Ruby 试图掌握一种新的符号.(inject(:) vs select(&:even?); 为什么有 &?) [英] Ruby trying to grasp a new notation. (inject(: ) vs select(&:even?); why one has &?)

问题描述

所以，我刚刚了解到，不要写这样的东西:

So, I've just learned that instead of writing things like:

[1,2,3,4,5].inject {|x,y| x + y} => 15

我会写

[1,2,3,4,5].inject(:+) => 15

我也学会了，而不是写作

I also learned that instead of writing

[1,2,3,4,5].select {|x| x.even?} => [2,4]

我会写

[1,2,3,4,5].select(&:even?) => [2,4]

我的问题是为什么一个 (select) 使用 & 而另一个 (inject) 没有.我确定 : 是因为 even? 和 + 被视为符号，但我很想澄清为什么 & 用于其中一个以及为什么使用 :.

My question is why one (select) uses the & and the other one (inject) doesn't. I'm sure that the : are because even? and + are treated at symbols, but I'd love clarification behind why the & is used in one and why the : are used.

此外，我知道我不仅可以在 inject 和 select 上使用这些符号.

Also, I'm aware that I could do these notations on more than just inject and select.

非常感谢！

推荐答案

& 运算符在这种情况下将符号转换为过程.所以代码在幕后做到了这一点:

& operator in this case converts a symbol to a proc. So the code does this under the hood:

[1,2,3,4,5].select {|elem| elem.send :even? } # => [2, 4]

inject 方法的实现认识到对这些快捷方法规范的需求，并为符号作为参数传递时添加了特殊处理.所以，在这种情况下，它基本上做 & 运算符所做的.

Implementation of inject method recognizes the need for these shortcut method specification and adds a special handling for when symbol is passes as a parameter. So, in this case, it basically does what & operator does.

为什么一个(选择)使用 &而另一个(注入)没有

why one (select) uses the & and the other one (inject) doesn't

因为没有人以这种方式实现select.Ruby 是开源的，他们甚至可以接受你的补丁.去解决这个问题:)

Because nobody implemented select this way. Ruby is open-source, they may even accept your patch. Go and fix this :)

PS:但如果由我决定，我会改为删除注入的特殊处理.使用 Symbol#to_proc(这就是 & 运算符使用的)感觉有点多余和混乱.

P.S.: But if it were up to me, I would instead remove inject's special handling. It feels a little bit redundant and confusing in presence of Symbol#to_proc (that's what & operator uses).

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