Maximum Length for scala queue

Question

I\'m curious if Scala has some gem hidden in its collection classes that I can use. Basically I\'m looking for something like a FIFO queue, but that has an upper-limit on i

Answer 1

An often preferable alternative to subclassing is the (unfortunately named) "pimp my library" pattern. You can use it to add an enqueueFinite method to Queue, like so:

import scala.collection.immutable.Queue
class FiniteQueue[A](q: Queue[A]) {
  def enqueueFinite[B >: A](elem: B, maxSize: Int): Queue[B] = {
    var ret = q.enqueue(elem)
    while (ret.size > maxSize) { ret = ret.dequeue._2 }
    ret
  }
}
implicit def queue2finitequeue[A](q: Queue[A]) = new FiniteQueue[A](q)

Whenever queue2finitequeue is in scope, you can treat Queue objects as though they have the enqueueFinite method:

val maxSize = 3
val q1 = Queue(1, 2, 3)
val q2 = q1.enqueueFinite(5, maxSize)
val q3 = q2.map(_+1)
val q4 = q3.enqueueFinite(7, maxSize)

The advantage of this approach over subclassing is that enqueueFinite is available to all Queues, including those that are constructed via operations like enqueue, map, ++, etc.

Update: As Dylan says in the comments, enqueueFinite needs also to take a parameter for the maximum queue size, and drop elements as necessary. I updated the code.