How to use flatMapConcat?

Question

I am trying to use flatMapConcat as the following:

Source.empty
      .flatMapConcat {
        Source.fromFuture(Future("hello"))
      }
      .runWith(Sink.foreach(println))
      .onComplete {
        case Success(_) =>
          println()
        case Failure(e) =>
          println(s"Thrown ${e.getMessage}")
      }

and the compiler complains:

Error:(31, 26) type mismatch;
 found   : akka.stream.scaladsl.Source[String,akka.NotUsed]
 required: ? => akka.stream.Graph[akka.stream.SourceShape[?],?]
        Source.fromFuture(Future("hello")) 

What am I doing wrong?

Answer 1

Method flatMapConcat has the following signature:

def flatMapConcat[T, M](f: (Out) => Graph[SourceShape[T], M]): Repr[T]

which, in the case of processing a Source of Strings, would expect a function like:

f: String => Source(Iterable[String])

Another issue with your sample code is that Source.empty[T] has no elements to be processed hence the subsequent flatMapConcat will never be executed.

Here's an example of using flatMapConcat to transform each element from a Source of names:

import akka.actor.ActorSystem
import akka.stream.ActorMaterializer
import akka.stream.scaladsl._

implicit val system = ActorSystem("system")
implicit val materializer = ActorMaterializer()

Source(List("alice", "bob", "jenn")).
  flatMapConcat{ name => Source(List(s"Hi $name", s"Bye $name")) }.
  runWith(Sink.foreach(println))
// Hi alice
// Bye alice
// Hi bob
// Bye bob
// Hi jenn
// Bye jenn

As a side note, one could replace flatMapConcat in the above example with mapConcat which expects a simpler function signature:

Source(List("alice", "bob", "jenn")).
  mapConcat{ name => List(s"Hi $name", s"Bye $name") }.
  runWith(Sink.foreach(println))