interface Promise

interface Promise {
    
    /**
     * Promise has not been fulfilled or rejected.
     */
    const PENDING = 'pending';
    
    /**
     * Promise has been fulfilled.
     */
    const FULFILLED = 'fulfilled';
    
    /**
     * Promise has been rejected.
     */
    const REJECTED = 'rejected';
    
    /**
     * Adds behavior for when the promise is resolved or rejected (response will be available, or error happens).
     *
     * If you do not care about one of the cases, you can set the corresponding callable to null
     * The callback will be called when the value arrived and never more than once.
     *
     * @param callable|null $onFulfilled called when a response will be available
     * @param callable|null $onRejected  called when an exception occurs
     *
     * @return Promise a new resolved promise with value of the executed callback (onFulfilled / onRejected)
     */
    public function then(?callable $onFulfilled = null, ?callable $onRejected = null);
    
    /**
     * Returns the state of the promise, one of PENDING, FULFILLED or REJECTED.
     *
     * @return string
     */
    public function getState();
    
    /**
     * Wait for the promise to be fulfilled or rejected.
     *
     * When this method returns, the request has been resolved and if callables have been
     * specified, the appropriate one has terminated.
     *
     * When $unwrap is true (the default), the response is returned, or the exception thrown
     * on failure. Otherwise, nothing is returned or thrown.
     *
     * @param bool $unwrap Whether to return resolved value / throw reason or not
     *
     * @return ($unwrap is true ? mixed : null) Resolved value, null if $unwrap is set to false
     *
     * @throws \Throwable the rejection reason if $unwrap is set to true and the request failed
     */
    public function wait($unwrap = true);

}