Co\Promise

A pure and well tested promise implementation designed for interoperability.

Casting a promise

Promises from other implementations can be cast to Co\Promise.

<?php
$promise = Co\Promise::cast($otherPromise);

Identifying a promise-like object

Since there is no canonical PHP promise interface, it is useful to have a simple way to check if an object appears to be a valid promise.

<?php
if (Co\Promise::isPromise($otherPromise)) {
    // promise has a valid `then()` method
}

Creating a promise

Promises can generally be created in two ways; with a resolver function or without a resolver function - in which case the promise must be resolved by calling the fulfill() or the reject() method.

With a resolver function*

<?php
$promise = new Promise(function(callable $fulfill, callable $reject) {
    $fulfill("Hello World");
});
$promise->then(function($value) {
    echo $value."\n";
});

Without a resolver function

<?php
$promise = new Promise();
$promise->then(function($value) {
    echo $value."\n";
});
$promise->resolve("Hello World");

Interoperability Notes

The interface is focused on the resolution of promises; it does not care about how a promise is created - the interface is for functions that accept a promise.

Most promise implementations have an extensive API, but a promise only needs to expose a then() function to be usable by 90% of libraries.

It is also very helpful to have a way to determine if a promise is already resolved or rejected - which generally is implemented using a method which returns a string "pending", "resolved", "rejected" or "fulfilled".

This library implements methods isPending(), isResolved() and isFulfilled() because these methods can be implemented regardless of how the underlying promise implementation records that state..

Many promise implementations have methods like otherwise(), finally() and similar - which may be convenient, but it reduces interoperability because the same features can be implemented in several ways and with different names.