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

szabgab has asked for the wisdom of the Perl Monks concerning the following question:

use strict;
use warnings;

use List::Util qw/reduce/;
print reduce { $a * $b } 1..6;
[download]

Genetates the following warnings:

Name "main::a" used only once: possible typo at x.pl line 6.
Name "main::b" used only once: possible typo at x.pl line 6.
[download]

OTOH

use strict;
use warnings;

print sort {$a <=> $b } 1..6;
[download]

does not generate the warnings. As expected. (running perl 5.14.2 and List::Util 1.27)

Why did the first example generate the warnings?
What is the recommended way to silence them?
Why is this not mentioned in List::Util?

Comment on Name "main::a" used only once: possible typo Select or Download Code

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Re: Name "main::a" used only once: possible typo by davido (Cardinal) on Mar 05, 2013 at 20:10 UTC
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;` [download] Update: I see that the List::Util RT suggests something like this: `reduce { our ( $a, $b ); $a * $b } 1 .. 6;` [download] Dave	[reply] [d/l] [select]
Re^2: Name "main::a" used only once: possible typo by ikegami (Patriarch) on Mar 06, 2013 at 04:48 UTC
or `reduce { our $a * our $b } 1 .. 6;` [download]	[reply] [d/l]
Re^3: Name "main::a" used only once: possible typo by szabgab (Priest) on Mar 06, 2013 at 05:16 UTC
I'd probably put the our ($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;` [download]	[reply] [d/l]
Re^4: Name "main::a" used only once: possible typo by AnomalousMonk (Archbishop) on Mar 11, 2013 at 17:27 UTC
Re: Name "main::a" used only once: possible typo by toolic (Bishop) on Mar 05, 2013 at 18:58 UTC
One way to silence the warnings is to disable them in a localized scope: `use warnings; use strict; use List::Util qw/reduce/; { no warnings qw(once); print reduce { $a * $b } 1..6; }` [download] Update: added "once" Update: Refer to the following discussion: https://rt.cpan.org/Public/Bug/Display.html?id=65629	[reply] [d/l]
Re: Name "main::a" used only once: possible typo (limitations of "sort" like functions) by LanX (Saint) on Mar 05, 2013 at 21:57 UTC
Hi Gabor > Why did the first example generate the warnings? Wrong question. Since sort is a built-in, it was able to silence the warnings! > What is the recommended way to silence them? Use them twice? > Why is this not mentioned in List::Util? Maybe because the author is frustrated about not beeing able to simulate a built-in? Seriously ... ... it's not that easy to design functional constructs with special variables in code-blocks like sort does. I tried it in hgrep and then I was confronted with many scoping problems, what if the block is defined within another package or if $a and $b are declared as lexicals? Look into the code of Hash::MostUtils to have an idea about the complications to solve all of this. I bet that List::Util couldn't solve all of this. UPDATE To prove my last guess: This prints 55, but after uncommenting the second line its only 0 ...oops! `use List::Util qw/reduce/; # my ($a,$b)=(0,0); print reduce { $b+=$a } 1..10;` [download] UPDATE and here the same problem for List::MoreUtils `use warnings; use strict; # my ($a,$b)=(0,0); use List::MoreUtils qw/pairwise/; my @a = (1 .. 5); my @b = (11 .. 15); my @x = pairwise { $a + $b } @a, @b; # returns 12, 14, 16, 18, 20 use Data::Dump; dd \@x;` [download] UPDATE At least sort simply dies `my ($a,$b)=(0,0); print sort { $b cmp $a } 1..10;` [download] `Can't use "my $b" in sort comparison at ... line 2.` [download] Update ...but `use warnings; use strict; our ($a,$b); package Tst; print sort { $b cmp $a } 1..10;` [download] `Use of uninitialized value $b in string comparison (cmp) at /home/lanx +/B/PL/PM/ScopeListUtils.pl line 9. Use of uninitialized value $a in string comparison (cmp) at /home/lanx +/B/PL/PM/ScopeListUtils.pl line 9. Use of uninitialized value $b in string comparison (cmp) at /home/lanx +/B/PL/PM/ScopeListUtils.pl line 9. Use of uninitialized value $a in string comparison (cmp) at /home/lanx +/B/PL/PM/ScopeListUtils.pl line 9. ...` [download] Cheers Rolf	[reply] [d/l] [select]
Re^2: Name "main::a" used only once: possible typo (limitations of "sort" like functions) by szabgab (Priest) on Mar 06, 2013 at 05:13 UTC
LanX, what I don't understand is why this warning, and the way to deal with it by the user of the module is not mentioned in the documentation as a limitation of perl/the module. If the situation I described is real, then I assume many people bump into it and many people think they are doing something wrong. So unless the code I wrote has the flaw, it would IMHO better to warn the user about the warning.	[reply]
