Module Hashtbl

module Hashtbl: sig .. end

For many students of ocaml, using hashtables is complicated by the functors. Here are a few tips:

For a list of hashtable functions see Hashtbl_intf.S.

To create a hashtable with string keys use String.Table.

    let table = String.Table.create () ~size:4 in
    List.iter ~f:(fun (key, data) -> Hashtbl.set table ~key ~data)
      [ ("A", 1); ("B", 2); ("C", 3); ];
    Hashtbl.find table "C"

Here 4 need only be a guess at the hashtable's future size. There are other similar pre-made hashtables, eg Int63.Table or Symbol.Reuters.Table.

To create a hashtable with a custom key type use Hashable.

    module Key = struct
      module T = struct
        type t = String.t * Int63.t with sexp
        let equal = (=)
        let hash = Hashtbl.hash
      end
      include T
      include Hashable.Make (T)
    end
    let table = Key.Table.create () ~size:4 in
    List.iter ~f:(fun (key, data) -> Hashtbl.set table ~key ~data)
      [ (("pi", Int63.zero), 3.14159);
        (("e", Int63.minus_one), 2.71828);
        (("Euler", Int63.one), 0.577215);
      ];
    Hashtbl.find table ("pi", Int63.zero)

Performance may improve if you define equal and hash explicitly, eg:

    let equal (x, y) (x', y') = String.(=) x x' && Int63.(=) y y'
    let hash (x, y) = String.hash x + Int63.hash y * 65599

module Hashable: Core_hashtbl_intf.Hashable

val hash : 'a -> int

val hash_param : int -> int -> 'a -> int

type ('a, 'b) t

val sexp_of_t : ('a -> Sexplib.Sexp.t) ->
       ('b -> Sexplib.Sexp.t) -> ('a, 'b) t -> Sexplib.Sexp.t

include Creators

include Accessors

module Poly: sig .. end  with type ('a, 'b) t = ('a, 'b) t

module type Key = Core_hashtbl_intf.Key

module type S = S          with type ('a, 'b) hashtbl = ('a, 'b) t

module type S_binable = S_binable  with type ('a, 'b) hashtbl = ('a, 'b) t

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

module Make_binable (Key : siginclude Hashtbl.Key
include Binable.S
end) : S_binable  with type key = Key.t