gistfile1.scala

object Unfold {

  /**
   * Builds a list by recursively applying `f`, until `f` returns None
   *
   * @param seed    the initial value passed to `f`
   * @param f       returns Some(Pair(element, nextSeed)) where `element` is added
   *                to the list and `nextSeed` is passed to `f` on the next invocation.
   */
  def buildList[T, R](seed: T)(f: T => Option[Pair[R, T]]): List[R] =
    f(seed) match {
      case None => List.empty
      case Some(Pair(e, succ)) => e :: buildList(succ)(f)
    }

  /**
   * Builds a stream by recursively applying `f`, until `f` returns None
   *
   * @param seed    the initial value passed to `f`
   * @param f       returns Some(Pair(element, nextSeed)) where `element` is added
   *                to the stream and `nextSeed` is passed to `f` on the next invocation.
   */
  def buildStream[T, R](seed: T)(f: T => Option[Pair[R, T]]): Stream[R] =
    f(seed) match {
      case None => Stream.empty
      case Some(Pair(e, succ)) => Stream.cons(e, buildStream(succ)(f))
    }
}