Assign in loop with and without declaration

andreas1234567 has asked for the wisdom of the Perl Monks concerning the following question:

What is it that prohibits assignment in loop to work when combined with declaration (my)? Why can't perl just DWIM (which is to set $i = 3 in both examples below)?

$ perl -l
use strict;
use warnings;

my $i = undef;
$i = $_ for (1 .. 3);
($i) ? print $i : warn "pre-defined - missing";

my $j = $_ for (1 ..3);
($j) ? print $j : warn "direct - missing";

__END__
3
direct - missing at - line 9.
[download]

I just wonder since, after all, it does what I mean without warnings and my:

$ perl -l
$j = $_ for (1 ..3);
($j) ? print $j : warn "no warnings - missing";
__END__
3
[download]

For the record:

This is perl, v5.8.5 built for i386-linux-thread-multi
[download]

--
No matter how great and destructive your problems may seem now, remember, you've probably only seen the tip of them. [1]

Comment on Assign in loop with and without declaration Select or Download Code

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Re: Assign in loop with and without declaration by ikegami (Patriarch) on May 22, 2008 at 08:56 UTC
Don't use a statement modifier on an `my`. It's a bug to do so. This is even explicitly mentioned in perlsyn. There is no answer to your "why?" since the docs specifically say that's Perl's behaviour in that situation can be anything. (That would include crashing.)	[reply] [d/l]
Re^2: Assign in loop with and without declaration by andreas1234567 (Vicar) on May 22, 2008 at 09:35 UTC
Thank you for directing me to the documentation that says: NOTE: The behaviour of a my statement modified with a statement modifier conditional or loop construct (e.g. `my $x if ...`) is undefined. The value of the my variable may be undef, any previously assigned value, or possibly anything else. Don't rely on it. Future versions of perl might do something different from the version of perl you try it out on. Here be dragons. My follow-up question is: Should there not at least be a warning issued when one attempts something as unpredictable as this? -- No matter how great and destructive your problems may seem now, remember, you've probably only seen the tip of them. [1]	[reply] [d/l]
Re^3: Assign in loop with and without declaration by Anonymous Monk on May 22, 2008 at 09:53 UTC
You're not the first to think of warnings:) Deprecated use of my() in false conditional	[reply]
Re^4: Assign in loop with and without declaration by andreas1234567 (Vicar) on May 22, 2008 at 10:34 UTC
Re^5: Assign in loop with and without declaration by rir (Vicar) on May 22, 2008 at 14:51 UTC
Re: Assign in loop with and without declaration by Anonymous Monk on May 22, 2008 at 08:19 UTC
because of scope, same as `my $i = undef; for(1..3){ $i = $_; } for(1..3){ my $j = $_; }` [download]	[reply] [d/l]
Re: Assign in loop with and without declaration by mwah (Hermit) on May 22, 2008 at 08:26 UTC
imho, it creates it's own scope level that includes the LHS of the assignment: `... { my $j; $j = $_ for (1 ..3) } ...` [download] Regards mwa	[reply] [d/l]
Re: Assign in loop with and without declaration by runrig (Abbot) on May 22, 2008 at 16:19 UTC
I just wonder since, after all, it does what I mean without warnings and my Because without strict and warnings, the variable is a package variable, not a lexical variable. The strict version of your last example would be: `use strict; use warnings; use vars qw($j); # or "our $j;" $j = $_ for (1 ..3); ($j) ? print $j : warn "no warnings - missing";` [download]	[reply] [d/l]


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