6.4.7. Declaring data types with explicit constructor signatures


Allow the use of GADT syntax in data type definitions (but not GADTs themselves; for this see GADTs)

When the GADTSyntax extension is enabled, GHC allows you to declare an algebraic data type by giving the type signatures of constructors explicitly. For example:

data Maybe a where
    Nothing :: Maybe a
    Just    :: a -> Maybe a

The form is called a “GADT-style declaration” because Generalised Algebraic Data Types, described in Generalised Algebraic Data Types (GADTs), can only be declared using this form.

Notice that GADT-style syntax generalises existential types (Existentially quantified data constructors). For example, these two declarations are equivalent:

data Foo = forall a. MkFoo a (a -> Bool)
data Foo' where { MKFoo :: a -> (a->Bool) -> Foo' }

Any data type that can be declared in standard Haskell 98 syntax can also be declared using GADT-style syntax. The choice is largely stylistic, but GADT-style declarations differ in one important respect: they treat class constraints on the data constructors differently. Specifically, if the constructor is given a type-class context, that context is made available by pattern matching. For example:

data Set a where
  MkSet :: Eq a => [a] -> Set a

makeSet :: Eq a => [a] -> Set a
makeSet xs = MkSet (nub xs)

insert :: a -> Set a -> Set a
insert a (MkSet as) | a `elem` as = MkSet as
                    | otherwise   = MkSet (a:as)

A use of MkSet as a constructor (e.g. in the definition of makeSet) gives rise to a (Eq a) constraint, as you would expect. The new feature is that pattern-matching on MkSet (as in the definition of insert) makes available an (Eq a) context. In implementation terms, the MkSet constructor has a hidden field that stores the (Eq a) dictionary that is passed to MkSet; so when pattern-matching that dictionary becomes available for the right-hand side of the match. In the example, the equality dictionary is used to satisfy the equality constraint generated by the call to elem, so that the type of insert itself has no Eq constraint.

For example, one possible application is to reify dictionaries:

data NumInst a where
  MkNumInst :: Num a => NumInst a

intInst :: NumInst Int
intInst = MkNumInst

plus :: NumInst a -> a -> a -> a
plus MkNumInst p q = p + q

Here, a value of type NumInst a is equivalent to an explicit (Num a) dictionary.

All this applies to constructors declared using the syntax of Existentials and type classes. For example, the NumInst data type above could equivalently be declared like this:

data NumInst a
   = Num a => MkNumInst (NumInst a)

Notice that, unlike the situation when declaring an existential, there is no forall, because the Num constrains the data type’s universally quantified type variable a. A constructor may have both universal and existential type variables: for example, the following two declarations are equivalent:

data T1 a
 = forall b. (Num a, Eq b) => MkT1 a b
data T2 a where
 MkT2 :: (Num a, Eq b) => a -> b -> T2 a

All this behaviour contrasts with Haskell 98’s peculiar treatment of contexts on a data type declaration (Section 4.2.1 of the Haskell 98 Report). In Haskell 98 the definition

data Eq a => Set' a = MkSet' [a]

gives MkSet' the same type as MkSet above. But instead of making available an (Eq a) constraint, pattern-matching on MkSet' requires an (Eq a) constraint! GHC faithfully implements this behaviour, odd though it is. But for GADT-style declarations, GHC’s behaviour is much more useful, as well as much more intuitive.

The rest of this section gives further details about GADT-style data type declarations.

  • The result type of each data constructor must begin with the type constructor being defined. If the result type of all constructors has the form T a1 ... an, where a1 ... an are distinct type variables, then the data type is ordinary; otherwise is a generalised data type (Generalised Algebraic Data Types (GADTs)).

