Scala interview questions hard

scala-interview2.scala

trait MyList[+A] { 
  def fold[B](k: Option[(A, B)] => B): B
  def map[B](f: A => B): MyList[B]             = sys.error("Implement me in terms of fold")
  def flatMap[B](f: A => MyList[B]): MyList[B] = sys.error("Implement me in terms of fold")
  def headOption: Option[B]                    = sys.error("Implement me in terms of fold")
  def tailOption: Option[MyList[B]]            = sys.error("Implement me in terms of fold")
  def isEmpty                                  = sys.error("Implement me in terms of fold")
  def length                                   = sys.error("Implement me in terms of fold")
}

object MyList {
  def nil[A] = new MyList[A] {
    def fold[B](k: Option[(A, B)] => B): B = sys.error("Implement me")
  }

  def cons[A](h: A, t: MyList[A]) = new MyList[A] {
    def fold[B](k: Option[(A, B)] => B): B = sys.error("Implement me")
  }
}