6.4.7. Declaring data types with explicit constructor signatures

GADTSyntax
Since

7.2.1

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.

6.4.7.1. Formal syntax for GADTs

To make more precise what is and what is not permitted inside of a GADT-style constructor, we provide a BNF-style grammar for GADT below. Note that this grammar is subject to change in the future.

gadt_con ::= conids '::' opt_forall opt_ctxt gadt_body

conids ::= conid
        |  conid ',' conids

opt_forall ::= <empty>
            |  'forall' tv_bndrs '.'

tv_bndrs ::= <empty>
          |  tv_bndr tv_bndrs

tv_bndr ::= tyvar
         |  '(' tyvar '::' ctype ')'

opt_ctxt ::= <empty>
          |  btype '=>'
          |  '(' ctxt ')' '=>'

ctxt ::= ctype
      |  ctype ',' ctxt

gadt_body ::= prefix_gadt_body
           |  record_gadt_body

prefix_gadt_body ::= '(' prefix_gadt_body ')'
                  |  return_type
                  |  opt_unpack btype '->' prefix_gadt_body

record_gadt_body ::= '{' fieldtypes '}' '->' return_type

fieldtypes ::= <empty>
            |  fieldnames '::' opt_unpack ctype
            |  fieldnames '::' opt_unpack ctype ',' fieldtypes

fieldnames ::= fieldname
            |  fieldname ',' fieldnames

opt_unpack ::= opt_bang
            :  {-# UNPACK #-} opt_bang
            |  {-# NOUNPACK #-} opt_bang

opt_bang ::= <empty>
          |  '!'
          |  '~'

Where:

  • btype is a type that is not allowed to have an outermost forall/=> unless it is surrounded by parentheses. For example, forall a. a and Eq a => a are not legal btypes, but (forall a. a) and (Eq a => a) are legal.

  • ctype is a btype that has no restrictions on an outermost forall/=>, so forall a. a and Eq a => a are legal ctypes.

  • return_type is a type that is not allowed to have foralls, =>s, or ->s.

This is a simplified grammar that does not fully delve into all of the implementation details of GHC’s parser (such as the placement of Haddock comments), but it is sufficient to attain an understanding of what is syntactically allowed. Some further various observations about this grammar:

  • GADT constructor types are currently not permitted to have nested foralls or =>s. (e.g., something like MkT :: Int -> forall a. a -> T would be rejected.) As a result, gadt_sig puts all of its quantification and constraints up front with opt_forall and opt_context. Note that higher-rank foralls and =>s are only permitted if they do not appear directly to the right of a function arrow in a prefix_gadt_body. (e.g., something like MkS :: Int -> (forall a. a) -> S is allowed, since parentheses separate the forall from the ->.)

  • Furthermore, GADT constructors do not permit outermost parentheses that surround the opt_forall or opt_ctxt, if at least one of them are used. For example, MkU :: (forall a. a -> U) would be rejected, since it would treat the forall as being nested.

    Note that it is acceptable to use parentheses in a prefix_gadt_body. For instance, MkV1 :: forall a. (a) -> (V1) is acceptable, as is MkV2 :: forall a. (a -> V2).

  • The function arrows in a prefix_gadt_body, as well as the function arrow in a record_gadt_body, are required to be used infix. For example, MkA :: (->) Int A would be rejected.

  • GHC uses the function arrows in a prefix_gadt_body and prefix_gadt_body to syntactically demarcate the function and result types. Note that GHC does not attempt to be clever about looking through type synonyms here. If you attempt to do this, for instance:

    type C = Int -> B
    
    data B where
      MkB :: C
    

    Then GHC will interpret the return type of MkB to be C, and since GHC requires that the return type must be headed by B, this will be rejected. On the other hand, it is acceptable to use type synonyms within the argument and result types themselves, so the following is permitted:

    type B1 = Int
    type B2 = B
    
    data B where
      MkB :: B1 -> B2
    
  • GHC will accept any combination of !/~ and {-# UNPACK #-}/{-# NOUNPACK #-}, although GHC will ignore some combinations. For example, GHC will produce a warning if you write {-# UNPACK #-} ~Int and proceed as if you had written Int.

6.4.7.2. GADT syntax odds and ends

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