Amp\Artax\IteratorWriter::writeNextElement PHP Method

    private function writeNextElement()
    {
        $current = $this->iterator->current();
        if (!$current instanceof Promise) {
            $this->finalizeEventualWriteElement($current);
            return;
        }
        $current->when(function ($error, $result) {
            if ($error) {
                $this->promisor->fail($error);
            } else {
                $this->finalizeEventualWriteElement($result);
            }
        });
    }