GHC.Generics

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GHC 7.2 includes a new generic deriving mechanism. This means you can have more classes that you can derive, like Show or Functor. This is accomplished through two new features, enabled with two new flags: DeriveRepresentable and DefaultSignatures. We'll show how this all works in a detailed example.

Serialization

Suppose you are writing a class for serialization of data. You have a type Bit representing bits, and a class Serialize:

data Bit = O | I

class Serialize a where
  put :: a -> [Bit]

You might have written some instances already:

instance Serialize Int where
  put i = serializeInt i

instance Serialize a => Serialize [a] where
  put []    = []
  put (h:t) = put h ++ put t

A user of your library, however, will have their his datatypes, like:

data UserTree a = Node a (UserTree a) (UserTree a) | Leaf

He will have to specify an instance Serialize (UserTree a) where ... himself. This, however, is tedious, especially because most instances will probably be rather trivial, and should be derived automatically.

It is here that generic programming can help you. If you are familiar with SYB you could use it at this stage, but now we'll see how to do this with the new generic deriving mechanism.

Generic serialization

First you have to tell the compiler how to serialize any datatype, in general. Since Haskell datatypes have a regular structure, this means you can just explain how to serialize a few basic datatypes.

Representation types

We can represent most Haskell datatypes using only the following primitive types:

-- | Void: used for datatypes without constructors
data V1 p

-- | 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

For starters, try to ignore the p parameter in all types; it's there just for future compatibility. The easiest way to understand how you can use these types to represent others is to see an example. Let's represent the UserTree type shown before:

type RepUserTree a =
  -- A UserTree is either a Leaf, which has no arguments
      U1
  -- ... or it is a Node, which has three arguments that we put in a product
  :+: a :*: UserTree a :*: UserTree a

Simple, right? Different constructors become alternatives of a sum, and multiple arguments become products. In fact, we want to have some more information in the representation, like datatype and constructor names, and to know if a product argument is a parameter or a type. We use the other primitives for this, and the representation looks more like:

type RealRepUserTree a =
  -- Information about the datatype
  M1 D Data_UserTree (
  -- Leaf, with information about the constructor
      M1 C Con_Leaf U1
  -- Node, with information about the constructor
  :+: M1 C Con_Node (
            -- Constructor argument, which could have information about a record selector label
            M1 S NoSelector (
              -- Argument, tagged with P because it is a parameter
              K1 P a) 
        -- Another argument, tagged with R because it is a recursive occurrence of a type
        :*: M1 S NoSelector (K1 R (UserTree a))
        -- Idem
        :*: M1 S NoSelector (K1 R (UserTree a))