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问题描述
是否可以指定类型族的所有实例必须满足的类型约束?
例如,给定以下声明,我将如何确保所有实例都是 Eq 的实例:数据族通道c :: *
非常感谢,
Michael
$ b { - #LANGUAGE FlexibleContexts,TypeFamilies,FlexibleInstances# - }- 类中的数据族,以便我们可以添加额外的Eq约束条件
class Eq(Channel c)=> MyClass c其中
数据通道c :: *
- 一个简单的玩具实例
实例MyClass Int其中
数据Channel Int = CI Int推导Eq
- 具有单独Eq实例的更复杂实例
实例MyClass Char其中
数据通道Char = CC Char
实例Eq(Channel Char)其中
(CC x)==(CC y)= x == y
Is it possible to specify a type class constraint that must be satisfied by all instances of a type family?
For example, given the following declaration, how would I ensure that all instances are also instances of Eq:
data family Channel c :: *
Many thanks,
Michael
解决方案
Is this what you are looking for?
{-# 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
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