Queue(3)                         OCaml library                        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 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


OCamldoc                          2024-02-29                          Queue(3)