Stdlib.List(3)

👑 Stdlib.List(3) — OCaml library 👑

💖 NAME 💖

Stdlib.List — no description. That’s literally the whole name section, bestie 😭💅

---

🧚 Module 🧚

Module Stdlib.List

---

🌸 Documentation 🌸

Module List : (module Stdlib__List)

Okay so first up, the type. It’s giving “lists are just lists,” and honestly? Iconic ✨

type 'a t = 'a list =
        | []
        | (::) of 'a * 'a list

This is an alias for the type of lists. Periodt 💅

val length : 'a list -> int

Returns the length, aka the number of elements, of the given list. Slay 🔥

val compare_lengths : 'a list -> 'b list -> int

Compares the lengths of two lists. compare_lengths l1 l2 is equivalent to compare (length l1) (length l2), except that the computation stops after reaching the end of the shortest list. Efficient queen behavior 👑

Since 4.05 ✨

val compare_length_with : 'a list -> int -> int

Compares the length of a list to an integer. compare_length_with l len is equivalent to compare (length l) len, except that the computation stops after at most len iterations on the list. Lowkey the same vibe as above but with an int 🫶

Since 4.05 ✨

val is_empty : 'a list -> bool

is_empty l is true if and only if l has no elements. It is equivalent to compare_length_with l 0 = 0. No cap, empty list check has never been cuter 💖

Since 5.1 ✨

val cons : 'a -> 'a list -> 'a list

cons x xs is x :: xs. That’s it, that’s the function 🎀

Since 4.03 (4.05 in ListLabels) ✨

val singleton : 'a -> 'a list

singleton x returns the one-element list [x]. Main character energy, one-element edition 👑

Since 5.4 ✨

val hd : 'a list -> 'a

Returns the first element of the given list. The head. The moment. The opener 💅

Raises Failure if the list is empty.

val tl : 'a list -> 'a list

Returns the given list without its first element. It’s the list after the head leaves the group chat 😭

Raises Failure if the list is empty.

val nth : 'a list -> int -> 'a

Returns the n-th element of the given list. The first element, aka the head of the list, is at position 0. Zero-indexed bestie, we been knew 🌸

Raises Failure if the list is too short.

Raises Invalid_argument if n is negative.

val nth_opt : 'a list -> int -> 'a option

Returns the n-th element of the given list. The first element, aka the head of the list, is at position 0. Returns None if the list is too short. Soft launch instead of crashing 🦋

Since 4.05 ✨

Raises Invalid_argument if n is negative.

val rev : 'a list -> 'a list

List reversal. Flip it, reverse it, slay it 💅

val init : int -> (int -> 'a) -> 'a list

init len f is [f 0; f 1; ...; f (len-1)], evaluated left to right. Build-a-list workshop ✨

Since 4.06 ✨

Raises Invalid_argument if len < 0.

val append : 'a list -> 'a list -> 'a list

append l0 l1 appends l1 to l0. Same function as the infix operator @. Glow-up from @ to a real function name 💖

Since 5.1 this function is tail-recursive. We love a responsible recursive icon 🫶

val rev_append : 'a list -> 'a list -> 'a list

rev_append l1 l2 reverses l1 and concatenates it with l2. This is equivalent to ( List.rev l1) @ l2. Reverse-then-append, no notes 🔥

val concat : 'a list list -> 'a list

Concatenates a list of lists. The elements of the argument are all concatenated together, in the same order, to give the result. Not tail-recursive; the cost is length of the argument + length of the longest sub-list. Cute but don’t make it do too much 😭

val flatten : 'a list list -> 'a list

Same as List.concat. Not tail-recursive; the cost is length of the argument + length of the longest sub-list. Literally concat’s twin 🎀

🌈 Comparison 🌈

val equal : ('a -> 'a -> bool) -> 'a list -> 'a list -> bool

equal eq [a1; ...; an] [b1; ..; bm] holds when the two input lists have the same length, and for each pair of elements ai, bi at the same position we have eq ai bi.

