build.sbt

lazy val root = (project in file(".")).
  settings(
    inThisBuild(List(
      organization := "io.github.natewave",
      scalaVersion := "2.12.3",
      version      := "0.1.0-SNAPSHOT"
    )),
    name := "AkkaStreamsSample",
    libraryDependencies ++= Seq(
      "com.typesafe.akka" %% "akka-stream" % "2.5.6",
      "com.typesafe.akka" %% "akka-stream-testkit" % "2.5.6" % Test
    ),
    scalacOptions in Test ++= Seq("-Yrangepos")
  )

GraphDSLExample.scala

package natewave

import akka.Done

import scala.concurrent.Future
import scala.util.{Failure, Success}

object GraphTest extends App {
  import akka.actor.ActorSystem
  import akka.stream._
  import akka.stream.scaladsl._

  override def main(args: Array[String]): Unit = {
    // implicit actor system
  implicit val system = ActorSystem()

  // implicit actor materializer
  implicit val materializer = ActorMaterializer()

  import system.dispatcher
    
    println("-- Main started! --")

    val in1 = Source(List("a", "b", "c"))
    val in2 = Source(List("d", "e", "f"))
    val out = Sink.foreach(println)

    import GraphDSL.Implicits._

    val graphShape = GraphDSL.create(out) { implicit b => sink =>
      val merge = b.add(Merge[String](2))
      in1 ~> merge.in(0)
      in2 ~> merge.in(1)
      merge.out ~> sink
      ClosedShape
    }

    val graph: RunnableGraph[Future[Done]] = RunnableGraph.fromGraph(graphShape)

    val run: Future[Done] = graph.run()

    run.onComplete {
      case Success(_) => system.terminate()
      case Failure(e) =>
        println(e.getMessage)
        system.terminate()
    }
  }
}