http://www.perlmonks.org?node_id=470586


in reply to sub that sets $_

It is possible to do what you want...by returning an object. See the implementation of IO::Prompt for an example.

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Re^2: sub that sets $_
by Animator (Hermit) on Jun 28, 2005 at 12:08 UTC

    That's not quite correct...

    IO::Prompt returns an object, in that object bool is overloaded, and bool sets $_. (which is run for each iteration of the while to decided if it's a true or false value)

    Which means that while ($z = prompt "") { will set $_ and $z. This is a bad idea. What if the code is used in an inner loop? when the outer loop uses $_ for example?

    If one wants to get the same behaviour then it would be enough to do a simple assignment in the subroutine to $_ and return the value of $_.

    Example code that is similar to the code in IO::Prompt

    Read more... (460 Bytes)

    Update: added relevant part of IO::Prompt

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      Which means that while ($z = prompt "") { will set $_ and $z. This is a bad idea. What if the code is used in an inner loop? when the outer loop uses $_ for example?
      Yes, that is a nuisance. I'm working on overcoming that problem, but it's difficult: Perl never quite gives you enough context information to detect the assignment. :-(
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        On further investigation, it appears that you can tell the two contexts apart using the Want module. Specifically, want('BOOL') knows the difference. For example: 
        my $flipflop = 0;

sub foo {
    use Want qw(want);

    print 'not ' if !want 'BOOL';
    print "bool\n";

    return $flipflop=!$flipflop;   # alternate true and false
}

for (1..3) {
    while (foo() ) {
        # print "here 1\n";
    }
    while ($answer = foo() ) {
        # print "here 2\n";
    }
}

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        Unfortunately, under 5.8.3 I get bus errors if there I uncomment the print statements, or if there is anything in the while blocks. :-(

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Re^2: sub that sets $_
by holli (Abbot) on Jun 28, 2005 at 12:29 UTC
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      First of all congratulations with the excelent idea to use DESTROY to set $_.

      Second, it still has a little bug though... it will set the value of $_ when the object gets destroyed... which is why it works and why it is bugged. Example: 

      $_ = "abc";
{
  my $line = $reader->chunk();
}
print $_;

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      $_ won't have the value "abc"...

      Update: since I'm not that familiar with overloading I re-looked at the overload pod... (to see if there is a function that is always called, so that the DESTROY could be made conditional/optional). In it I found an intresting overload option.

      You can overload <>. This would mean that you can overload it in your main module and use <$reader>. Which will set the appropriate variable. (Using < $reader->chunk > is not possible since it is a syntax error. You could ofcourse assign it to a temp var and use that but what would be wrong with using <$reader>?)

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