def unfold[A, S](z: S)(f: S => Option[(A, S)]): Stream[A] = f(z) match {
  case Some((a, s)) => Stream.cons(a, unfold(s)(f))
  case None => Stream.empty
}

trait F[C] {

  def map[A](a: List[C])(f: C => A): Stream[A] = {

    def foo(li: List[C]): Option[(A, List[C])] = li match {
      case head :: body => Some(f(head) -> body)
      case Nil => None
    }

    unfold(a)(foo)
  }

}

// Проверка
val stream = new F[Int]{}.map(List(1, 2, 3))(_ * 2)
stream.mkString(",")
//res0: String = 2,4,6