Typeclass.scala

//In library
trait Event
trait EventCombiner[A <: Event] {
    def initialValue: Int
    def combineEvents(value: Int, e2: A): Int
}

def foldEvents[A <: Event](list: Seq[A])(implicit C: EventCombiner[A]): Int =
    list.foldLeft(C.initialValue)(C.combineEvents)

//Outside library
  sealed trait HandledEvent extends Event with Product with Serializable
  object HandledEvent {
    implicit val handledEvent: EventCombiner[HandledEvent] = new EventCombiner[HandledEvent] {
      override def initialValue: Int = 0
      override def combineEvents(value: Int, e2: HandledEvent): Int = {
        e2 match {
          case Reset  => 0
          case Inc(i) => value + i
          case Noop   => value
        }
      }
    }
  }
  case object Reset         extends HandledEvent
  case class Inc(by: Int)   extends HandledEvent
  case object Noop          extends HandledEvent

  

  private val events = Seq(Inc(5), Inc(2), Inc(3), Reset, Inc(2))
  foldEvents(events)