AMP - not working

<?php

class Application
{
    public $socket;

    public function __construct()
    {
        return Amp\call(function () {
            $this->socket = yield connect("tcp://192.168.4.2:8080");
            return yield true;
        });
    }

    public function sendPrinterCommand($command)
    {
        return Amp\call(function () use ($command) {
            yield $this->socket->write($command . "\r");
            return yield $this->socket->read();
        });
    }

    public function run()
    {
        $result = yield $this->sendPrinterCommand('GJL'); // JBL|4|501|504|QR CODE 12mm|TESTE|
        if (strpos($result, 'JBL') !== 0) {
            die("\r\nGET JOB LIST: ERROR: {$result}\r\n");
        }

        $jobs = array_filter(explode('|', trim($result)));
        array_shift($jobs); // JBL
        array_shift($jobs); // job count

        // does something with job list
    }
}

Loop::run(function () {
    $app = yield new Application();
    yield $app->run();
    Loop::stop();
});