KindSignatures
-
Since: 6.8.1 Allow explicit kind signatures on type variables.
Haskell infers the kind of each type variable. Sometimes it is nice to be able to give the kind explicitly as (machine-checked) documentation, just as it is nice to give a type signature for a function. On some occasions, it is essential to do so. For example, in his paper “Restricted Data Types in Haskell” (Haskell Workshop 1999) John Hughes had to define the data type:
data Set cxt a = Set [a]
| Unused (cxt a -> ())
The only use for the Unused
constructor was to force the correct kind for the type variable cxt
.
GHC now instead allows you to specify the kind of a type variable directly, wherever a type variable is explicitly bound, with the extension KindSignatures
.
This extension enables kind signatures in the following places:
data
declarations:data Set (cxt :: Type -> Type) a = Set [a]
type
declarations:type T (f :: Type -> Type) = f Int
class
declarations:class (Eq a) => C (f :: Type -> Type) a where ...
forall
’s in type signatures:f :: forall (cxt :: Type -> Type). Set cxt Int
The parentheses are required.
As part of the same extension, you can put kind annotations in types as well. Thus:
f :: (Int :: Type) -> Int
g :: forall a. a -> (a :: Type)
The syntax is
atype ::= '(' ctype '::' kind ')
The parentheses are required.