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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilyDependencies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
-- A stripped-down version of singletons
module T17139a where
import Data.Kind (Type)
import Data.Proxy (Proxy)
data family Sing :: k -> Type
class SingI a where
sing :: Sing a
class SingKind k where
type Demote k = (r :: Type) | r -> k
toSing :: Demote k -> Proxy k
data TyFun :: Type -> Type -> Type
type a ~> b = TyFun a b -> Type
infixr 0 ~>
type family Apply (f :: k1 ~> k2) (x :: k1) :: k2
newtype instance Sing (f :: k1 ~> k2) =
SLambda { applySing :: forall t. Sing t -> Sing (Apply f t) }
instance (SingKind k1, SingKind k2) => SingKind (k1 ~> k2) where
type Demote (k1 ~> k2) = Demote k1 -> Demote k2
toSing = undefined
withSing :: SingI a => (Sing a -> b) -> b
withSing f = f sing
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