sequential futures!

gistfile1.scala

  def performSequentially[A](items: Seq[A])(f: A => Future[Unit]): Future[Unit] = {
    items.headOption match {
      case Some(nextItem) =>
        val fut = f(nextItem)
        fut.flatMap { _ =>
          // successful, let's move on to the next!
          performSequentially(items.tail)(f)
        }        
      case None =>
        // nothing left to process
        Future.successful(())
    }
  }
  
scala> val items = 0 to 5
items: scala.collection.immutable.Range.Inclusive = Range(0, 1, 2, 3, 4, 5)

scala> performSequentially(items) { i => future { Thread.sleep(10000); println("hello " + i) } }
res0: scala.concurrent.Future[Unit] = scala.concurrent.impl.Promise$DefaultPromise@250f9a46

// you will see the above output every 10 seconds, not at the same time!