
best described by example.
The following code forms the basis of a crush-like function which collects a list of values of type a from some datatype. As it currently is,
this function can only return the empty list, but it provides the basic
traversal which can be used in other functions.
type Collecthh?ii t
type Collecthhκ → νii t
= t → [a ]
= ∀u . Collecthhκii u
→ Collecthhνii (t u)
collectht :: κi
:: Collecthhκii t
= []
collecth:+:i mA mB (Inl a)
= mA a
collecth:+:i mA mB (Inr b)
= mB b
collecth:*:i mA mB (a :*: b) = mA a +
+ mB b
collecthCon ci m
(Con b) = m b
collecthChar i
= []
Now suppose we are working with the following datatypes and wish to
define a function which collects values of type Var , treating them as a
set rather than simply concatenating them together into a list.
data Var =
deriving Eq
data Type =
data Expr =
Type Type
Expr Expr
(Var , Type) Expr
(Var , Type) Expr Expr
Generic Haskell, Specifically
We can do this by adapting the function collect. First we need a more
specific type:
type VarCollecthh?ii t
type VarCollecthhκ → νii t
= t → [Var ]
= ∀u . VarCollecthhκii u
→ VarCollecthhνii (t u) .
We can now write the desired function in a few lines (the order of the
cases is irrelevant):
varcollectht :: κi
:: VarCollecthhκii t
varcollecthVar i v
= [v ]
varcollecth:*:i mA mB (a :*: b) = mA a ∪ mB b
= collecthci .
The line varcollecthci = collecthci is the default case, which has the effect
of copying the code from collect into the new generic function varcollect.
The line for varcollecthVar i specifies the desired additional functionality
for type Var . The line for varcollecth:*:i overrides the functionality
for :*:, using union instead of concatenation when accumulating the
Compilation is simple. For the function varcollect defined above the
compiler internally generates the following definition, and then compiles
this code as usual:
varcollectht :: κi
varcollecthVar i v
varcollecth:*:i mA mB (a :*: b)
varcollecthCon ci
varcollecthChar i
VarCollecthhκii t
[v ]
mA a ∪ mB b
collecthCon ci
collecthChar i .
The compiler keeps track of which named types the original generic
function is defined for, and updates and extends this collection depending on the cases specified in the new generic function.
We can also reuse cases from the existing definition in the new generic
definition, even cases which are overridden. The following function
collects just term variables from our expressions, using the function
termvar :: Var → Bool to determine whether a variable comes from
the syntactic category of term variables:
termcollectht :: κi
termcollecthVar i v
:: VarCollecthhκii t
= varcollecthci
= if termvar v then varcollecthVar i v else [ ] .
It is very important that the new function defined by means of the
default case only retains references to the original function in the righthand side of the cases, and that the specialisation mechanism proceeds
using solely the new function.
As an example, look at the definition of varcollect again. Because
there is no new case for :+:, the compiler will infer
varcollecth:+:i = collecth:+:i .
It would appear that varcollect, once called for the :+: case, will work
precisely as collect. But this is not true! The specialisation mechanism
will replace varcollecthA :+: B i as follows:
varcollecthA :+: B i = varcollecth:+:i varcollecthAi varcollecthB i
= collecth:+:i varcollecthAi varcollecthB i .
Thus the arguments to the :+: case will remain calls to varcollect.
As a consequence, varcollect will collect variables in a data structure
deeply, i.e. in a large system of mutually recursive data types all occurrences of the type Var will be found.
In the next section we modify varcollect to collect only the free occurrences of variables.
Constructor cases
The problem of dealing with large bananas is to write map-like or
crush-like functions for datatypes with a large number of constructors,
such as those which model the syntax of a real programming language
(Lämmel et al., 2000). The anticipated behaviour of such functions for
most constructors of a datatype is standard and can be coded using existing generic programming techniques. For certain constructors, however,
something different may be required, as when collecting free variables,
for example, which requires the variable scoping to be considered for
constructors such as Let and Lambda which denote binding constructs.
To do this using the machinery described thus far would require a
case for the type we are interested in, namely, Expr . But then we would
have to write the details of the traversal for each constructor of this
type – both the interesting cases and the plumbing – gaining nothing,
especially when there are a large number of constructors of which only
a few require special treatment. Thus the machinery described so far
cannot deal effectively with this problem.
To address this problem, Generic H SKELL now allows cases for specific constructors to be written. Using these constructor cases a generic
function can have special cases to deal with the constructors requiring
special treatment. Again we illustrate with examples.
Generic Haskell, Specifically
Our first example uses a copy line to extend the function for collecting
term variables with a constructor case for both the Let and Lambda
constructors of the Expr datatype to take account of variable scoping:
freecollectht :: κi :: VarCollecthhκii t
= termcollecthci
freecollecth case Lambda i (Lambda (v , t) e)
= filter (6 v ) (freecollecthExpr i e)
freecollecth case Let i (Let (v , t) e e 0 )
= freecollecthExpr i e
+ filter (6 v ) (freecollecthExpr i e 0 ) .
The case freecollecth case Lambda i, for example, is a constructor case.
This case will only be applied when the value of type Expr which is
actually encountered has the form Lambda (v , t) e. The case has been
written to exploit this knowledge. The case for Let is similar. Interestingly, when a constructor case produces a value, it need not produce
a value with the same constructor, but only of the correct type. This
comes in handy in our second example.
Specific constructor cases of a generic function can be called using
the syntax freecollecth case Lambda i. We expect that this will be most
useful when reusing existing code while overriding a constructor case.
As another example we give the following mini-optimiser for lambda
expressions, following in part (Lämmel et al., 2000). The function gmap
has the basic form of a map-like function. This can be extended with a
number of constructor cases to produce the following map-like function
which performs some minor optimisations on lambda expressions:
optimiseht :: κi
:: GMaphhκii t t
= gmaphci
optimiseh case Let i (Let (v , t) e e 0 ) =
optimisehExpr i $ (App (Lambda (v , t) (optimisehExpr i e 0 )) e)
optimiseh case App i (App e e 0 )
let oe = optimisehExpr i e in
let oe 0 = optimisehExpr i e 0 in
case oe of (Lambda (x , t) b) →
if not (elem x (freecollecthExpr i b))
then b
else App oe oe 0
This code performs a bottom-up traversal of an expression, converting
let expressions into an application of a lambda abstraction, and optimising applications of a function whose argument does not occur in the
body. One point of interest is that the Let case produces a constructor
different from Let.
The type of a constructor case is the same as the case for
the type from which the constructor comes. For example, the type of
optimisehExpr i is GMaphh?ii Expr Expr , that is, Expr → Expr . Thus
the type of optimiseh case Lambda i must also be Expr → Expr . As
described above, there is the added restriction that any data which this
function accepts must have the given constructor, otherwise pattern
matching will fail at run-time. This cannot be enforced in the current
system. A type system with constructor subtyping (Barthe and Frade,
1999) could help here.
Constructor cases are implemented using a slight
modification of the structure type encoding and specialisation process
introduced in Section 2. We demonstrate the scheme on our example
Expr datatype. For each constructor, a type synonym is introduced,
identifying the type of the constructor case with the type it is constructing:
type ConCase Var
= Expr
type ConCase App
= Expr
type ConCase Lambda = Expr
type ConCase Let
= Expr .
Whereas the old translation transformed the sum and the product structure of the datatype in one go, the modified translation proceeds in two
steps. Firstly, only the sum structure is revealed, making use of the new
type synonyms for the constructor cases:
type Expr ◦
Let .
Secondly, the types for the constructor cases are themselves given structure types reflecting the product structure of the constituent fields:
type ConCase
type ConCase
type ConCase
type ConCase
Var ◦
App ◦
Lambda ◦
Let ◦
Expr :*: Expr
(Var , Type) :*: Expr
(Var , Type) :*: Expr :*: Expr .
