Test.hs

{-# LANGUAGE RankNTypes #-}
module Test where

  import Prelude hiding (fst, snd)

  pair :: a -> b -> (forall c. (a -> b -> c) -> c)
  pair = flip . flip id

  fst :: (forall c. (a -> b -> c) -> c) -> a
  fst = ($ const)

  snd :: (forall c. (a -> b -> c) -> c) -> b
  snd = ($ const id)

  toPair :: (forall c. (a -> b -> c) -> c) -> (a, b)
  toPair p = (fst p, snd p)

  fromPair :: (a, b) -> (forall c. (a -> b -> c) -> c)
  fromPair = uncurry pair

  nil :: a -> a -> a
  nil = const

  -- 寫不出 cons 的 type
  cons :: forall b. a -> ((a -> ??? -> b) -> b) -> ((a -> ??? -> b) -> b)
  cons x xs = \_ cons' -> cons' x xs

  -- 再試著寫一遍
  type List a = forall b. (a -> List a -> b) -> b

  -- 用 newtype 做出來了可是我想要所有東西都是 function
  newtype List' a = List' {
    unlist :: forall b. (a -> List' a -> b) -> b
  }