红宝石:&&返回"vs “并返回"

问题描述

在 http://guides.rubyonrails 浏览 Rails 指南时.org/layouts_and_rendering.html#avoiding-double-render-errors ,我写了一个测试程序来测试Ruby的&&返回，我得到了这个奇怪的行为:

While going through the Rails guide at http://guides.rubyonrails.org/layouts_and_rendering.html#avoiding-double-render-errors ,I wrote a test program to test Ruby's && return, and I got this strange behavior:

def test1
  puts 'hello' && return
  puts 'world'
end

def test2
  puts 'hello' and return
  puts 'world'
end

这是结果输出:

irb(main):028:0> test1
=> nil
irb(main):029:0> test2
hello
world
=> nil

造成这种差异的原因是什么?

What accounts for the difference?

推荐答案

查看and 和 && 之间的区别.在示例中，您给出的方法 puts 在其参数周围没有括号的情况下被调用，并且优先级的差异改变了它的解析方式.

Check out the difference between and and &&. In the examples you give the method puts is called without parens around it's arguments and the difference in precedence changes how it is parsed.

在测试 1 中 && 比方法调用具有更高的优先级.所以实际发生的是puts('hello' && return).参数总是在调用它们的方法之前被评估——所以我们首先评估 'hello' &&返回.由于 'hello' 是真实的，布尔值不会短路并且 return 被评估.当返回时，我们不做任何其他事情就退出该方法:因此没有记录任何内容并且第二行不会运行.

In test 1 && has higher precedence than the method call. So what's actually happening is puts('hello' && return). Arguments are always evaluated before the methods they're called with -- so we first evaluate 'hello' && return. Since 'hello' is truthy the boolean does not short circuit and return is evaluated. When return we exit the method without doing anything else: so nothing is ever logged and the second line isn't run.

在测试 2 中 and 的优先级低于方法调用.所以发生的是puts('hello') 和return.puts 方法记录传递给它的内容，然后返回 nil.nil 是一个假值，所以 and 表达式短路，return 表达式永远不会被计算.我们只是移动到 puts 'world' 运行的第二行.

In test 2 and has a lower precedence than the method call. So what happens is puts('hello') and return. The puts method logs what is passed to it and then returns nil. nil is a falsey value so the and expression short circuits and the return expression is never evaluated. We just move to the second line where puts 'world' is run.

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