Question

这是我问过的早期question的后续内容。现在，我正在尝试修改toText方法，以便它还使用重载Maybe a函数处理toText。下面是一个编译和工作正常的代码 - 现在，我想为toText使用Maybe a定义本身为其他类型重载toText：

{-# LANGUAGE TypeFamilies, DataKinds, MultiParamTypeClasses, FlexibleInstances, ScopedTypeVariables, FlexibleContexts #-}

import Data.List
import Data.Text (unpack, pack, Text, empty)
import Data.Proxy
import Data.Maybe (maybe)

data ToTextMethod = TTMText | TTMShow | TTMMaybe

type family ToTextHow a where
     ToTextHow Text = TTMText
     ToTextHow (Maybe b) = TTMMaybe
     ToTextHow a = TTMShow

class ToTextC a b where
      toTextC :: a -> b -> Text

instance Show a => ToTextC a (Proxy TTMShow) where
      toTextC a _ = pack (show a)

instance Show a => ToTextC (Maybe a) (Proxy TTMMaybe) where
      -- Will like to replace this with
      -- toTextC a _ = maybe empty toText a 
      toTextC a _ = maybe empty (pack . show) a 

instance ToTextC Text (Proxy TTMText) where
      toTextC t _ = t

toText :: forall a. (Show a, ToTextC a (Proxy (ToTextHow a))) => a -> Text
toText x = toTextC x (Proxy :: Proxy (ToTextHow a))

如果我按以下评论中的说明更新toTextC类型的TTMMaybe定义：

instance Show a => ToTextC (Maybe a) (Proxy TTMMaybe) where
      toTextC a _ = maybe empty toText a

我得到了这个错误 - 会对如何解决这个重载的指点表示赞赏：

 Could not deduce (ToTextC a (Proxy (ToTextHow a)))
      arising from a use of ‘toText’
    from the context (Show a)
      bound by the instance declaration at Test.hs:21:10-53
    In the second argument of ‘maybe’, namely ‘toText’
    In the expression: maybe empty toText a
    In an equation for ‘toTextC’: toTextC a _ = maybe empty toText a

更新1：

根据评论中的建议，我使用额外的约束来修复它。但是，需要启用UndecidableInstances扩展程序！有没有更好的方法可以在不使用UndecidableInstances的情况下执行此操作？也许某种类型的运算符或函数来表达类型级计算？

instance (Show a,ToTextC a (Proxy (ToTextHow a))) => ToTextC (Maybe a) (Proxy TTMMaybe) where
      toTextC a _ = maybe empty toText a

重载Maybe类型的show方法

0 个答案: