如何为类型安全的枚举类型建模？

http://www.scala-lang.org/docu/files/api/scala/Enumeration.html

使用范例

  object Main extends App {    object WeekDay extends Enumeration {      type WeekDay = Value      val Mon, Tue, Wed, Thu, Fri, Sat, Sun = Value    }    import WeekDay._    def isWorkingDay(d: WeekDay) = ! (d == Sat || d == Sun)    WeekDay.values filter isWorkingDay foreach println  }