StdLabels.Array(3)

🌸 NAME 🌸

okay bestie, welcome to StdLabels.Array(3) from the OCaml library — manual name folded in near the top like a tiny crown, exactly as requested. 👑 The NAME section says StdLabels.Array and then, literally, no description. That’s it. No description. We’re going in blind and honestly that’s kind of iconic. 🫶

🧚 Module 🧚

Module Array
 : (module ArrayLabels)

Main character energy: this module is ArrayLabels, and the page is about to walk us through every single val like a group chat recap. 💅

✨ Documentation ✨

type 'a t = 'a array

An alias for the type of arrays. Same vibe, different name, no cap. 🦋

• val length : 'a array -> int — returns the length, aka the number of elements, of the array you hand it. Short, sweet, slay. ✨
• val get : 'a array -> int -> 'a — get a n returns the element number n of array a. The first element has number 0. The last element has number length a - 1. You can also write a.(n) instead of get a n. Iconic shorthand, iykyk. Raises Invalid_argument if n is outside the range 0 to (length a - 1).
• val set : 'a array -> int -> 'a -> unit — set a n x modifies array a in place, replacing element number n with x. You can also write a.(n) <- x instead of set a n x. In-place update, very main character. Raises Invalid_argument if n is outside the range 0 to length a - 1.
• val make : int -> 'a -> 'a array — make n x returns a fresh array of length n, initialized with x. All the elements of this new array are initially physically equal to x (in the sense of the == predicate). Consequently, if x is mutable, it is shared among all elements of the array, and modifying x through one of the array entries will modify all other entries at the same time. Shared mutable energy — be aware, bestie. 💀 Raises Invalid_argument if n < 0 or n > Sys.max_array_length. If the value of x is a floating-point number, then the maximum size is only Sys.max_array_length / 2.
• val create_float : int -> float array — create_float n returns a fresh float array of length n, with uninitialized data. Since 4.03 — vintage but still cute. 🎀
• val init : int -> f:(int -> 'a) -> 'a array — init n ~f returns a fresh array of length n, with element number i initialized to the result of f i. In other terms, init n ~f tabulates the results of f applied in order to the integers 0 to n-1. Raises Invalid_argument if n < 0 or n > Sys.max_array_length. If the return type of f is float, then the maximum size is only Sys.max_array_length / 2.
• val make_matrix : dimx:int -> dimy:int -> 'a -> 'a array array — make_matrix ~dimx ~dimy e returns a two-dimensional array (an array of arrays) with first dimension dimx and second dimension dimy. All the elements of this new matrix are initially physically equal to e. The element ( x,y ) of a matrix m is accessed with the notation m.(x).(y). Raises Invalid_argument if dimx or dimy is negative or greater than Sys.max_array_length. If the value of e is a floating-point number, then the maximum size is only Sys.max_array_length / 2.
• val init_matrix : dimx:int -> dimy:int -> f:(int -> int -> 'a) -> 'a array array — init_matrix ~dimx ~dimy ~f returns a two-dimensional array (an array of arrays) with first dimension dimx and second dimension dimy, where the element at index ( x,y ) is initialized with f x y. The element ( x,y ) of a matrix m is accessed with the notation m.(x).(y). Since 5.2 — fresh off the release notes. Raises Invalid_argument if dimx or dimy is negative or greater than Sys.max_array_length. If the return type of f is float, then the maximum size is only Sys.max_array_length / 2.
• val append : 'a array -> 'a array -> 'a array — append v1 v2 returns a fresh array containing the concatenation of the arrays v1 and v2. Two arrays walk into a function… and become one. Periodt. Raises Invalid_argument if length v1 + length v2 > Sys.max_array_length.
• val concat : 'a array list -> 'a array — Same as ArrayLabels.append, but concatenates a list of arrays. Group-chat merge, but make it arrays. 💖
• val sub : 'a array -> pos:int -> len:int -> 'a array — sub a ~pos ~len returns a fresh array of length len, containing the elements number pos to pos + len - 1 of array a. Raises Invalid_argument if pos and len do not designate a valid subarray of a; that is, if pos < 0, or len < 0, or pos + len > length a.
• val copy : 'a array -> 'a array — copy a returns a copy of a, that is, a fresh array containing the same elements as a. Clone era. 🧚
• val fill : 'a array -> pos:int -> len:int -> 'a -> unit — fill a ~pos ~len x modifies the array a in place, storing x in elements number pos to pos + len - 1. Raises Invalid_argument if pos and len do not designate a valid subarray of a.
• val blit : src:'a array -> src_pos:int -> dst:'a array -> dst_pos:int -> len:int -> unit — blit ~src ~src_pos ~dst ~dst_pos ~len copies len elements from array src, starting at element number src_pos, to array dst, starting at element number dst_pos. It works correctly even if src and dst are the same array, and the source and destination chunks overlap. Overlap? Handled. No cap. Raises Invalid_argument if src_pos and len do not designate a valid subarray of src, or if dst_pos and len do not designate a valid subarray of dst.
• val to_list : 'a array -> 'a list — to_list a returns the list of all the elements of a. Array to list pipeline, obsessed. 🦋
• val of_list : 'a list -> 'a array — of_list l returns a fresh array containing the elements of l. Raises Invalid_argument if the length of l is greater than Sys.max_array_length.

