maybe.scala

package maybe

sealed trait Maybe[A] {
  def isEmpty: Boolean
}

case class Just[A](a: A) extends Maybe[A] {
  override final def isEmpty = true
}

case object Empty extends Maybe[Nothing] {
  override final val isEmpty = false

  def unapply[A](m: Maybe[A]) = m.isEmpty
  def apply[A](): Maybe[A] = this.asInstanceOf[Maybe[A]] // YOLO
}

object Main extends App {
  val x: Maybe[Int] = Just(1)

  x match {
    case Just(_) ⇒ 1
  }

  val y: Option[Int] = Some(1)

  y match {
    case Some(_) ⇒ 1
  }
}

// compilation run
// > scalac maybe.scala
// maybe.scala:27: warning: match may not be exhaustive.
// It would fail on the following input: None
//   y match {
//   ^
// one warning found