is it Option? Maybe!..

MaybeImpl.scala

object MaybeImpl extends scala.App {

  sealed trait Maybe[+A] {
    def get: A
    def map[B >: A](f: A => B): Maybe[B]
    def flatMap[B >: A](f: A => Maybe[B]): Maybe[B]
    def filter(p: A => Boolean): Maybe[A]
    def isEmpty: Boolean
    def fold[B](ifEmpty: => B)(f: A => B): B
  }

  object Maybe {
    def spawn[A](value: A): Maybe[A] = Exists(value)
    def none: Maybe[Nothing]         = Void
  }

  case class Exists[A](value: A) extends Maybe[A] {
    override def get: A = value
    override def map[B >: A](f: A => B): Maybe[B] = Maybe.spawn(f(value))
    override def flatMap[B >: A](f: A => Maybe[B]): Maybe[B] = f(value)
    override def filter(p: A => Boolean): Maybe[A] = if (p(value)) this else Maybe.none
    override def isEmpty: Boolean = false
    override def fold[B](ifEmpty: => B)(f: A => B): B = f(value)
  }

  case object Void extends Maybe[Nothing] {
    override def get: Nothing = throw new NoSuchElementException("Void")
    override def map[B >: Nothing](f: Nothing => B): Maybe[B] = this
    override def flatMap[B >: Nothing](f: Nothing => Maybe[B]): Maybe[B] = this
    override def filter(p: Nothing => Boolean): Maybe[Nothing] = this
    override def isEmpty: Boolean = true
    override def fold[B](ifEmpty: => B)(f: Nothing => B): B = ifEmpty
  }

  val result = for {
    num1 <- Maybe.spawn(5)
    num2 <- Maybe.spawn(num1 + 15)
    num3 <- Maybe.spawn(num2).filter(_ % 2 == 0)
    num4 <- Maybe.spawn(Maybe.spawn(num3).fold(0)(v => v * v))
  } yield num4

  println("assertion..")
  assert(result == Exists(400))
  println("went fine!")

}