Note: the eq function may be called even if the lists have different length. If you know your equality function is costly, you may want to check List.compare_lengths first. Protect your expensive functions, bestie 💅

Since 4.12 ✨

val compare : ('a -> 'a -> int) -> 'a list -> 'a list -> int

compare cmp [a1; ...; an] [b1; ...; bm] performs a lexicographic comparison of the two input lists, using the same 'a -> 'a -> int interface as compare:

• a1 :: l1 is smaller than a2 :: l2 with a negative result if a1 is smaller than a2, or if they are equal with a 0 result and l1 is smaller than l2 -the empty list [] is strictly smaller than non-empty lists

Note: the cmp function will be called even if the lists have different lengths. It’s committed to the comparison, fr fr 😭

Since 4.12 ✨

🦋 Iterators 🦋

val iter : ('a -> unit) -> 'a list -> unit

iter f [a1; ...; an] applies function f in turn to [a1; ...; an]. It is equivalent to f a1; f a2; ...; f an. Side effects but make it fashion 💅

val iteri : (int -> 'a -> unit) -> 'a list -> unit

Same as List.iter, but the function is applied to the index of the element as first argument, counting from 0, and the element itself as second argument. Index girlies rise 🌸

Since 4.00 ✨

val map : ('a -> 'b) -> 'a list -> 'b list

map f [a1; ...; an] applies function f to a1, ..., an, and builds the list [f a1; ...; f an] with the results returned by f. Transformation slay 🔥

val mapi : (int -> 'a -> 'b) -> 'a list -> 'b list

Same as List.map, but the function is applied to the index of the element as first argument, counting from 0, and the element itself as second argument. Map’s indexed cousin 💖

Since 4.00 ✨

val rev_map : ('a -> 'b) -> 'a list -> 'b list

rev_map f l gives the same result as List.rev ( List.map f l), but is more efficient. Same result, better efficiency, main character optimization 🎀

val filter_map : ('a -> 'b option) -> 'a list -> 'b list

filter_map f l applies f to every element of l, filters out the None elements and returns the list of the arguments of the Some elements. Keep the good ones, drop the rest 💅

Since 4.08 ✨

val filter_mapi : (int -> 'a -> 'b option) -> 'a list -> 'b list

Same as List.filter_map, but the function is applied to the index of the element as first argument, counting from 0, and the element itself as second argument. Indexed filtering but make it cute 🌈

Since 5.5 ✨

val concat_map : ('a -> 'b list) -> 'a list -> 'b list

concat_map f l gives the same result as List.concat ( List.map f l). Tail-recursive. Combo move, no notes 👑

Since 4.10 ✨

val fold_left_map : ('acc -> 'a -> 'acc * 'b) -> 'acc -> 'a list -> 'acc * 'b list

fold_left_map is a combination of fold_left and map that threads an accumulator through calls to f. Two functions, one vibe, we ate 🍓

Since 4.11 ✨

val fold_left : ('acc -> 'a -> 'acc) -> 'acc -> 'a list -> 'acc

fold_left f init [b1; ...; bn] is f (... (f (f init b1) b2) ...) bn. Left fold queen, no cap 💖

val fold_right : ('a -> 'acc -> 'acc) -> 'a list -> 'acc -> 'acc

fold_right f [a1; ...; an] init is f a1 (f a2 (... (f an init) ...)). Not tail-recursive. Gorgeous but do not stack-overflow her 😭

🌟 Iterators on two lists 🌟

val iter2 : ('a -> 'b -> unit) -> 'a list -> 'b list -> unit

iter2 f [a1; ...; an] [b1; ...; bn] calls in turn f a1 b1; ...; f an bn. Two-list side effects, synchronized besties 🫶

Raises Invalid_argument if the two lists are determined to have different lengths.

val map2 : ('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list

map2 f [a1; ...; an] [b1; ...; bn] is [f a1 b1; ...; f an bn]. Pair them up and slay ✨

Raises Invalid_argument if the two lists are determined to have different lengths.

val rev_map2 : ('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list

rev_map2 f l1 l2 gives the same result as List.rev ( List.map2 f l1 l2), but is more efficient. Efficient and reversed, she contains multitudes 💅

val fold_left2 : ('acc -> 'a -> 'b -> 'acc) -> 'acc -> 'a list -> 'b list -> 'acc

fold_left2 f init [a1; ...; an] [b1; ...; bn] is f (... (f (f init a1 b1) a2 b2) ...) an bn. Two-list left fold, locked in 🔥

Raises Invalid_argument if the two lists are determined to have different lengths.

val fold_right2 : ('a -> 'b -> 'acc -> 'acc) -> 'a list -> 'b list -> 'acc -> 'acc

fold_right2 f [a1; ...; an] [b1; ...; bn] init is f a1 b1 (f a2 b2 (... (f an bn init) ...)). Not tail-recursive, so be nice to the stack 😭

Raises Invalid_argument if the two lists are determined to have different lengths. Not tail-recursive.

