做出Maybe a的约束，其中Eq a [英] Making a constraint of Maybe a where Eq a

问题描述

我如何将Maybe限制为Eq a?它必须是*-> Constraint

How can I constrain to Maybe a where Eq a? It needs to be of kind * -> Constraint

我尝试过的事情:

class (a ~ Maybe b, Eq b) => K a where
instance (a ~ Maybe b, Eq b) => K a where

错误:

Not in scope: type variable ‘b’

示例用法:

data Test c = forall a. (c a) => Test a
r :: Test K -> Maybe Bool
r (Test o) = (==) <$> o <*> o -- I need GHC to infer that o is Maybe Eq

有效的情况:

pp :: Test ((~) String) -> String
pp (Test o) = o ++ "X" -- GHC infers that o is a string

hh :: Test Eq -> Bool
hh (Test o) = o == o -- GHC infers that o is Eq

此处的通用答案:推荐答案

以下内容在我的计算机上编译.不知道这有多明智.

The following compiles on my machine. No idea how sensible it is.

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE ConstraintKinds #-}
class (Eq (UnMaybe a), a ~ Maybe (UnMaybe a)) => EqMaybe a where
    type UnMaybe a

instance Eq a => EqMaybe (Maybe a) where
    type UnMaybe (Maybe a) = a

data Test c = forall a. c a => Test a
r :: Test EqMaybe -> Maybe Bool
r (Test o) = (==) <$> o <*> o
f :: Test Eq -> Test EqMaybe
f (Test o) = Test (Just o)

这篇关于做出Maybe a的约束，其中Eq a的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！