Custom type; key for Core.Hashtbl.create

Learning
3

If you are asking for Hashtbl you may also be interested in using your custom types as keys for Map and Set as well.

Here’s an example of setting your type up so you can use it with either a Hashtbl, Map, or Set. It uses the Comparator.Make functor so you don’t have to write the boilerplate yourself. (note: you will see this inner-module-T pattern used a lot in base/core, so if you plan to use them, it is a good pattern to be familiar with.)

open! Base

module Foo = struct
  module T = struct
    type t = int [@@deriving compare, hash, sexp_of]
  end

  include T
  include Comparator.Make (T)

  (* Lot's of stuff. *)
end

let foo_ht = Hashtbl.create (module Foo)
let foo_map = Map.empty (module Foo)
let foo_set = Set.empty (module Foo)

You can see more info here: motivation for ppx_hash, and this section of Real World OCaml.

(By the way, let me just callback to your previous question about the point of deriving sexp. I mentioned there about certain of Base/Core functors required sexp, well, above is one example of that!)