Haskell实例模拟？ [英] Haskell instanceof analogue?

问题描述

我想要一个函数

  show2 ::（Show a）=> a  - >字符串
  

这将返回 show a a ，但 a 如果a本身 String 。
我如何实现它？

如果这个函数已经在某个地方实现了，但是我仍然希望看到一个实现的例子。

用这段肮脏而危险的代码来做这件事：

  class显示a where 
 show2 :: a  - >字符串
 
实例显示字符串其中
 show2 = id 
 
实例（显示a）=> Showable a where 
 show2 = show 
  

您需要 -XOverlappingInstances - XFlexibleInstances -XUndecidableInstances 来编译和使用它。

  * Main> show2abc
abc
 * Main> show2 3 
3
  

I am new to Haskell, so my question is probably stupid.

I want a function

show2 :: (Show a) => a -> String

which would return show a for any a, but a if a is itself String. How can I implement it?

P.S. it is great if this function is already implemented somewhere, but I still want to see an example of the implementation.

解决方案

You can do this with this piece of dirty and dangerous code:

class Showable a where
  show2 :: a -> String

instance Showable String where
  show2 = id

instance (Show a) => Showable a where
  show2 = show

You need -XOverlappingInstances -XFlexibleInstances -XUndecidableInstances to compile and use it.

*Main> show2 "abc"
"abc"
*Main> show2 3
"3"

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