BQ.scala

import scala.collection.immutable.Queue
import scalaz.Monoid

case class BQ[A](private val q: Queue[A], n: Int) {

  private def bound(qq: Queue[A]): Queue[A] = {
    val s = qq.size
    if (s > n)
      qq.drop(n - s)
    else
      qq
  }

  def enqueue(a: A): BQ[A] =
    new BQ[A](bound(q.enqueue(a)), n)

  def append(x: BQ[A]): BQ[A] =
    new BQ[A](bound(q ++ x.q), n)
}

object BQ {
  def empty[A](n: Int) = new BQ(Queue.empty[A], n)

  implicit def bqMonoid[A] = new Monoid[A] {
    override def zero: A = ???

    override def append(f1: A, f2: => A): A = ???
  }
}