|Perl: the Markov chain saw|
It *is* a feature: mod_perl and 'my $x if 0'by MeowChow (Vicar)
|on Jul 15, 2001 at 01:59 UTC||Need Help??|
The bug recently discussed in Unusual Closure Behaviour is considered something of an oddity, with little practical value, but here's what I believe is a valid and defensible use of it, specifically with regards to mod_perl, and more generally with regards to any tool that wraps scripts into callable subroutines/hooks.
Those of you who've developed for mod_perl know that it's a bad idea to create global lexical variables in your Apache::Registry scripts, since they do not stay shared when mod_perl wraps your code into a subroutine. For example, under mod_perl, the following script is transformed from:
into a subroutine like this:
As you see, foo() becomes one of those pesky inner named subroutines, which means that upon successive invocations of the script (or more precisely, the handler subroutine), the $fname and $lname variables of the inner sub will remain bound to their values from the first invocation, and will not reflect new values. This issue is well documented in the mod_perl guide, along with several remedies for the problem, none of which I like too much.
Looking over that discussion, it occured to me that the 'my $x if 0' construct can resolve this issue:
What's happening here is that successive invocations simply overwrite the values bound upon the initial invocation. Since we've cheated Perl out of enacting the run-time effect of my $x, this allows lexical variables to stay shared between outer and inner named subroutines (despite -w's admonitions to the contrary).
I don't think I would do something like this in production code. I'll probably stick to using package globals, perhaps with our-scoping, but it's certainly worth thinking about and perhaps incorporating as a bonified language feature in future perl releases, eg:
MeowChow s aamecha.s a..a\u$&owag.print