实施“展示”功能 [英] Implementation of "show" for function

问题描述

我想为（二进制）函数实现 show 方法，并使其能够将endofunctions （a - > a）。

类似于伪哈斯克尔代码：

  instance显示（a-> b）其中
 show fun =<< Endofunction>>如果a == b 
 show fun =<< Function>>如果a \ = b 
  

如何区分这两种情况？

$ b $您需要启用一些扩展功能：

  { - ＃LANGUAGE OverlappingInstances，FlexibleInstances＃ - } 
模块FunShow其中
 
实例显示（（ - >））aa）其中
显示_ =<< Endofunction>> ; 
 
实例显示（（ - >）a b）其中
显示_ =<<函数>> 
  

您需要 OverlappingInstances ，因为实例 a - >由于语言标准要求实例声明中的类型变量是不同的，因此也匹配endofunctions，因此存在重叠，并且需要 FlexibleInstances p>



  * FunShow> show not 
<< Endofunction>> 
 * FunShow>显示fst 
<<功能>> 
 * FunShow> show id 
<< Endofunction>> 
  




I would like to implement the show method for (binary) functions and make it able to distingish endofunctions (a -> a).

Something like the pseudo-haskell code:

instance Show (a->b) where
    show fun = "<<Endofunction>>" if a==b
    show fun = "<<Function>>" if a\=b

How can I distinguish the two cases?

解决方案

You need to enable some extensions:

{-# LANGUAGE OverlappingInstances, FlexibleInstances #-}
module FunShow where

instance Show ((->) a a) where
    show _ = "<<Endofunction>>"

instance Show ((->) a b) where
    show _ = "<<Function>>"

You need OverlappingInstances since the instance a -> b also matches endofunctions, so there's overlap, and you need FlexibleInstances because the language standard mandates that the type variables in instance declarations are distinct.

*FunShow> show not
"<<Endofunction>>"
*FunShow> show fst
"<<Function>>"
*FunShow> show id
"<<Endofunction>>"

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