The mappings to convert between a type and its structure type must
also be adapted – in fact, simplified – to reflect the modified shape of
Generic Haskell, Specifically
the structure type. In addition, we now need a mapping for each of the
constructors to convert between the constructor type synonym and its
structure type.
Let us look at the first part more closely: for the Expr datatype, the
mappings between Expr and Expr ◦ could naı̈vely be implemented by the
following functions from and to:
e@(Var )
) =
Expr → Expr ◦
Inl (Con e)
Inr (Inl (Con e))
Inr (Inr (Inl (Con e)))
Inr (Inr (Inr (Con e)))
(Inl (Con e@(Var )))
(Inr (Inl (Con e@(App
(Inr (Inr (Inl (Con e@(Lambda
))))) =
(Inr (Inr (Inr (Con e@(Let
Expr ◦ → Expr
The sum structure is created and removed on top of the type, the value
remains completely intact. But when a generic function changes a constructor, as in optimise above, we get a run-time pattern matching error in the to function, because the sum structure encountered does not
match the constructor that is present in the value.
In the current system, we cannot enforce that the argument and result
values have the same outermost constructor. On the other hand, the
constructor rewrite functionality proves to be useful in many places, as
the optimise example shows. Therefore, we relax the implementation of
to in order to ignore the sum structure that is present, remove it and
recover the value stored inside.
(Inl (Con e))
(Inr (Inl (Con e)))
(Inr (Inr (Inl (Con e))))
(Inr (Inr (Inr (Con e))))
With these preparations made, specialisation can now be handled exactly
as described in Section 2.3.3, where constructors are treated as if they
were additional types.
If a function is specialised for type Expr , then a specialisation for
Expr ◦ will be generated together with a wrapper function that lifts the
isomorphism between Expr and Expr ◦ to the specialisation’s type. But,
the specialisation Expr ◦ will now refer to the specialisations of the same
function for all four constructor type synonyms. Thus, specialisations
for the four constructors must also be generated.
A constructor case in the generic function definition now plays the
role of a special case for an ordinary named type: a constructor case
takes the place of the specialisation, thereby overriding the default behaviour. Otherwise, the specialisation is generated in the usual way for
the structure type of the constructor, together with another wrapper
In the function definition of optimise, there are two constructor cases
present, one for both Lambda and Let. Thus, when optimise is specialised to Expr , the functions
optimise Expr ◦ , optimise Expr
(for the type Expr ) ,
optimise ConCase Var ◦ , optimise ConCase Var
(for the constructor Var ) ,
optimise ConCase App ◦ , optimise ConCase App
(for the constructor App)
are generated in the usual manner, whereas optimise ConCase Lambda
and optimise ConCase Let will be taken directly from the definition of
the two constructor cases.
Generic Abstraction
In addition to copy lines, Generic H SKELL offers another possibility
to define generic functions in terms of other generic functions, namely
using generic abstractions.
In a full generic function definition, one is often forced to be more
general than one intends to be. For instance, it is impossible to write
a generic function that does not have function type when applied to
a type of kind ? → ?. This is because the specialisation mechanism
interprets abstraction and application on the type level as abstraction
and application on the value level, and thus forces generic function types
to follow a specific pattern.
Generic abstraction lifts all restrictions that are normally imposed on
the type of a generic function. With generic abstraction we can define
a function which takes a single type argument which can be used generically within the body of the function. The abstracted type parameter
is, however, restricted to types of a fixed kind. The resulting function
can be then applied to any type having the appropriate kind. Generic
abstractions can be used to write variations, simplifications and special
cases of other generic functions.
Generic Haskell, Specifically
The syntax of generic abstractions is similar to ordinary generic definitions, with two important differences:
the type signature is restricted to a fixed kind, and thus no kind
variable is introduced; and
there is exactly one case, which has a type variable as its type
argument, rather than being indexed by constant named types.
