How do I prevent List::MoreUtils from warning about using $a and $b only once?

Question

The List::MoreUtils module indicates that you use the variables $a and $b when supplying the BLOCK that goes with the pairwise function. For example:

use strict;
use warnings;
use List::MoreUtils qw'pairwise';

my @x = ( 1 ..  5);
my @y = (11 .. 15);
my @sums = pairwise { $a + $b } @x, @y;

But when I do that, I get warnings like this:

Name "main::b" used only once: possible typo at try.pl line 7.
Name "main::a" used only once: possible typo at try.pl line 7.

Is there an elegant way to deal with this problem?

Answer 1

I have the same problem with a similar module I'm writing. The only solution I've found (other than using functions that use $a and $b twice, of course) is to put this line somewhere in your code:

$a = $b; # hack to disable warnings about "main::a" used only once

It basically does nothing, but it does disable the warning. Consider keeping the comment so future maintainers don't have to read your mind.

Answer 2

Depends on what you consider elegant.

no warnings qw(once);
our ($a, $b);

One of these two will suffice. You can even limit their scope pretty easily.

my @sums = pairwise { no warnings qw(once); $a + $b } @x, @y;
my @sums = pairwise { our $a + our $b } @x, @y;

Explicitly specifying the package will suppress the warning too. If you're in main,

my @sums = pairwise { $::a + $::b } @x, @y;

Answer 3

Add this near top of your program:

use vars qw( $a $b );

or, if you don't like the "obsolete" part of perldoc vars, simply add:

our ( $a, $b );

Answer 4

This is probably a bug in List::Util.

Turning off warnings globally is probably not a good idea, however you could do something like this:

{
  no warnings 'once';
  return join("_", @monsters) if @monsters && List::Util::reduce { $a && $b // 0 > 0 } 1,@monsters;
}

This would turn off the relevant warning category for that part of the code only.

Answer 5

Yep, it's not you. You can no warnings 'once'; or you can predeclare $a and $b so that they will not be used once anymore.

our ($a, $b);

does the trick. I tend to prefer that because it doesn't turn off warnings for anything else, and it's a bit more descriptive.