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object Main{
  def main(args: Array[String]){
    val head = List(1, 2, 3) match {
      case (head :: tail) => head
      case Nil => -1
    }

    val head2 = List() match {
      case (head :: tail) => head
      // 空のときのパターンを書かないとコンパイラが警告を出す。
      // Test.scala:8: warning: match may not be exhaustive.
      // It would fail on the following input: Nil
      //     val head2 = List() match {
      //     one warning found
      //
      //
    }
  }
}