add method dequeueOption to immutable.Queue

This allows immutable.Queue to be used conveniently in functional code.
Method is in the spirit of `headOption` on seqs or `get` on maps. Also
adds a unit test for immutable.Queue.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/library/scala/collection/immutable/Queue.scala b/src/library/scala/collection/immutable/Queue.scala
index 05c5dd799d..df1484c4ab 100644
--- a/src/library/scala/collection/immutable/Queue.scala
+++ b/src/library/scala/collection/immutable/Queue.scala
@@ -118,6 +118,13 @@ class Queue[+A] protected(protected val in: List[A], protected val out: List[A])
     case x :: xs            => (x, new Queue(in, xs))
     case _                  => throw new NoSuchElementException("dequeue on empty queue")
   }
+  
+  /** Optionally retrieves the first element and a queue of the remaining elements.
+   *
+   * @return A tuple of the first element of the queue, and a new queue with this element removed.
+   *         If the queue is empty, `None` is returned.
+   */
+  def dequeueOption: Option[(A, Queue[A])] = if(isEmpty) None else Some(dequeue)
 
   /** Returns the first element in the queue, or throws an error if there
    *  is no element contained in the queue.