Kraken\Promise\Partial\PromiseTrait::reduce PHP Method

    public static function reduce($promisesOrValues, callable $reduceFunc, $initialValue = null)
    {
        $cancellationQueue = new CancellationQueue();
        return new Promise(function ($resolve, $reject, $cancel) use($promisesOrValues, $reduceFunc, $initialValue, $cancellationQueue) {
            self::doResolve($promisesOrValues)->done(function ($array) use($reduceFunc, $initialValue, $resolve, $reject, $cancel, $cancellationQueue) {
                if (!is_array($array)) {
                    $array = [];
                }
                $total = count($array);
                $i = 0;
                // Wrap the supplied $reduceFunc with one that handles promises and then delegates to the supplied.
                //
                $wrappedReduceFunc = function (PromiseInterface $current, $val) use($reduceFunc, $cancellationQueue, $total, &$i) {
                    $cancellationQueue->enqueue($val);
                    return $current->then(function ($c) use($reduceFunc, $total, &$i, $val) {
                        return self::doResolve($val)->then(function ($value) use($reduceFunc, $total, &$i, $c) {
                            return $reduceFunc($c, $value, $i++, $total);
                        });
                    });
                };
                $initialValue = self::doResolve($initialValue);
                $cancellationQueue->enqueue($initialValue);
                array_reduce($array, $wrappedReduceFunc, $initialValue)->done($resolve, $reject, $cancel);
            }, $reject, $cancel);
        }, $cancellationQueue);
    }