CompositionPost2

FirstAttempt.scala

// we'll start by applying f1 and mapping the future result by pattern-matching Left or Right: 
def processRequest(a: A): Future[Result] = f1(a).flatMap {
  case Right(b) => getC(b)
  case Left(f) => Future.successful(f)
}

// Now we do something similar to the result, this time applying f2:
private def getC(b: B): Future[Result] = f2(b).flatMap {
  case Right(c) => getD(c)
  case Left(f) => Future.successful(f)
}

// and again, for f3:
private def getD(c: C): Future[Result] = f3(c).flatMap {
  case Right(d) => getResult(d)
  case Left(f) => Future.successful(f)
}

// lastly, applying f4 and mappting the returned Either into a Result:
private def getResult(d: D): Future[Result] =
  f4(d).map(either => either.getOrElse(either.left.get))