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{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE UndecidableInstances #-}
module T7019a where
newtype Free c a = Free { runFree :: forall r. c r => (a -> r) -> r }
type C c = forall a. c (Free c a)
-- Notice that C is a synonym, illegal!
instance C c => Monad (Free c) where
return a = Free ($ a)
Free f >>= g = f g
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