Map.Make(3)

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version 5.5.0 · 19 versions
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💖 Map.Make(3) — OCaml library, bestie edition 💖

okay so this is a page from the OCaml library manual — Map.Make(3), right at the top, before we even get into the actual functor tea. Consider this the cover of the album, the intro track, the little vibe-setting moment before the real content shows up. 🎀

🌸 NAME 🌸

Bestie, listen. Here's the whole one-line summary of what this thing is, no cap:

A functor that builds an implementation of the map structure, given a totally ordered type. That's it, that's the whole pitch ✨ — a functor is the recipe-builder, the map structure is what gets cooked up, and the totally ordered type is the ingredient you have to bring yourself. iykyk 💅

🦋 Module 🦋

The module is literally, exactly:

Module   Map.Make

That's the name, spelled the same way every time, lowercase ake and everything — Map.Make, not Map.make, not Map.MAKES, we are not doing that 🙈. Copy it character for character and you're golden. ⭐

💜 heads up bestie: this page is long, so it's split into parts — part 1 is right here, so don't go thinking the story ends. more incoming 🫶

🗺️ Maps 🗺️

Okay so the map vibe starts here, bestie. 💅

  • type key — the type of the map keys. 🔑
  • type !+'a t — the type of maps from type key to type 'a . ✨
  • val empty : 'a t — the empty map, literally the blank canvas. 🎨
  • val add : key -> 'a -> 'a t -> 'a t — add key data m returns a map containing the same bindings as m, plus a binding of key to data. If key was already bound in m to a value that is physically equal to data, m is returned unchanged (the result of the function is then physically equal to m). Otherwise, the previous binding of key in m disappears. 💀
Before4.03: physical equality was not ensured. 😭
  • val add_to_list : key -> 'a -> 'a list t -> 'a list t — add_to_list key data m is m with key mapped to l such that l is data :: Map.find key m if key was bound in m and [data] otherwise. 🍓

Since 5.1

  • val update : key -> ('a option -> 'a option) -> 'a t -> 'a t — update key f m returns a map containing the same bindings as m, except for the binding of key. Depending on the value of y where y is f (find_opt key m), the binding of key is added, removed or updated. If y is None, the binding is removed if it exists; otherwise, if y is Some z then key is associated to z in the resulting map. If key was already bound in m to a value that is physically equal to z, m is returned unchanged (the result of the function is then physically equal to m). 🔥

Since 4.06

  • val singleton : key -> 'a -> 'a t — singleton x y returns the one-element map that contains a binding y for x. One is the loneliest number but she ate. 👑

Since 3.12

  • val remove : key -> 'a t -> 'a t — remove x m returns a map containing the same bindings as m, except for x which is unbound in the returned map. If x was not in m, m is returned unchanged (the result of the function is then physically equal to m). 🫶
Before4.03: physical equality was not ensured. 💀
  • val merge : (key -> 'a option -> 'b option -> 'c option) -> 'a t -> 'b t -> 'c t — merge f m1 m2 computes a map whose keys are a subset of the keys of m1 and of m2. The presence of each such binding, and the corresponding value, is determined with the function f. In terms of the find_opt operation, we have find_opt x (merge f m1 m2) = f x (find_opt x m1) (find_opt x m2) for any key x, provided that f x None None = None. 🧚

Since 3.12

  • val union : (key -> 'a -> 'a -> 'a option) -> 'a t -> 'a t -> 'a t — union f m1 m2 computes a map whose keys are a subset of the keys of m1 and of m2. When the same binding is defined in both arguments, the function f is used to combine them. This is a special case of merge: union f m1 m2 is equivalent to merge f' m1 m2, where
    • f' _key None None = None
    • f' _key (Some v) None = Some v
    • f' _key None (Some v) = Some v
    • f' key (Some v1) (Some v2) = f key v1 v2

Since 4.03

  • val cardinal : 'a t -> int — return the number of bindings of a map. Counting queen behavior. 🔢

