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    <h3>Editing: Coroutine.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php

namespace GuzzleHttp\Promise;

use Exception;
use Generator;
use Throwable;

/**
 * Creates a promise that is resolved using a generator that yields values or
 * promises (somewhat similar to C#'s async keyword).
 *
 * When called, the Coroutine::of method will start an instance of the generator
 * and returns a promise that is fulfilled with its final yielded value.
 *
 * Control is returned back to the generator when the yielded promise settles.
 * This can lead to less verbose code when doing lots of sequential async calls
 * with minimal processing in between.
 *
 *     use GuzzleHttp\Promise;
 *
 *     function createPromise($value) {
 *         return new Promise\FulfilledPromise($value);
 *     }
 *
 *     $promise = Promise\Coroutine::of(function () {
 *         $value = (yield createPromise('a'));
 *         try {
 *             $value = (yield createPromise($value . 'b'));
 *         } catch (\Exception $e) {
 *             // The promise was rejected.
 *         }
 *         yield $value . 'c';
 *     });
 *
 *     // Outputs &quot;abc&quot;
 *     $promise-&gt;then(function ($v) { echo $v; });
 *
 * @param callable $generatorFn Generator function to wrap into a promise.
 *
 * @return Promise
 *
 * @link https://github.com/petkaantonov/bluebird/blob/master/API.md#generators inspiration
 */
final class Coroutine implements PromiseInterface
{
    /**
     * @var PromiseInterface|null
     */
    private $currentPromise;

    /**
     * @var Generator
     */
    private $generator;

    /**
     * @var Promise
     */
    private $result;

    public function __construct(callable $generatorFn)
    {
        $this-&gt;generator = $generatorFn();
        $this-&gt;result = new Promise(function () {
            while (isset($this-&gt;currentPromise)) {
                $this-&gt;currentPromise-&gt;wait();
            }
        });
        try {
            $this-&gt;nextCoroutine($this-&gt;generator-&gt;current());
        } catch (\Exception $exception) {
            $this-&gt;result-&gt;reject($exception);
        } catch (Throwable $throwable) {
            $this-&gt;result-&gt;reject($throwable);
        }
    }

    /**
     * Create a new coroutine.
     *
     * @return self
     */
    public static function of(callable $generatorFn)
    {
        return new self($generatorFn);
    }

    public function then(
        callable $onFulfilled = null,
        callable $onRejected = null
    ) {
        return $this-&gt;result-&gt;then($onFulfilled, $onRejected);
    }

    public function otherwise(callable $onRejected)
    {
        return $this-&gt;result-&gt;otherwise($onRejected);
    }

    public function wait($unwrap = true)
    {
        return $this-&gt;result-&gt;wait($unwrap);
    }

    public function getState()
    {
        return $this-&gt;result-&gt;getState();
    }

    public function resolve($value)
    {
        $this-&gt;result-&gt;resolve($value);
    }

    public function reject($reason)
    {
        $this-&gt;result-&gt;reject($reason);
    }

    public function cancel()
    {
        $this-&gt;currentPromise-&gt;cancel();
        $this-&gt;result-&gt;cancel();
    }

    private function nextCoroutine($yielded)
    {
        $this-&gt;currentPromise = Create::promiseFor($yielded)
            -&gt;then([$this, '_handleSuccess'], [$this, '_handleFailure']);
    }

    /**
     * @internal
     */
    public function _handleSuccess($value)
    {
        unset($this-&gt;currentPromise);
        try {
            $next = $this-&gt;generator-&gt;send($value);
            if ($this-&gt;generator-&gt;valid()) {
                $this-&gt;nextCoroutine($next);
            } else {
                $this-&gt;result-&gt;resolve($value);
            }
        } catch (Exception $exception) {
            $this-&gt;result-&gt;reject($exception);
        } catch (Throwable $throwable) {
            $this-&gt;result-&gt;reject($throwable);
        }
    }

    /**
     * @internal
     */
    public function _handleFailure($reason)
    {
        unset($this-&gt;currentPromise);
        try {
            $nextYield = $this-&gt;generator-&gt;throw(Create::exceptionFor($reason));
            // The throw was caught, so keep iterating on the coroutine
            $this-&gt;nextCoroutine($nextYield);
        } catch (Exception $exception) {
            $this-&gt;result-&gt;reject($exception);
        } catch (Throwable $throwable) {
            $this-&gt;result-&gt;reject($throwable);
        }
    }
}
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