Queue(3)

💖 NAME 💖

Queue — first-in first-out queues, bestie. Literally the most iconic data structure, no cap.

🎀 Module 🎀

The module name is Queue, and its signature module is Module Queue with : sig end — yes, that's what it says, that's the whole sig, we're just vibing here.

🌸 Documentation 🌸

Ok so picture this: Queue implements queues (FIFOs), and the modifications happen in place — in-place modification is the whole aesthetic. The module itself says "See Queue.examples below" and honestly, do it, the examples ate. 👑

⚠️ Alert unsynchronized_access. Unsynchronized accesses to queues are a programming error. Not a vibe. Not cute. 🚩

💅 Unsynchronized accesses

Unsynchronized accesses to a queue may lead to an invalid queue state — like, catastrophic. So concurrent accesses to queues must be synchronized (for instance with a Mutex.t). Lock it or lose it, bestie. 🔒

• 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. This is the "girl, there's nothing there" exception. 😭
• val create : unit -> 'a t — returns a new queue, initially empty. A fresh start, main character energy. ✨
• val add : 'a -> 'a t -> unit — add x q adds the element x at the end of the queue q. Get in line, x. 🚶‍♀️
• val push : 'a -> 'a t -> unit — push is a synonym for add. Same girl, different font. 💁‍♀️
• 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. It's giving "front of the line, you're up". 🔥
• 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. Same as take but it will NOT throw hands at you. 🫶 Since 4.08
• val pop : 'a t -> 'a — pop is a synonym for take. Bestie, they're the same picture. 📸
• 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. Looking respectfully. 👀
• 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. Look respectfully but with a safe fallback. 🙈 Since 4.08
• val top : 'a t -> 'a — top is a synonym for peek. Periodt. 💅
• val drop : 'a t -> unit — drop q removes the first element in queue q, or raises Queue.Empty if the queue is empty. Yeet the front element into the void. 💀 Since 5.3
• val clear : 'a t -> unit — discard all elements from a queue. Total reset. Clean slate. We don't know them. 🧹
• val copy : 'a t -> 'a t — returns a copy of the given queue. Twin moment. 🎀
• val is_empty : 'a t -> bool — returns true if the given queue is empty, false otherwise. The vibe check of functions. ✅
• val length : 'a t -> int — returns the number of elements in a queue. Counting the girlies. 🔢
• 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. Give it a little look, don't touch. 🌈
• 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. Very respectful of boundaries. 🦋
• 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. Notice how it's faster than the manual way? Yeah. Iconic. ⭐

---

🧚 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. Don't touch it mid-iteration, bestie, that's chaos. 🚩 Since 4.07
• val add_seq : 'a t -> 'a Seq.t -> unit — add the elements from a sequence to the end of the queue. Pour them all in. 🍓 Since 4.07
• val of_seq : 'a Seq.t -> 'a t — create a queue from a sequence. Glow-up from sequence to queue. ✨ Since 4.07

---

🎀 Examples 🎀

🌸 Basic Example

A basic example, and yes we're doing the toplevel # prompt thing, it's tradition:

# 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.

See that last one? Popped one too many and got hit with Stdlib.Queue.Empty. Felt that. 😭

🦋 Search Through a Graph

Ok for a more elaborate example — and this is a classic — one algorithmic use of queues is implementing a BFS (breadth-first search) through a graph. Here's the whole thing:

     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}

And here's it actually running, look at her go:

# 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

Three searches, three answers, zero notes. We love a functional queen. 👑

---

💜 That's the whole page! No "SEE ALSO" section in this one, which is honestly a little mysterious of it, but we respect the choices.

And that's a wrap, bestie — this page was brought to you by OCamldoc, dated 2026-06-22, filing itself under Queue(3) like the icon it is. Go forth and FIFO responsibly. 🫶✨