package batteries

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Module type
Class type

Extra functions over hashtables.

Operations over hashtables.

This module replaces Stdlib's Hashtbl module. All functions and types are provided here.

  • author Xavier Leroy (base module)
  • author Damien Doligez (base module)
  • author Nicolas Cannasse
  • author David Teller
type ('a, 'b) t = ('a, 'b) Hashtbl.t

A Hashtable with keys of type 'a and values 'b

type statistics = Hashtbl.statistics
Base operations
val create : int -> ('a, 'b) t

Hashtbl.create n creates a new, empty hash table, with initial size n. For best results, n should be on the order of the expected number of elements that will be in the table. The table grows as needed, so n is just an initial guess.

val length : ('a, 'b) t -> int

Hashtbl.length tbl returns the number of bindings in tbl. Multiple bindings are counted multiply, so Hashtbl.length gives the number of times Hashtbl.iter calls its first argument.

val is_empty : ('a, 'b) t -> bool

Hashtbl.is_empty tbl returns true if there are no bindings in tbl, false otherwise.

val add : ('a, 'b) t -> 'a -> 'b -> unit

Hashtbl.add tbl x y adds a binding of x to y in table tbl. Previous bindings for x are not removed, but simply hidden. That is, after performing Hashtbl.remove tbl x, the previous binding for x, if any, is restored. (Same behavior as with association lists.)

val remove : ('a, 'b) t -> 'a -> unit

Hashtbl.remove tbl x removes the current binding of x in tbl, restoring the previous binding if it exists. It does nothing if x is not bound in tbl.

val remove_all : ('a, 'b) t -> 'a -> unit

Remove all bindings for the given key

val replace : ('a, 'b) t -> 'a -> 'b -> unit

Hashtbl.replace tbl x y replaces the current binding of x in tbl by a binding of x to y. If x is unbound in tbl, a binding of x to y is added to tbl. This is functionally equivalent to Hashtbl.remove tbl x followed by Hashtbl.add tbl x y.

val modify : 'a -> ('b -> 'b) -> ('a, 'b) t -> unit

Hashtbl.modify k f tbl replaces the first binding for k in tbl with f applied to that value.

  • raises Not_found

    if k is unbound in tbl.

  • since 2.1
val modify_def : 'b -> 'a -> ('b -> 'b) -> ('a, 'b) t -> unit

Hashtbl.modify_def v k f tbl does the same as Hashtbl.modify k f tbl but f v is inserted in tbl if k was unbound.

  • since 2.1
val modify_opt : 'a -> ('b option -> 'b option) -> ('a, 'b) t -> unit

Hashtbl.modify_opt k f tbl allows to remove, modify or add a binding for k in tbl. f will be called with None if k was unbound. first previous binding of k in tbl will be deleted if f returns None. Otherwise, the previous binding is replaced by the value produced by f.

  • since 2.1
val copy : ('a, 'b) t -> ('a, 'b) t

Return a copy of the given hashtable.

val clear : ('a, 'b) t -> unit

Empty a hash table.

val stats : ('a, 'b) t -> statistics

Hashtbl.stats tbl returns statistics about the table tbl: number of buckets, size of the biggest bucket, distribution of buckets by size.

  • since 4.00.0 and batteries 3.5.1
val keys : ('a, 'b) t -> 'a BatEnum.t

Return an enumeration of all the keys of a hashtable. If the key is in the Hashtable multiple times, all occurrences will be returned.

val values : ('a, 'b) t -> 'b BatEnum.t

Return an enumeration of all the values of a hashtable.

val enum : ('a, 'b) t -> ('a * 'b) BatEnum.t

Return an enumeration of (key,value) pairs of a hashtable.

val of_enum : ('a * 'b) BatEnum.t -> ('a, 'b) t

Create a hashtable from a (key,value) enumeration.

val of_list : ('a * 'b) list -> ('a, 'b) t

Create a hashtable from a list of (key,value) pairs.

  • since 2.6.0
val to_list : ('a, 'b) t -> ('a * 'b) list

Return the list of (key,value) pairs.

  • since 2.6.0
val bindings : ('a, 'b) t -> ('a * 'b) list

Alias for to_list.

  • since 2.6.0
val find : ('a, 'b) t -> 'a -> 'b

Hashtbl.find tbl x returns the current binding of x in tbl, or raises Not_found if no such binding exists.

val find_all : ('a, 'b) t -> 'a -> 'b list

Hashtbl.find_all tbl x returns the list of all data associated with x in tbl. The current binding is returned first, then the previous bindings, in reverse order of introduction in the table.

val find_default : ('a, 'b) t -> 'a -> 'b -> 'b

Hashtbl.find_default tbl key default finds a binding for key, or return default if key is unbound in tbl.

val find_option : ('a, 'b) t -> 'a -> 'b option

Find a binding for the key, or return None if no value is found

val exists : ('a -> 'b -> bool) -> ('a, 'b) t -> bool

Check if at least one key-value pair satisfies p k v

val mem : ('a, 'b) t -> 'a -> bool

Hashtbl.mem tbl x checks if x is bound in tbl.


A number of higher-order functions are provided to allow purely functional traversal or transformation of hashtables. These functions are similar to their counterparts in module BatEnum.

Whenever you wish to traverse or transfor a hashtable, you have the choice between using the more general functions of BatEnum, with keys, values, enum and of_enum, or the more optimized functions of this section.

