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Edit File : README.md \| Size : 14.94 KB
# 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" ``` Back

Edit File : README.md | Size : 14.94 KB

# 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"
```

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