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{-# LANGUAGE TypeFamilies, GADTs, ConstraintKinds, RankNTypes #-}
module T5655 where
import Data.Kind (Type, Constraint)
class Show a => Twice a where twice :: a -> a
instance Twice Int where twice = (*2)
data ETwice where ETwice :: Twice a => a -> ETwice
class E e where
type C e :: Type -> Constraint
ap :: (forall a. C e a => a -> r) -> e -> r
instance E ETwice where
type C ETwice = Twice
ap f (ETwice a) = f a
f :: (E e, C e ~ Twice) => e -> ETwice
f = ap (ETwice . twice)
foo :: ETwice
foo = ETwice (5 :: Int)
bar :: IO ()
bar = ap print (f foo)
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