Partitioning on SubType for akka streams

Question

I have an akka stream where I have an ADT of the form.

sealed trait Message
sealed trait ThisMessage extends Message
sealed trait ThatMessage extends Message

Now I have a This Message Handler Flow and a That Message Handler Flow. I have an inlet flow that accepts type Message.

In order to create a split, I have the following partitioner. I have the following definition for a partitioner function.

 /**
  * Creates a Partition stage that, given a type A, makes a decision to whether to partition to subtype B or subtype C
  *
  * @tparam A type of input
  * @tparam B type of output on the first outlet.
  * @tparam C type of output on the second outlet.
  *
  * @return A partition stage
  */
  def binaryPartitionByType[A, B <: A, C <: A](): Graph[FanOutShape2[A, B, C], NotUsed] =
GraphDSL.create[FanOutShape2[A, B, C]]() { implicit builder =>
  import GraphDSL.Implicits._

  // This is wrong, but I have no idea how to write this.
  val partitioner: UniformFanOutShape[A, A] = builder.add(Partition[A](2, {
    case _: B => 0
    case _: C => 1
  }))

  new FanOutShape2(partitioner.in, partitioner.out(0).outlet, partitioner.out(1).outlet)
}

I wish to use the above method, and use the ADT in the type params to initialize a partitioner.

The compiler throws this error.

Error:(63, 7) type mismatch;
 found   : akka.stream.FanOutShape2[A,A,A]
 required: akka.stream.FanOutShape2[A,B,C]
      new FanOutShape2(partitioner.in, partitioner.out(0).outlet, 
partitioner.out(1).outlet)

From what I understand the partition object only has the Inlet (in this case A, a parametrized type.

Anyone have any ideas how I can fix this?

Answer 1

Here's one way to instantiate a FanOutShape2[A, B<:A, C<:A] from UniformFanOutShape[A, A] generated by builder.add(Partition[A]()):

import akka.stream.scaladsl._
import akka.stream.{Graph, FanOutShape2}
import akka.NotUsed
import scala.reflect.ClassTag

def binaryPartitionByType[A, B <: A : ClassTag, C <: A : ClassTag](): Graph[FanOutShape2[A, B, C], NotUsed] =
  GraphDSL.create[FanOutShape2[A, B, C]]() { implicit builder =>
    import GraphDSL.Implicits._

    val partitioner = builder.add(Partition[A](2, {
      case _: B => 0
      case _: C => 1
    }))

    val partitionB = builder.add(Flow[A].collect{ case b: B => b })
    val partitionC = builder.add(Flow[A].collect{ case c: C => c })

    partitioner.out(0) ~> partitionB
    partitioner.out(1) ~> partitionC

    new FanOutShape2(partitioner.in, partitionB.out, partitionC.out)
}

// binaryPartitionByType: [A, B <: A, C <: A]()(
//   implicit evidence$1: scala.reflect.ClassTag[B], implicit evidence$2: scala.reflect.ClassTag[C]
// ) akka.stream.Graph[akka.stream.FanOutShape2[A,B,C],akka.NotUsed]

Note that ClassTag is needed to avoid type erasure.