Re: [Caml-list] Typing Dynamic Typing in ocaml?

[Mauricio Fernandez <mfp@acm.org>]

On Wed, Dec 17, 2008 at 02:45:01PM -0500, Jacques Carette wrote:
> I have two (related) questions:
> 1) Has anyone transcribed the TypeRep library into ocaml?
> http://people.cs.uu.nl/arthurb/dynamic.html
>
> 2) How do I embed 'dynamically known' data into a single ocaml  
> data-structure?
>
> More specifically, I am experimenting with a (new) language which allows  
> deduction and computations to be performed with equal ease on its terms.  
> This language uses ocaml has the host meta-language (ie the interpreter 
> is written in ocaml).  I would like to be able to use arbitrary ocaml 
> data-structures to represent some of my terms, when these terms are known 
> to come from specific theories.  For example, I would like to use Bigint 
> to represent integers, but without exposing that per se.  Perhaps a 
> better way to phrase this would be to say that I want to have a "generic 
> external data container" type in my language terms, which I can 
> instantiate in multiple different ways (in the same program), with data 
> handled in different modules, without having to change the 'generic' 
> data-structure everytime I add a new module.

Does this signature correspond to what you need? (This is a "property list",
typically used to decorate an AST in multiple passes.)

type 'a t
type ('a, 'b) property
val create : unit -> 'a t
val new_property : unit -> ('a, 'b) property
val get : 'a t -> ('b, 'a) property -> 'b option
val set : 'a t -> ('b, 'a) property -> 'b -> unit

Your data structure holds a  'a t  value, and the different modules
declare their own properties (of different types, not reflected in the
property list).

Here's a fairly efficient implementation of mine:

type 'a t = (int, unit -> unit) Hashtbl.t
type ('a, 'b) property = { 
  set : 'b t -> 'a -> unit;
  get : 'b t -> 'a option;
}

let create () = Hashtbl.create 13

let new_id : unit -> int =
  let id = ref 0 in
    fun () -> incr id; !id

let new_property () =
  let id = new_id () in
  let v = ref None in

  let set t x =
    Hashtbl.replace t id (fun () -> v := Some x) in

  let get t =
    try
      (Hashtbl.find t id) ();
      match !v with
          Some x as s -> v := None; s
        | None -> None
    with Not_found -> None
  in { set = set; get = get }

let set t p x = p.set t x
let get t p = p.get t

-- 
Mauricio Fernandez  -   http://eigenclass.org