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# Guzzle Promises

[Promises/A+](https://promisesaplus.com/) implementation that handles promise

chaining and resolution iteratively, allowing for "infinite" promise chaining

while keeping the stack size constant. Read [this blog post](https://blog.domenic.me/youre-missing-the-point-of-promises/)

for a general introduction to promises.

- [Features](#features)

- [Quick start](#quick-start)

- [Synchronous wait](#synchronous-wait)

- [Cancellation](#cancellation)

- [API](#api)

  - [Promise](#promise)

  - [FulfilledPromise](#fulfilledpromise)

  - [RejectedPromise](#rejectedpromise)

- [Promise interop](#promise-interop)

- [Implementation notes](#implementation-notes)

# Features

- [Promises/A+](https://promisesaplus.com/) implementation.

- Promise resolution and chaining is handled iteratively, allowing for

  "infinite" promise chaining.

- Promises have a synchronous `wait` method.

- Promises can be cancelled.

- Works with any object that has a `then` function.

- C# style async/await coroutine promises using

  `GuzzleHttp\Promise\coroutine()`.

# Quick start

A *promise* represents the eventual result of an asynchronous operation. The

primary way of interacting with a promise is through its `then` method, which

registers callbacks to receive either a promise's eventual value or the reason

why the promise cannot be fulfilled.

## Callbacks

Callbacks are registered with the `then` method by providing an optional

`$onFulfilled` followed by an optional `$onRejected` function.

```php

use GuzzleHttp\Promise\Promise;

$promise = new Promise();

$promise->then(

    // $onFulfilled

    function ($value) {

        echo 'The promise was fulfilled.';

    },

    // $onRejected

    function ($reason) {

        echo 'The promise was rejected.';

    }

);

```

*Resolving* a promise means that you either fulfill a promise with a *value* or

reject a promise with a *reason*. Resolving a promises triggers callbacks

registered with the promises's `then` method. These callbacks are triggered

only once and in the order in which they were added.

## Resolving a promise

Promises are fulfilled using the `resolve($value)` method. Resolving a promise

with any value other than a `GuzzleHttp\Promise\RejectedPromise` will trigger

all of the onFulfilled callbacks (resolving a promise with a rejected promise

will reject the promise and trigger the `$onRejected` callbacks).

```php

use GuzzleHttp\Promise\Promise;

$promise = new Promise();

$promise

    ->then(function ($value) {

        // Return a value and don't break the chain

        return "Hello, " . $value;

    })

    // This then is executed after the first then and receives the value

    // returned from the first then.

    ->then(function ($value) {

        echo $value;

    });

// Resolving the promise triggers the $onFulfilled callbacks and outputs

// "Hello, reader".

$promise->resolve('reader.');

```

## Promise forwarding

Promises can be chained one after the other. Each then in the chain is a new

promise. The return value of a promise is what's forwarded to the next

promise in the chain. Returning a promise in a `then` callback will cause the

subsequent promises in the chain to only be fulfilled when the returned promise

has been fulfilled. The next promise in the chain will be invoked with the

resolved value of the promise.

```php

use GuzzleHttp\Promise\Promise;

$promise = new Promise();

$nextPromise = new Promise();

$promise

    ->then(function ($value) use ($nextPromise) {

        echo $value;

        return $nextPromise;

    })

    ->then(function ($value) {

        echo $value;

    });

// Triggers the first callback and outputs "A"

$promise->resolve('A');

// Triggers the second callback and outputs "B"

$nextPromise->resolve('B');

```

## Promise rejection

When a promise is rejected, the `$onRejected` callbacks are invoked with the

rejection reason.

```php

use GuzzleHttp\Promise\Promise;

$promise = new Promise();

$promise->then(null, function ($reason) {

    echo $reason;

});

$promise->reject('Error!');

// Outputs "Error!"

```

## Rejection forwarding

If an exception is thrown in an `$onRejected` callback, subsequent

`$onRejected` callbacks are invoked with the thrown exception as the reason.

```php

use GuzzleHttp\Promise\Promise;

$promise = new Promise();

$promise->then(null, function ($reason) {

    throw new \Exception($reason);

})->then(null, function ($reason) {

    assert($reason->getMessage() === 'Error!');

});

$promise->reject('Error!');

```

You can also forward a rejection down the promise chain by returning a

`GuzzleHttp\Promise\RejectedPromise` in either an `$onFulfilled` or

`$onRejected` callback.

```php

use GuzzleHttp\Promise\Promise;

use GuzzleHttp\Promise\RejectedPromise;

$promise = new Promise();

$promise->then(null, function ($reason) {

    return new RejectedPromise($reason);

})->then(null, function ($reason) {

    assert($reason === 'Error!');

});

$promise->reject('Error!');

