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Lexical $_ in given/when vs. BLOCK arguments

by mbethke (Hermit)
on Oct 25, 2011 at 09:35 UTC ( #933579=perlquestion: print w/ replies, xml ) Need Help??
mbethke has asked for the wisdom of the Perl Monks concerning the following question:

Hi guys,

I just noticed that the given/when construct's way of introducing a lexical $_ plays badly with functions that take BLOCKs such as List::MoreUtils::any. The following does not print anything:

use 5.010; use strict; use warnings; use List::MoreUtils qw/ any /; my $s = "foo"; my @list = ( 1, undef, 2 ); given($s) { when('foo') { say "found undef" if any { ! defined($_) } @list; } }

Within the block, $_ is always 'foo' as it was bound during the block's definition.

Now this must be a known problem, right? For my inner peace, could someone explain me the rationale for making "given" so subtly different from say "for"? And for my productivity, is there a way around it short of "don't use 'given'"?

Thanks very much!

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Re: Lexical $_ in given/when vs. BLOCK arguments
by Anonymous Monk on Oct 25, 2011 at 09:52 UTC
    Beats me, but you can use $::_ to refer to the global $_
        or even any { our $_; ... }

        The old standby local $_; will fail with

        Can't localize lexical variable $_ at junk line 13. (#1) (F) You used local on a variable name that was previously declared + as a lexical variable using "my" or "state". This is not allowed. If +you want to localize a package variable of the same name, qualify it with the package name.

        I think local $_ should just work, for those who can't remember to use our $_;

        At a minimum, I think the perldiag entry for this error should say to use our $_;

Re: Lexical $_ in given/when vs. BLOCK arguments
by moritz (Cardinal) on Oct 25, 2011 at 10:02 UTC

      I have to admit I didn't even search because after a look at the trivial code of any() (and pasting it in my script to make sure it's not the fault of some XS foo) I was pretty convinced it can't possibly be the function's fault. It's written exactly as I would do it and as every book I've seen recommends it.

      Thanks to everybody who recommended "our" etc.---I'd only tried local which unsurprisingly didn't work. Still, having to think about something like this is a major PITA, particularly when it's inconsistent and e.g. works with builtins like grep. I think for me it boils down to "given is evil". for/when should do fine as long as there are no continues.

      List::* and similar modules could probably work around that with some bytecode hack as suggested in a reply to the bug but having to introduce such extreme ugliness to restore sane behavior suggests something else needs fixing

      So I think this is at least a documentation bug of given/when. Why the lexical $_ I still don't know; making

      given($foo) { when('bar') { ... } }
      equivalent (well, with some added magic for the last/continue stuff) to
      do { if($foo ~~ 'bar') { ... } }
      would seem to make more sense IMHO. But if it behaves like it does, it should carry a big fat warning saying "If you define any blocks or callbacks within given(), don't expect $_ to behave like everywhere else, use explicit "our" or there will be weeping and gnashing of teeth!!!11!"

        I think for me it boils down to "given is evil".

        I concur. I tried to use given/when on a few occasions and always reverted to something simpler. Altogether just to damn magical for its own good. I seriously doubt I will ever make use of it again.

        I much prefer the simple, reliable semantics of use Switch::Perlish 'C';

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
Re: Lexical $_ in given/when vs. BLOCK arguments
by ikegami (Pope) on Oct 25, 2011 at 17:42 UTC

    It's a known problem with lexical $_, but lexical $_ is not a bug. The relevant developers of Perl hadn't yet clued in about this issue when given was added.

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