Queue(3)

NAME
Queue - First-in first-out queues.

Module
Module Queue

Documentation

Module Queue
 : sig end

First-in first-out queues.

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