Greetings fellow monks,

This evening I was in the process of doing some work on a GUI I've been developing, when I came across a very strange phenomenon that I can't explain.  The test program here exhibits the same symptoms I found in the (much longer) original program.  I've kept it as a GUI, but I'm guessing it would be quite easy to duplicate this as a 10-15 line simple Perl script as well.

First of all, here's the program:

#!/usr/bin/perl -w
#
#  070502 liverpole -- Demonstrates a weirdness which I don't understa
+nd
#

# Strict
use strict;
use warnings;

# Libraries
use Tk;

##################
## Main program ##
##################

# Construct the GUI
my $mw = new MainWindow();
my $f1 = $mw->Frame()->pack;

my $chosen = "";
my $fr1 = $f1->Frame()->pack(-side => 'left');
my $fr2 = $f1->Frame()->pack(-side => 'left');
button($fr1, 'green', ' Exit GUI   ', sub { $mw->destroy() });
button($fr1, 'green', ' Start test ', sub { start_test($fr2) });
$fr1->Label(-width => 16, -textvar => \$chosen, -bg => 'white')->pack;

MainLoop();


#################
## Subroutines ##
#################

sub button {
    my ($w, $bg, $text, $pcmd) = @_;
    my $b = $w->Button(-bg => $bg, -text => $text);
    $b->configure(-width => 16, -command => $pcmd);
    $b->pack;
    return $b;
}


sub randomize_list {
    my $plist = shift;
    my $prandom = [ ];
    while (@$plist) {
        push @$prandom, splice(@$plist, int(rand(@$plist)), 1);
    }
    return $prandom;
}


sub start_test {
    my ($frame) = @_;

    # Create a random list of 9 items { 1 ... 9 }
    my @values  = qw( 1 2 3 4 5 6 7 8 9 );
    my $pvalues = randomize_list([ @values ]);

    my $chosen_idx = -1;

    foreach my $num (@$pvalues) {
        $chosen = $num;

        # Create a duplicate list (not including the chosen number)
        my @dups  = grep { $_ ne $num } @values;
        my $pdups = randomize_list([ @dups ]);;

        # Insert chosen number at a random place { 0 ... 3 } in the li
+st
        my $correct_idx = int(rand(4));
        $pdups->[$correct_idx] = $num;

        my $pbuttons = [ ];

        for (my $i = 0; $i < 4; $i++) {
            my $dup = $pdups->[$i];
            my $icopy = $i;

            # The closure where I'm expecting $chosen_idx to get
            # assigned to $i = {0,1,2,3}, but it seems to always
            # get assigned to 4! (or whatever the final value of
            # $i is.)  However, if I set $chosen_idx to $icopy:
            #
            #     my $psub = sub { $chosen_idx = $icopy };
            #
            # then everything works ... ???
            #
            my $psub = sub { $chosen_idx = $i };

            push @$pbuttons, button($frame, 'skyblue', $dup, $psub);
        }

        $chosen_idx = -1;
        while ($chosen_idx < 0) {
            $mw->update();
        }

        # Delete the buttons
        map { $_->destroy() } @$pbuttons;

        # Why on EARTH is this showing $chosen_idx as 4 every time?!
        print "correct idx[$correct_idx] ... chosen_idx[$chosen_idx]\n
+";
    }
}
[download]

When I run it, it creates a small GUI where you click on the "Start test" button, and it displays a single random value from 1-9 in a Label widget on the left, and 4 buttons on the right (one of which contains the same value as in the Label).

Okay, so far so good.  But now, when you click on one of the 4 buttons, it prints to STDOUT what the correct index of the chosen value is (from 0 to 3), and the index of the button you pressed.  However, the second index is always a 4?!

I don't understand why this should be true.  I'm constructing a closure $psub around each value of $i in the loop, which can be verified by printing the value of $i in the button subroutine:

sub button {
    my ($w, $bg, $text, $pcmd, $ival) = @_;
    (defined $ival) and print "Value of \$i is $ival\n";
    my $b = $w->Button(-bg => $bg, -text => $text);
    $b->configure(-width => 16, -command => $pcmd);
    $b->pack;
    return $b;
}

# ... and later, when calling button() ...
    push @$pbuttons, button($frame, 'skyblue', $dup, $psub, $i);

# The above correctly displays each value of $i
[download]

I've also tried making $i global in case it was somehow related to its scope within the for statement, but that didn't change anything.

The workaround I'm now using is to make a copy of $i called $icopy, and assign to it instead:

    my $icopy = $i;
    # ...
    my $psub = sub { $chosen_idx = $icopy };
[download]

And it's clearly somehow related to the final value of $i at the end of the for loop (to verify this, try changing the loop to for (my $i = 0; $i < 7; $i++) {, for example).

But, can some wise monk please explain to me why the closure is not working the way I'd expect it to, with respect to the interim values if $i??