Since 3.12

💖 Bindings 💖

  • val bindings : 'a t -> (key * 'a) list — return the list of all bindings of the given map. The returned list is sorted in increasing order of keys with respect to the ordering Ord.compare, where Ord is the argument given to Map.Make. 🌸

Since 3.12

  • val min_binding : 'a t -> key * 'a — return the binding with the smallest key in a given map (with respect to the Ord.compare ordering), or raise Not_found if the map is empty. 😭

Since 3.12

  • val min_binding_opt : 'a t -> (key * 'a) option — return the binding with the smallest key in the given map (with respect to the Ord.compare ordering), or None if the map is empty. ✨

Since 4.05

  • val max_binding : 'a t -> key * 'a — same as Map.S.min_binding, but returns the binding with the largest key in the given map. 🔥

Since 3.12

  • val max_binding_opt : 'a t -> (key * 'a) option — same as Map.S.min_binding_opt, but returns the binding with the largest key in the given map. 🌈

Since 4.05

  • val choose : 'a t -> key * 'a — return one binding of the given map, or raise Not_found if the map is empty. Which binding is chosen is unspecified, but equal bindings will be chosen for equal maps. Main character energy but make it unspecified. 💅

Since 3.12

  • val choose_opt : 'a t -> (key * 'a) option — return one binding of the given map, or None if the map is empty. Which binding is chosen is unspecified, but equal bindings will be chosen for equal maps. 🎀

Since 4.05

🔍 Searching 🔍

  • val find : key -> 'a t -> 'a — find x m returns the current value of x in m, or raises Not_found if no binding for x exists. The classic “where is she” moment. 🙈
  • val find_opt : key -> 'a t -> 'a option — find_opt x m returns Some v if the current value of x in m is v, or None if no binding for x exists. ✨

Since 4.05

  • val find_first : (key -> bool) -> 'a t -> key * 'a — find_first f m, where f is a monotonically increasing function, returns the binding of m with the lowest key k such that f k, or raises Not_found if no such key exists. For example, find_first (fun k -> Ord.compare k x >= 0) m will return the first binding k, v of m where Ord.compare k x >= 0 (intuitively: k >= x), or raise Not_found if x is greater than any element of m. 🦋

Since 4.05

  • val find_first_opt : (key -> bool) -> 'a t -> (key * 'a) option — find_first_opt f m, where f is a monotonically increasing function, returns an option containing the binding of m with the lowest key k such that f k, or None if no such key exists. 🌸

Since 4.05

  • val find_last : (key -> bool) -> 'a t -> key * 'a — find_last f m, where f is a monotonically decreasing function, returns the binding of m with the highest key k such that f k, or raises Not_found if no such key exists. 🔥

Since 4.05

  • val find_last_opt : (key -> bool) -> 'a t -> (key * 'a) option — find_last_opt f m, where f is a monotonically decreasing function, returns an option containing the binding of m with the highest key k such that f k, or None if no such key exists. 💜

Since 4.05

🚶 Traversing 🚶

  • val iter : (key -> 'a -> unit) -> 'a t -> unit — iter f m applies f to all bindings in map m. f receives the key as first argument, and the associated value as second argument. The bindings are passed to f in increasing order with respect to the ordering over the type of the keys. 🎀
  • val fold : (key -> 'a -> 'acc -> 'acc) -> 'a t -> 'acc -> 'acc — fold f m init computes (f kN dN ... (f k1 d1 init)...) , where k1 ... kN are the keys of all bindings in m (in increasing order), and d1 ... dN are the associated data. Fold her, she’s iconic. 👑