Suppose now we want a function generic in types f ::? → ? which takes
a data structure containing monadic actions and performs the monadic
action stored at each point in the data structure. This function, which we
call thread , is a generalisation of the Haskell prelude function sequence,
which has type Monad m ⇒ [m a ] → m [b ], from lists to arbitrary type
constructors. It is defined using generic abstraction as follows:
thread hf :: ? → ?i :: (Functor m, Monad m) ⇒ f (m a) → m (f a)
thread hf i
= mapMl hf i id
This declares a function called thread which has type parameter f of kind
? → ?. The right-hand side of this definition uses this type parameter
when applying the generic function mapMl , which is a monadic map
function from the Generic H SKELL library. We reproduce mapMl here
for completeness:
type MapM hh?ii t1 t2 m
= t1 → m t 2
type MapM hhκ → νii t1 t2 m
∀u1 u2 . MapM hhκii u1 u2 m → MapM hhνii (t1 u1 ) (t2 u2 ) m
mapMl ht :: κi :: (Functor m, Monad m) ⇒ MapM hhκii t t m
mapMl hUniti
= return Unit
mapMl h:+:i mA mB (Inl a)
= do {a2 ← mA a; return (Inl a2 )}
mapMl h:+:i mA mB (Inr b)
= do {b2 ← mB b; return (Inr b2 )}
mapMl h:*:i mA mB (a :*: b) = do {a2 ← mA a; b2 ← mB b;
return (a2 :*: b2 )}
mapMl hCon ci mA (Con a) = do {a2 ← mA a;
return (Con a2 )} .
Generic functions can be used to – at least partially – remove the need
for type classes. Generic abstractions then help redefining functions that
previously required a class constraint. For instance, the Eq and Ord type
classes can be replaced by generic equality and ordering functions that
can be applied to all types. Similarly, Functor can be replaced by the
gmap function. Now, there are quite a number of useful functions that
are usually implemented as ad-hoc polymorphic functions referring to
one of these classes. An example from the Haskell prelude is
lookup :: (Eq a) ⇒ a → [(a, b)] → Maybe b
Written using generic abstraction, this function can be implemented to
make use of the generic equality function equal :
glookupha :: ?i
:: a → [(a, b)] → Maybe b
glookuphai key [ ]
= Nothing
glookuphai key ((x , y) : xys)
| equal hai key x
= Just y
| otherwise
= glookuphai key xys .
Compilation. We cannot naı̈vely compile a generic function defined
via generic abstraction using the techniques described in Section 2.3.3.
A specialisation to a type expression cannot always be reduced to a
set of specialisations on named types only. In particular, assume that
F :: ? → ? → ? and A :: ?. We cannot apply the simplification rule
thread hF Ai = thread hF i thread hAi ,
since we have cases for neither ? → ? → ? nor ? in the definition of
thread . Instead we produce a new function to denote thread hF Ai, and
then, unravelling the definition
thread hF Ai = mapMl hF Ai id ,
we set the new function’s right-hand side to be mapMl hF Ai id .
Recall that right-hand sides of generic abstractions may contain calls
to other generic abstractions, not only to regular generic functions. Furthermore, the type arguments of the calls on the right-hand side are not
necessarily simpler than the type-argument on the left-hand side. Thus
the translation mechanism for generic abstractions is not guaranteed to
terminate. This does not seem a big problem in practice, though, because we do not yet know of any set of useful generic functions that call
themselves recursively with type arguments of increasing complexity. In
the current implementation, the number of iterations performed to determine the specialisations needed can be bounded. It remains to be
investigated whether it would be more practical to impose restrictions
on the right-hand sides of generic abstractions so that termination could
be guaranteed.
A larger example
As the final part of this section, we present a larger example consisting of multiple generic functions that interact and make use of all the
features presented in the preceeding subsections.
Generic Haskell, Specifically
Consider the problem of comparing two expressions up to alphaequivalence: two expressions are equal if they can be transformed into
each other only by renaming variables bound in let or lambda expressions.
