How to iterate inner objects in a defined object in Scala

Question

My question is how to iterate inner objects in a defined object in Scala using Scala reflection package?

object Units {

    val values = CITIZEN :: WORKER :: Nil // I need something reflectional to list all of the case objects

    case object CITIZEN extends Population
    case object WORKER extends Population
}

score 5 · Accepted Answer · answered Aug 04 '13 at 14:36

There are lots of ways to do this, but I'd suggest using macros (compile-time reflection) over runtime reflection, for the sake of both performance and (more importantly) type safety.

Here's a quick implementation with macros:

import scala.language.experimental.macros

object MacroUtils {
  import scala.reflect.macros.Context

  def values = macro MacroUtils.values_impl
  def values_impl(c: Context) = {
    import c.universe._

    val objs = c.enclosingClass.collect {
      case ModuleDef(mods, name, _) if mods hasFlag Flag.CASE => Ident(name)
    }

    c.Expr[List[Any]](
      Apply(Select(reify(List).tree, newTermName("apply")), objs.toList)
    )
  }
}

trait Population

object Units {
  val values = MacroUtils.values
  case object CITIZEN extends Population
  case object WORKER extends Population
}

And then, for example:

scala> val populations: List[Population] = Units.values
populations: List[Population] = List(CITIZEN, WORKER)

Note that the compiler knows that the list of case objects can be statically typed as a list of populations.

There is a problem, macro implementation must be in statically accessible object in `def values = macro MacroUtils.values_impl` — Pooya, Aug 04 '13 at 14:56
Did you move the definition of the `values` method? It works for me exactly as pasted above. — Travis Brown, Aug 04 '13 at 14:58

score 1 · Answer 2 · answered Aug 04 '13 at 14:25

I'm not sure if this is the best way to do it, but here it is:

object Units {

    val values = CITIZEN :: WORKER :: Nil

    trait Population

    case object CITIZEN extends Population

    case object WORKER extends Population

    val reflectionValues = {
        import scala.reflect.runtime.{universe => ru}
        val mirror = ru.runtimeMirror(getClass.getClassLoader)
        val objects = ru.typeOf[Units.type].declarations.filter(_.isModule)
        objects.map(o => mirror.reflectModule(o.asModule).instance.asInstanceOf[Population]).toList
    }

    def main(args: Array[String]) {
        assert(values == reflectionValues)
    }
}

How to iterate inner objects in a defined object in Scala

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