Scala waiting for sequence of futures

Question

I was hoping code like follows would wait for both futures, but it does not.

object Fiddle {
  val f1 = Future {
    throw new Throwable(\"baaa\") // emulat         


        

Answer 1

We can enrich Seq[Future[T]] with its own onComplete method through an implicit class:

  def lift[T](f: Future[T])(implicit ec: ExecutionContext): Future[Try[T]] =
    f map { Success(_) } recover { case e => Failure(e) }

  def lift[T](fs: Seq[Future[T]])(implicit ec: ExecutionContext): Seq[Future[Try[T]]] =
    fs map { lift(_) }

  implicit class RichSeqFuture[+T](val fs: Seq[Future[T]]) extends AnyVal {
    def onComplete[U](f: Seq[Try[T]] => U)(implicit ec: ExecutionContext) = {
      Future.sequence(lift(fs)) onComplete {
        case Success(s) => f(s)
        case Failure(e) => throw e // will never happen, because of the Try lifting
      }
    }
  }

Then, in your particular MWE, you can do:

  val f1 = Future {
    throw new Throwable("baaa") // emulating a future that bumped into an exception
  }

  val f2 = Future {
    Thread.sleep(3000L) // emulating a future that takes a bit longer to complete
    2
  }

  val lf = List(f1, f2)

  lf onComplete { _ map {
    case Success(v) => ???
    case Failure(e) => ???
  }}

This solution has the advantage of allowing you to call an onComplete on a sequence of futures as you would on a single future.