Ruby 中的 :+ 和 &:+ 是什么? [英] What are :+ and &:+ in Ruby?

问题描述

我已经看过好几次了，但我不知道如何使用它们.镐说这些是特殊的快捷方式，但我找不到语法描述.

I've seen these several times but I can't figure out how to use them. The pickaxe says that these are special shortcuts but I wasn't able to find the syntactical description.

我在这种情况下见过它们:

I've seen them in such contexts:

[1,2,3].inject(:+)

例如计算总和.

推荐答案

让我们从一个更简单的例子开始.假设我们有一个想要大写的字符串数组:

Let's start with an easier example. Say we have an array of strings we want to have in caps:

['foo', 'bar', 'blah'].map { |e| e.upcase }
# => ['FOO', 'BAR', 'BLAH']

此外，您还可以创建所谓的 Proc 对象(闭包):

Also, you can create so called Proc objects (closures):

block = proc { |e| e.upcase }
block.call("foo") # => "FOO"

您可以使用 & 将这样的过程传递给方法.语法:

You can pass such a proc to a method with the & syntax:

block = proc { |e| e.upcase }
['foo', 'bar', 'blah'].map(&block)
# => ['FOO', 'BAR', 'BLAH']

这样做是在块上调用 to_proc，然后为每个块调用它:

What this does, is call to_proc on block and then calls that for every block:

some_object = Object.new
def some_object.to_proc
  proc { |e| e.upcase }
end

['foo', 'bar', 'blah'].map(&some_object)
# => ['FOO', 'BAR', 'BLAH']

现在，Rails 首先将 to_proc 方法添加到 Symbol 中，该方法后来被添加到 ruby​​ 核心库中:

Now, Rails first added the to_proc method to Symbol, which later has been added to the ruby core library:

:whatever.to_proc # => proc { |e| e.whatever }

因此您可以这样做:

['foo', 'bar', 'blah'].map(&:upcase)
# => ['FOO', 'BAR', 'BLAH']

此外，Symbol#to_proc 更智能，因为它实际上执行以下操作:

Also, Symbol#to_proc is even smarter, as it actually does the following:

:whatever.to_proc # => proc { |obj, *args| obj.send(:whatever, *args) }

这意味着

[1, 2, 3].inject(&:+)

等于

[1, 2, 3].inject { |a, b| a + b }

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