使用“类型系列”限制输出

Question

我正在尝试使用Haskells Type Families添加约束的代码。我想强制输出始终是一个翻转，这个代码可以吗？

{-# LANGUAGE GADTs, TypeFamilies #-}

module TF where

data Even
data Odd

data Coin where
  Up :: Coin
  Down :: Coin

type family Flip n :: *
type instance Flip Even = Odd
type instance Flip Odd = Even

up = Up 
down = Down

flip Up   = Down
flip Down = Up

Answer 1

将我的评论写入答案：

{-# LANGUAGE GADTs, DataKinds, TypeFamilies #-}

data Parity = Even | Odd

data Coin :: Parity -> * where
    Up   :: Coin Even
    Down :: Coin Odd

type family Flip (p :: Parity) :: Parity where
    Flip Even = Odd
    Flip Odd  = Even

flip :: Coin p -> Coin (Flip p)
flip Up   = Down
flip Down = Up