how do I reflect on Scalavro union types in a base trait with abstract type variable?

Question

I have a trait EsperClassification defined as follows:

trait EsperClassification {
   import com.gensler.scalavro.util.Union
   import scala.reflect.runtime.{currentMirror => m}
   type EsperEvents

   // what I want to do is this, but this fails:
   val u = new Union[EsperEvents]
   // so that I can do this:
   u.memberTypes() foreach { t=> registerType(t.typeSymbol.Name,m.runtimeClass(t))}

   private def registerType(name:String, clz: Class[_ <: Any]) = ...
}

This trait is used as follows:

class EsperEventBus extends ActorEVentBus with EsperClassification {
  type EsperEvents = union[Buy] #or [Sell]

  … more stuff
}

this fails at the instantiation of Union with the scala compiler complaining about missing TypeTags.

However, if I instantiate the Union in the concrete EsperEventBus it works fine:

trait EsperClassification {
  import com.gensler.scalavro.util.Union
  import scala.reflect.runtime.{currentMirror => m}
  type EsperEvents
  def esperEventsUnion: Union[EsperEVents] // abstract!

  // this works:
  esperEventsUnion.memberTypes() foreach { 
      t=> registerType(t.typeSymbol.Name,m.runtimeClass(t))}

  private def registerType(name:String, clz: Class[_ <: Any]) = ...
}

trait EsperEventBus extends ActorEventBus with EsperClassification {
  type EsperEvents = union[Buy] #or [Sell]

  def esperEventsUnion = new Union[EsperEvents] 
}

What I would like to know is if and how I can instantiate the Union in the trait itself?

Thanks,

Frank

Answer 1

Unfortunately, Union requires resolution of an implicit TypeTag for the underlying disjunction of types. The only workaround is as you've done: delay instantiating the Union until such evidence is available to scalac.