  • As with other type signatures, you can give a single signature for several data constructors. In this example we give a single signature for T1 and T2:

    data T a where
      T1,T2 :: a -> T a
      T3 :: T a
  • The type signature of each constructor is independent, and is implicitly universally quantified as usual. In particular, the type variable(s) in the “data T a where” header have no scope, and different constructors may have different universally-quantified type variables:

    data T a where        -- The 'a' has no scope
      T1,T2 :: b -> T b   -- Means forall b. b -> T b
      T3 :: T a           -- Means forall a. T a
  • A constructor signature may mention type class constraints, which can differ for different constructors. For example, this is fine:

    data T a where
      T1 :: Eq b => b -> b -> T b
      T2 :: (Show c, Ix c) => c -> [c] -> T c

    When pattern matching, these constraints are made available to discharge constraints in the body of the match. For example:

    f :: T a -> String
    f (T1 x y) | x==y      = "yes"
               | otherwise = "no"
    f (T2 a b)             = show a

    Note that f is not overloaded; the Eq constraint arising from the use of == is discharged by the pattern match on T1 and similarly the Show constraint arising from the use of show.

  • Unlike a Haskell-98-style data type declaration, the type variable(s) in the “data Set a where” header have no scope. Indeed, one can write a kind signature instead:

    data Set :: Type -> Type where ...

    or even a mixture of the two:

    data Bar a :: (Type -> Type) -> Type where ...

    The type variables (if given) may be explicitly kinded, so we could also write the header for Foo like this:

    data Bar a (b :: Type -> Type) where ...
  • You can use strictness annotations, in the obvious places in the constructor type:

    data Term a where
        Lit    :: !Int -> Term Int
        If     :: Term Bool -> !(Term a) -> !(Term a) -> Term a
        Pair   :: Term a -> Term b -> Term (a,b)
  • You can use a deriving clause on a GADT-style data type declaration. For example, these two declarations are equivalent

    data Maybe1 a where {
        Nothing1 :: Maybe1 a ;
        Just1    :: a -> Maybe1 a
      } deriving( Eq, Ord )
    data Maybe2 a = Nothing2 | Just2 a
         deriving( Eq, Ord )
  • The type signature may have quantified type variables that do not appear in the result type:

    data Foo where
       MkFoo :: a -> (a->Bool) -> Foo
       Nil   :: Foo

    Here the type variable a does not appear in the result type of either constructor. Although it is universally quantified in the type of the constructor, such a type variable is often called “existential”. Indeed, the above declaration declares precisely the same type as the data Foo in Existentially quantified data constructors.

    The type may contain a class context too, of course:

    data Showable where
      MkShowable :: Show a => a -> Showable
  • You can use record syntax on a GADT-style data type declaration:

    data Person where
        Adult :: { name :: String, children :: [Person] } -> Person
        Child :: Show a => { name :: !String, funny :: a } -> Person

    As usual, for every constructor that has a field f, the type of field f must be the same (modulo alpha conversion). The Child constructor above shows that the signature may have a context, existentially-quantified variables, and strictness annotations, just as in the non-record case. (NB: the “type” that follows the double-colon is not really a type, because of the record syntax and strictness annotations. A “type” of this form can appear only in a constructor signature.)

  • Record updates are allowed with GADT-style declarations, only fields that have the following property: the type of the field mentions no existential type variables.

  • As in the case of existentials declared using the Haskell-98-like record syntax (Record Constructors), record-selector functions are generated only for those fields that have well-typed selectors. Here is the example of that section, in GADT-style syntax:

    data Counter a where
        NewCounter :: { _this    :: self
                      , _inc     :: self -> self
                      , _display :: self -> IO ()
                      , tag      :: a
                      } -> Counter a

    As before, only one selector function is generated here, that for tag. Nevertheless, you can still use all the field names in pattern matching and record construction.

  • In a GADT-style data type declaration there is no obvious way to specify that a data constructor should be infix, which makes a difference if you derive Show for the type. (Data constructors declared infix are displayed infix by the derived show.) So GHC implements the following design: a data constructor declared in a GADT-style data type declaration is displayed infix by Show iff (a) it is an operator symbol, (b) it has two arguments, (c) it has a programmer-supplied fixity declaration. For example

    infix 6 (:--:)
    data T a where
      (:--:) :: Int -> Bool -> T Int