One way of solving this problem is to introduce a normalisation step
and then use standard equality on the normalised values. We choose
to transform expressions to use de Bruijn indices for their bound variables: instead of a name, each bound variable is replaced by a number
that indicates how many binding constructs are between the binding
occurrence and the variable.
Recall the datatypes used before: variables occurring in expressions
are of type
data Var = V String deriving Eq .
We could now extend this type to allow both free variables (strings) and
bound variables (numbers) to be represented. However, for simplicity we
will keep the original type and simply replace bound variables with string
representations of numbers. In the binding occurrences the variables are
superfluous after the numbering, hence there is no longer any need to
store the name. We will model this by storing the empty string. As we
plan to do no more with the numbered expression than compare them,
this simplified representation satisfies our requirements.
The function computing the de Bruijn numbering from an expression
is derived from the monadic map and uses a simple state monad to keep
track of the list of currently known bound variables. The position of a
variable in the list indicates its nesting level.
We assume the following interface to the abstract state monad type:
data State s a
-- abstract
instance Monad (State s)
instance Functor (State s)
readstate :: State s s
writestate :: s → State s ()
:: State s a → s → a .
We now implement deBruijn by extending mapMl using a default case.
We implement only three additional cases. For a variable, check whether
it is currently bound. If it is, replace by the correct index. For lambda
and let expressions, the newly bound variable is added to the state before
proceeding to the body of the construct, and removed again afterwards.
deBruijnht :: κi
:: MapM hhκii t t (State [String ])
= mapMl hti
deBruijnhVar i (V a) = do s ← readstate
case findIndex ( a) s of
Nothing → return $ V a
Just n → return $ V $ show (n + 1)
deBruijnh case Lambda i (Lambda (V a, t) e)
= do e 0 ← boundinhExpr i a e
return (Lambda (V "", t) e 0 )
deBruijnh case Let i (Let (V a, t) e1 e2 )
= do e1 0 ← deBruijnhExpr i
e2 0 ← boundinhExpr i a e2
return (Let (V "", t) e1 0 e2 0 ) .
The helper function boundin is implemented using generic abstraction,
although we only use it with type Expr here. It calls deBruijn, first
temporarily adding a variable name to the state.
boundinha :: ?i
boundinhai var body
:: String → a → State [String ] a
= do s ← readstate
writestate (a : s)
body 0 ← deBruijnhai body
writestate s
return body 0 .
Note that even though we explicitly refer to Var as well as the constructors Lambda and Let of Expr , this is still a generic function. Programming this way has its advantages: not only is the code less sensitive to
future extensions or modifications in the expression datatype, we also
get the same function for free working on types containing values of type
Expr , from simple lists or trees of expressions to “real” extensions such
as statements or function declarations.
Once again making use of generic abstraction, we hide the state monad
by supplying an initial empty list of bound variables.
normha :: ?i :: a → a
normhai x
= runstate (deBruijnhai x ) [ ] .
Finally, we implement the equality up to alpha-conversion using normal
generic equality:
alphaequal ha :: ?i
:: a → a → Bool
alphaequal hai x1 x2 = equal hai (normhai x1 ) (normhai x2 ) .
Generic Haskell, Specifically
Alternatively, we could implement alphaequal using a default case, extending the original equality function, using the relaxed notion of equality only for occurrences of the Expr datatype:
alphaequal ht :: κi :: Equal h?i t t
alphaequal hti
= equal hti
alphaequal hExpr i = equal hExpr i
(normhExpr i x1 ) (normhExpr i x2 ) .
Limitations and Future Work
Although the enhancements described in this paper seem to make
Generic H SKELL suitable for practical programming, room for improvement still remains. In this section we pinpoint a few problems that will
be subject of further investigation and research.
