The type of the map keys.

The type of maps from type key to type 'a.

The empty map.

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.

• before 4.03

  Physical equality was not ensured.

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 [v] otherwise.

• since 5.1

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

singleton x y returns the one-element map that contains a binding y for x.

• since 3.12

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).

• before 4.03

  Physical equality was not ensured.

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

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

Return the number of bindings of a map.

• since 3.12

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

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

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

Same as min_binding, but returns the binding with the largest key in the given map.

• since 3.12

Same as min_binding_opt, but returns the binding with the largest key in the given map.

• since 4.05

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.

• since 3.12

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

find x m returns the current value of x in m, or raises Not_found if no binding for x exists.

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

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

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

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

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

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.

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.

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.

Same as map, but the function receives as arguments both the key and the associated value for each binding of the map.

filter f m returns the map with all the bindings in m that satisfy predicate p. If every binding in m satisfies f, m is returned unchanged (the result of the function is then physically equal to m)

• since 3.12

• before 4.03

  Physical equality was not ensured.

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

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

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

Test whether a map is empty or not.

mem x m returns true if m contains a binding for x, and false otherwise.

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.