💖 Comparison 💖

• val equal : eq:('a -> 'a -> bool) -> 'a array -> 'a array -> bool — equal eq a b is true if and only if a and b have the same length n and for all i in [ 0 ; n-1 ], eq a.(i) b.(i) is true. Since 5.4 — new-ish, and checking all the vibes. 💅
• val compare : cmp:('a -> 'a -> int) -> 'a array -> 'a array -> int — compare cmp a b compares a and b according to the shortlex order, that is, shorter arrays are smaller and equal-sized arrays are compared in lexicographic order using cmp to compare elements. Since 5.4 — shortlex is the moment. 🔥

🔥 Iterators 🔥

• val iter : f:('a -> unit) -> 'a array -> unit — iter ~f a applies function f in turn to all the elements of a. It is equivalent to f a.(0); f a.(1); ...; f a.(length a - 1); (). Just doing laps through the array, bestie.
• val iteri : f:(int -> 'a -> unit) -> 'a array -> unit — Same as ArrayLabels.iter, but the function is applied to the index of the element as first argument, and the element itself as second argument. Index + element = duo.
• val map : f:('a -> 'b) -> 'a array -> 'b array — map ~f a applies function f to all the elements of a, and builds an array with the results returned by f: [| f a.(0); f a.(1); ...; f a.(length a - 1) |]. Glow-up every element. 💖
• val map_inplace : f:('a -> 'a) -> 'a array -> unit — map_inplace ~f a applies function f to all elements of a, and updates their values in place. Since 5.1 — same array, new you. 🦋
• val mapi : f:(int -> 'a -> 'b) -> 'a array -> 'b array — Same as ArrayLabels.map, but the function is applied to the index of the element as first argument, and the element itself as second argument. Index-aware mapping, very iykyk.
• val mapi_inplace : f:(int -> 'a -> 'a) -> 'a array -> unit — Same as ArrayLabels.map_inplace, but the function is applied to the index of the element as first argument, and the element itself as second argument. Since 5.1 — in-place and indexed. 🎀
• val fold_left : f:('acc -> 'a -> 'acc) -> init:'acc -> 'a array -> 'acc — fold_left ~f ~init a computes f (... (f (f init a.(0)) a.(1)) ...) a.(n-1), where n is the length of the array a. Accumulator threading, left to right. Slay.
• val fold_left_map : f:('acc -> 'a -> 'acc * 'b) -> init:'acc -> 'a array -> 'acc * 'b array — fold_left_map is a combination of ArrayLabels.fold_left and ArrayLabels.map that threads an accumulator through calls to f. Since 4.13 — two icons in one function. 👑
• val fold_right : f:('a -> 'acc -> 'acc) -> 'a array -> init:'acc -> 'acc — fold_right ~f a ~init computes f a.(0) (f a.(1) ( ... (f a.(n-1) init) ...)), where n is the length of the array a. Right-to-left fold, still ate.

🦋 Iterators on two arrays 🦋

• val iter2 : f:('a -> 'b -> unit) -> 'a array -> 'b array -> unit — iter2 ~f a b applies function f to all the elements of a and b. Since 4.05. Raises Invalid_argument if the arrays are not the same size.
• val map2 : f:('a -> 'b -> 'c) -> 'a array -> 'b array -> 'c array — map2 ~f a b applies function f to all the elements of a and b, and builds an array with the results returned by f: [| f a.(0) b.(0); ...; f a.(length a - 1) b.(length b - 1)|]. Since 4.05. Raises Invalid_argument if the arrays are not the same size.

