A Source.SplitAfter Akka example extra output

[vasily-kirichenko]

I'm trying to port an example from https://doc.akka.io/docs/akka/snapshot/stream/stream-substream.html

use system = ActorSystem.Create("test")
let mat = system.Materializer()

let text = "This is the first line.\nThe second line.\nThere is also the 3rd line\n"

Source.From(text.ToCharArray())
    .SplitAfter(fun x -> x = '\n')
    .Where(fun x -> x <> '\n')
    .Select(fun _ -> 1)
    .Aggregate(0, fun s x -> s + x)
    .To(Sink.ForEach(fun x -> Console.WriteLine (sprintf "%A" x)))
    .Run(mat)
    |> ignore
       
Console.ReadKey() |> ignore

As the docs says, it should output

23
16
26

but it outputs

26
0
16
23

Where does the 0 come from?

[vasily-kirichenko]

If I replace SplitAfter with SplitWhen, the result is not changed.

[vasily-kirichenko]

A simpler example.

Akka

val xs =
    List(
      Some(1), Some(2), None,
      Some(3), Some(4), None,
      Some(5), None)

  Source(xs)
    .splitAfter(_.isEmpty)
    .filter(_.isDefined)
    .fold(List[Int]())((x, r) => r.get +: x)
    .to(Sink.foreach(println))
    .run()

List(4, 3)
List(5)
List(2, 1)

Akka.NET

use system = ActorSystem.Create("test")
    let mat = system.Materializer()
    
    let xs =
        [ Ok 1; Ok 2; Error 0
          Ok 3; Ok 4; Error 0
          Ok 5; Error 0 ]
    
    Source.From(xs)
        .SplitAfter(fun x -> match x with Ok _ -> false | _ -> true)
        .Where(fun x -> match x with Ok _ -> true | _ -> false)
        .Aggregate([], fun r (Ok x) -> x :: r)
        .To(Sink.ForEach(fun x -> printfn "%A" x))
        .Run(mat)
        |> ignore

[]
[5]
[4; 3]
[2; 1]

(here I use F# Result type because it's a struct whereas Option is a ref type and None is represented as null, which causes Akka.NET crashes at runtime, see #3223).

[marcpiechura]

Problem ist that SplitAfter creates 4 substreams, the last one is empty an no element get's pushed but the Aggregate stage pushes the aggration when upstream finishes and since the aggregate function wasn't called for this substream it pushes the zero element downstream, i.e. the empty list. Need to investigate further why SplitAfter creates a stream too much.

[marcpiechura]

@vasily-kirichenko should be fixed in the next release, thx for reporting.

[vasily-kirichenko]

Thank you very much :)