🔍 List scanning 🔍

val for_all : ('a -> bool) -> 'a list -> bool

for_all f [a1; ...; an] checks if all elements of the list satisfy the predicate f. That is, it returns (f a1) && (f a2) && ... && (f an) for a non-empty list and true if the list is empty. All-elements vibe check ✅

val exists : ('a -> bool) -> 'a list -> bool

exists f [a1; ...; an] checks if at least one element of the list satisfies the predicate f. That is, it returns (f a1) || (f a2) || ... || (f an) for a non-empty list and false if the list is empty. Any-element vibe check, we only need one 💖

val for_all2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool

Same as List.for_all, but for a two-argument predicate. Two lists, one vibe check 🎀

Raises Invalid_argument if the two lists are determined to have different lengths.

val exists2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool

Same as List.exists, but for a two-argument predicate. Two lists, one “do we have a winner?” moment ✨

Raises Invalid_argument if the two lists are determined to have different lengths.

val mem : 'a -> 'a list -> bool

mem a set is true if and only if a is equal to an element of set. Membership check, structural equality edition 🌸

val memq : 'a -> 'a list -> bool

Same as List.mem, but uses physical equality instead of structural equality to compare list elements. Same vibe, different equality 💅

🕵️ List searching 🕵️

val find : ('a -> bool) -> 'a list -> 'a

find f l returns the first element of the list l that satisfies the predicate f. First match gets the crown 👑

Raises Not_found if there is no value that satisfies f in the list l.

val find_opt : ('a -> bool) -> 'a list -> 'a option

find_opt f l returns the first element of the list l that satisfies the predicate f. Returns None if there is no value that satisfies f in the list l. Soft launch, no exception thrown 🦋

Since 4.05 ✨

val find_index : ('a -> bool) -> 'a list -> int option

find_index f xs returns Some i, where i is the index of the first element of the list xs that satisfies f x, if there is such an element. It returns None if there is no such element. Index of the first match, cute and optional 🎀

Since 5.1 ✨

val find_map : ('a -> 'b option) -> 'a list -> 'b option

find_map f l applies f to the elements of l in order, and returns the first result of the form Some v, or None if none exist. First Some wins, literally 💖

Since 4.10 ✨

val find_mapi : (int -> 'a -> 'b option) -> 'a list -> '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. Indexed find_map, she’s thorough 💅

Since 5.1 ✨

val filter : ('a -> bool) -> 'a list -> 'a list

filter f l returns all the elements of the list l that satisfy the predicate f. The order of the elements in the input list is preserved. Keep the keepers, preserve the order 🌈

val find_all : ('a -> bool) -> 'a list -> 'a list

find_all is another name for List.filter. Same function, different name, still slaying 💅

val filteri : (int -> 'a -> bool) -> 'a list -> 'a list

Same as List.filter, but the predicate is applied to the index of the element as first argument, counting from 0, and the element itself as second argument. Filtering with index receipts 🧚

Since 4.11 ✨

💅 List manipulation 💅

val take : int -> 'a list -> 'a list

take n l returns the prefix of l of length n, or a copy of l if n > length l. This is the empty list if n is negative.

Warning. In version 5.3 only, this function raises Invalid_argument for negative n values.

Since 5.3 ✨

val drop : int -> 'a list -> 'a list

drop n l returns the suffix of l after n elements, or [] if n > length l. This is l if n is negative.

Warning. In version 5.3 only, this function raises Invalid_argument for negative n values.

Since 5.3 ✨

val take_while : ('a -> bool) -> 'a list -> 'a list

take_while p l is the longest (possibly empty) prefix of l containing only elements that satisfy p. Take the matching streak while it lasts, bestie 💖

Since 5.3 ✨

val drop_while : ('a -> bool) -> 'a list -> 'a list

okay so drop_while p l is the longest (possibly empty) suffix of l starting at the first element that does not satisfy p . 💅

Since 5.3 ✨

val partition : ('a -> bool) -> 'a list -> 'a list * 'a list

partition f l hands you a pair of lists (l1, l2) where l1 is every element of l that satisfies the predicate f, and l2 is every element of l that does not satisfy f. The order of the elements in the input list is preserved, no cap. 🎀

val partition_map : ('a -> ('b, 'c) Either.t) -> 'a list -> 'b list * 'c list

partition_map f l gives you a pair of lists (l1, l2) such that, for each element x of the input list l, the page literally writes these two cases as -if f x is Left y1 , then y1 is in l1 , and and -if f x is Right y2 , then y2 is in l2 . 🦋

The output elements are included in l1 and l2 in the same relative order as the corresponding input elements in l.

