봉인된 트레이트

트레이트란 무엇인가요?

기본 트레이트

trait Shape
case class Circle(r: Double) extends Shape
case class Square(side: Double) extends Shape
object Main {
  def main(args: Array[String]): Unit = {
    val s: Shape = Circle(2.0)
    println(s)
  }
}

sealed는 무엇을 의미하나요?

sealed trait Color
case object Red extends Color
case object Green extends Color
case object Blue extends Color
object Main {
  def main(args: Array[String]): Unit = {
    val c: Color = Green
    println(c)
  }
}

변형으로 사용하는 케이스 객체

sealed trait Direction
case object North extends Direction
case object South extends Direction
object Main {
  def main(args: Array[String]): Unit = {
    val d: Direction = North
    println(d)
  }
}

객체와 클래스 섞어 사용하기

sealed trait Event
case object Started extends Event
case class Failed(reason: String) extends Event
object Main {
  def main(args: Array[String]): Unit = {
    val e: Event = Failed("timeout")
    println(e)
  }
}

봉인된 트레이트에 매칭하기

sealed trait Color
case object Red extends Color
case object Green extends Color
case object Blue extends Color
object Main {
  def hex(c: Color): String = c match {
    case Red   => "#FF0000"
    case Green => "#00FF00"
    case Blue  => "#0000FF"
  }
  def main(args: Array[String]): Unit = {
    println(hex(Green))
  }
}

컴파일러 경고

트레이트에서 멤버 선언하기

sealed trait Animal {
  def sound: String
}
case object Dog extends Animal { def sound = "Woof" }
case object Cat extends Animal { def sound = "Meow" }
object Main {
  def main(args: Array[String]): Unit = {
    println(Dog.sound)
    println(Cat.sound)
  }
}

왜 sealed를 사용하나요?

계층 구조 사용하기

sealed trait Payment
case class Cash(amount: Int) extends Payment
case class Card(number: String) extends Payment
object Main {
  def summary(p: Payment): String = p match {
    case Cash(a) => s"cash $a"
    case Card(n) => s"card ending ${n.takeRight(4)}"
  }
  def main(args: Array[String]): Unit = {
    println(summary(Cash(50)))
    println(summary(Card("123456789012")))
  }
}

하나로 합쳐 보기

sealed trait Result
case object Pending extends Result
case class Success(value: Int) extends Result
case class Error(msg: String) extends Result
object Main {
  def show(r: Result): String = r match {
    case Pending    => "waiting"
    case Success(v) => s"ok: $v"
    case Error(m)   => s"fail: $m"
  }
  def main(args: Array[String]): Unit = {
    List(Pending, Success(7), Error("boom")).foreach(r => println(show(r)))
  }
}

빠른 확인

복습