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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
-- Exercise 10.2-ii
-- Defunctionalize `foldr :: (a -> b -> b) -> b -> [a] -> b`.
import Data.Kind (Type)
type Exp a = a -> Type
type family Eval (e :: Exp a) :: a
data Foldr :: (a -> b -> Exp b) -> b -> [a] -> Exp b
type instance Eval (Foldr f z '[]) = z
type instance Eval (Foldr f z (x ': xs)) = Eval (f x (Eval (Foldr f z xs)))
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