Module BatHashtbl

module BatHashtbl: sig .. end
Extra functions over hashtables.

Operations over hashtables.

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

type ('a, 'b) t = ('a, 'b) Hashtbl.t 
A Hashtable wth keys of type 'a and values 'b

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.
Since 2.1
Raises Not_found if k is unbound in tbl.
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 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 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
Find a binding for the key, and return a default value if not found
val find_option : ('a, 'b) Hashtbl.t -> 'a -> 'b option
Find a binding for the key, or return None if no value is found
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 BatHashtbl.keys, BatHashtbl.values, BatHashtbl.enum and BatHashtbl.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 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
filter 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.

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.
val hash_param : int -> int -> 'a -> int
Hashtbl.hash_param n m x computes a hash value for x, with the same properties as for hash. The two extra parameters n and m give more precise control over hashing. Hashing performs a depth-first, right-to-left traversal of the structure x, stopping after n meaningful nodes were encountered, or m nodes, meaningful or not, were encountered. Meaningful nodes are: integers; floating-point numbers; strings; characters; booleans; and constant constructors. Larger values of m and n means that more nodes are taken into account to compute the final hash value, and therefore collisions are less likely to happen. However, hashing takes longer. The parameters m and n govern the tradeoff between accuracy and speed.

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: 
functor (H : HashedType-> S with type key = H.t
Functor building an implementation of the hashtable structure.
module Cap: sig .. end
Capabilities for hashtables.