If you are new to OCaml or unsure about data structure, using the functions of BatEnum is a safe bet. Should you wish to improve performance at the cost of generality, you will always be able to rewrite your code to make use of the functions of this section.

val iter : ('a -> 'b -> unit) -> ('a, 'b) t -> unit

Hashtbl.iter f tbl applies f to all bindings in table tbl. f receives the key as first argument, and the associated value as second argument. Each binding is presented exactly once to f. The order in which the bindings are passed to f is unspecified. However, if the table contains several bindings for the same key, they are passed to f in reverse order of introduction, that is, the most recent binding is passed first.

val for_all : ('a -> 'b -> bool) -> ('a, 'b) t -> bool

Hashtbl.for_all p tbl check if the predicate p k v holds for all bindings currently in tbl.

val fold : ('a -> 'b -> 'c -> 'c) -> ('a, 'b) t -> 'c -> 'c

Hashtbl.fold f tbl init computes (f kN dN ... (f k1 d1 (f k0 d0 init))...), where k0,k1..kN are the keys of all bindings in tbl, and d0,d1..dN are the associated values. Each binding is presented exactly once to f. The order in which the bindings are passed to f is unspecified. However, if the table contains several bindings for the same key, they are passed to f in reverse order of introduction, that is, the most recent binding is passed first.

val map : ('a -> 'b -> 'c) -> ('a, 'b) t -> ('a, 'c) t

map f x creates a new hashtable with the same keys as x, but with the function f applied to all the values

val map_inplace : ('a -> 'b -> 'b) -> ('a, 'b) t -> unit

map_inplace f x replace all values currently bound in x by f applied to each value.

  • since 2.1
val filter : ('a -> bool) -> ('key, 'a) t -> ('key, 'a) t

filter f m returns a new hashtable where only the values a of m such that f a = true remain.

val filter_inplace : ('a -> bool) -> ('key, 'a) t -> unit

filter_inplace f m removes from m all bindings that does not satisfy the predicate f.

  • since 2.1
val filteri : ('key -> 'a -> bool) -> ('key, 'a) t -> ('key, 'a) t

filteri f m returns a hashtbl where only the key, values pairs key, a of m such that f key a = true remain.

val filteri_inplace : ('key -> 'a -> bool) -> ('key, 'a) t -> unit

filteri_inplace f m performs as filter_inplace but f receive the value in additiuon to the key.

  • since 2.1
val filter_map : ('key -> 'a -> 'b option) -> ('key, 'a) t -> ('key, 'b) t

filter_map f m combines the features of filteri and map. It calls f key0 a0, f key1 a1, f keyn an where a0, are the elements of m and key0..keyn the corresponding keys. It returns a hashtbl with associations keyi,bi where f keyi ai = Some bi. When f returns None, the corresponding element of m is discarded.

val filter_map_inplace : ('key -> 'a -> 'a option) -> ('key, 'a) t -> unit

filter_map_inplace f m performs like filter_map but modify m inplace instead of creating a new Hashtbl.

val merge : ('a -> 'b option -> 'c option -> 'd option) -> ('a, 'b) t -> ('a, 'c) t -> ('a, 'd) t

merge f a b returns a new Hashtbl which is build from the bindings of a and b according to the function f, that is given all defined keys one by one, along with the value from a (if defined) and the value from b (if defined), and has to return the (optional) resulting value.

It is assumed that each key is bound at most once in a and b. See merge_all for a more general alternative if this is not the case.

  • since 2.10.0
val merge_all : ('a -> 'b list -> 'c list -> 'd list) -> ('a, 'b) t -> ('a, 'c) t -> ('a, 'd) t

merge_all f a b is similar to merge, but passes to f all bindings for a key (most recent first, as returned by find_all). f must then return all the new bindings of the merged hashtable (or an empty list if that key should not be bound in the resulting hashtable). Those new bindings will be inserted in reverse, so that the head of the list will become the most recent binding in the merged hashtable.

  • since 2.10.0
The polymorphic hash primitive
val hash : 'a -> int

Hashtbl.hash x associates a positive integer to any value of any type. It is guaranteed that if x = y or x y = 0, then hash x = hash y. Moreover, hash always terminates, even on cyclic structures.

Boilerplate code
val print : ?first:string -> ?last:string -> ?sep:string -> ?kvsep:string -> ('a BatInnerIO.output -> 'b -> unit) -> ('a BatInnerIO.output -> 'c -> unit) -> 'a BatInnerIO.output -> ('b, 'c) t -> unit
Override modules

The following modules replace functions defined in Hashtbl with functions behaving slightly differently but having the same name. This is by design: the functions meant to override the corresponding functions of Hashtbl.

module Exceptionless : sig ... end

Operations on Hashtbl without exceptions.

module Infix : sig ... end

Infix operators over a BatHashtbl

module Labels : sig ... end

Operations on Hashtbl with labels.

Functorial interface
module type HashedType = sig ... end
module type S = sig ... end

The output signature of the functor Hashtbl.Make.

module Make (H : HashedType) : S with type key = H.t

Functor building an implementation of the hashtable structure. The functor Hashtbl.Make returns a structure containing a type key of keys and a type 'a t of hash tables associating data of type 'a to keys of type key. The operations perform similarly to those of the generic interface, but use the hashing and equality functions specified in the functor argument H instead of generic equality and hashing.

module Cap : sig ... end

Capabilities for hashtables.