Add Exists2, Exists3... ?

[m-bock]

In case you want to hide two type arguments, the best I could figure out to do with the existing Exist is:

data Sample a b = Sample a b

newtype Sample' a = Sample' (Exists (Sample a))

type Sample'' = Exists Sample'

Does anyone know a better way? If not, I would suggest to add something like this:

foreign import data Exists2 :: (Type -> Type -> Type) -> Type

type role Exists2 representational

mkExists2 :: forall f a b. f a b -> Exists2 f
mkExists2 = unsafeCoerce

runExists2 :: forall f r. (forall a b. f a b -> r) -> Exists2 f -> r
runExists2 = unsafeCoerce

You can use it like this:

data Sample a b = Sample a b
type Sample' = Exists2 Sample