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 7.5.3, “Deriving clause for extra classes (Typeable, Data, etc)”), 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. 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: