zero cost nullable type for scala

nullable.scala

object N {

  type Nullable[T <: AnyRef]

  object Impl {
    def apply[T <: AnyRef](t: T): Nullable[T] =
      t.asInstanceOf[Nullable[T]]

    def empty[T <: AnyRef]: Nullable[T] =
      null.asInstanceOf[Nullable[T]]

    private def unwrap[T <: AnyRef](t: Nullable[T]): T = t.asInstanceOf[T]

    def flatMap[A <: AnyRef, B <: AnyRef](a: Nullable[A])(fn: A => Nullable[B]): Nullable[B] =
      if (a == null) empty[B] else fn(unwrap(a))

    def map[A <: AnyRef, B <: AnyRef](a: Nullable[A])(fn: A => B): Nullable[B] =
      if (a == null) empty[B] else apply(fn(unwrap(a)))

    // a strict version of getOrElse
    def getOrThat[A <: AnyRef](a: Nullable[A], orElse: A): A =
      if (a == null) orElse else unwrap(a)
  }


  def someJavaMethod(f: String): String = null

  val m: Nullable[String] = Impl(someJavaMethod("foo"))

  // won't compile, Nullable[T] is not <: AnyRef
  //Impl(m)

  assert(Impl.getOrThat(m, "bar") == "bar")
}