How to pattern match a scala immutable queue?

Question

I want to use immutable.Queue and in particular

1. I want to access the head of the queue without dequeuing it. Immutable queue is implemented using two immutable lists/stacks, and from the code it looks like this operation is not constant time (see this line), although dequeue is (amortized constant time). Can someone confirm or correct me?

2. I like the pattern matching syntax for List (e.g., list match { case head :: tail => ... }). Do we have something similar for Queue as well?

Answer 1

You can use the general purpose matcher +: on Seq:

val q = Queue.empty[Int]

q match {
  case x +: xs => // non-empty case
  case _ => // empty case
}

You can also use Queue's unapplySeq:

q match {
  case Queue(x, _*) => // non-empty case
  case Queue() => // empty case
}

Note that both of these are potentially more inefficient than Queue.head, since they have to construct the dequeued Queue as well, which is only O(1) in amortized time.