amp/lib/Coroutine.php

<?php

namespace Amp;

use React\Promise\PromiseInterface as ReactPromise;

/**
 * Creates a promise from a generator function yielding promises.
 *
 * When a promise is yielded, execution of the generator is interrupted until the promise is resolved. A success
 * value is sent into the generator, while a failure reason is thrown into the generator. Using a coroutine,
 * asynchronous code can be written without callbacks and be structured like synchronous code.
 */
final class Coroutine implements Promise {
    use Internal\Placeholder;

    /**
     * Maximum number of immediate coroutine continuations before deferring next continuation to the loop.
     *
     * @internal
     */
    const MAX_CONTINUATION_DEPTH = 3;

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

    /** @var callable(\Throwable|null $exception, mixed $value): void */
    private $onResolve;

    /** @var int */
    private $depth = 0;

    /**
     * @param \Generator $generator
     */
    public function __construct(\Generator $generator) {
        $this->generator = $generator;

        /**
         * @param \Throwable|null $exception Exception to be thrown into the generator.
         * @param mixed           $value Value to be sent into the generator.
         */
        $this->onResolve = function ($exception, $value) {
            if ($this->depth > self::MAX_CONTINUATION_DEPTH) { // Defer continuation to avoid blowing up call stack.
                Loop::defer(function () use ($exception, $value) {
                    ($this->onResolve)($exception, $value);
                });
                return;
            }

            try {
                if ($exception) {
                    // Throw exception at current execution point.
                    $yielded = $this->generator->throw($exception);
                } else {
                    // Send the new value and execute to next yield statement.
                    $yielded = $this->generator->send($value);
                }

                if (!$yielded instanceof Promise) {
                    if (!$this->generator->valid()) {
                        $this->resolve($this->generator->getReturn());
                        $this->onResolve = null;
                        return;
                    }

                    $yielded = $this->transform($yielded);
                }

                ++$this->depth;
                $yielded->onResolve($this->onResolve);
                --$this->depth;
            } catch (\Throwable $exception) {
                $this->fail($exception);
                $this->onResolve = null;
            }
        };

        try {
            $yielded = $this->generator->current();

            if (!$yielded instanceof Promise) {
                if (!$this->generator->valid()) {
                    $this->resolve($this->generator->getReturn());
                    $this->onResolve = null;
                    return;
                }

                $yielded = $this->transform($yielded);
            }

            ++$this->depth;
            $yielded->onResolve($this->onResolve);
            --$this->depth;
        } catch (\Throwable $exception) {
            $this->fail($exception);
            $this->onResolve = null;
        }
    }

    /**
     * Attempts to transform the non-promise yielded from the generator into a promise, otherwise returns an instance
     * `Amp\Failure` failed with an instance of `Amp\InvalidYieldError`.
     *
     * @param mixed $yielded Non-promise yielded from generator.
     *
     * @return \Amp\Promise
     */
    private function transform($yielded): Promise {
        try {
            if (\is_array($yielded)) {
                return Promise\all($yielded);
            }

            if ($yielded instanceof ReactPromise) {
                return Promise\adapt($yielded);
            }

            // No match, continue to returning Failure below.
        } catch (\Throwable $exception) {
            // Conversion to promise failed, fall-through to returning Failure below.
        }

        return new Failure(new InvalidYieldError(
            $this->generator,
            \sprintf(
                "Unexpected yield; Expected an instance of %s or %s or an array of such instances",
                Promise::class,
                ReactPromise::class
            ),
            $exception ?? null
        ));
    }
}