use strict;
use warnings;

# setup some iterators
my ($i, $j, $k, $l) = (0) x 4;
my @iter = (
   sub { return $i++ unless $i == 2; $i = 0; return },
   sub { return $j++ unless $j == 3; $j = 0; return },
   sub { return $k++ unless $k == 1; $k = 0; return },
   sub { return $l++ unless $l == 2; $l = 0; return },
);

# build the "nested loop" for them
my $while_iter = nl_for_iters(@iter);

# use the nested iterators
while ( my @values = $while_iter->() ) {
    print join(', ', @values ) . "\n";
}

# now how to build nested loops from iterators
sub nl_for_iters {
    my @iter = @_;
    my @value = ();
    my $ptr = 0;
    return sub {
      LOOP: {
        if ( !defined $value[$ptr] || $ptr == $#iter ) {
            if ( defined ($value[$ptr] = $iter[$ptr]->()) ) {
                $ptr += 1 if ($ptr < $#iter);
                return @value;
            }
            else {
                pop @value;         
                $ptr -= 1;          
                return if $ptr < 0; 
                pop @value;         
                redo LOOP;          
           }
        }
      } # LOOP
    }
}
__END__
0
0, 0
0, 0, 0
0, 0, 0, 0
0, 0, 0, 1
0, 1
0, 1, 0
0, 1, 0, 0
0, 1, 0, 1
0, 2
0, 2, 0
0, 2, 0, 0
0, 2, 0, 1
1
1, 0
1, 0, 0
1, 0, 0, 0
1, 0, 0, 1
1, 1
1, 1, 0
1, 1, 0, 0
1, 1, 0, 1
1, 2
1, 2, 0
1, 2, 0, 0
1, 2, 0, 1
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