# Syntax sugar
sub Iterator (&) { $_[0] }

# Takes a sub followed by an iterator
sub igrep (&$) {
   my ($check, $iterator) = @_;
   return Iterator {
       # While the source iterator is not exhausted
       while (defined( local $_ = $iterator->() )) {
            return $_ if $check->();
       }
       return undef; # we are exhausted
   }
}