Question

I just wanted to know if it is possible to iterate over a sealed trait in Scala?
If not, why is it not possible? Since the trait is sealed it should be possible no?

What I want to do is something like that:

sealed trait ResizedImageKey {

  /**
   * Get the dimensions to use on the resized image associated with this key
   */
  def getDimension(originalDimension: Dimension): Dimension

}

case class Dimension(width: Int,  height: Int)

case object Large extends ResizedImageKey {
  def getDimension(originalDimension: Dimension) = Dimension(1000,1000)
}

case object Medium extends ResizedImageKey{
  def getDimension(originalDimension: Dimension) = Dimension(500,500)
}

case object Small extends ResizedImageKey{
  def getDimension(originalDimension: Dimension) = Dimension(100,100)
}

What I want can be done in Java by giving an implementation to the enum values. Is there an equivalent in Scala?

Answer 1

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 See my answer here for a related (but now slightly out-of-date) example, or just use something like this: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.