🌈 Array scanning 🌈

• val for_all : f:('a -> bool) -> 'a array -> bool — for_all ~f [|a1; ...; an|] checks if all elements of the array satisfy the predicate f. That is, it returns (f a1) && (f a2) && ... && (f an). Since 4.03 — all-clear energy. ✅
• val exists : f:('a -> bool) -> 'a array -> bool — exists ~f [|a1; ...; an|] checks if at least one element of the array satisfies the predicate f. That is, it returns (f a1) || (f a2) || ... || (f an). Since 4.03 — one match is enough. ✨
• val for_all2 : f:('a -> 'b -> bool) -> 'a array -> 'b array -> bool — Same as ArrayLabels.for_all, but for a two-argument predicate. Since 4.11. Raises Invalid_argument if the two arrays have different lengths.
• val exists2 : f:('a -> 'b -> bool) -> 'a array -> 'b array -> bool — Same as ArrayLabels.exists, but for a two-argument predicate. Since 4.11. Raises Invalid_argument if the two arrays have different lengths.
• val mem : 'a -> set:'a array -> bool — mem a ~set is true if and only if a is structurally equal to an element of set (i.e. there is an x in set such that compare a x = 0). Since 4.03 — structural equality checking. 💖
• val memq : 'a -> set:'a array -> bool — Same as ArrayLabels.mem, but uses physical equality instead of structural equality to compare array elements. Since 4.03 — physical equality, no cap. 💅
• val find_opt : f:('a -> bool) -> 'a array -> 'a option — find_opt ~f a returns the first element of the array a that satisfies the predicate f, or None if there is no value that satisfies f in the array a. Since 4.13.
• val find_index : f:('a -> bool) -> 'a array -> int option — find_index ~f a returns Some i, where i is the index of the first element of the array a that satisfies f x, if there is such an element. It returns None if there is no such element. Since 5.1 — index hunting era. 🔥
• val find_map : f:('a -> 'b option) -> 'a array -> 'b option — find_map ~f a applies f to the elements of a in order, and returns the first result of the form Some v, or None if none exist. Since 4.13.
• val find_mapi : f:(int -> 'a -> 'b option) -> 'a array -> 'b option — Same as find_map, but the predicate is applied to the index of the element as first argument (counting from 0), and the element itself as second argument. Since 5.1 — indexed and searching. 🦋

🎀 Arrays of pairs 🎀

• val split : ('a * 'b) array -> 'a array * 'b array — split [|(a1,b1); ...; (an,bn)|] is ([|a1; ...; an|], [|b1; ...; bn|]). Since 4.13 — pair separation, iconic. ✂️
• val combine : 'a array -> 'b array -> ('a * 'b) array — combine [|a1; ...; an|] [|b1; ...; bn|] is [|(a1,b1); ...; (an,bn)|]. Raise Invalid_argument if the two arrays have different lengths. Since 4.13 — zipping them together. 🎀

👑 Sorting and shuffling 👑

• val sort : cmp:('a -> 'a -> int) -> 'a array -> unit

and that’s exactly where this part cuts off — part 2 is pulling up with the rest. 💅

okay so sort is the whole vibe: it sorts an array in increasing order according to a comparison function. The comparison function has to return 0 if its arguments compare as equal, a positive integer if the first is greater, and a negative integer if the first is smaller (there's a complete spec below, bestie). For example, compare is a suitable comparison function. After calling sort, the array is sorted in place in increasing order. sort is guaranteed to run in constant heap space and (at most) logarithmic stack space.

The current implementation uses Heap Sort. It runs in constant stack space. 💅

Specification of the comparison function: let a be the array and cmp the comparison function. The following must be true for all x, y, z in a:

• cmp x y > 0 if and only if cmp y x < 0
• if cmp x y >= 0 and cmp y z >= 0 then cmp x z >= 0

When sort returns, a contains the same elements as before, reordered in such a way that for all i and j valid indices of a:

• cmp a.(i) a.(j) >= 0 if i >= j

---

• val stable_sort : cmp:('a -> 'a -> int) -> 'a array -> unit — same as ArrayLabels.sort, but the sorting algorithm is stable (i.e. elements that compare equal are kept in their original order) and not guaranteed to run in constant heap space.