(Mutually) recursive generic functions and types.
a generic function makes use of one or more other generic functions. We
have seen examples in the form of generic abstractions, but what about a
full-fledged generic function with multiple cases which refers to another
generic using a variable type argument? Consider a generic function
that collects information from a value of a generic type, which descends
further into the branch of a product case only when a certain condition
testcollecth:*:i tcA tcB (a :*: b) =
let rA = if . . . then tcA a else [ ]
rB = if . . . then tcB b else [ ]
in rA +
+ rB .
We want the test, indicated by the ellipses, to depend generically on
the types of a and of b. But since we cannot access these types on the
right-hand side, we cannot include a call to the generic function that
performs the test.
This limitation stems from the fact that we are using MPC-style, where
generic function definitions are seen as catamorphisms: the recursive
calls of the function to the children are given as parameters, but neither the types of the children itself nor the calls to other functions are
available on the right-hand side.
There is a solution: we can tuple different functions into a single
bundle and define them together. Instead of getting the recursive calls
for just one function, we then get a tuple containing all the required
functions. Although a tenable solution, this approach is difficult to use
by hand: the programs written in this style are confusing and error-
prone. For example, applying tupling to our fragment results in:
testcollecth:*:i (tA, cA) (tB , cB ) (a :*: b) =
(testh:*:i tA tB
, let rA = if tA a then cA a else [ ]
rB = if tB a then cB b else [ ]
in rA +
+ rB
Another style of generic function definitions, also introduced by Hinze
(Hinze, 2000b), removes the problem by making the type arguments
of the named types in the function cases explicit. Thus, there is the
possibility to syntactically refer to other functions. However, these socalled POPL-style function definitions bear a number of limitations that
MPC definitions do not share, most notably the restriction to types of
a single kind and the requirement that a different set of structure type
constructors is used for each different kind.
We are working on a combination that allows a programming syntax
similar to POPL-style, but uses MPC-style functions behind the scenes,
thereby allowing the interaction of multiple generic functions without
losing the generality offered by the current implementation.
Type inference.
The usage of generic functions is less convenient
than one could expect because it always requires the type arguments to
be explicitly specified. This restriction could be lifted using some form
of type inference. Another solution is to integrate generic functions with
type classes as done in the generic extensions to the Glasgow Haskell
Compiler (Hinze and Peyton Jones, 2001) or to Clean (Alimarine and
Plasmeijer, 2001).
As long as we limit generic functions to specific kinds, the problem of
assigning types is closely related to the type inference and checking of
type classes and is therefore quite well understood. However, for MPCstyle generic functions in their full generality, type inference becomes
more complicated, involving functions applied at multiple kinds, having
types of different shapes, and thus requires further research.
In many situations, it would be desirable to quantify
over a group of types. In the gmap function, for example, it would be
more concise to be able to write that gmaphT i = id , for all types T :: ?.
Instead, though, one has to write a separate line for each base type,
and when a new abstract type is added, the function definition must be
Generic Haskell, Specifically
One possible approach to solving this problem might be to use the
Haskell type class system. One could allow cases of generic functions to
be specified for all members of a class. The current implementation does
not keep track of classes and instances though.
Related Work
The field of generic programming has grown considerably in recent
years. Much of this work stems from so-called theories of datatypes, for
example (Bird et al., 1996), which are often based on category theoretic
notions such as initial algebras. These approaches focus on generic control constructs, such as folds, which are derived from datatypes. A good
overview is available (Backhouse et al., 1999).
Other approaches more or less define functions inductively on the
structure of types. The work of Hinze, upon which Generic H SKELL
is based, is just one variant. Chen and Appel, for instance, describe
an implementation of polytypic polymorphism using dictionary passing
(Chen and Appel, 2001). They do not, however, give any indication of
how to code polytypic functions, working instead with built-in polytypic
functions such as pretty printing and equality test. Weirich and others (Weirich, 2002) (and the earlier work on intensional polymorphism
(Crary et al., 1998), for example) employ a typecase construct which
performs run-time tests on types to implement polytypic functions of
a expressiveness similar to Hinze’s. Ruehr’s structural polymorphism
(Ruehr, 1992) adopts similar type tests. Generic H SKELL distinguishes
itself from these approaches as it does not require type interpretation at
run-time. In Functorial ML the algorithm for map, for example, is defined using combinators to find the data upon which argument function
will apply (Jay et al., 1998). While supporting type inference, Functorial ML programming is at a rather low level and lacks the simplicity of
Hinze’s approach.
