为什么没有函数的 Show 实例? [英] Why is there no Show instance for functions?

问题描述

只是一个简单的概念性问题，我目前正在尝试更好地学习和理解 Haskell.

Just a quick conceptual question, I am currently trying to learn and understand Haskell better.

我知道 Show 函数用于将值转换为字符串，但为什么函数类型不能与 show 一起使用?

I know the Show function is used to convert values to strings, but why can't function types be used with show?

Prelude> (x -> x*3)

<interactive>:7:1:
    No instance for (Show (a0 -> a0))
      arising from a use of `print'
    Possible fix: add an instance declaration for (Show (a0 -> a0))
    In a stmt of an interactive GHCi command: print it
Prelude>

推荐答案

不是他们不能，而是通常没有充分的理由这样做.

It's not that they can't, but that there's not usually a good reason to.

但如果您愿意，绝对可以:

But if you'd like, you definitely can:

Prelude> :{
Prelude| instance Show (a -> b) where
Prelude|    show _ = "A function."
Prelude| :}
Prelude> print (x -> x + 7)
A function.
Prelude> print (a b c -> a + b + c)
A function.

如果您想显示函数的文本表示，那么 - 您不能那样做.与 Ruby、JS 等元编程语言不同，Haskell 对其内部结构知之甚少.

If you'd like to show the textual representation of the function, well - you can't do that. Unlike metaprogramming languages like Ruby, JS, etc, Haskell code very little knowledge of its own internals.

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