The current implementation uses Merge Sort. It uses a temporary array of length n/2, where n is the length of the array. It is usually faster than the current implementation of ArrayLabels.sort. 🔥

• val stable_sort_sub : cmp:('a -> 'a -> int) -> 'a array -> pos:int -> len:int -> unit — stable_sort_sub ~cmp a ~pos ~len sorts the subarray of the array a delimited by the start position pos and by the length len. The data in this subarray is sorted in increasing order according to the comparison function cmp. The data outside of this subarray is unaffected. The sorting algorithm is stable; it is the same as in ArrayLabels.stable_sort.

Since 5.5

🚨 Raises Invalid_argument if pos and len do not designate a valid subarray of a; that is, if pos < 0, or len < 0, or pos + len > length a.

• val fast_sort : cmp:('a -> 'a -> int) -> 'a array -> unit — same as ArrayLabels.sort or ArrayLabels.stable_sort, whichever is faster on typical input. 🏁
• val shuffle : rand:(int -> int) -> 'a array -> unit — shuffle ~rand a randomly permutes a's elements using rand for randomness. The distribution of permutations is uniform. 🎲

rand must be such that a call to rand n returns a uniformly distributed random number in the range [ 0 ; n-1 ]. Random.int can be used for this (do not forget to Random.self_init the generator).

Since 5.2

🦋 Arrays and Sequences 🦋

• val to_seq : 'a array -> 'a Seq.t — iterate on the array, in increasing order. Modifications of the array during iteration will be reflected in the sequence. Since 4.07
• val to_seqi : 'a array -> (int * 'a) Seq.t — iterate on the array, in increasing order, yielding indices along elements. Modifications of the array during iteration will be reflected in the sequence. Since 4.07
• val of_seq : 'a Seq.t -> 'a array — create an array from the generator. Since 4.07

🫶 Arrays and concurrency safety 🫶

Care must be taken when concurrently accessing arrays from multiple domains: accessing an array will never crash a program, but unsynchronized accesses might yield surprising (non-sequentially-consistent) results. no cap.

💅 Atomicity

Every array operation that accesses more than one array element is not atomic. This includes iteration, scanning, sorting, splitting and combining arrays.

For example, consider the following program:

let size = 100_000_000
let a = ArrayLabels.make size 1
let d1 = Domain.spawn (fun () ->
   ArrayLabels.iteri ~f:(fun i x -> a.(i) <- x + 1) a
)
let d2 = Domain.spawn (fun () ->
  ArrayLabels.iteri ~f:(fun i x -> a.(i) <- 2 * x + 1) a
)
let () = Domain.join d1; Domain.join d2

After executing this code, each field of the array a is either 2, 3, 4 or 5. If atomicity is required, then the user must implement their own synchronization (for example, using Mutex.t). 🔒

💀 Data races 💀

If two domains only access disjoint parts of the array, then the observed behaviour is the equivalent to some sequential interleaving of the operations from the two domains.

A data race is said to occur when two domains access the same array element without synchronization and at least one of the accesses is a write. In the absence of data races, the observed behaviour is equivalent to some sequential interleaving of the operations from different domains.

Whenever possible, data races should be avoided by using synchronization to mediate the accesses to the array elements.

Indeed, in the presence of data races, programs will not crash but the observed behaviour may not be equivalent to any sequential interleaving of operations from different domains. Nevertheless, even in the presence of data races, a read operation will return the value of some prior write to that location (with a few exceptions for float arrays). 😭

🌊 Float arrays 🌊

Float arrays have two supplementary caveats in the presence of data races.

First, the blit operation might copy an array byte-by-byte. Data races between such a blit operation and another operation might produce surprising values due to tearing: partial writes interleaved with other operations can create float values that would not exist with a sequential execution.

For instance, at the end of

let zeros = Array.make size 0.
let max_floats = Array.make size Float.max_float
let res = Array.copy zeros
let d1 = Domain.spawn (fun () -> Array.blit zeros 0 res 0 size)
let d2 = Domain.spawn (fun () -> Array.blit max_floats 0 res 0 size)
let () = Domain.join d1; Domain.join d2

the res array might contain values that are neither 0. nor max_float. 💀

Second, on 32-bit architectures, getting or setting a field involves two separate memory accesses. In the presence of data races, the user may observe tearing on any operation. ✨

later bestie 💖 this whole thing is from OCamldoc, dated 2026-06-22, and the page is StdLabels.Array(3) — periodt 🎀✨