phantom-types.scala

sealed trait NodeState
trait Follower extends NodeState
trait Candidate extends NodeState
trait Leader extends NodeState

final class NotLeader extends NodeState with Follower with Candidate
final class NotFollower extends NodeState with Candidate with Leader
final class NotCandidate extends NodeState with Follower with Candidate

class Node[State <: NodeState] private () {
  // Only a non leader can become a candidate
  def becomeCandidate[T >: State <: NotLeader]() = this.asInstanceOf[Node[Candidate]]

  // become follower
  def becomeFollower[T >: State <: NotFollower]() = this.asInstanceOf[Node[Follower]]

  // only a non leader can become a leader
  def becomeLeader[T >: State <: NotLeader]() = this.asInstanceOf[Node[Leader]]
}

object Node {
  // A node starts as a Follower
  def create() = new Node[Follower]
}