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

One common approach to waiting for all results (failed or not) is to "lift" failures into a new representation inside the future, so that all futures complete with some result (although they may complete with a result that represents failure). One natural way to get that is lifting to a Try.

Twitter's implementation of futures provides a liftToTry method that makes this trivial, but you can do something similar with the standard library's implementation:

import scala.util.{ Failure, Success, Try }

val lifted: List[Future[Try[Int]]] = List(f1, f2).map(
  _.map(Success(_)).recover { case t => Failure(t) }
)

Now Future.sequence(lifted) will be completed when every future is completed, and will represent successes and failures using Try.

And so, a generic solution for waiting on all original futures of a sequence of futures may look as follows, assuming an execution context is of course implicitly available.

  import scala.util.{ Failure, Success, Try }

  private def lift[T](futures: Seq[Future[T]]) = 
    futures.map(_.map { Success(_) }.recover { case t => Failure(t) })

  def waitAll[T](futures: Seq[Future[T]]) =
    Future.sequence(lift(futures)) // having neutralized exception completions through the lifting, .sequence can now be used

  waitAll(SeqOfFutures).map { 
    // do whatever with the completed futures
  }