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Postings from January 2022
On Fri, Jan 21, 2022 at 6:58 PM Ed Avis <ed.avis@pgim.com> wrote:
> On the subject of passing aliases to subroutines, I will note there is a
> longstanding wart with passing regexp capture variables.
>
> our $CONFIG = 'logging level 5';
>
> sub foo {
> if ($CONFIG =~ /logging level (\d+)/) { say "set logging to $1" }
> say "string is $_[0]";
> }
>
> $_ = 'hello';
> /(.+)/ or die;
> foo($1);
>
> The contents of @_ get unexpectedly changed by what looks like a read-only
> regexp operation on some other variable. This gotcha does lead to bugs in
> real code. I think it would be better for perl to take a copy of $1, etc,
> when passing them to subroutines, rather than passing an alias. After all,
> that's the behaviour we expect for other magic variables like $_.
>
It actually isn't. Passing $_ around can lead to similar issues due to its
superglobal nature. The only protection is the implicit localization done
by foreach/map/grep when using it, most commonly bugs arise from using
while-readline which clobbers $_ and does not localize it, as with regular
assignment.
-Dan
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