simple-state-machine-2.kt

abstract class Transition<InputState: State, OutputState: State> {
  abstract fun invoke(state: InputState) : OutputState
}

class ValidateRequest: Transition<State.RequestReceived, State.RequestValidated>() {
  override fun invoke(state: State.RequestReceived): State.RequestValidated {
    // let's just check something here
    return State.RequestValidated(state.request > 0, state)
  }
}

class ProcessRequest: Transition<State.RequestValidated, State.RequestProcessed>() {
  override fun invoke(state: State.RequestValidated): State.RequestProcessed {
    return State.RequestProcessed(state.previousState.request * 2, state)
  }
}

class ValidateResponse: Transition<State.RequestProcessed, State.ResponseValidated>() {
  override fun invoke(state: State.RequestProcessed): State.ResponseValidated {
    // let's just check something here
    return State.ResponseValidated(state.response > 0, state)
  }
}