a non working scala monad

Determine.scala

sealed abstract class Determine[+A] {
  def map[B](f: A => B): Determine[B]
  def flatMap[B](f: A => Determine[B]): Determine[B]
  def filter(p: A => Boolean): Determine[A]
  def foreach(b: A => Unit): Unit
}
final case class Known[+A](value: A) extends Determine[A] {
  def map[B](f: A => B): Determine[B] = Known(f(value))
  def flatMap[B](f: A => Determine[B]): Determine[B] = f(value)
  def filter(p: A => Boolean): Determine[A] = if (p(value)) this else Unknown
  def foreach(b: A => Unit): Unit = b(value)
}
final case class TBD[A](definer : () => A) extends Determine[A] {
  private var value: Known[A] = _
  private var decided: Boolean = false
  private var listener: List[A => Any] = Nil

  def map[B](f: A => B): Determine[B] = {
    if (decided)
      value.map(f)
    else
      null
  }

  def flatMap[B](f: A => Determine[B]): Determine[B] = {
    null
  }

  def filter(p: A => Boolean): Determine[A] = {
    null
  }

  def foreach(b: A => Unit): Unit = {
    
  }

  def set(value: A): Unit = { this.value = Known(value) }
}
case object Unknown extends Determine[Nothing] {
  def map[B](f: Nothing => B): Determine[B] = this
  def flatMap[B](f: Nothing => Determine[B]): Determine[B] = this
  def filter(p: Nothing => Boolean): Determine[Nothing] = this
  def foreach(b: Nothing => Unit): Unit = {}
}