The basic theoretical work has manifest itself in several programming
languages and language extensions. Functorial ML and other work on
shape theory has resulted in the programming language FISh (Jay, 1999).
Based on initial algebra semantics, Charity (Cockett and Fukushima,
1992) automatically generates folds and so forth for datatypes on demand, but it otherwise does not allow the programmer to write her own
generic functions. PolyP (Jansson and Jeuring, 1997) extends Haskell
with a special construct for defining generic functions, also based on
initial algebras. In addition to the cases we consider, the programmer
must define cases for type composition and type recursion. Although
PolyP permits type inference, it supports only regular data types, not
mutually recursive, multiparametered, nested types, or types containing
function spaces.
The programming language Haskell supports deriving clauses for a
certain number of built-in classes (Eq, Show , etc) (Peyton Jones et al.,
1999). This facilitates the automatic overloading of certain functions
for datatypes which have the deriving clause specified. Hinze and Peyton Jones explored an extension, following Hinze’s earlier ideas (Hinze,
2000b), which integrated generics with Haskell’s type class system (Hinze
and Peyton Jones, 2001). This system suffers from some limitations
due to the interaction with the type class system. G’Caml (Furuse,
2001a; Furuse, 2001b) presents a generic programming extension for
O’Caml (Leroy et al., 2001). The proposal does not aim to cover all
datatypes, and as such can be seen as a way of achieving Haskell-style
overloading in O’Caml. The generic extension for Clean is also based on
Hinze’s work (Alimarine and Plasmeijer, 2001). This proposal is more
closely integrated with the type class system, but does not include any
of the extensions described here. Generic H SKELL, on the other hand,
takes the approach of exploring generic programming in isolation, as an
extension to the Haskell language.
In the earlier days of generic programming the number of example
programs was rather small: equality tests, pretty printing and parsing,
map, fold, and so forth. As research continues, more examples are being
unearthed. Pretty printing and parsing were reincarnated as data compression (Jansson and Jeuring, 2001). Type-indexed datatypes allow
many new possibilities: digital searching, pattern matching, and a generalised zipper (Hinze et al., 2002). Unification (Jansson and Jeuring,
1998), term rewriting (Jansson and Jeuring, 2000), and other program
transformations can also be seen as generic in the term datatype.
Generic program transformation has been the realm over which Stratego reigns (Visser et al., 1999). But Stratego is untyped. A number of approaches have been proposed for doing program transformations in a typed environment, these include RhoStratego (Dolstra and
Visser, 2001), updateable algebras (Lämmel et al., 2000), and typed
generic traversals (Lämmel and Visser, 2002; Lämmel, 2002). We hope
to achieve such expressiveness within our framework and have described
in this paper some steps in this direction.
Generic H SKELL implements a new approach to generic programming due to Hinze (Hinze, 2000c) which promises to relieve Haskell programmers of the burden of writing the same functions over and over
Generic Haskell, Specifically
again for different datatypes. Its main advantages are that generic definitions are simple to write and can be applied to Haskell 98 types of all
To gain better leverage from this technology, we have introduced a few
new features to Generic H SKELL which are useful when writing generic
functions. These extensions are copy lines, constructor cases, and generic
abstraction. Although these are relatively small modifications to the
existing machinery, their interaction seems to open up new application
areas for Generic H SKELL.
As we experiment with Generic H SKELL and approach problems in
different ways, we continually discover new ways of using the existing
machinery and stumble upon new challenges. These we hope to address
from both theoretical and practical perspectives in the near future.
The extensions described herein have been implemented in the Generic
H SKELL compiler, which is freely available for Unix, Windows, and
MacOS X based platforms from
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