Promises

The basic unit of concurrency in Amp applications is the Amp\Promise. These objects should be thought of as placeholders for values or tasks that aren’t yet complete. By using placeholders we’re able to reason about the results of concurrent operations as if they were already complete variables.

Amp’s Promise interface does not conform to the “Thenables” abstraction common in JavaScript promise implementations. Chaining .then() calls is a suboptimal method for avoiding callback hell in a world with generator coroutines. Instead, Amp utilizes PHP generators to “synchronize” concurrent task execution.

However, as ReactPHP is another wide-spread implementation, we also accept any React\Promise\PromiseInterface where we accept instances of Amp\Promise. In case of custom implementations not implementing React\Promise\PromiseInterface, Amp\Promise\adapt can be used to adapt any object having a then or done method.

Promise Consumption

interface Promise {
    public function onResolve(callable $onResolve);
}

In its simplest form the Amp\Promise aggregates callbacks for dealing with results once they eventually resolve. While most code will not interact with this API directly thanks to coroutines, let’s take a quick look at the one simple API method exposed on Amp\Promise implementations:

Parameter Callback Signature
$onResolve function ($error = null, $result = null)

Amp\Promise::onResolve() accepts an error-first callback. This callback is responsible for reacting to the eventual result represented by the promise placeholder. For example:

<?php

$promise = someFunctionThatReturnsAPromise();
$promise->onResolve(function (Throwable $error = null, $result = null) {
    if ($error) {
        printf(
            "Something went wrong:\n%s\n",
            $error->getMessage()
        );
    } else {
        printf(
            "Hurray! Our result is:\n%s\n",
            print_r($result, true)
        );
    }
});

Those familiar with JavaScript code generally reflect that the above interface quickly devolves into “callback hell”, and they’re correct. We will shortly see how to avoid this problem in the coroutines section.

Promise Creation

Amp\Deferred is the abstraction responsible for resolving future values once they become available. A library that resolves values asynchronously creates an Amp\Deferred and uses it to return an Amp\Promise to API consumers. Once the async library determines that the value is ready it resolves the promise held by the API consumer using methods on the linked promisor.

final class Deferred {
    public function promise(): Promise;
    public function resolve($result = null);
    public function fail(Throwable $error);
}

promise()

Returns the corresponding Promise instance. Deferred and Promise are separated, so the consumer of the promise can’t fulfill it. You should always return $deferred->promise() to API consumers. If you’re passing Deferred objects around, you’re probably doing something wrong.

This separation of concerns is generally a good thing. However, creating two objects instead of one for each fundamental placeholder is a measurable performance penalty. For that reason, this separation only exists if assertions are enabled to ensure the code does what it’s expected to do. Deferred directly implements Promise if assertions are disabled.

resolve()

Resolves the promise with the first parameter as value, otherwise null. If a Amp\Promise is passed, the resolution will wait until the passed promise has been resolved. Invokes all registered Promise::onResolve() callbacks.

fail()

Makes the promise fail. Invokes all registered Promise::onResolve() callbacks with the passed Throwable as $error argument.

Here’s a simple example of an async value producer asyncMultiply() creating a deferred and returning the associated promise to its API consumer.

<?php // Example async producer using promisor

use Amp\Loop;

function asyncMultiply($x, $y) {
    // Create a new promisor
    $deferred = new Amp\Deferred;

    // Resolve the async result one second from now
    Loop::delay($msDelay = 1000, function () use ($deferred, $x, $y) {
        $deferred->resolve($x * $y);
    });

    return $deferred->promise();
}

$promise = asyncMultiply(6, 7);
$result = Amp\Promise\wait($promise);
var_dump($result); // int(42)

Success and Failure

Sometimes values are immediately available. This might be due to them being cached, but can also be the case if an interface mandates a promise to be returned to allow for async I/O but the specific implementation always having the result directly available. In these cases Amp\Success and Amp\Failure can be used to construct an immediately resolved promise. Amp\Success accepts a resolution value. Amp\Failure accepts an exception as failure reason.