What are Sealed classes in Scala?

Traits and classes can be marked sealed which means all subtypes must be declared in the same file. This assures that all subtypes are known.

sealed abstract class Communication
case class VoiceCall() extends Communication
case class Message() extends Communication

def findPlaceToSit(commtype: Communication): String = commtype match {
  case a: VoiceCall => "Call him"
  case b: Message => "Message him"
}

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