Ruby 中的 Nil 和布尔值 [英] Nil and boolean in Ruby

问题描述

有人可以解释一下原因吗?刚刚花了 30 分钟试图弄清楚为什么我的布尔方法返回 nil 并在 Ruby 中发现了这一点:

Can someone explain the reasoning being this? Just spent 30 mins trying to figure out why my boolean method returned nil and found out that in Ruby:

2.2.1 :001 > nil && true
 => nil
2.2.1 :002 > nil && false
 => nil

由于 nil 是一个假值，我本来期望 nil && 的输出真 为假.似乎也违背了条件运算符应该返回布尔值的想法.

Since nil is a falsey value, I would have expected the output of nil && true to be false. Also it seems to go against the idea that conditional operators should return a boolean value.

这背后的原理是什么?

布尔运算符不可交换是有道理的:

It makes sense that the boolean operator is not commutative:

nil && false != false && nil

对于看到这一点的其他人，我的问题是在 Rails 中我有如下声明:

For others seeing this, my issue was that in rails I had a statement like:

def some_method?
  object.attr && object.attr > something
end

但是当 object.attr 为 nil 时，函数将为 nil.在大多数情况下这很好，但是当将布尔方法链接在一起时，就没有那么多了.我只是把它改成了这个:

But when object.attr is nil, the function will be nil. Which is fine in most cases but when chaining boolean methods together, not so much. I just changed it to this instead:

def some_method?
  object.attr.present? && object.attr > something
end

我可以在 vanilla Ruby 中做同样的事情:

I could do the same thing in vanilla Ruby with:

def some_method?
  !!object.attr && object.attr > something
end

推荐答案

该语句按顺序遍历条件，得到假结果时停止并返回上次执行的求值的值.

The statement goes through the conditions in order, will stop when a falsy result is obtained and return the value of the last evaluation performed.

与 && 不同的是，|| 会停在一个真值处.

In contrary to && which stops at a falsy value, || will stop at a truthy value instead.

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