问题描述
为什么下面的代码没有报错?
Why does the following code not throw an error?
if false
x = 0
end
x #=> nil
而以下确实会引发错误:
Whereas the following does throw an error:
y # NameError: undefined local variable or method `y' for main:Object
同样的事情发生在 unless &case 语句.
The same thing happens with unless & case statements.
推荐答案
这是因为 Ruby 解析器的工作方式.变量由解析器定义，解析器逐行遍历代码，无论是否实际执行.
It's because of how the Ruby parser works. Variables are defined by the parser, which walks through the code line-by-line, regardless of whether it will actually be executed.
一旦解析器看到x =，它就会在当前范围内定义局部变量x(值为nil).由于 if/unless/case/for/while 不创建新的作用域，x 被定义并且在代码块之外可用.并且由于内部块永远不会被评估为条件为假，x 没有分配给(因此是 nil).
Once the parser sees x =, it defines the local variable x (with value nil) henceforth in the current scope. Since if/unless/case/for/while do not create a new scope, x is defined and available outside the code block. And since the inner block is never evaluated as the conditional is false, x is not assigned to (and is thus nil).
这是一个类似的例子:
defined?(x) and x = 0
x #=> nil
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