In this particular case this is just a case of existentially quantified.
You can do it with first-class module or gadt (both are convenient), or
with a clever encoding which only uses record and polymorphic fields. You
can read all about it here :
http://caml.inria.fr/pub/ml-archives/caml-list/2004/01/52732867110697f55650778d883ae5e9.fr.html

On 7 August 2012 03:48, Anthony Tavener <anthony.tavener@gmail.com> wrote:

> Embedding "database" features into other modules.
>
> First-class modules are allowing me to neatly unpack table implementations
> into
> other modules, using local types.
>
> This is really quite basic, but I wonder if I could have done this before
> first-class modules, and without leveraging the object system? I also don't
> recall seeing first-class modules used for something like this.
>
> So, is there another way to do this, aside from the object system? It's
> very
> much like creating a basic object.
>
> This is an extracted and simplified example...
> --------------------
>
> module Db = struct
>
>   (* Signature for a single 'table' in the database, elements of type t. *)
>   module type S = sig
>     (* Full implementation relies on a Key module for different key types
> than 'int' *)
>     type t
>     val get : int -> t
>     val set : int -> t -> unit
>     val del : int -> unit
>     val iter : (int -> t -> unit) -> unit
>     val fold : (int -> t -> 'a -> 'a) -> 'a -> 'a
>   end
>
>   (* Instantiates storage for a table, and returns FC module to interact
> with the store. *)
>   let create_with_default (type s) ?(size=19) default =
>     (* Full implementation is parameterized by Key and Table modules *)
>     let h = Hashtbl.create size in
>     let module H = struct
>       type t = s
>       let get (id:int) =
>         try Hashtbl.find h id
>         with Not_found -> default
>       let set id (v:t) = Hashtbl.replace h id v
>       let del id = Hashtbl.remove h id
>       let iter f = Hashtbl.iter f h
>       let fold f init = Hashtbl.fold f h init
>     end in
>     (module H : S with type t = s)
>
> end
>
> (* An example table... *)
> module Location = struct
>   let unknown = "Unknown"
>   include (val (Db.create_with_default unknown) : S with type t = string)
>   (* Location might have a bunch of other functionality as well... *)
> end
>
> (* and basic usage... *)
> # Location.get 1;;
> - : Location.t = "Unknown"
> # Location.set 1 "Mars";;
> - : unit = ()
> # Location.get 1;;
> - : Location.t = "Mars"
>
> --------------------
> Some background on what this is for: (skip unless you're interested!)
>
> I use a "component architecture" with most games -- basically a database of
> properties keyed off "game object IDs". I thought this was a very powerful
> feature in old MUDs/MUSHs. It's one of the first things I tried making
> when I
> started in OCaml, but I had some difficulties and ended up explicity
> instantiating hashtables or maps in the global context of various modules.
> Sloppy, but workable. (The reason I had difficulty is because I was trying
> to
> create a database of tables which were created at runtime -- not statically
> known.)
>
> Recently I decided to fix this mess. I had many modules, each which tended
> to
> have a corresponding "table". Eg. Characteristics, Size, Wounds, Faction,
> Inventory, etc. So ideally I wanted a convenient way to embed "database"
> functions into such modules while declaring the implementation of the
> underlying datastore (hashtable, map, whatever).
>
> This might seem a bit ugly from a functional-programming perspective, but
> I've
> found components to be quite powerful, and overall helps to constrain where
> and how mutation happens. "Game state" is generally in flux -- well, it is
> everything variable, and can be compared closely with save-game state.
> Most code
> which doesn't update game state can be functional. Actually, it feels
> creepy to
> have a variable assignment in the code, since mutation is generally to
> game-state
> and that's handled through a database. So the resulting style is
> functional+database.
>
>  -Tony
>
>