akka-stream - How to treat the last element of a stream differently in a Flow/Graph

Question

I'm trying to implement an Akka Streams Flow that will convert a stream of JSON objects to a stream of a single array of JSON objects. I can use Concat to add an "[" before and "]" after, as well as Zip to insert commas in between elements, but I can't figure out how to not insert the final comma.

The code I have so far is:

trait JsonStreamSupport {

  protected def toJsonArrayString[T : Writes] = Flow[T].map(Json.toJson(_)).map(_.toString()).via(jsonArrayWrapper)


  private[this] val jsonArrayWrapper: Flow[String, String, NotUsed] = Flow.fromGraph(GraphDSL.create() { implicit b =>
    import GraphDSL.Implicits._
    val start = Source.single("[")
    val comma = Source.repeat(",")
    val end = Source.single("]")
    val concat = b.add(Concat[String](3))
    val zip = b.add(Zip[String,String])

    comma ~> zip.in1
    start ~> concat.in(0)
    zip.out.map({case (msg,delim) => msg + delim}) ~> concat.in(1)
    end ~> concat.in(2)
    FlowShape(zip.in0, concat.out)
  })
}

Currently the output is:
[{"key":"value},{"key","value"},]
but I need it to be
[{"key":"value},{"key","value"}] (without final comma),
where each element of the array is still a distinct element of the stream so can be, for example, sent over chunked HTTP separately.

Answer 1

just found out about intersperse which is exactly what you need, and much simpler than what I suggested in the first place:

http://doc.akka.io/api/akka/2.4.4/index.html#akka.stream.scaladsl.Flow@intersperse[T%3E:Out]%28start:T,inject:T,end:T%29:FlowOps.this.Repr[T]