foreach $1

When answering a question on StackOverflow, I noticed an interesting anti-idiom (antidiom?) I haven't seen before (and haven't thought of trying):

foreach $1 (@array)
[download]

Interestingly, you can use it under strict and warnings and Perl doesn't protest. But $1 is aliased to the elements of the array and loses its magic behaviour:

#!/usr/bin/perl
use warnings;
use strict;

for $1 ('a' .. 'h') {
    print $1 if "A" =~ /(.)/;
}
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Can you guess what the output is?

So, if you ever get the idea of using special variables for something else than what they were designed to, stop.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

Comment on foreach $1 Select or Download Code

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Re: foreach $1 by LanX (Saint) on Jun 06, 2020 at 12:29 UTC
Interesting, thanks for showing! ++ Can anyone guess of a special var other than $_ where aliasing might make sense? Otherwise shouldn't we make a feature request to throw a compilation error? (At least under strict or warnings?) edit Hmm ... Should be noted here that a special var makes the whole typeglob special. I.e. if $a is a special var then @a and %a will act special too, even without carrying a meaning. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply]
Re^2: foreach $1 by kcott (Archbishop) on Jun 06, 2020 at 17:50 UTC
G'day Rolf, "Can anyone guess of a special var other than $_ where aliasing might make sense?" I made a quick scan through perlvar and nothing leapt out at me as being appropriate for this. "Otherwise shouldn't we make a feature request to throw a compilation error? (At least under strict or warnings?)" I'm not sold on making it a fatal error but I do think a warning may be appropriate. I had a look at "warnings: Category Hierarchy" but didn't think any existing category was really suitable: maybe create a new top-level category or a new sub-category of "syntax". — Ken	[reply]
Re^3: foreach $1 by LanX (Saint) on Jun 06, 2020 at 21:52 UTC
Well I'm not sure yet. I've tested with $a and the loop var didn't interfere with the sort block. Probably we need to decide case by case... Though my gut feeling says all non alphabetic symbols should cause a warning when used as a loop alias. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply]
Re^4: foreach $1 by choroba (Cardinal) on Jun 06, 2020 at 23:47 UTC
Re^5: foreach $1 -- all special variables by Discipulus (Canon) on Jun 07, 2020 at 19:36 UTC
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Re^5: foreach $1 by LanX (Saint) on Jun 07, 2020 at 02:19 UTC
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Re^2: foreach $1 by ikegami (Patriarch) on Jun 08, 2020 at 22:25 UTC
if $a is a special var then @a and %a will act special too, even without carrying a meaning. Well, they are considered "declared", but that's the extent of it.	[reply]
Re^3: foreach $1 by LanX (Saint) on Jun 08, 2020 at 23:55 UTC
> I.e. if $a is a special var then @a and %a will act special too, even without carrying a meaning. > > Well, they are considered "declared", but that's the extent of it. Nope, e.g. $% is a special var which is global, @% isn't documented anywhere, but still global `DB<1> x @%=1..3 0 1 1 2 2 3 DB<2> x {package A; @%} 0 1 1 2 2 3 DB<3> x @a=1..3 # not global 0 1 1 2 2 3 DB<4> x {package A; @a} empty array DB<5>` [download] Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply] [d/l]

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