This is actually in my opinion an appropriate use case for 2.10 macros: you want access to information that you know the compiler has, but isn’t exposing, and macros give you a (reasonably) easy way to peek inside. See my answer here for a related (but now slightly out-of-date) example, or just use something like this:
import language.experimental.macros
import scala.reflect.macros.Context
object SealedExample {
def values[A]: Set[A] = macro values_impl[A]
def values_impl[A: c.WeakTypeTag](c: Context) = {
import c.universe._
val symbol = weakTypeOf[A].typeSymbol
if (!symbol.isClass) c.abort(
c.enclosingPosition,
"Can only enumerate values of a sealed trait or class."
) else if (!symbol.asClass.isSealed) c.abort(
c.enclosingPosition,
"Can only enumerate values of a sealed trait or class."
) else {
val children = symbol.asClass.knownDirectSubclasses.toList
if (!children.forall(_.isModuleClass)) c.abort(
c.enclosingPosition,
"All children must be objects."
) else c.Expr[Set[A]] {
def sourceModuleRef(sym: Symbol) = Ident(
sym.asInstanceOf[
scala.reflect.internal.Symbols#Symbol
].sourceModule.asInstanceOf[Symbol]
)
Apply(
Select(
reify(Set).tree,
newTermName("apply")
),
children.map(sourceModuleRef(_))
)
}
}
}
}
Now we can write the following:
scala> val keys: Set[ResizedImageKey] = SealedExample.values[ResizedImageKey]
keys: Set[ResizedImageKey] = Set(Large, Medium, Small)
And this is all perfectly safe—you’ll get a compile-time error if you ask for values of a type that isn’t sealed, has non-object children, etc.