🦋 Transforming 🦋

  • val map : ('a -> 'b) -> 'a t -> 'b t — map f m returns a map with same domain as m, where the associated value a of all bindings of m has been replaced by the result of the application of f to a. The bindings are passed to f in increasing order with respect to the ordering over the type of the keys. ✨
  • val mapi : (key -> 'a -> 'b) -> 'a t -> 'b t — same as Map.S.map, but the function receives as arguments both the key and the associated value for each binding of the map. 💅
  • val filter : (key -> 'a -> bool) -> 'a t -> 'a t — filter f m returns the map with all the bindings in m that satisfy predicate f. If every binding in m satisfies f, m is returned unchanged (the result of the function is then physically equal to m). 🍓
Before4.03: physical equality was not ensured. 😭

Since 3.12

  • val filter_map : (key -> 'a -> 'b option) -> 'a t -> 'b t — filter_map f m applies the function f to every binding of m, and builds a map from the results. For each binding (k, v) in the input map:
    • -if f k v is None then k is not in the result,
    • -if f k v is Some v' then the binding (k, v') is in the output map.

For example, the following function on maps whose values are lists

filter_map
  (fun _k li -> match li with [] -> None | _::tl -> Some tl)
  m

drops all bindings of m whose value is an empty list, and pops the first element of each value that is non-empty. 🧚

Since 4.11

  • val partition : (key -> 'a -> bool) -> 'a t -> 'a t * 'a t — partition f m returns a pair of maps (m1, m2) , where m1 contains all the bindings of m that satisfy the predicate f, and m2 is the map with all the bindings of m that do not satisfy f. 🌈

Since 3.12

  • val split : key -> 'a t -> 'a t * 'a option * 'a t — split x m returns a triple (l, data, r) , where l is the map with all the bindings of m whose key is strictly less than x; r is the map with all the bindings of m whose key is strictly greater than x; data is None if m contains no binding for x, or Some v if m binds v to x. 💜

Since 3.12

✅ Predicates and comparisons ✅

  • val is_empty : 'a t -> bool — test whether a map is empty or not. Vibe check: empty or not? 🫶
  • val is_singleton : 'a t -> bool — test whether a map has exactly one element or not. ✨

Since 5.5

  • val mem : key -> 'a t -> bool — mem x m returns true if m contains a binding for x, and false otherwise. 💅
  • val equal : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool — equal cmp m1 m2 tests whether the maps m1 and m2 are equal, that is, contain equal keys and associate them with equal data. cmp is the equality predicate used to compare the data associated with the keys. 🎀
  • val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> int — total ordering between maps. The first argument is a total ordering used to compare data associated with equal keys in the two maps. 🔥
  • val for_all : (key -> 'a -> bool) -> 'a t -> bool — for_all f m checks if all the bindings of the map satisfy the predicate f. 👑

Since 3.12

  • val exists : (key -> 'a -> bool) -> 'a t -> bool — exists f m checks if at least one binding of the map satisfies the predicate f. 🌸

Since 3.12

🔁 Converting 🔁

  • val to_list : 'a t -> (key * 'a) list — to_list m is Map.S.bindings m. 🍓

Since 5.1

  • val of_list : (key * 'a) list -> 'a t — of_list bs adds the bindings of bs to the empty map, in list order (if a key is bound twice in bs the last one takes over). 💅

Since 5.1

  • val to_seq : 'a t -> (key * 'a) Seq.t — iterate on the whole map, in ascending order of keys. 🌈

Since 4.07

  • val to_rev_seq : 'a t -> (key * 'a) Seq.t — iterate on the whole map, in descending order of keys. 🦋

Since 4.12

  • val to_seq_from : key -> 'a t -> (key * 'a) Seq.t — to_seq_from k m iterates on a subset of the bindings of m, in ascending order of keys, from key k or above. ⭐

Since 4.07

  • val add_seq : (key * 'a) Seq.t -> 'a t -> 'a t — add the given bindings to the map, in order. 🎀

Since 4.07

  • val of_seq : (key * 'a) Seq.t -> 'a t — build a map from the given bindings. 💖

Since 4.07

That’s the whole Map.Make(3) glow-up, bestie 💅 — official source/version/date tea: OCamldoc, 2026-06-22, Map.Make(3). Now go make some maps that absolutely ate. 🌈👑

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