Scala for expression study

Poption.scala

object Main extends App {
  val poPoption = Poption("Po")
  val popoPoption = Poption("PoPo")
  
  val popopo = for {
    po <- poPoption
    popo <- popoPoption
  } yield { po + popo }
  
  println(popopo)
}

object Poption {
  def apply[A](x: A): Poption[A] = if (x == null) Pone else Pome(x)
}

sealed abstract class Poption[+A] {
  self =>
  
  def isEmpty: Boolean

  def get: A

  def map[B](f: A => B): Poption[B] =
    if (isEmpty) Pone else Pome(f(this.get))
  
  def flatMap[B](f: A => Poption[B]): Poption[B] =
    if (isEmpty) Pone else f(this.get)

  def flatten[B](implicit ev: A <:< Poption[B]): Poption[B] =
    if (isEmpty) Pone else ev(this.get)

  def filter(p: A => Boolean): Poption[A] =
    if (isEmpty || p(this.get)) this else Pone

  def withFilter(p: A => Boolean): WithFilter = new WithFilter(p)

  class WithFilter(p: A => Boolean) {
    def map[B](f: A => B): Poption[B] = self filter p map f
    def flatMap[B](f: A => Poption[B]): Poption[B] = self filter p flatMap f
    def foreach[U](f: A => U): Unit = self filter p foreach f
    def withFilter(q: A => Boolean): WithFilter = new WithFilter(x => p(x) && q(x))
  }

  def foreach[U](f: A => U) {
    if (!isEmpty) f(this.get)
  }
}

final case class Pome[+A](value: A) extends Poption[A] {
  def isEmpty = false
  def get = value
}

case object Pone extends Poption[Nothing] {
  def isEmpty = true
  def get = throw new NoSuchElementException("Pone.get")
}