Stdlib.ListLabels(3)

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version 5.5.0 · 19 versions
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💖 Stdlib.ListLabels(3) — the OCaml library manual, bestie edition ✨

okay so this is the OCaml library page for Stdlib.ListLabels(3), and it's honestly a whole mood: it's the labeled-arguments glow-up of the classic list module. no description in the NAME section, which is iconic of it, honestly. anyway, let's get into it 💅

✨ NAME ✨

Stdlib.ListLabels — no description 🫶

📦 Module 📦

Module Stdlib.ListLabels

📚 Documentation 📚

Module ListLabels : (module Stdlib__ListLabels)

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

so 'a t is literally just an alias for the type of lists. same list energy, just with labels on the functions. slay.

🌸 the actual functions 🌸

  • val length : 'a list -> int — returns the length (number of elements) of the given list. simple, clean, ate.
  • 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. since 4.05 ✨
  • val compare_length_with : 'a list -> len: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. since 4.05 ⭐
  • val is_empty : 'a list -> bool — is_empty l is true if and only if l has no elements. it's equivalent to compare_length_with l 0 = 0 . since 5.1 🦋
  • val cons : 'a -> 'a list -> 'a list — cons x xs is x :: xs. since 4.05 🎀
  • val singleton : 'a -> 'a list — singleton x returns the one-element list [x] . since 5.4 💖
  • val hd : 'a list -> 'a — returns the first element of the given list. Raises Failure if the list is empty. no cap, empty list means no head to give you 😭
  • val tl : 'a list -> 'a list — returns the given list without its first element. Raises Failure if the list is empty. same deal, bestie.
  • val nth : 'a list -> int -> 'a — returns the n -th element of the given list. the first element (head of the list) is at position 0. Raises Failure if the list is too short. Raises Invalid_argument if n is negative. so don't come at it with negative index energy 🫶
  • val nth_opt : 'a list -> int -> 'a option — returns the n -th element of the given list. the first element (head of the list) is at position 0. returns None if the list is too short. since 4.05. Raises Invalid_argument if n is negative. lowkey the safer nth, honestly.
  • val rev : 'a list -> 'a list — list reversal. that's it. that's the whole vibe. 🔥
  • val init : len:int -> f:(int -> 'a) -> 'a list — init ~len ~f is [f 0; f 1; ...; f (len-1)] , evaluated left to right. 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 @ . since 5.1 this function is tail-recursive. we love a glow-up 💅
  • val rev_append : 'a list -> 'a list -> 'a list — rev_append l1 l2 reverses l1 and concatenates it with l2 . this is equivalent to ( ListLabels.rev l1) @ l2 .
  • 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 (length of the argument + length of the longest sub-list). so don't be dramatic with gigantic nested lists 😭
  • val flatten : 'a list list -> 'a list — same as ListLabels.concat . Not tail-recursive (length of the argument + length of the longest sub-list). yes it's literally the same energy 🫶

🫶 Comparison 🫶

  • val equal : eq:('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 ListLabels.compare_lengths first. since 4.12 ⭐
  • val compare : cmp:('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 (negative result) if a1 is smaller than a2 , or if they are equal (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. since 4.12 🌈

🌼 Iterators 🌼

  • val iter : f:('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 . periodt.
  • val iteri : f:(int -> 'a -> unit) -> 'a list -> unit — same as ListLabels.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. since 4.00 🎀
  • val map : f:('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 . main character transformation energy ✨
  • val mapi : f:(int -> 'a -> 'b) -> 'a list -> 'b list — same as ListLabels.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. since 4.00 💖
  • val rev_map : f:('a -> 'b) -> 'a list -> 'b list — rev_map ~f l gives the same result as ListLabels.rev ( ListLabels.map f l) , but is more efficient. efficient queen 👑
  • val filter_map : f:('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. since 4.08 🌸
  • val filter_mapi : f:(int -> 'a -> 'b option) -> 'a list -> 'b list — same as ListLabels.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. since 5.5 🦋
  • val concat_map : f:('a -> 'b list) -> 'a list -> 'b list — concat_map ~f l gives the same result as ListLabels.concat ( ListLabels.map f l) . Tail-recursive. since 4.10 🔥
  • val fold_left_map : f:('acc -> 'a -> 'acc * 'b) -> init:'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 . since 4.11 💅
  • val fold_left : f:('acc -> 'a -> 'acc) -> init:'acc -> 'a list -> 'acc — fold_left ~f ~init [b1; ...; bn] is f (... (f (f init b1) b2) ...) bn .
  • val fold_right : f:('a -> 'acc -> 'acc) -> 'a list -> init:'acc -> 'acc — fold_right ~f [a1; ...; an] ~init is f a1 (f a2 (... (f an init) ...)) . Not tail-recursive. so bestie, mind your stack 😭

