Get rid of Scala Future nesting

Question

Again and again I am struggling when a function relies on some future results. This usually boils down to a result like Future[Seq[Future[MyObject]]]

To get rid of that

Answer 1

The Akka documentation has a nice overview on how to deal with a compositions of futures. In general, it outlines four methods in scala.concurrent.Future that can be used to reduce a composition of futures into a single Future instance:

• Future.sequence
• Future.traverse
• Future.fold
• Future.reduce

Answer 2

Two handy functions on the Future companion object you should know could help here, the first, and easier to wrap your head around is Future.sequence. It takes a sequnce of futures and returns a Future of a sequence. If are ending up with a Future[Seq[Future[MyObject]]], lets call that result. then you can change this to a Future[Future[Seq[MyObject]]] with result.map(Future.sequence(_))

Then to collapse a Future[Future[X]] for any X, you can run "result.flatMap(identity)", in fact, you can do this for any M[M[X]] to create a M[X] as long as M has flatMap.

Another useful function here is Future.traverse. It is basically the result of taking a Seq[A], mapping it to a Seq[Future[B]], then running Future.sequence to get a Future[Seq[B]] So in your example, you'd have:

ideas.map{ Future.traverse(_){ idea =>
    /*something that returns a Future[JsObject]*/
} }.flatMap(identity)

However, many times when you are running flatMap(identity), you could be turning a map into a flatMap, and this is the case here:

ideas.flatMap{ Future.traverse(_) { idea =>
    /*something that returns a Future[JsOjbect]*/
} }