在Haskell中，"@"是什么意思? [英] What does '@' mean in Haskell?

问题描述

我尝试使用Google谷歌搜索，但失败了.我通过阅读一些文章来增强我的Haskell知识，并且偶然发现了一种使用以前从未见过的语法的文章.一个例子是:

I've tried googling but come up short. I am furthering my Haskell knowledge by reading some articles and I came across one that uses a syntax I've never seen before. An example would be:

reconstruct node@(Node a b c l r) parent@(Node b d le ri)

我以前从未见过这些@.我尝试在网上搜索答案，但结果很短.这仅仅是嵌入标签以帮助使事情变得更清晰的一种方法，还是对代码有实际影响?

I've never seen these @'s before. I tried searching online for an answer but came up short. Is this simply a way to embed tags to help make things clearer, or do they have an actual impact on the code?

推荐答案

它用于模式匹配.现在， node 变量将引用参数 Node a b c l r 的整个 Node 数据类型.因此，您可以使用 node 代替将其传递给 Node a b c l r 作为功能，而不是传递给该函数.

It is used in pattern matching. Now node variable will refer to the entire Node data type for the argument Node a b c l r. So instead of passing to the function as Node a b c l r, you can use node instead to pass it up.

一个简单得多的示例来演示它:

A much simpler example to demonstrate it:

data SomeType = Leaf Int Int Int | Nil deriving Show

someFunction :: SomeType -> SomeType
someFunction leaf@(Leaf _ _ _) = leaf
someFunction Nil = Leaf 0 0 0

someFunction 也可以写为:

someFunction :: SomeType -> SomeType
someFunction (Leaf x y z) = Leaf x y z
someFunction Nil = Leaf 0 0 0

看看第一个版本有多简单?

See how simpler was the first version ?

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