Custom Scala enum, most elegant version searched

Question

For a project of mine I have implemented a Enum based upon

trait Enum[A] {
  trait Value { self: A =>
    _values :+= this
  }
  private var _values = List.empty[A]
  def values = _values
}

sealed trait Currency extends Currency.Value
object Currency extends Enum[Currency] {
  case object EUR extends Currency
  case object GBP extends Currency
}

from Case objects vs Enumerations in Scala. I worked quite nice, till I run into the following problem. Case objects seem to be lazy and if I use Currency.value I might actually get an empty List. It would have been possible to make a call against all Enum Values on startup so that the value list would be populated, but that would be kind of defeating the point.

So I ventured into the dark and unknown places of scala reflection and came up with this solution, based upon the following SO answers. Can I get a compile-time list of all of the case objects which derive from a sealed parent in Scala? and How can I get the actual object referred to by Scala 2.10 reflection?

import scala.reflect.runtime.universe._

abstract class Enum[A: TypeTag] {
  trait Value

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

  def values = (sealedDescendants getOrElse Set.empty).map(
    symbol => symbol.owner.typeSignature.member(symbol.name.toTermName)).map(
    module => reflect.runtime.currentMirror.reflectModule(module.asModule).instance).map(
    obj => obj.asInstanceOf[A]
  )
}

The amazing part of this is that it actually works, but it is ugly as hell and I would be interested if it would be possible to make this simpler and more elegant and to get rid of the asInstanceOf calls.

Answer 1

Here is a simple macro based implementation:

import scala.language.experimental.macros
import scala.reflect.macros.blackbox

abstract class Enum[E] {
  def values: Seq[E] = macro Enum.caseObjectsSeqImpl[E]
}

object Enum {
  def caseObjectsSeqImpl[A: c.WeakTypeTag](c: blackbox.Context) = {
    import c.universe._

    val typeSymbol = weakTypeOf[A].typeSymbol.asClass
    require(typeSymbol.isSealed)
    val subclasses = typeSymbol.knownDirectSubclasses
      .filter(_.asClass.isCaseClass)
      .map(s => Ident(s.companion))
      .toList
    val seqTSymbol = weakTypeOf[Seq[A]].typeSymbol.companion
    c.Expr(Apply(Ident(seqTSymbol), subclasses))
  }
}

With this you could then write:

sealed trait Currency
object Currency extends Enum[Currency] {
  case object USD extends Currency
  case object EUR extends Currency
}

so then

Currency.values == Seq(Currency.USD, Currency.EUR)

Since it's a macro, the Seq(Currency.USD, Currency.EUR) is generated at compile time, rather than runtime. Note, though, that since it's a macro, the definition of the class Enum must be in a separate project from where it is used (i.e. the concrete subclasses of Enum like Currency). This is a relatively simple implementation; you could do more complicated things like traverse multilevel class hierarchies to find more case objects at the cost of greater complexity, but hopefully this will get you started.