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{-# LANGUAGE DataKinds, PolyKinds, KindSignatures #-}
{-# LANGUAGE ExistentialQuantification, UndecidableInstances, TypeFamilies,
FlexibleContexts #-}
module Test where
import Data.Kind (Type)
-- Kind-level proxies.
data {-kind-} K (a :: Type) = KP
-- A type with 1 kind-polymorphic type argument.
data T (n :: k)
-- A type with 1 kind argument.
data F (kp :: K k)
-- A class with 1 kind argument.
class (kp ~ KP) => C (kp :: K k) where
f :: T (a :: k) -> F kp
-- A type with 1 kind argument.
-- Contains an existentially quantified type-variable of this kind.
data SomeT (kp :: K k) = forall (n :: k). Mk (T n)
-- Show `SomeT` by converting it to `F`, using `C`.
instance (C kp, Show (F kp)) => Show (SomeT kp) where
show (Mk x) = show (f x)
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