The ideas behind this extension are described in detail in "Derivable type classes", Ralf Hinze and Simon Peyton Jones, Haskell Workshop, Montreal Sept 2000, pp94-105. An example will give the idea:
import Generics
class Bin a where
toBin :: a -> [Int]
fromBin :: [Int] -> (a, [Int])
toBin {| Unit |} Unit = []
toBin {| a :+: b |} (Inl x) = 0 : toBin x
toBin {| a :+: b |} (Inr y) = 1 : toBin y
toBin {| a :*: b |} (x :*: y) = toBin x ++ toBin y
fromBin {| Unit |} bs = (Unit, bs)
fromBin {| a :+: b |} (0:bs) = (Inl x, bs') where (x,bs') = fromBin bs
fromBin {| a :+: b |} (1:bs) = (Inr y, bs') where (y,bs') = fromBin bs
fromBin {| a :*: b |} bs = (x :*: y, bs'') where (x,bs' ) = fromBin bs
(y,bs'') = fromBin bs'
This class declaration explains how toBin and fromBin
work for arbitrary data types. They do so by giving cases for unit, product, and sum,
which are defined thus in the library module Generics:
data Unit = Unit data a :+: b = Inl a | Inr b data a :*: b = a :*: b
Now you can make a data type into an instance of Bin like this:
instance (Bin a, Bin b) => Bin (a,b) instance Bin a => Bin [a]
That is, just leave off the "where" clause. Of course, you can put in the where clause and over-ride whichever methods you please.
To use generics you need to
Use the flags -fglasgow-exts (to enable the extra syntax),
-XGenerics (to generate extra per-data-type code),
and -package lang (to make the Generics library
available.
Import the module Generics from the
lang package. This import brings into
scope the data types Unit,
:*:, and :+:. (You
don't need this import if you don't mention these types
explicitly; for example, if you are simply giving instance
declarations.)
Note that the type constructors :+: and :*:
can be written infix (indeed, you can now use
any operator starting in a colon as an infix type constructor). Also note that
the type constructors are not exactly as in the paper (Unit instead of 1, etc).
Finally, note that the syntax of the type patterns in the class declaration
uses "{|" and "|}" brackets; curly braces
alone would ambiguous when they appear on right hand sides (an extension we
anticipate wanting).
Terminology. A "generic default method" in a class declaration is one that is defined using type patterns as above. A "polymorphic default method" is a default method defined as in Haskell 98. A "generic class declaration" is a class declaration with at least one generic default method.
Restrictions:
Alas, we do not yet implement the stuff about constructor names and field labels.
A generic class can have only one parameter; you can't have a generic multi-parameter class.
A default method must be defined entirely using type patterns, or entirely without. So this is illegal:
class Foo a where
op :: a -> (a, Bool)
op {| Unit |} Unit = (Unit, True)
op x = (x, False)
However it is perfectly OK for some methods of a generic class to have generic default methods and others to have polymorphic default methods.
The type variable(s) in the type pattern for a generic method declaration scope over the right hand side. So this is legal (note the use of the type variable ``p'' in a type signature on the right hand side:
class Foo a where
op :: a -> Bool
op {| p :*: q |} (x :*: y) = op (x :: p)
...
The type patterns in a generic default method must take one of the forms:
a :+: b
a :*: b
Unit
where "a" and "b" are type variables. Furthermore, all the type patterns for
a single type constructor (:*:, say) must be identical; they
must use the same type variables. So this is illegal:
class Foo a where
op :: a -> Bool
op {| a :+: b |} (Inl x) = True
op {| p :+: q |} (Inr y) = False
The type patterns must be identical, even in equations for different methods of the class. So this too is illegal:
class Foo a where
op1 :: a -> Bool
op1 {| a :*: b |} (x :*: y) = True
op2 :: a -> Bool
op2 {| p :*: q |} (x :*: y) = False
(The reason for this restriction is that we gather all the equations for a particular type constructor into a single generic instance declaration.)
A generic method declaration must give a case for each of the three type constructors.
The type for a generic method can be built only from:
Function arrows
Type variables
Tuples
Arbitrary types not involving type variables
Here are some example type signatures for generic methods:
op1 :: a -> Bool
op2 :: Bool -> (a,Bool)
op3 :: [Int] -> a -> a
op4 :: [a] -> Bool
Here, op1, op2, op3 are OK, but op4 is rejected, because it has a type variable inside a list.
This restriction is an implementation restriction: we just haven't got around to implementing the necessary bidirectional maps over arbitrary type constructors. It would be relatively easy to add specific type constructors, such as Maybe and list, to the ones that are allowed.
In an instance declaration for a generic class, the idea is that the compiler will fill in the methods for you, based on the generic templates. However it can only do so if
The instance type is simple (a type constructor applied to type variables, as in Haskell 98).
No constructor of the instance type has unboxed fields.
(Of course, these things can only arise if you are already using GHC extensions.) However, you can still give an instance declarations for types which break these rules, provided you give explicit code to override any generic default methods.
The option -ddump-deriv dumps incomprehensible stuff giving details of
what the compiler does with generic declarations.
Just to finish with, here's another example I rather like:
class Tag a where
nCons :: a -> Int
nCons {| Unit |} _ = 1
nCons {| a :*: b |} _ = 1
nCons {| a :+: b |} _ = nCons (bot::a) + nCons (bot::b)
tag :: a -> Int
tag {| Unit |} _ = 1
tag {| a :*: b |} _ = 1
tag {| a :+: b |} (Inl x) = tag x
tag {| a :+: b |} (Inr y) = nCons (bot::a) + tag y