Re^2: sub that sets $_

[Animator]

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

package Example;

use overload q{bool} => sub { $_="def"; }; 

sub new { bless {}; } 
sub test { return $_[0]; } 

package main;
my $z=new Example;
$_="ghi";
while ($m = $z->test()) { 
  print; 
  print $m; 
  last; 
};

Update: added relevant part of IO::Prompt

[TheDamian]

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. :-(

[TheDamian]

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";
    }
}

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. :-(