alternative idea for toDictionary implementation

gistfile1.php

<?php

namespace Ginq;

require_once "PHPUnit/Framework/IncompleteTestError.php";
require_once dirname(dirname(__FILE__)) . "/src/Ginq.php";

class GinqTest2 extends \PHPUnit_Framework_TestCase {
    public static function main()
    {
        require_once "PHPUnit/TextUI/TestRunner.php";
        $suite  = new \PHPUnit_Framework_TestSuite("GinqTest2");
        $result = \PHPUnit_TextUI_TestRunner::run($suite);
    }
    
    /**
     * testGetIterator().
     */
    public function testToDictionary() {
        $source = array(
            array(1, "aaa"), array(2, "aaa"), array(3, "bbb"), array(4, "aaa")
        );
        
        $expected = array(
            "aaa" => array(1, 2, 4),
            "bbb" => array(3)
        );
        
        $groups = \Ginq::from($source)
            ->groupBy(
                function ($v) { return $v[1]; },
                function ($v) { return $v[0]; }
            )
        ;
        
        $actual = $this
            ->toDictionary(
                $groups,
                function ($group, $k) { return $k; },
                function ($group, $k) { return $group->toArray(); }
            )
        ;
        
        var_dump($actual);exit;
        
        
    }
    
    public function toDictionary(\Ginq $ginq, $keySelector, $valueSelector = null) {
        if (is_null($valueSelector)) {
            $valueSelector = function ($v, $k) { return $v; };
        }
        
        return $ginq->fold(
            array(),
            function ($acc, $v, $k) use($keySelector, $valueSelector) {
                // 必要ならここで、二重キーのチェック
                $acc[$keySelector($v, $k)] = $valueSelector($v, $k);
                
                return $acc;
            }
        );
    }
    
}