phantomTypesDemo.scala

object FSM {

  sealed trait State

  object State {
    sealed trait State1 extends State
    sealed trait State2 extends State
    sealed trait State3 extends State
  }

  def state1 = new FSM[State.State1]
  def state2 = new FSM[State.State2]
  def state3 = new FSM[State.State3]
}

class FSM[S <: FSM.State] {

  import FSM.State._

  def to2()(implicit ev: S =:= State1) = {
    println("State1 => State2")
    new FSM[State2]
  }

  def to3()(implicit ev: S =:= State2) = {
    println("State2 => State3")
    new FSM[State3]
  }

  def to1()(implicit ev: S =:= State3) = {
    println("State3 => State1")
    new FSM[State1]
  }
}

FSM.state1.to2().to3().to1().to2()