Scala Workflow using Any and Nothing types

Workflow.scala

import scala.language.existentials

// Simple laboratory exercise that uses incompatible parameter types, as the
// [[State]] type parameter is bound to [[Nothing]], and nothing above. That
// does not make sense at all.

object Lab {

  /** ************** Types */

  trait StateDef {
    def id: String
  }
  case class State[+CV](override val id: String, context: CV) extends StateDef

  trait Action[-P, +R]

  abstract class Condition[-T] extends Function1[T, Boolean] {
    override def apply(contextState: T): Boolean
  }

  case class Transition[-P, +R](tailState: StateDef, headState: StateDef, action: Action[P, R], condition: Option[Condition[P]] = None)

  case class Workflow private(transitions: List[Transition[Nothing, Any]]) {
    def getTransitions(tail: StateDef, head: StateDef): List[Transition[Nothing, Any]] = {
      transitions.filter(t => true)
    }

    def matchTransition[P <: Nothing](tail: State[P], head: StateDef): Option[Transition[Nothing, Any]] = {
      val value = tail.context
      val transitions: List[Transition[Nothing, Any]] = getTransitions(tail, head)
      transitions.find(t => t.condition.forall(cond => cond(value)))
    }
  }

  /** ************** Fixtures */

  // Model
  case class CreateImage()
  case class Image()

  // States
  class Init extends State[CreateImage]("INIT", CreateImage())
  val init = new Init()

  case object Published extends StateDef {
    override def id: String = "PUBLISHED"
  }

  // Action
  class CreateTipAction extends Action[CreateImage, Image]

  /** ************** Demo */

  val transitions = List(
    Transition(init, Published, new CreateTipAction())
  )
  val w = Workflow(transitions)
  val t = w.matchTransition(init, Published)

}