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

UPDATE: else: is a label. my bad.

So I'm learning Python. I always disdained it because no curlies, but once you start doing stuff in it you get used to it. Sort of

But I still use Perl when I want to cobble something up quick. Using multiple languages has it's issues, though. I get confused. Here's one case where Perl seems to be confused, too, at 5.26.1. I installed perlbrew and it still happens at 5.032000 and 5.033001 

#!/usr/bin/perl

print "Perl version is: $]\n";

use strict;
use warnings;

if ( 1 )
{
    warn "there can only be one, and it's in the IF clause\n"
}
else:   # beware that colon!!!! that's Python, not Perl!!!
{
    warn "there can only be one, and it's in the ELSE clause\n"
}

[download]

produces

Perl version is: 5.026001
there can only be one, and it's in the IF clause
there can only be one, and it's in the ELSE clause

Perl version is: 5.032000
there can only be one, and it's in the IF clause
there can only be one, and it's in the ELSE clause

Perl version is: 5.033001
there can only be one, and it's in the IF clause
there can only be one, and it's in the ELSE clause

[download]

No warnings, no syntax errors, nothing except the (seemingly) impossible behavior

Removing the colon from the else statement restores correct behavior. gvim didn't catch it either - it's syntax highlighting is as if this was perfectly valid.

Is this a bug, or am i being persnickety?

--Bob

All code given here is UNTESTED unless otherwise stated.

Replies are listed 'Best First'.
Re: Python gave me a colonic
by Paladin (Vicar) on Aug 24, 2020 at 22:46 UTC
    It is valid code. else: is a label, and LABEL BLOCK is valid perl syntax.
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      Correct, but

      > LABEL BLOCK is valid perl syntax.

      to be more precise

      LABEL: and { NAKED BLOCK } are already valid in isolation, they don't need to be compounded.

      Cheers Rolf
      (addicted to the Perl Programming Language :)
      Wikisyntax for the Monastery

[reply]
      OK, then, yes, it's a label. I saw else with a colon and didn't even think about labels, even though I've used them many times over the years.

      Actually, it seems a little odd that a keyword is permitted to be used in this fashion, but whatever.

      Thanks for the explanation. I'm gonna blame it on Guido, lol.

      --Bob Niederman, http://bob-n.com

      All code given here is UNTESTED unless otherwise stated.

[reply]
Re: Python gave me a colonic
by LanX (Saint) on Aug 25, 2020 at 00:14 UTC
    To elaborate more ...

    > No warnings, no syntax errors, nothing except the (seemingly) impossible behavior

    Like already explained, because of the colon else: becomes a label, so it's quite possible.

    BUT it's good practice - or at least my practice - to only use uppercase WORDS for labels.

    So a linter could easily catch this already.

    Furthermore it's pretty hard to use a keyword like else as a label b/c next else; and alike would be syntax errors.

    To make it work you'd need to hide it inside an expression like with goto ("else")[0];

    So yes, there are potential ways to catch unwanted labels and warn about this. (lowercase and or reserved keyword)

    POC: this runs fine

    use strict;
use warnings;

my $x;

else:
{
    print ++$x;
    redo ("else")[0] if $x <3;
}

    [download]

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery

    update

    in hindsight I realized that putting the label into quotes is already enough

    redo "else" if $x <3;

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Re: Python gave me a colonic (solved - brainfart)
by perlfan (Parson) on Aug 25, 2020 at 02:14 UTC
    >Python gave me a colonic

    LOL. Sounds like you got a labotomy to boot! This has given me some hilarious ideas for obfuscation research.

[reply]

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