in reply to Re: Short-circuiting a map list. (This works! But ...)
in thread Short-circuiting a map list.

In 5.12.2 I get as you, 0,1,2,3,4,5,6,7,8,9, with 0,1,2,3,4

If I explicitly add return map{} I get only 0,1,2,3,4,5,6,7,8,9, without 0,1,2,3,4

In perl 5.6 I get 0,1,2,3,4,5,6,7,8,9, without 0,1,2,3,4

In perl 5.14.1 I get 0,1,2,3,4,5,6,7,8,9, BUT with 0,0,0,0

perlsub says: If no return is found and if the last statement is an expression, its value is returned. If the last statement is a loop control structure like a foreach or a while , the returned value is unspecified.

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Re^3: Short-circuiting a map list. (This works! But ...)
by BrowserUk (Patriarch) on Oct 08, 2011 at 15:21 UTC

    Okay. If what you are telling me is that I am utilising unspecified behaviour, then how about this version which produces identical output but, as far as I can tell, does nothing that is either prohibited or unspecified? 

    sub a{
    my $x = shift;

    return do{
        {
            print 'a';
            map {
                print 'b';
                last if $_ == $x;
                $_;
            } @_;
            print 'c';
        }
    };
}

my @data = 0 .. 9;

print for a( 5, @data );

    [download]

    Does that mean uncovered an obscure bug?

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]

      Does that mean uncovered an obscure bug?

      Probability is high :)

      do does say "do BLOCK" does *not* count as a loop, so the loop control statements "next", "last", or "redo" cannot be used to leave or restart the block. See perlsyn for alternative strategies.

[reply]
        do does say "do BLOCK" does *not* count as a loop,

        I don't think that is relevant as it is the anonymous block inside the do block that is the target of the last?

        I can also get the same output with the same weirdness whilst avoiding the do block:

        sub b{
    my $x = shift;

    goto sub {
        {
            print 'ba';
            map {
                print 'bb';
                last if $_ == $x;
                $_;
            } @_;
            print 'bc';
        }
    };
}

my @data = 0 .. 9;

print for b( 5, @data );

        [download]

        I am completely at a loss to explain the return values being stacked on top of the passes values?

        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]
      The behaviour is just as unspecified for a bare loop ({ ... }) as it is for a while loop (while (...) { ... }).
[reply]
[d/l]
[select]
        If the last statement is a loop control structure like a foreach or a while , the returned value is unspecified.

        But the bare block is not the last statement.

        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

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