###
### dir_walk-iterator
###

## Chapter 4 section 2.2

# iterator version
sub dir_walk {          
  my @queue = shift;
  return Iterator {
    if (@queue) {
      my $file = shift @queue;
      if (-d $file) { 
        opendir my $dh, $file or next;
        my @newfiles = grep {$_ ne "." && $_ ne ".."} readdir $dh;
        push @queue, map "$file/$_", @newfiles;
      }
      return $file;
    } else {
      return;
    }
  };
}