Scala：在模式匹配中混合特征和案例类

Question

我想匹配一些案例类。如果我不认识它们，我想匹配类必须扩展的指定特征。这看起来像

trait Event  //root trait
trait Status extends Event  //special trait
trait UIEvent extends Event //special trait

case class Results extends Event   //concrete case class
case class Query extends Event     //concrete case class

case class Running extends Status  //concrete case class
case class Finished extends Status //concrete case class

case class Update extends UIEvent  //concrete case class

我运行以下测试

  val events = List(Results, Query, Running, Finished, Update)
    events foreach {
      case Results => println("Got a Results")
      case Running => println("Got a Running")
      case s:Status => println("Got some StatusEvent")
      case ui:UIEvent => println("Got some UIEvent")
      case e: Event => println("Generic Event")
      case x => println("Didn't matched at all " + x)
    }
    println("############################")
    val STATUS = classOf[Status]
    val EVENT = classOf[Event]
    val UIEVENT = classOf[UIEvent]
    val RESULTS = classOf[Results]
    val eventsClass = events map (_.getClass)
    eventsClass foreach {
      case RESULTS => println("Got a Results")
      case STATUS => println("Got some StatusEvent")
      case UIEVENT =>  println("Got some UIEvent")
      case EVENT => println("Generic Event")
      case x => println("Didn't matched at all " + x)
    }

导致以下输出

Got a Results
Didn't match at all Query
Got a Running
Didn't match at all Finished
Didn't match at all Update
############################
Didn't match at all class de.mukis.scala.test.main.Results$
Didn't match at all class de.mukis.scala.test.main.Query$
Didn't match at all class de.mukis.scala.test.main.Running$
Didn't match at all class de.mukis.scala.test.main.Finished$
Didn't match at all class de.mukis.scala.test.main.Update$

为什么我不能在案例类和特征上进行模式匹配，或者只在类上进行模式匹配？

Answer 1

问题在于您指的是案例类的伴随对象，而不是它们的特定实例。因此，REPL 应该已经向您提供了弃用警告。

解决办法是加几个括号：

sealed abstract trait Event
sealed abstract trait Status extends Event
sealed abstract trait UIEvent extends Event

case class Results() extends Event
case class Query() extends Event

case class Running() extends Status
case class Finished() extends Status

case class Update() extends UIEvent

和

val events = List(Results(), Query(), Running(), Finished(), Update())
events foreach {
  case Results() => println("Got a Results")
  case Running() => println("Got a Running")
  case s:Status => println("Got some StatusEvent")
  case ui:UIEvent => println("Got some UIEvent")
  case e: Event => println("Generic Event")
  case x => println("Didn't match at all " + x)
}

或者，正如 didierd 建议的那样，使用 case objects

sealed abstract trait Event
sealed abstract trait Status extends Event
sealed abstract trait UIEvent extends Event

case object Results extends Event
case object Query extends Event

case object Running extends Status
case object Finished extends Status

case object Update extends UIEvent

和

val events = List(Results, Query, Running, Finished, Update)
events foreach {
  case Results => println("Got a Results")
  case Running => println("Got a Running")
  case s:Status => println("Got some StatusEvent")
  case ui:UIEvent => println("Got some UIEvent")
  case e: Event => println("Generic Event")
  case x => println("Didn't match at all " + x)
}

Answer 2

您的问题是没有括号的案例类（现在已弃用）。案例类意味着创建伴随对象。当您在列表和模式匹配中编写不带括号的结果时，它表示伴随对象。

你可以试试

define sortOut(x: Any) = x match {
  case Results => "companion object"
  case Results() => "instance"
}

sortOut(Results) // returns companion object
sortout(Results()) // returns instance

这解释了第二部分中的行为。由于Results是伴生对象，Results.getClass()不是classOf[Results]，它是实例的类，而是伴生对象的（合成）类，Results$

如果case class没有参数，多数情况下表示无法区分各个实例，应该使用case object。否则，放括号。