Can I get a compile-time list of all of the case objects which derive from a sealed parent in Scala?

Question

As has been discussed many times on SO, a Scala match will warn you if you don\'t exhaustively list all of the types deriving from a sealed class.

What I want is a

Answer 1

Update. Since 2.10.0-M7 we're exposing the methods mentioned in this answer as a part of public API. isSealed is ClassSymbol.isSealed and sealedDescendants is ClassSymbol.knownDirectSubclasses.

This is not going to be an answer to your question.

But, if you're willing to settle for something more like Enumeration.values(), and you're using a recent milestone of 2.10, and you're willing to muck about with some ugly casting-to-internal-APIs business, you can write the following:

import scala.reflect.runtime.universe._

def sealedDescendants[Root: TypeTag]: Option[Set[Symbol]] = {
  val symbol = typeOf[Root].typeSymbol
  val internal = symbol.asInstanceOf[scala.reflect.internal.Symbols#Symbol]
  if (internal.isSealed)
    Some(internal.sealedDescendants.map(_.asInstanceOf[Symbol]) - symbol)
  else None
}

Now if you've got a hierarchy like this:

object Test {
  sealed trait Parent
  case object A extends Parent
  case object B extends Parent
  case object C extends Parent
}

You can get the type symbols for the members of the sealed type hierarchy like this:

scala> sealedDescendants[Test.Parent] getOrElse Set.empty
res1: Set[reflect.runtime.universe.Symbol] = Set(object A, object B, object C)

It's hideous, but I don't think you're going to get what you actually want without writing a compiler plugin.