类型族实例上的类型类约束

这就是你要找的吗?

{-# 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