Perl:虽然没有条件

问题描述

根据 doc，while只要表达式为 true，语句就会执行块.我想知道为什么它变成了一个带有空表达式的无限循环:

According to the doc, the while statement executes the block as long as the expression is true. I wonder why it becomes an infinite loop with an empty expression:

while () { # infinite loop
 ...
}

这只是文档不准确吗?

推荐答案

$ perl -MO=Deparse -e 'while () { }'
while (1) {
    ();
}
-e syntax OK

看起来while () {} 和while (1) {} 是等价的.还要注意空括号*被插入到空块中.

It seems that while () {} and while (1) {} are equivalent. Also note that empty parens* are inserted in the empty block.

预定义编译器行为的另一个示例:

Another example of pre-defined compiler behaviour:

$ perl -MO=Deparse -e 'while (<>) { }'
while (defined($_ = <ARGV>)) {
    ();
}
-e syntax OK

我会说这只是没有报告特殊情况的文档.

I would say that this is just the docs not reporting a special case.

* —准确地说，插入了 stub 操作码.它什么都不做，但为 enterloop 操作码提供了一个 goto 目标.没有真正的理由要注意这一点.Deparse 使用空括号表示这个 stub 操作，因为括号不生成代码.

* — To be precise, the stub opcode is inserted. It does nothing, but serves a goto target for the enterloop opcode. There's no real reason to note this. Deparse denotes this stub op using empty parens, since parens don't generate code.

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