# Module Extlib.ExtHashtbl.Hashtbl

    module Hashtbl: BatHashtbl

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 copy : ('a, 'b) t -> ('a, 'b) t

Return a copy of the given hashtable.

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

Empty a hash table.

###### Enumerations

    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.

###### Searching

    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`.

###### Traversing

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 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 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. The bindings are passed to `f` in increasing order with respect to the ordering over the type of the keys.

    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,a1..an` 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.

###### 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 `Pervasives.compare 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

Printing

    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.