Use of generalised type constraints to implement Option.flatten

option-flatten.scala

sealed trait Option[+A] {
  def isDefined: Boolean
  def flatten[B](implicit ev: A <:< Option[B]): Option[B]
}

case object None extends Option[Nothing] {
  def isDefined = false
  def flatten[B](implicit ev: Nothing <:< Option[B]): Option[B] = None
}

case class Some[A](value: A) extends Option[A] {
  def isDefined = true
  def flatten[B](implicit ev: A <:< Option[B]): Option[B] = value
}

println((Some(Some(1)).flatten, Some(None).flatten)) //(Some(1),None)