Introduction to GHC.Generics

2018-05-11

Generic functions traverse the structure of a type to produce the body of a function with the goal of reducing code reptition.

Imagine a simple record type and a function that converts any type to a String.

data Person
  = MkPerson
  { name :: String
  , age :: Int
  }
  
genericShow :: a -> String
genericShow = undefined

Here is a simplified version of how we traverse over the structure of the data type to produce the body of the genericShow function. The following is pseudo code and does not actually compile. It intends to emulate the way GHC.Generics works in a simple, readable manner.

genericShow :: a -> String
genericShow a =
  case getTypeRepresentation a of
    TypeName name rest = show name ++ genericShow rest

    Constructor constructor rest = show constructor ++ genericShow rest

    NamedField (Just fieldName) typ = show fieldName ++ show type
    
    UnnamedField Nothing type = show typ
    
    ConcatFields field1 field2 = genericShow field1 ++ genericShow field2
    
    ConcatConstructors constructor1 constructor2 = genericShow constructor1 ++ genericShow constructor2
    
    TypeParameter a = genericShow a
	
    NoFields = ""

Examples of Generic Representations

Product (Record with Fields)

data Person
  = MkPerson
  { name :: String
  , age :: Int
  }

TypeName "Person"
  (Constructor "MkPerson"
    (NamedField (Just "name") "String" 
	`ConcatFields`
	 NamedField (Just "age") "Int"))

Sum Type without Values

data Empty

TypeName "Empty"
  (Constructor "Empty" NoFields)

Sum Type with One Value

data Wrapper = Wrapper Int

TypeName "Wrapper"
  (Constructor "Wrapper" (NamedField Nothing "Int"))

Sum Type with Multiple Constructors and Type Parameters

data Either a b
  = Left a
  | Right b
  
TypeName "Either"
  ((Constructor "Left" (TypeParameter a))
   `ConcatConstructors`
   (Constructor "Right" (TypeParameter b)))

GHC.Generic Types

We can view use our simple representation as type synonyms for the real types in GHC.Generics.

type TypeName = D1 ('MetaSelector name moduleName packageName isNewType) rest

type Constructor = C1 ('MetaCons constructor prefixOrInfix isRecord) rest

type NamedField = S1 ('MetaSel ('Just selector) unpackednesss sourceStrictness decidedStrictness)

type UnnamedField = S1 ('MetaSel Nothing unpackednesss sourceStrictness decidedStrictness)

type ConcatFields = (:*:)

type ConcatConstructors = (:+:)

type TypeParameter a = Rec0 a

type NoFields = U1

GHC.Generic Representations

Now we will take a look at the real generic repsentations of the types from above. In GHCi, you can see the representations by calling :kind! Rep on any type that derives Generic

Product (Record with Fields)

:set -XDeriveGeneric
import GHC.Generics (Generic, Rep)

data Person
  = MkPerson
  { name :: String
  , age :: Int
  } deriving Generic

:kind! Rep Person

Rep Person :: * -> *
= D1
    ('MetaData "Person" "Ghci2" "interactive" 'False)
    (C1
       ('MetaCons "MkPerson" 'PrefixI 'True)
       (S1
          ('MetaSel
             ('Just "name")
             'NoSourceUnpackedness
             'NoSourceStrictness
             'DecidedLazy)
          (Rec0 String)
        :*: S1
          ('MetaSel
		      ('Just "age")
              'NoSourceUnpackedness
			  'NoSourceStrictness
			  'DecidedLazy)
		  (Rec0 Int)))

Sum Type without Values

Rep Empty :: * -> *
= D1
    ('MetaData "Empty" "Ghci3" "interactive" 'False)
    V1

Sum Type with One Value

data Wrapper = Wrapper Int deriving Generic

Rep Wrapper :: * -> *
= D1
    ('MetaData "Wrapper" "Ghci4" "interactive" 'False)
    (C1
       ('MetaCons "Wrapper" 'PrefixI 'False)
       (S1
          ('MetaSel
             'Nothing
             'NoSourceUnpackedness
             'NoSourceStrictness
             'DecidedLazy)
          (Rec0 Int)))

Sum Type with Multiple Constructors and Type Parameters

data Either a b
  = Left a
  | Right b
  deriving Generic


:kind! Rep (Either Int String)

Rep (Either Int String) :: * -> *
= D1
    ('MetaData "Either" "Ghci5" "interactive" 'False)
    (C1
       ('MetaCons "Left" 'PrefixI 'False)
       (S1
          ('MetaSel
             'Nothing
             'NoSourceUnpackedness
             'NoSourceStrictness
             'DecidedLazy)
          (Rec0 Int))
     :+: C1
       ('MetaCons "Right" 'PrefixI 'False)
       (S1
          ('MetaSel
             'Nothing
             'NoSourceUnpackedness
             'NoSourceStrictness
             'DecidedLazy)
          (Rec0 [Char])))

You can also see the generic representations when you build the package: stack build --ghc-options=-ddump-deriv.