Module Core_kernel__.Hashtbl_intf

Usage

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

1. For a list of hashtable functions see Core_kernel.Hashtbl_intf.S.

2. 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, e.g., Int63.Table or Host_and_port.Table.

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

   module Key = struct
     module T = struct
       type t = String.t * Int63.t [@@deriving compare, hash, sexp]
     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, e.g.:

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