```

If an exception is not thrown in a `$onRejected` callback and the callback

does not return a rejected promise, downstream `$onFulfilled` callbacks are

invoked using the value returned from the `$onRejected` callback.

```php

use GuzzleHttp\Promise\Promise;

use GuzzleHttp\Promise\RejectedPromise;

$promise = new Promise();

$promise

    ->then(null, function ($reason) {

        return "It's ok";

    })

    ->then(function ($value) {

        assert($value === "It's ok");

    });

$promise->reject('Error!');

```

# Synchronous wait

You can synchronously force promises to complete using a promise's `wait`

method. When creating a promise, you can provide a wait function that is used

to synchronously force a promise to complete. When a wait function is invoked

it is expected to deliver a value to the promise or reject the promise. If the

wait function does not deliver a value, then an exception is thrown. The wait

function provided to a promise constructor is invoked when the `wait` function

of the promise is called.

```php

$promise = new Promise(function () use (&$promise) {

    $promise->resolve('foo');

});

// Calling wait will return the value of the promise.

echo $promise->wait(); // outputs "foo"

```

If an exception is encountered while invoking the wait function of a promise,

the promise is rejected with the exception and the exception is thrown.

```php

$promise = new Promise(function () use (&$promise) {

    throw new \Exception('foo');

});

$promise->wait(); // throws the exception.

```

Calling `wait` on a promise that has been fulfilled will not trigger the wait

function. It will simply return the previously resolved value.

```php

$promise = new Promise(function () { die('this is not called!'); });

$promise->resolve('foo');

echo $promise->wait(); // outputs "foo"

```

Calling `wait` on a promise that has been rejected will throw an exception. If

the rejection reason is an instance of `\Exception` the reason is thrown.

Otherwise, a `GuzzleHttp\Promise\RejectionException` is thrown and the reason

can be obtained by calling the `getReason` method of the exception.

```php

$promise = new Promise();

$promise->reject('foo');

$promise->wait();

```

> PHP Fatal error:  Uncaught exception 'GuzzleHttp\Promise\RejectionException' with message 'The promise was rejected with value: foo'

## Unwrapping a promise

When synchronously waiting on a promise, you are joining the state of the

promise into the current state of execution (i.e., return the value of the

promise if it was fulfilled or throw an exception if it was rejected). This is

called "unwrapping" the promise. Waiting on a promise will by default unwrap

the promise state.

You can force a promise to resolve and *not* unwrap the state of the promise

by passing `false` to the first argument of the `wait` function:

```php

$promise = new Promise();

$promise->reject('foo');

// This will not throw an exception. It simply ensures the promise has

// been resolved.

$promise->wait(false);

```

When unwrapping a promise, the resolved value of the promise will be waited

upon until the unwrapped value is not a promise. This means that if you resolve

promise A with a promise B and unwrap promise A, the value returned by the

wait function will be the value delivered to promise B.

**Note**: when you do not unwrap the promise, no value is returned.

# Cancellation

You can cancel a promise that has not yet been fulfilled using the `cancel()`

method of a promise. When creating a promise you can provide an optional

cancel function that when invoked cancels the action of computing a resolution

of the promise.

# API

## Promise

When creating a promise object, you can provide an optional `$waitFn` and

`$cancelFn`. `$waitFn` is a function that is invoked with no arguments and is

expected to resolve the promise. `$cancelFn` is a function with no arguments

that is expected to cancel the computation of a promise. It is invoked when the

`cancel()` method of a promise is called.

```php

use GuzzleHttp\Promise\Promise;

$promise = new Promise(

    function () use (&$promise) {

        $promise->resolve('waited');

    },

    function () {

        // do something that will cancel the promise computation (e.g., close

        // a socket, cancel a database query, etc...)

    }

);

assert('waited' === $promise->wait());

```

A promise has the following methods:

- `then(callable $onFulfilled, callable $onRejected) : PromiseInterface`

  Appends fulfillment and rejection handlers to the promise, and returns a new promise resolving to the return value of the called handler.

- `otherwise(callable $onRejected) : PromiseInterface`

  Appends a rejection handler callback to the promise, and returns a new promise resolving to the return value of the callback if it is called, or to its original fulfillment value if the promise is instead fulfilled.

- `wait($unwrap = true) : mixed`

  Synchronously waits on the promise to complete.

  `$unwrap` controls whether or not the value of the promise is returned for a

  fulfilled promise or if an exception is thrown if the promise is rejected.

  This is set to `true` by default.

