A array_filter alternative for iterables

iterator_filter.php

<?php

describe(strict_types = 1);

/**
 * Like array_filter() but over an iterable, and it returns a new iterable with
 * filtering instead of a filtered array. The callable, if non-null, should
 * take arguments like iterator_map(). When the callable is null, null values
 * will be filtered out (NOT falsey values, just x === null).
 */
function iterator_filter(?callable $cb, iterable $itr): iterable {
   $cb = $cb ?: function($x) {
      return $x !== null;
   };
   foreach ($itr as $key => $value) {
      $keep = $cb($value, $key);
      if ($keep) {
         yield $value;
      }
   }
}