Getting case object instances which derive from a sealed parent in Scala

Question

This is essentially my question, but the accepted answer stops at returning their symbols rather than the case objects themselves.

In theory, this should be easy:

def getCaseObjects(enumType: Type) = {
    val m = ru.runtimeMirror(getClass.getClassLoader)
    enumType.typeSymbol.asClass.knownDirectSubclasses map { subclass =>
        val enumObjMirror = m.reflectClass(subclass.asClass.toType.typeSymbol.asClass)
        enumObjMirror.reflectConstructor(subclass.asClass.toType.decl(ru.termNames.CONSTRUCTOR).asMethod)()
    }
}

And this works!

...Except that they are entirely new instances compared to the ones contained in their Parent sealed trait; hooray, I've busted the "case objects are singleton" assumption!

I could override equals and hashCode in my Parent sealed trait and be done with it, but I'd prefer a way to get those particular case objects rather than ones that happen to look like them. Is this possible? I'm on 2.11 if that makes any difference.

score 3 · Accepted Answer · answered Aug 28 '15 at 21:06

Assuming you use the sealedDescendants method in the post you reference, I believe you should be able to get the underlying objects like:

import scala.reflect.runtime.universe._
import scala.reflect.runtime.{ universe => ru }
val m = ru.runtimeMirror(getClass.getClassLoader)

val descendants = sealedDescendants[Parent] getOrElse Set.empty

val objects = descendants.map({ desc =>
  val mod = m.staticModule(desc.asClass.name.decoded)
  m.reflectModule(mod).instance
})

Spot on! Thank you very much. – helgridly Aug 29 '15 at 00:11 — helgridly, Aug 29 '15 at 00:11

Getting case object instances which derive from a sealed parent in Scala

1 Answers1