On Aug 26, 12:25 pm, shlo...@shlomifish.org (Shlomi Fish) wrote:
> ...
> The problem starts to happen when you try to declare $a and $b using my. This
> program:
>
> [CODE]
> #!/usr/bin/perl
>
> use strict;
> use warnings;
>
> my $a = 5;
> my $b = 6;
>
> print map { "$_\n" } sort { $a <=> $b } (9,100,5,6,70,3,4,98,28,27);
> [/CODE]
>
> Yields this error:
>
> [CODE]
> Can't use "my $a" in sort comparison at test.pl line 9.
> [/CODE]
>
> This is perl-5.14.1 - previous versions of Perl may behave more
> erratically when executing this. $a and $b are built-ins plain and simple, and
> should be treated as such, due to their use by perldoc -f sort and other
> functions from List::Util, List::MoreUtils, etc.
The use of $a,$b elsewhere should definitely ring an
alarm. Remembering: "use diagnostics qw/-verbose/
though makes the problem/solution very clear:
(F) The global variables $a and $b are reserved for sort
comparisons. You mentioned $a or $b in the same
line as the <=> or cmp operator, and the variable had
earlier been declared as a lexical variable. Either
qualify the sort variable with the package name, or
rename the lexical variable.
Rob's follow-on remarks:
------------------------------
> > Overly-careful warnings can have the opposite of the
desired effect,
> > especially on beginner programmers, and make it seem
like the language
> > is rife with pitfalls and gotchas, especially when these
apply to
> > ubiquitous core concepts like $_. I hope people will think
twice about
> > the ideas that they are conveying.
I tend to agree here that a cautionary -- rather
than dogmatic -- tone is the better choice.
--
Charles DeRykus
--
To unsubscribe, e-mail: beginners-unsubscr...@perl.org
For additional commands, e-mail: beginners-h...@perl.org
http://learn.perl.org/
