haskelltype-familiestype-kinds

Type family forcing parameter to have the kind *

This simple code doesn't compile

import Data.Kind
type family F (k :: Type) :: (t :: k) -> Type

The error message is

• Expected a type, but ‘t’ has kind ‘k’
• In the kind ‘(t :: k) -> Type’

I get in some sense that this actually defines a "family of type families" but I don't really understand why this limitation would exist.

type family F (k :: Type) (t :: k) :: Type

does work but it doesn't have the same semantics and can't be used the same.

Solution

  • There is no need to name t in the resulting type. You can can simply use

    type family F (k :: Type) :: k -> Type