zippers.scala

object Zippers {
  case class Zipper[A](before: List[A], focus: A, after: List[A]) {
    /**
     * Focus the element to the left, if any.
     */
    def left: Option[Zipper[A]] = this match {
      case Zipper(List(), _, _) => None
      case Zipper(x :: xs, y, zs) => Some(Zipper(xs, x, y :: zs))
    }

    /**
     * Focus the element to the right, if any.
     */
    def right: Option[Zipper[A]] = this match {
      case Zipper(_, _, List()) => None
      case Zipper(xs, y, z :: zs) => Some(Zipper(y :: xs, z, zs))
    }

    /**
     * Replace the focused element.
     */
    def update(a: A): Zipper[A] = {
      Zipper(before, a, after)
    }

    /**
     * Delete the focused element.
     */
    def delete: Zipper[A] = this match {
      case Zipper(h @ List(), _, x :: xs) => Zipper(h, x, xs)
      case Zipper(x :: xs, _, h) => Zipper(xs, x, h)
    }

    /**
     * Insert to the left of the focus and set focus on the inserted element.
     */
    def insertLeft(a: A): Zipper[A] = {
      Zipper(before, a, focus :: after)
    }

    /**
     * Insert to the right of the focus and set focus on the inserted element.
     */
    def insertRight(a: A): Zipper[A] = {
      Zipper(focus :: before, a, after)
    }
  }

  def example(): Unit = {
    val zipper = Zipper(List(4,3,2,1), 5, List(6,7,8,9))

    val result = for {
      a <- zipper.left       // focused: 4
      b <- a.left            // focused: 3
      c <- b.left            // focused: 2
      d = c.delete           // deletes: 2
      e = d.insertRight(42)  // inserts: 42 after 1
      f = e.insertLeft(41) // inserts: 41 before 42
    } yield f

    println(result)
  }
}