- `cancel()`

  Attempts to cancel the promise if possible. The promise being cancelled and

  the parent most ancestor that has not yet been resolved will also be

  cancelled. Any promises waiting on the cancelled promise to resolve will also

  be cancelled.

- `getState() : string`

  Returns the state of the promise. One of `pending`, `fulfilled`, or

  `rejected`.

- `resolve($value)`

  Fulfills the promise with the given `$value`.

- `reject($reason)`

  Rejects the promise with the given `$reason`.

## FulfilledPromise

A fulfilled promise can be created to represent a promise that has been

fulfilled.

```php

use GuzzleHttp\Promise\FulfilledPromise;

$promise = new FulfilledPromise('value');

// Fulfilled callbacks are immediately invoked.

$promise->then(function ($value) {

    echo $value;

});

```

## RejectedPromise

A rejected promise can be created to represent a promise that has been

rejected.

```php

use GuzzleHttp\Promise\RejectedPromise;

$promise = new RejectedPromise('Error');

// Rejected callbacks are immediately invoked.

$promise->then(null, function ($reason) {

    echo $reason;

});

```

# Promise interop

This library works with foreign promises that have a `then` method. This means

you can use Guzzle promises with [React promises](https://github.com/reactphp/promise)

for example. When a foreign promise is returned inside of a then method

callback, promise resolution will occur recursively.

```php

// Create a React promise

$deferred = new React\Promise\Deferred();

$reactPromise = $deferred->promise();

// Create a Guzzle promise that is fulfilled with a React promise.

$guzzlePromise = new \GuzzleHttp\Promise\Promise();

$guzzlePromise->then(function ($value) use ($reactPromise) {

    // Do something something with the value...

    // Return the React promise

    return $reactPromise;

});

```

Please note that wait and cancel chaining is no longer possible when forwarding

a foreign promise. You will need to wrap a third-party promise with a Guzzle

promise in order to utilize wait and cancel functions with foreign promises.

## Event Loop Integration

In order to keep the stack size constant, Guzzle promises are resolved

asynchronously using a task queue. When waiting on promises synchronously, the

task queue will be automatically run to ensure that the blocking promise and

any forwarded promises are resolved. When using promises asynchronously in an

event loop, you will need to run the task queue on each tick of the loop. If

you do not run the task queue, then promises will not be resolved.

You can run the task queue using the `run()` method of the global task queue

instance.

```php

// Get the global task queue

$queue = \GuzzleHttp\Promise\queue();

$queue->run();

```

For example, you could use Guzzle promises with React using a periodic timer:

```php

$loop = React\EventLoop\Factory::create();

$loop->addPeriodicTimer(0, [$queue, 'run']);

```

*TODO*: Perhaps adding a `futureTick()` on each tick would be faster?

# Implementation notes

## Promise resolution and chaining is handled iteratively

By shuffling pending handlers from one owner to another, promises are

resolved iteratively, allowing for "infinite" then chaining.

```php

<?php

require 'vendor/autoload.php';

use GuzzleHttp\Promise\Promise;

$parent = new Promise();

$p = $parent;

for ($i = 0; $i < 1000; $i++) {

    $p = $p->then(function ($v) {

        // The stack size remains constant (a good thing)

        echo xdebug_get_stack_depth() . ', ';

        return $v + 1;

    });

}

$parent->resolve(0);

var_dump($p->wait()); // int(1000)

```

When a promise is fulfilled or rejected with a non-promise value, the promise

then takes ownership of the handlers of each child promise and delivers values

down the chain without using recursion.

When a promise is resolved with another promise, the original promise transfers

all of its pending handlers to the new promise. When the new promise is

eventually resolved, all of the pending handlers are delivered the forwarded

value.

## A promise is the deferred.

Some promise libraries implement promises using a deferred object to represent

a computation and a promise object to represent the delivery of the result of

the computation. This is a nice separation of computation and delivery because

consumers of the promise cannot modify the value that will be eventually

delivered.

One side effect of being able to implement promise resolution and chaining

iteratively is that you need to be able for one promise to reach into the state

of another promise to shuffle around ownership of handlers. In order to achieve

this without making the handlers of a promise publicly mutable, a promise is

also the deferred value, allowing promises of the same parent class to reach

into and modify the private properties of promises of the same type. While this

does allow consumers of the value to modify the resolution or rejection of the

deferred, it is a small price to pay for keeping the stack size constant.

```php

$promise = new Promise();

$promise->then(function ($value) { echo $value; });

// The promise is the deferred value, so you can deliver a value to it.

$promise->resolve('foo');

// prints "foo"

```