Simplified Future Flatmap

SimpleFutureFlatmap.scala

trait Future[+T] {

  def flatMap[S](f: T => Future[S])
                (implicit executor: ExecutionContext): Future[S] = {

    val promise = new Promise[S]()

    this.onComplete {

      // The first Future (this) failed
      case Failure(t) => promise.failure(t)

      case Success(v1) =>

        // Apply the flatMap function (f) to the first Future's result
        Try(f(v1)) match {

          // The flatMap function (f) threw an exception
          case Failure(t) => promise.failure(t)

          case Success(future2) =>
            future2.onComplete {

              // The second Future failed
              case Failure(t) => promise.failure(t)

              // Both futures succeeded - Complete the promise
              // successfully with the second Future's result.
              case Success(v2) => promise.success(v2)
            }
        }
    }
    promise.future
  }
}