Missing parameter type for expanded function for Unit in scala

Question

I'm trying to wrap some actor receive code in db transaction to build idempotent persistent view that handles events by updating the information in database.

class TrajectoryView extends IdempotentView {


  override def viewId: String = "trajectoryView"

  override def persistenceId: String = "sample-id-1"

  override def handleEvent(event: Event): Unit = {
      case ProcessDetectedEvent(time, processData, id, activityType) =>
        val coords = Db.save(processData.coordinates)
        Db.save(CoordinatesWrapper(id, coords))

      case ProcessUpdatedEvent(id, processData, time) =>
        val coords = Db.save(processData.coordinates)
        Db.save(CoordinatesWrapper(id, coords))
  }
}

abstract class IdempotentView extends PersistentView {

  def handleEvent(event: Event): Unit = ???
  var maxSeenSeqNumber = Db.query[SeqNumberWrapper].fetchOne().getOrElse(SeqNumberWrapper(0, viewId)).seqNumber

  def receive: Receive = {
    case event: Event =>
      if (lastSequenceNr > maxSeenSeqNumber) {
        Db.transaction {
          Db.save(SeqNumberWrapper(lastSequenceNr, viewId))
          handleEvent(event)
        }
      }

    case _: Unit =>
      println("Unknown message")
  }
}



Error:(22, 50) missing parameter type for expanded function The argument types of an anonymous function must be fully known. (SLS 8.5) 

Expected type was: Unit   override def handleEvent(event: Event): Unit = { 

What should I do? I don't event understand what this error means exactly. From my point of view I have written all types, the compiler doesn't need to infer anything.

Answer 1

Choose one of

override def handleEvent:  Event => Unit = {
...
}

or

override def handleEvent(event: Event): Unit = event match {
...
}

If more precisely your code inside braces is suitable for defining PartialFunction[T,Unit] which is convertible to Unit . So generally your code is correct, but compiler could not infer type T and whines about it. But if it could result would be a function returning another function and not what you are expecting.