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

Hi monks,

A bit of background: I'm using Filter::Simple to inline a couple of methods for speed - I replace a method call with a do block. I started having a really weird problem -  return inlinesub(); and my $ret = inlinesub(); return $ret; would behave differently, even though the function was called in scalar context. I've drilled it down to a small example: 
use strict;
sub bar{
        if (1){
                return do {
                                #do block will return 1
                                if (1){
                                        1;
                                }
                                1;
                        }
        ;
        }
}
sub foo{
        if (1){
                return do {
                                #do block will return 1
                                1;
                        } ;
        }
}

print ( "foo said '".foo()."'\n");
print ( "bar said '".bar()."'\n");

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output: 
localhost:~# perl /tmp/z.pl
foo said '1'
bar said ''
localhost:~# perl -v

This is perl, v5.8.8 built for i486-linux-gnu-thread-multi

Copyright 1987-2006, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using "man perl" or "perldoc perl".  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.

localhost:~#

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This is all on debian - I get the same behaviour in stable (5.8.8-7) and unstable (5.8.8-11.1) - am I doing something wrong, or is this a bug? Thanks!

Replies are listed 'Best First'.
Re: Strange behaviour when returning the value of "do {}" - possible perl bug?
by kyle (Abbot) on Nov 02, 2007 at 17:13 UTC

    I notice that bar will return 1 if you introduce a scalar context explicitly. 

    sub bar{
    if (1){
        return scalar do {
            #do block will return 1
            if (1){
                1;
            }
            1;
        };
    }
}

    [download]

    I notice also that B::Deparse turns 'if(1)' into do, so you also get '1' if you give the optimizer more to chew on: 

    sub bar{
    my $truth = 'beauty';
    if ($truth){
        return do {
            #do block will return 1
            if ($truth) {
                1;
            }
            1;
        };
    }
}

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    My guess is that the scalar context is getting lost in all those nested do blocks, but even so, I can't see why that would be a problem. It looks like a bug to me.

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Re: Strange behaviour when returning the value of "do {}" - possible perl bug?
by gamache (Friar) on Nov 02, 2007 at 17:48 UTC
    Some simple test cases. Perl v5.8.8 on Linux and WinXP acted the same. 
    my @testsubs;

## test 1
push @testsubs, sub {
    return do {
        1;
        1;
    }
};
 
## test 2
push @testsubs, sub {
    if (1) {
        return do {
            1;
        }
    }
};

## test 3
push @testsubs, sub {
    if (1) {
        return do {
            1;
            1;
        }
    }
};

## test 4
push @testsubs, sub {
    if (1) {
        return scalar do {
            1;
            1;
        }
    }
};


for (1..@testsubs) {
    print "test $_ ", 
        $testsubs[$_-1]->()==1 ? 'passes' : 'fails', 
        "\n";
}

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    Output: 
    test 1 passes
test 2 passes
test 3 fails
test 4 passes

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Re: Strange behaviour when returning the value of "do {}" - possible perl bug?
by samtregar (Abbot) on Nov 02, 2007 at 17:06 UTC
    Sure looks like a bug to me. I'd try it with the latest 5.9.x dev code - if it's still broken there then report it to perl5-porters. You can find details on how to do this in perlhack and perlbug.

    -sam

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Re: Strange behaviour when returning the value of "do {}" - possible perl bug?
by moritz (Cardinal) on Nov 02, 2007 at 17:20 UTC
    I just tested it with blead (revision 32212) on Debian GNU/Linux 4.0 "Etch", it produces the same result.
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Re: Strange behaviour when returning the value of "do {}" - possible perl bug?
by Prof Vince (Friar) on Nov 02, 2007 at 22:48 UTC
    Crossposted from p5p :

    This reduces to :
    use strict;
use warnings;

sub foo {
 if (1) {
  return do {
   1;
  }
 }
}
sub bar {
 if (1) {
  return do {
   my $x;
   1;
  }
 }
}
sub baz {
 if (1) {
  return scalar do {
   my $x;
   1;
  }
 }
}

my $x = foo;
my $y = bar;
my $z = baz;

print "#$x#\n";
print "#$y#\n";
print "#$z#\n";

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    This looks like a duplicate of "my" old bug #38809. If you turn warnings on, you'll notice that bar returns undef. This seems to have to do with the return context, since changing to return scalar do fixes the problem.

    Edit: When ran into taint mode, foo fails too.

    Edit^2: Bah, exactly what kyle said !
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Re: Strange behaviour when returning the value of "do {}" - possible perl bug?
by RenalPete (Acolyte) on Nov 02, 2007 at 18:48 UTC
    Thank you all, I have perlbuged it
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Re: Strange behaviour when returning the value of "do {}" - possible perl bug?
by ambrus (Abbot) on Nov 03, 2007 at 16:57 UTC
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