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Re: Name "main::a" used only once: possible typo

by davido (Archbishop)
on Mar 05, 2013 at 20:10 UTC ( #1021896=note: print w/replies, xml ) Need Help??


in reply to Name "main::a" used only once: possible typo

This is a common problem with no known great solution (if there is a great solution I hope someone will show it so that we can submit patches). It also annoys anyone using List::MoreUtils pairwise, for about the same reason. Several ideas were tossed around in the List::MoreUtils RT as to how to fix the problem, but none were ideal, and none fixed it in 100% of use cases (including my attempt).

The easiest solution is still annoying, and looks like this:

reduce { no warnings 'once'; $a * $b } 1 .. 6;

Update: I see that the List::Util RT suggests something like this:

reduce { our ( $a, $b ); $a * $b } 1 .. 6;

Dave

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Re^2: Name "main::a" used only once: possible typo
by ikegami (Pope) on Mar 06, 2013 at 04:48 UTC
      I'd probably put the our ($a,$b) outside of that block, but this is what I had in mind. I was just interested in more options. - Thanks
      out ($a, $b); reduce { $a * $b } 1 .. 6;
        I was just interested in more options.

        This is kind of old-school and amounts to the same thing as sticking an  our ($a, $b); statement somewhere, but it's another option:

        >perl -wMstrict -le "use List::Util qw/reduce/; use vars qw($a $b); ;; print reduce { $a * $b } 1..6; " 720

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[Corion]: hippo: If you have a Unicode-wise Perl then likely some zero-width characters in $x would work. Maybe $x = "\x{200b}" works.
[Corion]: Hmm - no, that outputs 1 for me on 5.14 - perl -wle "my $x = qq(\x{200b}); warn $x; warn length $x"
[hippo]: Smart - I'll give that a go. Thanks.
[hippo]: Ah
[Corion]: But maybe there is some other Unicode string that will be true but have a zero width
[hippo]: For explanation, I've seen this construct in someone else's code (no names, no pack drill) and couldn't think of a situation to trigger it.
[Corion]: You'll have to look somewhere esoteric for that. Maybe some tied variable or special dualvar can also trigger that. But it's certainly not a common occurrence
[Corion]: And on 5.20, the following also outputs no find:perl -wle 'for my $x ("\x{2000}".."\ x{1fffff}") { if( $x && ! length $x ) { warn qq(<$x>); warn length $x; die } }'
[Corion]: (this time on Unix)
[hippo]: Understood. I'll have to go through the code and see if it's doing anything fancy with ties, dual-vars or non-scalars. In the end, it's probably a bug though.

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