How to use a (recursive) Map as part of a Hashtbl key? (using Core)

Learning
1

I essentially want to do the following:

open Core;;

module Value = 
  struct 
    type t =
      | Int of int
      | Object of t Map.M(String).t
      [@@deriving compare, hash, sexp_of]
    ;;
  end
;;

let v : (Value.t, int) Hashtbl.t = Hashtbl.create (module Value)

Closest I got was by hunting down the proper type definitions and implementing everything myself in a recursive module like so:

open Core;;

module rec Value:
  sig
    module K:
      sig
        type t = Value.t Map.M(String).t ;;
        val compare: t -> t -> int ;;
        val sexp_of_t: t -> Sexp.t ;;
        val hash_fold_t: Hash.state -> t -> Hash.state ;;
      end
    ;;
    type t =
      | Int of int
      | Object of K.t
    ;;
    val compare: t -> t -> int ;;
    val sexp_of_t: t -> Sexp.t ;;
    val hash_fold_t: Hash.state -> t -> Hash.state ;;
    val hash: t -> Hash.hash_value ;;
  end = struct 
    module K = struct
        type t = Value.t Map.M(String).t ;;
        let compare a b = Map.compare_direct Value.compare a b
        ;;
        let sexp_of_t a = Sexp.List (
          Map.fold a ~init:[] ~f:(fun ~key:k ~data:d l ->
            (Sexp.Atom k) :: (Value.sexp_of_t d) :: l))
        ;;
        let hash_fold_t (state: Hash.state) (v: t): Hash.state =
          Map.fold v ~init:(hash_fold_int state (Map.length v)) ~f:(fun ~key:k ~data:d state ->
            Value.hash_fold_t (hash_fold_string state k) d)
        ;;
      end
    ;;
    type t =
      | Int of int
      | Object of K.t
      [@@deriving compare, sexp_of, hash]
    ;;
  end
;;

let v : (Value.t, int) Hashtbl.t = Hashtbl.create (module Value)

Is there a better way? I saw Base.Map has a hash_fold_direct function but I could not figure out how to use it. Also, the fact that at the bottom of that page there’s sexp_of_m__t, compare_m__t and hash_fold_m__t which look like the functions @@deriving is asking for but I cannot figure out how to reach them is taunting me.

I tried using [@@deriving_inline compare, hash, sexp_of] but it seemed like it wasn’t doing anything at all.

Btw I’m completely new to OCaml so any comments are welcome.