Haskell 中的模式匹配等效变量，例如 Prolog [英] Pattern matching equivalent variables in Haskell, like in Prolog

问题描述

在 prolog 中，我们可以这样做:

In prolog, we can do something like the following:

myFunction a (a:xs) = ...

也就是说，当 myFunction 的第一个参数与第二个参数中的列表的第一项相同时，此函数将评估为 ....

This is, when the 1st argument of myFunction is the same as the first item of the list that's in the 2nd argument, this function will evaluate to ....

我现在的问题是......如何在 Haskell 中完成类似的事情?我认为 Prolog 的模式匹配比 Haskell 的更具表现力.我一直在尝试在 Haskell 中编写代码，但遇到了麻烦——要么我使用了无效的语法，要么上面的技巧根本行不通.

My question now is... how to accomplish a similar thing in Haskell? I have the idea that Prolog's Pattern Matching is more expressive than Haskell's. I've been trying to code that in Haskell and I'm having trouble -- either I am using invalid syntax or the above trick will simply not do.

推荐答案

Haskell 不做这种变量匹配".您必须明确表示:

Haskell doesn't do this kind of "variable matching". You'll have to explicitly put a guard on:

myFunction a (x:xs)
    | x == a = ...

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