Introducing Futures

[leocavalcante]

This PR introduces Futures as part of Swoole's standard library features.

Futures are abstractions on top of Coroutines providing alternative syntax for building concurrent programs with Swoole.

Main differences from Coroutines are:

Futures are lazy

They don't evaluate until you call await:

$future = Future\async(static function (): void {
    echo "are lazy\n";
});

Coroutine::create(static function (): void {
    echo 'Futures ';
});

echo $future->await();

Futures can return values

The Channel handling is done under the hood:

$future = Future\async(static function (): string {
    return Http\get('https://httpbin.org/get')->getBody();
});

echo $future->await();

Futures propagates exceptions

$future = Future\async(static function (): string {
    throw new RuntimeException('Futures propagates exceptions');
});

try {
    $future->await();
} catch (Throwable $e) {
    echo $e->getMessage();
}

Futures synchronization is similar

It uses Coroutine::join under the hood:

$future1 = \Swoole\Future::create(static function () {
    return \Swoole\Coroutine\Http\get('https://httpbin.org/delay/2')->getBody();
});

$future2 = \Swoole\Future::create(static function () {
    return \Swoole\Coroutine\Http\get('https://httpbin.org/delay/2')->getBody();
});

echo implode(PHP_EOL, \Swoole\Future::join([$future1, $future2]));

[KennedyTedesco]

Your implementation is good and readable. But, I would put some minor tweaks (not functional ones, just a different style and one less variable to decrement) into it:

   public static function join(array $futures, float $timeout = -1): array
    {
        $ch = new Channel(count($futures));

        Coroutine::join(array_map(static function (Future $future) use ($ch): int {
            return $future->run($ch);
        }, $futures));

        $rets = [];
        while ($ret = $ch->pop($timeout)) {
            if ($ret instanceof Throwable) {
                throw $ret;
            }

            $rets[] = $ret;
        }

        return $rets;
    }

[johanjanssens]

@leocavalcante This is really nice work!