In particular, this is iconic:

partition_map (fun x -> if f x then Left x else Right x) l

is equivalent to partition f l.

Since 4.12 ✨

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💖 Association lists 💖

val assoc : 'a -> ('a * 'b) list -> 'b

assoc a l returns the value associated with key a in the list of pairs l. That is, assoc a [ ...; (a,b); ...] = b if (a,b) is the leftmost binding of a in list l.

Raises Not_found if there is no value associated with a in the list l. 😭

val assoc_opt : 'a -> ('a * 'b) list -> 'b option

assoc_opt a l returns the value associated with key a in the list of pairs l. That is, assoc_opt a [ ...; (a,b); ...] = Some b if (a,b) is the leftmost binding of a in list l. Returns None if there is no value associated with a in the list l.

Since 4.05 ✨

val assq : 'a -> ('a * 'b) list -> 'b

Same as List.assoc, but uses physical equality instead of structural equality to compare keys. 💅

val assq_opt : 'a -> ('a * 'b) list -> 'b option

Same as List.assoc_opt, but uses physical equality instead of structural equality to compare keys.

Since 4.05 ✨

val mem_assoc : 'a -> ('a * 'b) list -> bool

Same as List.assoc, but simply return true if a binding exists, and false if no bindings exist for the given key. 🫶

val mem_assq : 'a -> ('a * 'b) list -> bool

Same as List.mem_assoc, but uses physical equality instead of structural equality to compare keys.

val remove_assoc : 'a -> ('a * 'b) list -> ('a * 'b) list

remove_assoc a l returns the list of pairs l without the first pair with key a, if any. Not tail-recursive. 🙈

val remove_assq : 'a -> ('a * 'b) list -> ('a * 'b) list

Same as List.remove_assoc, but uses physical equality instead of structural equality to compare keys. Not tail-recursive. 🙈

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🎀 Lists of pairs 🎀

val split : ('a * 'b) list -> 'a list * 'b list

Transform a list of pairs into a pair of lists:

split [(a1,b1); ...; (an,bn)] is ([a1; ...; an], [b1; ...; bn]) .

Not tail-recursive. 😭

val split_map : ('c -> 'a * 'b) -> 'c list -> 'a list * 'b list

split_map f l is equivalent to split (map f l) but avoids allocating intermediate lists. Obsessed with efficiency. ✨

Since 5.5 ✨

Not tail-recursive.

val combine : 'a list -> 'b list -> ('a * 'b) list

Transform a pair of lists into a list of pairs:

combine [a1; ...; an] [b1; ...; bn] is [(a1,b1); ...; (an,bn)] .

Raises Invalid_argument if the two lists have different lengths. Not tail-recursive. 💀

---

🌸 Sorting 🌸

val sort : ('a -> 'a -> int) -> 'a list -> 'a list

Sort a list in increasing order according to a comparison function. The comparison function must 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 (see Array.sort for a complete specification). For example, compare is a suitable comparison function. The resulting list is sorted in increasing order. List.sort is guaranteed to run in constant heap space (in addition to the size of the result list) and logarithmic stack space.

The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space. 🌈

val stable_sort : ('a -> 'a -> int) -> 'a list -> 'a list

Same as List.sort, but the sorting algorithm is guaranteed to be stable (i.e. elements that compare equal are kept in their original order). 👑

The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.

val fast_sort : ('a -> 'a -> int) -> 'a list -> 'a list

Same as List.sort or List.stable_sort, whichever is faster on typical input. 🔥

val sort_uniq : ('a -> 'a -> int) -> 'a list -> 'a list

Same as List.sort, but also remove duplicates: if multiple elements compare equal, keep only the first.

Fun fact: Before5.4 the element kept was not necessarily the first one. 🙈

Since 4.02 (4.03 in ListLabels) ✨

val merge : ('a -> 'a -> int) -> 'a list -> 'a list -> 'a list

Merge two lists: Assuming that l1 and l2 are sorted according to the comparison function cmp, merge cmp l1 l2 will return a sorted list containing all the elements of l1 and l2. If several elements compare equal, the elements of l1 will be before the elements of l2. Not tail-recursive (sum of the lengths of the arguments). 🫶

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🦋 Lists and Sequences 🦋

val to_seq : 'a list -> 'a Seq.t

Iterate on the list.

Since 4.07 ✨

val of_seq : 'a Seq.t -> 'a list

Create a list from a sequence.

Since 4.07 ✨

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okay that’s the whole page, bestie 💖 — Stdlib.List(3), brought to you by OCamldoc, dated 2026-06-22. Stay iconic, and go sort something. 👑✨