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Rephrasing of dynamic module selection problem
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Date: -- (:)
From: Martin Jambon <martin_jambon@e...>
Subject: Re: "ocaml_beginners"::[] Rephrasing of dynamic module selection problem
On Mon, 20 Feb 2006, Nathan Cooprider wrote:

> So I am still trying to get modules to be dynamically (run-time)
> selectable instead of only statically (compile-time). The closest I have
> come to so far is bellow. I want to be able to choose between a set of
> modules (hello1 and hello2 in this example) fairly transparently.
>
> [coop@ender example]$ cat hello1.ml
> type t = int
> let of_int i =
>  i
> let print i =
>  print_int i;
>  print_string " says Hello1\n"
>
> [coop@ender example]$ cat hello2.ml
> type t = float
> let of_int i =
>  float_of_int i
> let print i =
>  print_float i;
>  print_string " says Hello2\n"
>
> [coop@ender example]$ cat main.ml
> module Hello1 = struct
>  #include "hello1.ml"
> end ;;
> module Hello2 = struct
>  #include "hello2.ml"
> end ;;
> (* This works . . . *)
> module H = Hello1
> (* But I would like this to be something like this instead:
> let parameter = 1
> module H =
>  match parameter with
>    1 -> Hello1
>  | _ -> Hello2
> *)

That's not possible because Hello1 and Hello2 don't have the same "type" 
(same module signature though): what I mean is that if you represent them 
as objects, the objects would have different types since Hello1.t and 
Hello2.t are incompatible.

Try the following code (which fails when I try to mix the objects):

module type HELLO =
sig
   type t
   val of_int : int -> t
   val print : t -> unit
   val obj : < of_int : int -> t; print : t -> unit >
end

module Hello1 : HELLO =
struct
   type t = int
   let of_int i =
     i
   let print i =
     print_int i;
     print_string " says Hello1\n"

   let obj =
   object
     method of_int = of_int
     method print = print
   end
end

module Hello2 : HELLO =
struct
   type t = float
   let of_int i =
     float_of_int i
   let print i =
     print_float i;
     print_string " says Hello2\n"

   let obj =
   object
     method of_int = of_int
     method print = print
   end
end

let param = 2 (* defined at runtime *)
let obj =
   match param with
       1 -> Hello1.obj
     | 2 -> Hello2.obj
     | _ -> assert false;;

         Characters 63-73:
       | 2 -> Hello2.obj
              ^^^^^^^^^^
This expression has type
   < of_int : int -> Hello2.t; print : Hello2.t -> unit >
but is here used with type
   < of_int : int -> Hello1.t; print : Hello1.t -> unit >
Types for method of_int are incompatible


So if you want to do that, you must give the same type to your objects.
That would work if your modules Hello* all use a common type t instead of 
their own. In this case you just have to include the same code at the end 
of each Hello* module so that obj is defined. You can also add a line 
which places obj in a global table, e.g.
   let obj = ...
   let _ = Hashtbl.add Hello_modules.tbl 1 obj

And your main program would be:

   let param = ...
   let obj = Hashtbl.find Hello_modules.tbl param in
   ...


Martin

--
Martin Jambon, PhD
http://martin.jambon.free.fr

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