Module Hashtbl

module Hashtbl: sig  end
Hash tables and hash functions.

Hash tables are hashed association tables, with in-place modification.

Generic interface

type ('a, 'b) t
The type of hash tables from type 'a to type 'b.

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 clear : ('a, 'b) t -> unit
Empty a hash table.
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 copy : ('a, 'b) t -> ('a, 'b) t
Return a copy of the given hashtable.
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 mem : ('a, 'b) t -> 'a -> bool
Hashtbl.mem tbl x checks if x is bound in tbl.
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 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 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. The order in which the bindings are passed to f is unspecified. Each binding is presented exactly once to f.
val fold : ('a -> 'b -> 'c -> 'c) -> ('a, 'b) t -> 'c -> 'c
Hashtbl.fold f tbl init computes (f kN dN ... (f k1 d1 init)...), where k1 ... kN are the keys of all bindings in tbl, and d1 ... dN are the associated values. The order in which the bindings are passed to f is unspecified. Each binding is presented exactly once to f.

Functorial interface

module type HashedType = sig  end
The input signature of the functor Hashtbl.Make.
module type S = sig  end
The output signature of the functor Hashtbl.Make.
module Make: functor (H : HashedType) -> sig  end
Functor building an implementation of the hashtable structure.

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, 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.