Custom iteration in C# and Scala

enumerator.cs

  public IEnumerator<Tuple2<T, Direction>> GetEnumerator() {
    if (top != null) 
      yield return new Tuple2<T, Direction>(top, Direction.Top);
    if (right != null) 
      yield return new Tuple2<T, Direction>(right, Direction.Right);
    if (down != null) 
      yield return new Tuple2<T, Direction>(down, Direction.Down);
    if (left != null)
      yield return new Tuple2<T, Direction>(left, Direction.Left);
  }

iteratable.scala

  object Iterator {
    def Accessors[A]: List[(Neighbourhood[A] => Option[A], Direction)] = List(
      (_.top, Top), (_.right, Right), (_.down, Down), (_.left, Left)
    )
  }
  class Iterator[A](neighbourhood: Neighbourhood[A]) 
  extends scala.Iterator[(A, Neighbourhood.Direction)] {
    private[this] var tail = Iterator.Accessors[A]
    private[this] var _next: Option[(A, Direction)] = None
    fetchNext()

    def hasNext = _next.isDefined
    def next() = {
      val current = _next.get
      fetchNext()
      current
    }

    private[this] def fetchNext() {
      tail match {
        case (fetcher, direction) :: tl =>
          tail = tl
          _next = fetcher(neighbourhood).map { a => (a, direction) }
          if (_next.isEmpty) fetchNext()
        case Nil =>
          _next = None
      }
    }
  }