# Module Queue

    module Queue: sig .. end

First-in first-out queues.

This module implements queues (FIFOs), with in-place modification.

Warning This module is not thread-safe: each `Queue.t` value must be protected from concurrent access (e.g. with a `Mutex.t`). Failure to do so can lead to a crash.

    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 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 : ('b -> 'a -> 'b) -> 'b -> 'a t -> 'b

`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 defined 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