View.scala

import scala.annotation.tailrec

sealed abstract class Queue[F[_], A, B] {

  def :+[C](f: B => F[C]): Queue[F, A, C] =
    new Node(this, new Leaf(f))

  def ++[C](that: Queue[F, B, C]): Queue[F, A, C] =
    new Node(this, that)

  def view: View[F, A, B]

}

case class Leaf[F[_], A, B](arrow: A => F[B]) extends Queue[F, A, B] {

  def view: View[F, A, B] =
    new One(arrow)

}

case class Node[F[_], A, B, T](left: Queue[F, A, T], right: Queue[F, T, B]) extends Queue[F, A, B] {

  def view: View[F, A, B] = {
    @tailrec
    def go[F[_], A, B](x: Queue[F, A, Any], y: Queue[F, Any, B]): View[F, A, B] = {
      x match {
        case Leaf(v) => new Cons(v, y)
        case Node(l, r) => go[F, A, B](l, new Node(r, y))
      }
    }
    go(left.asInstanceOf[Queue[F, A, Any]], right.asInstanceOf[Queue[F, Any, B]])
  }

}

sealed abstract class View[F[_], A, B]

case class One[F[_], A, B](arrow: A => F[B]) extends View[F, A, B]

case class Cons[F[_], A, B, T](arrow: A => F[T], queue: Queue[F, T, B]) extends View[F, A, B]