MultipleIterator, add a default value

Hi internals, I propose to ameliorate the MultipleIterator class. When a MultipleIterator instance has the MIT_NEED_ANY flag, and when one of the attached iterators has reached its end, the returned value is NULL. I propose to change this behavior by allowing a different default value. Example with no default value:

    $it = new \MultipleIterator(
        \MultipleIterator::MIT_NEED_ANY
      | \MultipleIterator::MIT_KEYS_ASSOC
    );
    $it->attachIterator(new \ArrayIterator(['a', 'b', 'c']), 'foo');
    $it->attachIterator(new \ArrayIterator(['x']),           'bar');

    foreach($it as $value)
        echo $value['foo'], ' => ', $value['bar'], "\n";

    /**
     * Will output:
     *     a => x
     *     b =>
     *     c =>
     */

Example with 42 as a default value (given as a third parameter of the MultipleIterator::attachIterator method):

    $it->attachIterator(new \ArrayIterator(['a', 'b', 'c']), 'foo', 42);
    $it->attachIterator(new \ArrayIterator(['x']),           'bar', 42);

    foreach($it as $value)
        echo $value['foo'], ' => ', $value['bar'], "\n";

    /**
     * Will output:
     *     a => x
     *     b => 42
     *     c => 42
     */

It is clearly useful when one of the iterators returns an array, used directly in a loop. To avoid a "NULL pointer exception", we can specify an empty array as a default value. A PHP implementation is pretty easy to do. Naively, we are able to write https://github.com/hoaproject/Iterator/commit/59dcff7f331a7338644cf9a8965d2a934a38661e. If you think it can be useful, I propose to write a RFC with a patch. Thoughts? -- Ivan Enderlin Developer of Hoa http://hoa-project.net/ PhD. student at DISC/Femto-ST (Vesontio) and INRIA (Cassis) http://disc.univ-fcomte.fr/ and http://www.inria.fr/ Member of HTML and WebApps Working Group of W3C http://w3.org/