Future of Etither monad transformer

FutureOfEitherT.scala

object FutureOfEitherT {
  
  type Result[T] = Either[Throwable, T]

  implicit class FutureOps[T](val f: Future[T]) extends AnyVal {

    implicit def orFail: FutureOfEither[T] =
      FutureOfEither(f map Right.apply recover { case NonFatal(e) => Left(e) })
  }

  final case class FutureOfEither[T](value: Future[Result[T]]) {

    def map[S](f: T => S)(implicit executor: ExecutionContext): FutureOfEither[S] = {
      val p = Promise[Result[S]]()
      value.onComplete {
        case f: Failure[_] =>
          p complete f.asInstanceOf[Failure[Result[S]]]
        case Success(v) => v match {
          case Left(failure) => p complete Success(Either.left(failure))
          case Right(a) => p complete Success(Right(f(a)))
        }
      }
      FutureOfEither(p.future)
    }

    def flatMap[S](f: T => FutureOfEither[S])(implicit executor: ExecutionContext): FutureOfEither[S] = {
      val p = Promise[Result[S]]()
      value.onComplete {
        case f: Failure[_] =>
          p complete f.asInstanceOf[Failure[Result[S]]]
        case Success(v) => v match {
          case Left(failure) => p complete Success(Either.left(failure))
          case Right(a) => f(a).value.onComplete(v => p complete v)
        }
      }
      FutureOfEither(p.future)
    }

    def fold[S](f: Result[T] => S): Future[S] = value map f

  }
}