枚举

使用 sealed trait 和 case objects 的方法是首选，因为 Scala 枚举有一些问题：

1. 擦除后枚举具有相同的类型。
2. 编译器没有抱怨匹配并不详尽，如果错过了它将在运行时失败 3：

def isWeekendWithBug(day: WeekDays.Value): Boolean = day match {
  case WeekDays.Sun | WeekDays.Sat => true
}

isWeekendWithBug(WeekDays.Fri)
scala.MatchError: Fri (of class scala.Enumeration$Val)

与之比较：

def isWeekendWithBug(day: WeekDay): Boolean = day match {
  case WeekDay.Sun | WeekDay.Sat => true
}

Warning: match may not be exhaustive.
It would fail on the following inputs: Fri, Mon, Thu, Tue, Wed
def isWeekendWithBug(day: WeekDay): Boolean = day match {
                                          ^

有关 Scala Enumeration 的文章中提供了更详细的说明。