Using a combination of -XDeriveGeneric
(Section 7.5.4, “Deriving Typeable instances”),
-XDefaultSignatures (Section 7.6.1.4, “Default method signatures”),
and -XDeriveAnyClass (Section 7.5.6, “Deriving any other class”),
you can easily do datatype-generic
programming using the GHC.Generics framework. This section
gives a very brief overview of how to do it.
Generic programming support in GHC allows defining classes with methods that
do not need a user specification when instantiating: the method body is
automatically derived by GHC. This is similar to what happens for standard
classes such as Read and Show, for
instance, but now for user-defined classes.
The first thing we need is generic representations. The
GHC.Generics module defines a couple of primitive types
that are used to represent Haskell datatypes:
-- | Unit: used for constructors without arguments
data U1 p = U1
-- | Constants, additional parameters and recursion of kind *
newtype K1 i c p = K1 { unK1 :: c }
-- | Meta-information (constructor names, etc.)
newtype M1 i c f p = M1 { unM1 :: f p }
-- | Sums: encode choice between constructors
infixr 5 :+:
data (:+:) f g p = L1 (f p) | R1 (g p)
-- | Products: encode multiple arguments to constructors
infixr 6 :*:
data (:*:) f g p = f p :*: g p
The Generic and Generic1 classes mediate
between user-defined datatypes and their internal representation as a
sum-of-products:
class Generic a where -- Encode the representation of a user datatype type Rep a :: * -> * -- Convert from the datatype to its representation from :: a -> (Rep a) x -- Convert from the representation to the datatype to :: (Rep a) x -> a class Generic1 f where type Rep1 f :: * -> * from1 :: f a -> Rep1 f a to1 :: Rep1 f a -> f a
Generic1 is used for functions that can only be defined over
type containers, such as map.
Instances of these classes can be derived by GHC with the
-XDeriveGeneric (Section 7.5.4, “Deriving Typeable instances”), and are
necessary to be able to define generic instances automatically.
For example, a user-defined datatype of trees data UserTree a = Node a
(UserTree a) (UserTree a) | Leaf gets the following representation:
instance Generic (UserTree a) where
-- Representation type
type Rep (UserTree a) =
M1 D D1UserTree (
M1 C C1_0UserTree (
M1 S NoSelector (K1 R a)
:*: M1 S NoSelector (K1 R (UserTree a))
:*: M1 S NoSelector (K1 R (UserTree a)))
:+: M1 C C1_1UserTree U1)
-- Conversion functions
from (Node x l r) = M1 (L1 (M1 (M1 (K1 x) :*: M1 (K1 l) :*: M1 (K1 r))))
from Leaf = M1 (R1 (M1 U1))
to (M1 (L1 (M1 (M1 (K1 x) :*: M1 (K1 l) :*: M1 (K1 r))))) = Node x l r
to (M1 (R1 (M1 U1))) = Leaf
-- Meta-information
data D1UserTree
data C1_0UserTree
data C1_1UserTree
instance Datatype D1UserTree where
datatypeName _ = "UserTree"
moduleName _ = "Main"
instance Constructor C1_0UserTree where
conName _ = "Node"
instance Constructor C1_1UserTree where
conName _ = "Leaf"
This representation is generated automatically if a
deriving Generic clause is attached to the datatype.
Standalone deriving can also be
used.
A generic function is defined by creating a class and giving instances for
each of the representation types of GHC.Generics. As an
example we show generic serialization:
data Bin = O | I class GSerialize f where gput :: f a -> [Bin] instance GSerialize U1 where gput U1 = [] instance (GSerialize a, GSerialize b) => GSerialize (a :*: b) where gput (x :*: y) = gput x ++ gput y instance (GSerialize a, GSerialize b) => GSerialize (a :+: b) where gput (L1 x) = O : gput x gput (R1 x) = I : gput x instance (GSerialize a) => GSerialize (M1 i c a) where gput (M1 x) = gput x instance (Serialize a) => GSerialize (K1 i a) where gput (K1 x) = put x
Typically this class will not be exported, as it only makes sense to have instances for the representation types.
The only thing left to do now is to define a "front-end" class, which is exposed to the user:
class Serialize a where put :: a -> [Bin] default put :: (Generic a, GSerialize (Rep a)) => a -> [Bit] put = gput . from
Here we use a default signature
to specify that the user does not have to provide an implementation for
put, as long as there is a Generic
instance for the type to instantiate. For the UserTree type,
for instance, the user can just write:
instance (Serialize a) => Serialize (UserTree a)
The default method for put is then used, corresponding to the
generic implementation of serialization.
If you are using -XDeriveAnyClass, the same instance is
generated by simply attaching a deriving Serialize clause
to the UserTree datatype declaration.
For more examples of generic functions please refer to the
generic-deriving
package on Hackage.
For more details please refer to the HaskellWiki page or the original paper:
Jose Pedro Magalhaes, Atze Dijkstra, Johan Jeuring, and Andres Loeh. A generic deriving mechanism for Haskell. Proceedings of the third ACM Haskell symposium on Haskell (Haskell'2010), pp. 37-48, ACM, 2010.