Re^3: Name "main::a" used only once: possible typo (limitations of "sort" like functions) by LanX (Saint) on Mar 06, 2013 at 09:42 UTC
> LanX, what I don't understand is why this warning, and the way to deal with it by the user of the module is not mentioned in the documentation as a limitation of perl/the module. I can only suppose that the problems are so overwhelming, that the authors don't know how to describe them. Cheers Rolf	[reply]
Re: Name "main::a" used only once: possible typo by Khen1950fx (Canon) on Mar 06, 2013 at 03:33 UTC
Maybe this is satisfactory? `#!/usr/bin/perl -l use strict; use warnings FATAL => 'syntax'; use List::Util qw/reduce/; print reduce( sub { $a * $b; }, 1..6);` [download]	[reply] [d/l]
Re: Name "main::a" used only once: possible typo by kcott (Archbishop) on Mar 08, 2013 at 08:07 UTC
G'day szabgab, I see you've got explanations for why this is happening. In terms of a workaround, I would probably add `$a \|\| $b \|\| 1;` after the `use List::Util ...` line. This would help to indicate why that code was there through association and would also remove the clutter of `no warnings ...` or `our ...` from the calls to `reduce()`. Furthermore, changes to code would not require any additional workaround changes. Here's some examples: `$ perl -Mstrict -Mwarnings -E ' use List::Util qw/reduce/; say reduce { $a * $b } 1..6; ' Name "main::a" used only once: possible typo at -e line 3. Name "main::b" used only once: possible typo at -e line 3. 720` [download] `$ perl -Mstrict -Mwarnings -E ' use List::Util qw/reduce/; $a \|\| $b \|\| 1; say reduce { $a * $b } 1..6; ' 720` [download] `$ perl -Mstrict -Mwarnings -E ' use List::Util qw/reduce sum/; $a \|\| $b \|\| 1; say reduce { $a * $b } 1..6; say sum 1..6; ' 720 21` [download] -- Ken	[reply] [d/l] [select]
Re: Name "main::a" used only once: possible typo by subogero (Initiate) on Sep 12, 2017 at 15:56 UTC
The best solution I found so far is this `$SIG{__WARN__} = sub { my $w = shift; warn $w unless $w =~ /::[ab]" used only once:/; };` [download] EDIT: An even better one, preserving an already existing signal handler. It still does not guarantee ours won't be overwritten. `my $orig_warn = $SIG{__WARN__}; $SIG{__WARN__} = $orig_warn ? sub { my $w = shift; $orig_warn->($w) if $w !~ /::[ab]" used onl +y once/ } : sub { my $w = shift; warn $w if $w !~ /::[ab]" used only once/ } +;` [download] EDIT: an even shorter one: `my $abwarn = $SIG{__WARN__} // sub { warn shift }; $SIG{__WARN__} = sub { my $w = shift; $abwarn->($w) if $w !~ /::[ab]" used only once:/; };` [download]	[reply] [d/l] [select]
Re: Name "main::a" used only once: possible typo by mithaldu (Monk) on Mar 11, 2013 at 13:23 UTC
I am confused that nobody suggested: `print reduce { $_[0] * $_[1] } 1..6;`	[reply] [d/l]
Re^2: Name "main::a" used only once: possible typo by toolic (Bishop) on Mar 11, 2013 at 13:38 UTC
Perhaps because it produces the incorrect result and 8 more warning messages: #!/usr/bin/env perl use warnings; use strict; use List::Util qw/reduce/; print reduce { $_[0] * $_[1] } 1..6; __END__ Use of uninitialized value in multiplication () at x.pl line 7. Use of uninitialized value in multiplication () at x.pl line 7. Use of uninitialized value in multiplication () at x.pl line 7. Use of uninitialized value in multiplication () at x.pl line 7. Use of uninitialized value in multiplication () at x.pl line 7. Use of uninitialized value in multiplication () at x.pl line 7. Use of uninitialized value in multiplication () at x.pl line 7. Use of uninitialized value in multiplication () at x.pl line 7. Use of uninitialized value in multiplication () at x.pl line 7. Use of uninitialized value in multiplication () at x.pl line 7. 0 [download]	[reply] [d/l]
Re^2: Name "main::a" used only once: possible typo by LanX (Saint) on Mar 11, 2013 at 13:40 UTC
cause it doesn't work? `use List::Util qw/reduce/; print reduce { $_[0] * $_[1] } 1..6; __END__ Use of uninitialized value in multiplication (*) at /home/lanx/B/PL/PM +/ScopeListUtils.pl line 23. ...` [download] Cheers Rolf	[reply] [d/l]
Re^3: Name "main::a" used only once: possible typo by mithaldu (Monk) on Mar 11, 2013 at 17:48 UTC
Well damn. I was sure i had used that on List::Util stuff to get around $a/b warnings. Guess i misremembered.	[reply]
Re^2: Name "main::a" used only once: possible typo by subogero (Initiate) on Sep 13, 2017 at 10:42 UTC
I guess List::Util::reduce does not pass the parameter pair at all to your function, it just localizes $a and $b with them.	[reply]

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