Question

I am lost in scala generics.

I need a method storeUnit which accepts Unit‘s subclasses’s instances (e.q. Visit) and returns StoredUnit‘s subclasses’s instances (e.q. StoredVisit) but I am getting compilation errors.

trait StatsUnit { val ip: String } 
case class Visit(ip: String) extends StatsUnit
case class Click(ip: String) extends StatsUnit

trait StoredStatsUnit extends StatsUnit { val id: String }
case class StoredVisit(id: String, ip: String) extends StoredStatsUnit
case class StoredClick(id: String, ip: String) extends StoredStatsUnit

def storeUnit[A <: StatsUnit, B <: StoredStatsUnit](statsUnit: A): B = {
  statsUnit match {
    case x: Visit => StoredVisit("myid", x.ip)
    case x: Click => StoredClick("myid", x.ip)
  }
}

/tmp/1.scala:11: error: type mismatch;
 found   : this.StoredVisit
 required: B
    case x: Visit => StoredVisit("myid", x.ip)
                                    ^
/tmp/1.scala:12: error: type mismatch;
 found   : this.StoredClick
 required: B
    case x: Click => StoredClick("myid", x.ip)

Answer 1

A comment first:

1. Don’t name your trait Unit! Unit has specific meaning in Scala – it’s equivalent to Java’s void – and shadowing that definition is only going to cause troubles!

However, the problem here is that you specify that your method will return an instance of B and then you try to return something of type StoredVisit. You don’t need B at all in this example, so the following will work fine:

def storeUnit[A <: StatsUnit](unit: A): StoredStatsUnit = {
  StoredVisit("myid", unit.ip)
}