haskell - 类型族实例的类型类约束

Question

是否可以指定一个类型族的所有实例都必须满足的类型类约束？

例如，给定以下声明，我如何确保所有实例也是 Eq 的实例？ :
data family Channel c :: *
非常感谢，

迈克尔

Answer 1

这是你想要的？

{-# LANGUAGE FlexibleContexts, TypeFamilies, FlexibleInstances #-}

-- Data family inside a class so that we can add an extra Eq constraint
class Eq (Channel c) => MyClass c where
    data Channel c :: *

-- A simple toy instance
instance MyClass Int where
    data Channel Int = CI Int deriving Eq

-- A more complex instance with separate Eq instance
instance MyClass Char where
    data Channel Char = CC Char

instance Eq (Channel Char) where
   (CC x) == (CC y) = x == y