# Module Queue

    module Queue: sig .. end

First-in first-out queues.

This module implements queues (FIFOs), with in-place modification. See the example section below.

- Alert unsynchronized_access. Unsynchronized accesses to queues are a programming error.

Unsynchronized accesses

Unsynchronized accesses to a queue may lead to an invalid queue state. Thus, concurrent accesses to queues must be synchronized (for instance with a `Mutex.t`).

    type !'a t

The type of queues containing elements of type `'a`.

    exception Empty

Raised when `Queue.take` or `Queue.peek` is applied to an empty queue.

    val create : unit -> 'a t

Return a new queue, initially empty.

    val add : 'a -> 'a t -> unit

`add x q` adds the element `x` at the end of the queue `q`.

    val push : 'a -> 'a t -> unit

`push` is a synonym for `add`.

    val take : 'a t -> 'a

`take q` removes and returns the first element in queue `q`, or raises `Queue.Empty` if the queue is empty.

    val take_opt : 'a t -> 'a option

`take_opt q` removes and returns the first element in queue `q`, or returns `None` if the queue is empty.

- Since 4.08

    val pop : 'a t -> 'a

`pop` is a synonym for `take`.

    val peek : 'a t -> 'a

`peek q` returns the first element in queue `q`, without removing it from the queue, or raises `Queue.Empty` if the queue is empty.

    val peek_opt : 'a t -> 'a option

`peek_opt q` returns the first element in queue `q`, without removing it from the queue, or returns `None` if the queue is empty.

- Since 4.08

    val top : 'a t -> 'a

`top` is a synonym for `peek`.

    val drop : 'a t -> unit

`drop q` removes the first element in queue `q`, or raises `Queue.Empty` if the queue is empty.

- Since 5.3

    val clear : 'a t -> unit

Discard all elements from a queue.

    val copy : 'a t -> 'a t

Return a copy of the given queue.

    val is_empty : 'a t -> bool

Return `true` if the given queue is empty, `false` otherwise.

    val length : 'a t -> int

Return the number of elements in a queue.

    val iter : ('a -> unit) -> 'a t -> unit

`iter f q` applies `f` in turn to all elements of `q`, from the least recently entered to the most recently entered. The queue itself is unchanged.

    val fold : ('acc -> 'a -> 'acc) -> 'acc -> 'a t -> 'acc

`fold f accu q` is equivalent to `List.fold_left f accu l`, where `l` is the list of `q`'s elements. The queue remains unchanged.

    val transfer : 'a t -> 'a t -> unit

`transfer q1 q2` adds all of `q1`'s elements at the end of the queue `q2`, then clears `q1`. It is equivalent to the sequence `iter (fun x -> add x q2) q1; clear q1`, but runs in constant time.

## Iterators

    val to_seq : 'a t -> 'a Seq.t

Iterate on the queue, in front-to-back order. The behavior is not specified if the queue is modified during the iteration.

- Since 4.07

    val add_seq : 'a t -> 'a Seq.t -> unit

Add the elements from a sequence to the end of the queue.

- Since 4.07

    val of_seq : 'a Seq.t -> 'a t

Create a queue from a sequence.

- Since 4.07

## Examples

### Basic Example

A basic example:

        # let q = Queue.create ()
        val q : '_weak1 Queue.t = <abstr>

        # Queue.push 1 q; Queue.push 2 q; Queue.push 3 q
        - : unit = ()

        # Queue.length q
        - : int = 3

        # Queue.pop q
        - : int = 1

        # Queue.pop q
        - : int = 2

        # Queue.pop q
        - : int = 3

        # Queue.pop q
        Exception: Stdlib.Queue.Empty.

### Search Through a Graph

For a more elaborate example, a classic algorithmic use of queues is to implement a BFS (breadth-first search) through a graph.

         type graph = {
           edges: (int, int list) Hashtbl.t
         }

        (* Search in graph [g] using BFS, starting from node [start].
           It returns the first node that satisfies [p], or [None] if
           no node reachable from [start] satisfies [p].
        *)
        let search_for ~(g:graph) ~(start:int) (p:int -> bool) : int option =
          let to_explore = Queue.create() in
          let explored = Hashtbl.create 16 in

          Queue.push start to_explore;
          let rec loop () =
            if Queue.is_empty to_explore then None
            else
              (* node to explore *)
              let node = Queue.pop to_explore in
              explore_node node

          and explore_node node =
            if not (Hashtbl.mem explored node) then (
              if p node then Some node (* found *)
              else (
                Hashtbl.add explored node ();
                let children =
                  Hashtbl.find_opt g.edges node
                  |> Option.value ~default:[]
                in
                List.iter (fun child -> Queue.push child to_explore) children;
                loop()
              )
            ) else loop()
          in
          loop()

        (* a sample graph *)
        let my_graph: graph =
          let edges =
            List.to_seq [
              1, [2;3];
              2, [10; 11];
              3, [4;5];
              5, [100];
              11, [0; 20];
            ]
            |> Hashtbl.of_seq
          in {edges}

        # search_for ~g:my_graph ~start:1 (fun x -> x = 30)
        - : int option = None

        # search_for ~g:my_graph ~start:1 (fun x -> x >= 15)
        - : int option = Some 20

        # search_for ~g:my_graph ~start:1 (fun x -> x >= 50)
        - : int option = Some 100