Cannot use higher-rank kinds with type families
This ticket came out of a discussion with Richard in this mailing list post: https://mail.haskell.org/pipermail/haskell-cafe/2016-March/123462.html
Here's the code that should work, but doesn't:
import Data.Kind
data TyFun :: * -> * -> *
type a ~> b = TyFun a b -> *
type family (f :: a ~> b) @@ (x :: a) :: b
data Null a = Nullable a | NotNullable a
type family ((f :: b ~> c) ∘ (g :: a ~> b)) (x :: a) :: c where
(f ∘ g) x = f @@ (g @@ x)
type family BaseType (k :: forall a. a ~> Type) (x :: b) :: Type where -- this fails :(
-- BaseType k x = (@@) k x