Finite State Machine with state-specific commands in Scala using Akka

another-example.scala

import akka.actor.typed.Behavior
import akka.actor.typed.scaladsl.Behaviors

object Test extends App {

  sealed trait AnimalCommand

  sealed trait DogCommand extends AnimalCommand
  object Bark extends DogCommand

  sealed trait HumanCommand extends AnimalCommand
  object Sing extends HumanCommand

  object Sit extends DogCommand with HumanCommand

  def humanBehavior: Behavior[HumanCommand] =
    Behaviors.receiveMessage(_ => animalBehavior.narrow[HumanCommand])

  def dogBehavior: Behavior[DogCommand] =
    Behaviors.receiveMessage(_ => animalBehavior.narrow[DogCommand])

  def humanCommandHandler(command: HumanCommand): Unit =
    command match {
      case Sing => println("la la la")
      case Sit => println("uh, okay")
    }

  def dogCommandHandler(command: DogCommand): Unit =
    command match {
      case Bark => println("woof")
      case Sit => println("...")
    }

  def animalBehavior: Behavior[AnimalCommand] =
    Behaviors.receiveMessage {
      case command: DogCommand =>
        dogCommandHandler(command)
        Behaviors.same

      case command: HumanCommand =>
        humanCommandHandler(command)
        Behaviors.same
    }

}

specialised-actors.scala

import akka.actor.typed.Behavior
import akka.actor.typed.scaladsl.Behaviors

object Test extends App {

  object AppleState {

    sealed trait AppleStateCommand
    object DoAppleThing extends AppleStateCommand

    def apply(): Behavior[AppleStateCommand] =
      Behaviors.receiveMessage {
        case DoAppleThing => ???
        case command: FruitCommand => command match {
          case DoFruitThing => ???
        }
      }
  }

  object BananaState {

    sealed trait BananaStateCommand
    object DoBananaThing extends BananaStateCommand

    def apply(): Behavior[BananaStateCommand] =
      Behaviors.receiveMessage {
        case DoBananaThing => ???
        case command: FruitCommand => command match {
          case DoFruitThing => ???
        }
      }
  }

  import AppleState._
  import BananaState._

  sealed trait FruitCommand extends BananaStateCommand with AppleStateCommand

  object DoFruitThing extends FruitCommand

  def juicer[T <: FruitCommand](fruit: Behavior[T]) = ???

  juicer(AppleState())
  juicer(BananaState())

  def mysteryFruit: Behavior[FruitCommand] = ???
  juicer(mysteryFruit)

}