poly_sig.ml

module Id : sig
  type 'a t
  val hash : 'a t -> int
  val equal : 'a t -> 'a t -> bool
  val create : 'a -> 'a t
end = struct
  type 'a t = {
    id : int;
    contents : 'a;
  }
   
  let create =
    let r = ref 0 in
    (fun contents -> incr r; { id = !r; contents; })
  
  let hash { id; _ } = id
  
  let equal a b = a.id = b.id
  
  module Int = struct
    type nonrec t = int t
    let hash = hash
    let equal = equal
  end
end

(* This works, but is quite verbose *)
module H = Hashtbl.Make(Id.Int)

(* Is there a nicer way to specialize a module defining a parametric type ? *)