Haskell下划线与显式变量 [英] Haskell underscore vs. explicit variable

问题描述

我已经学习Haskell几周了，我对使用下划线(_)作为函数参数有疑问.我想通过一个具体的例子可以更好地提出我的问题.假设我想定义一个函数，该函数根据提供的索引提取列表的元素，是的，我知道(!!)已经预先定义了.我可以通过以下两种方法定义函数(我敢肯定还有更多方法):

I've been learning Haskell for a few weeks, and I have a question about the use of the underscore (_) as a function parameter. I think my question will be better asked with a specific example. Let's say I want to define a function that extracts the element of a list based on the provided index—yes, I realize (!!) is already pre-defined. The two ways that I can define the function (I'm sure there are more) are the following:

版本1

indexedElement             :: [a] -> Int -> a
indexedElement xs n | n < 0 = error "Index can not be negative."
indexedElement [] n         = error "Index must be smaller than the length of the list."
indexedElement (x:xs) 0     = x
indexedElement (x:xs) n     = indexedElement xs (n - 1)

版本2

indexedElement            :: [a] -> Int -> a
indexedElement _ n | n < 0 = error "Index can not be negative."
indexedElement [] _        = error "Index must be smaller than the length of the list."
indexedElement (x:_) 0     = x
indexedElement (_:xs) n    = indexedElement xs (n - 1)

两个版本显然非常相似.两者之间的唯一区别是使用显式变量或下划线.对我来说，_意味着实际上可以在其中写入任何内容，而像n这样的显式变量使该参数必须是整数更加明显.因此，我更喜欢版本1.但是(!!)的GHC源代码的编写方式与第2版类似.第二版在功能上有优势吗?如果不是这样，核心" Haskell程序员是否会对版本1产生疑问?我了解使用一致的代码编写方式的重要性，因此我尝试遵循未编写的规则"来进行特定语言的编程.这是一个示例，尽管我更喜欢第一个版本，但我认为它不会使代码更难阅读.我不知道这是由于我的纯数学背景还是什么原因引起的，但我想听听您这些经验更丰富的Haskell兽医的想法.

The two versions are obviously very similar. The only difference between the two is the use of an explicit variable or an underscore. To me _ means that literally anything can be written there while an explicit variable like n makes it more obvious that the argument must be an integer. For that reason, I prefer Version 1; but the GHC source code for (!!) is written like Version 2. Is there a functional advantage of the second version? If not, would "hardcore" Haskell programmers take issue with Version 1? I understand the importance of having a consistent way of writing code, so I try to follow the "unwritten rules" for programming in a particular language. This is an example where I much prefer the first version though, and I don't think it makes the code any more difficult to read. I don't know if it's due to my background in pure math or what, but I'd like to hear what you more-seasoned-Haskell vets think.

推荐答案

第二个版本有功能上的优势吗?

Is there a functional advantage of the second version?

我认为它们在操作上没有任何区别.但是我发现第二个版本更具可读性. _表示根本没有使用它.因此，在阅读代码时，我可以忽略它，而只专注于其他参数.而在第一个版本中，我会认为已经定义了n，但是作者可能忘记使用它了?或者也许不需要该参数.第二个版本只是避免了这种精神上的负担.但这只是我的意见. :)

I don't think they have any operational difference. But I find the second version more readable. _ indicates that it isn't used at all. So while reading the code, I can just ignore it and just concentrate on the other parameters. Whereas in the first version, I will be thinking that n is defined but maybe the author forgot to use it ? Or maybe the argument isn't required. The second version just avoids this kind of mental overload. But this is just my opinion. :)

实际上，如果启用警告标志(-Wall)并编译代码，它将为您的第一个版本引发警告:

In fact, if you enable the warning flag (-Wall) and compile your code, it will throw warning for your first version:

[1 of 1] Compiling Main             ( code.hs, code.o )

code.hs:2:16: Warning: Defined but not used: ‘xs’

code.hs:3:19: Warning: Defined but not used: ‘n’

code.hs:4:19: Warning: Defined but not used: ‘xs’

code.hs:5:17: Warning: Defined but not used: ‘x’

code.hs:8:17: Warning: Defined but not used: ‘xs’

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