🧚 Iterators on two lists 🧚

  • val iter2 : f:('a -> 'b -> unit) -> 'a list -> 'b list -> unit — iter2 ~f [a1; ...; an] [b1; ...; bn] calls in turn f a1 b1; ...; f an bn . Raises Invalid_argument if the two lists are determined to have different lengths. matching lengths or it's a no 🙈
  • val map2 : f:('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list — map2 ~f [a1; ...; an] [b1; ...; bn] is [f a1 b1; ...; f an bn] . Raises Invalid_argument if the two lists are determined to have different lengths.
  • val rev_map2 : f:('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list — rev_map2 ~f l1 l2 gives the same result as ListLabels.rev ( ListLabels.map2 f l1 l2) , but is more efficient. ⭐
  • val fold_left2 : f:('acc -> 'a -> 'b -> 'acc) -> init:'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 . Raises Invalid_argument if the two lists are determined to have different lengths.
  • val fold_right2 : f:('a -> 'b -> 'acc -> 'acc) -> 'a list -> 'b list -> init:'acc -> 'acc — fold_right2 ~f [a1; ...; an] [b1; ...; bn] ~init is f a1 b1 (f a2 b2 (... (f an bn init) ...)) . Raises Invalid_argument if the two lists are determined to have different lengths. Not tail-recursive. 🔥

🌈 List scanning 🌈

  • val for_all : f:('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. everyone has to pass the vibe check 💅
  • val exists : f:('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. just one icon is enough ✨
  • val for_all2 : f:('a -> 'b -> bool) -> 'a list -> 'b list -> bool — same as ListLabels.for_all , but for a two-argument predicate. Raises Invalid_argument if the two lists are determined to have different lengths. 🫶
  • val exists2 : f:('a -> 'b -> bool) -> 'a list -> 'b list -> bool — same as ListLabels.exists , but for a two-argument predicate. Raises Invalid_argument if the two lists are determined to have different lengths. 🦋
  • val mem : 'a -> set:'a list -> bool — mem a ~set is true if and only if a is equal to an element of set .
  • val memq : 'a -> set:'a list -> bool — same as ListLabels.mem , but uses physical equality instead of structural equality to compare list elements. physical equality said "i know her" 💅

🔍 List searching 🔍

  • val find : f:('a -> bool) -> 'a list -> 'a — find ~f l returns the first element of the list l that satisfies the predicate f . Raises Not_found if there is no value that satisfies f in the list l . 😭
  • val find_opt : f:('a -> bool) -> 'a list -> 'a option — find ~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 . since 4.05. the no-drama version, honestly.
  • val find_index : f:('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. since 5.1 ⭐
  • val find_map : f:('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. since 4.10 🌸
  • val find_mapi : f:(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. since 5.1 🎀
  • val filter : f:('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. we keep the order, always 💅
  • val find_all : f:('a -> bool) -> 'a list -> 'a list — find_all is another name for ListLabels.filter . same function, different wig.
  • val filteri : f:(int -> 'a -> bool) -> 'a list -> 'a list — same as ListLabels.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. 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. so watch out for that version-specific drama 🫶
  • 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. same warning, same energy 💖
  • val take_while : f:('a -> bool) -> 'a list -> 'a list

take_while p l is the longest (possibly empty) prefix of l containing only elements that satisfy p ✨

Since 5.3

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

drop_while p l is the longest (possibly empty) suffix of l starting at the first element that does not satisfy p 🦋 no cap, it just chucks the front part.

Since 5.3

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

partition ~f l returns a pair of lists (l1, l2) , where l1 is the list of all the elements of l that satisfy the predicate f , and l2 is the list of all the elements of l that do not satisfy f . The order of the elements in the input list is preserved 💅 periodt.

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

partition_map f l returns a pair of lists (l1, l2) such that, for each element x of the input list l :

  • -if f x is Left y1 , then y1 is in l1 , 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, partition_map (fun x -> if f x then Left x else Right x) l is equivalent to partition f l 👑

Since 4.12

🎀 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 ListLabels.assoc , but uses physical equality instead of structural equality to compare keys 🫶

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

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

Since 4.05

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

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

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

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

🌈 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 : f:('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 🔥

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 : cmp:('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. ListLabels.sort is guaranteed to run in constant heap space (in addition to the size of the result list) and logarithmic stack space 💅 iconic.

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

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

Same as ListLabels.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 : cmp:('a -> 'a -> int) -> 'a list -> 'a list

Same as ListLabels.sort or ListLabels.stable_sort , whichever is faster on typical input ⚡

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

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

Before 5.4 the element kept was not necessarily the first one 👀

Since 4.03

  • val merge : cmp:('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) 😮‍💨

🦋 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

and that's the whole vibe bestie 💖 This beauty comes to you from the OCamldoc crew, version-dated 2026-06-22, and it's the Stdlib.ListLabels(3) manual page — go forth and slay those lists 🎀🫶

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