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Amp ⋅ Coroutines

Coroutines are interruptible functions. In PHP they can be implemented using generators.

While generators are usually used to implement simple iterators and yielding elements using the yield keyword, Amp uses yield as interruption points. When a coroutine yields a value, execution of the coroutine is temporarily interrupted, allowing other tasks to be run, such as I/O handlers, timers, or other coroutines.

// Fetches a resource with Artax and returns its body.
$promise = Amp\call(function () use ($http) {
    try {
        // Yield control until the generator resolves
        // and return its eventual result.
        $response = yield $http->request("https://example.com/");

        $body = yield $response->getBody();

        return $body;
    } catch (HttpException $e) {
        // If promise resolution fails the exception is
        // thrown back to us and we handle it as needed.
    }
});

Every time a promise is yielded, the coroutine subscribes to the promise and automatically continues it once the promise resolved. On successful resolution the coroutine will send the resolution value into the generator using Generator::send(). On failure it will throw the exception into the generator using Generator::throw(). This allows writing asynchronous code almost like synchronous code.

Note that no callbacks need to be registered to consume promises and errors can be handled with ordinary catch clauses, which will bubble up to the calling context if uncaught in the same way exceptions bubble up in synchronous code.

Use Amp\call() to always return a promise instead of a \Generator from your public APIs. Generators are an implementation detail that shouldn’t be leaked to API consumers.

Yield Behavior

All yields in a coroutine must be one of the following three types:

Yieldable Description
Amp\Promise Any promise instance may be yielded and control will be returned to the coroutine once the promise resolves. If resolution fails the relevant exception is thrown into the generator and must be handled by the application or it will bubble up. If resolution succeeds the promise’s resolved value is sent back into the generator.
React\Promise\PromiseInterface Same as Amp\Promise. Any React promise will automatically be adapted to an Amp promise.
array Yielding an array of promises combines them implicitly using Amp\Promise\all(). An array with elements not being promises will result in an Amp\InvalidYieldError.

Yield vs. Yield From

yield is used to “await” promises, yield from can be used to delegate to a sub-routine. yield from should only be used to delegate to private methods, any public API should always return promises instead of generators.

When a promise is yielded from within a \Generator, \Generator will be paused and continue as soon as the promise is resolved. Use yield from to yield another \Generator. Instead of using yield from, you can also use yield new Coroutine($this->bar()); or yield call([$this, "bar"]);.

An example:

class Foo
{
    public function delegationWithCoroutine(): Amp\Promise
    {
        return new Amp\Coroutine($this->bar());
    }

    public function delegationWithYieldFrom(): Amp\Promise
    {
        return Amp\call(function () {
            return yield from $this->bar();
        });
    }

    public function delegationWithCallable(): Amp\Promise
    {
        return Amp\call([$this, 'bar']);
    }

    public function bar(): Generator
    {
        yield new Amp\Success(1);
        yield new Amp\Success(2);
        return yield new Amp\Success(3);
    }
}

Amp\Loop::run(function () {
    $foo = new Foo();
    $r1 = yield $foo->delegationWithCoroutine();
    $r2 = yield $foo->delegationWithYieldFrom();
    $r3 = yield $foo->delegationWithCallable();
    var_dump($r1);
    var_dump($r2);
    var_dump($r3);
});

Outputs:

int(3)
int(3)
int(3)

For further information about yield from, consult the PHP manual.