使用密封的 trait 和 case 对象以及 allValues-macro

这只是密封特征变体的扩展，其中宏在编译时生成包含所有实例的集合。这很好地省略了开发人员可以向枚举添加值但又忘记将其添加到 allElements 集的缺点。

这种变体特别适合大型枚举。

import EnumerationMacros._

sealed trait WeekDay
object WeekDay {
  case object Mon extends WeekDay
  case object Tue extends WeekDay
  case object Wed extends WeekDay
  case object Thu extends WeekDay
  case object Fri extends WeekDay
  case object Sun extends WeekDay
  case object Sat extends WeekDay
  val allWeekDays: Set[WeekDay] = sealedInstancesOf[WeekDay]
}

为此，你需要这个宏：

import scala.collection.immutable.TreeSet
import scala.language.experimental.macros
import scala.reflect.macros.blackbox

/**
A macro to produce a TreeSet of all instances of a sealed trait.
Based on Travis Brown's work:
http://stackoverflow.com/questions/13671734/iteration-over-a-sealed-trait-in-scala
CAREFUL: !!! MUST be used at END OF code block containing the instances !!!
*/
object EnumerationMacros {
  def sealedInstancesOf[A]: TreeSet[A] = macro sealedInstancesOf_impl[A]

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

    val symbol = weakTypeOf[A].typeSymbol.asClass

    if  (!symbol.isClass || !symbol.isSealed)
      c.abort(c.enclosingPosition, "Can only enumerate values of a sealed trait or class.")
    else {

      val children = symbol.knownDirectSubclasses.toList

      if (!children.forall(_.isModuleClass)) c.abort(c.enclosingPosition, "All children must be objects.")
      else c.Expr[TreeSet[A]] {

        def sourceModuleRef(sym: Symbol) = Ident(sym.asInstanceOf[scala.reflect.internal.Symbols#Symbol
          ].sourceModule.asInstanceOf[Symbol]
        )

        Apply(
          Select(
            reify(TreeSet).tree,
            TermName("apply")
          ),
          children.map(sourceModuleRef(_))
        )
      }
    }
  }
}