CompositionPost3

ComposedFlipImpl.scala

// Introduce a flip method: it looks tricky, but as usual it just "follows the types":
private def flip[L, R](eitherFuture: Either[L, Future[Either[L, R]]]): Future[Either[L, R]] = {
  eitherFuture.fold[Future[Either[L, Either[L, R]]]](
    left => Future.successful[Either[L, Either[L, R]]](Left(left)),
    right => right.map(Right(_))
  ).map(_.joinRight)
}