Scala Future flatmap

FutureFlatmap.scala

trait Future[+T] extends Awaitable[T] {
  def flatMap[S](f: T => Future[S])(implicit executor: ExecutionContext): Future[S] = {
    import impl.Promise.DefaultPromise
    val p = new DefaultPromise[S]()
    onComplete {
      case f: Failure[_] => p complete f.asInstanceOf[Failure[S]]
      case Success(v) => try f(v) match {
        // If possible, link DefaultPromises to avoid space leaks
        case dp: DefaultPromise[_] => dp.asInstanceOf[DefaultPromise[S]].linkRootOf(p)
        case fut => fut.onComplete(p.complete)(internalExecutor)
      } catch { case NonFatal(t) => p failure t }
    }
    p.future
  }
}