在这个 Perl 子程序中使用 vars 有什么意义?

问题描述

在 掌握 Perl 的一章中，brian d foy 展示了来自 List::Util:

In one of the chapters in Mastering Perl, brian d foy shows this snippet from List::Util:

sub reduce(&@) {
    my $code = shift;
    no strict "refs";
    return shift unless @_ > 1;
    use vars qw($a $b);
    my $caller = caller;
    local(*{$caller . "::a"}) = \my $a;
    local(*{$caller . "::b"}) = \my $b;
    $a = shift;
    foreach(@_) {
        $b = $_;
        $a = &{$code}();
    }
    $a;
}

我不明白 use vars qw($a $b) 行的意义何在.即使我评论它，我也会得到相同的输出 &警告.

I don't understand what's the point of the use vars qw($a $b) line. Even if I comment it, I get the same output & warnings.

推荐答案

这样做是因为 List::Util 在内部使用了 reduce() 函数.

This is done because List::Util uses reduce() function internally.

在没有use vars的情况下，使用该函数时给出如下警告:

In the abscence of use vars, the following warning is given when the function is used:

Name "List::MyUtil::a" used only once: possible typo at a.pl line 35.
Name "List::MyUtil::b" used only once: possible typo at a.pl line 35.

您可以通过运行以下代码亲眼看到这一点:

You can see this for yourself by running the following code:

use strict;
use warnings;

package List::MyUtil;

sub reduce (&@) {
   # INSERT THE TEXT FROM SUBROUTINE HERE - deleted to save space in the answer
}

sub x {
    return reduce(sub {$a+$b}, 1,2,3);
}

package main;
my $res = List::MyUtil::x();
print "$res\n";

然后在禁用 use vars 的情况下再次运行它.

And then running it again with use vars disabled.

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