[Caml-list] How OCaml objects of sum types can be passed to a C/C++ functions?

[Caml-list] How OCaml objects of sum types can be passed to a C/C++ functions?

[Claudio Trento]

on the subject of interfacing OCaml with C/C++, it is not clear
to me whether and how OCaml objects of sum types can be passed
to a C/C++ function.  For example, suppose I have the declaration

type expr =
     Variable of int
   | Coefficient of int
   | UPlus of expr
   | UMinus of expr
   | Sum of (expr * expr)
   | Difference of (expr * expr)
   | Product of (expr * expr)

What is the best way to pass an expr object to C/C++?
How can C/C++ code visit such a structure?
Many thanks for your time and patience,

     Claudio



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Re: [Caml-list] How OCaml objects of sum types can be passed to a C/C++ functions?

[Richard Jones]

On Wed, Jul 07, 2004 at 10:56:04AM +0200, Claudio Trento wrote:
> 
> on the subject of interfacing OCaml with C/C++, it is not clear
> to me whether and how OCaml objects of sum types can be passed
> to a C/C++ function.  For example, suppose I have the declaration
> 
> type expr =
>      Variable of int
>    | Coefficient of int
>    | UPlus of expr
>    | UMinus of expr
>    | Sum of (expr * expr)
>    | Difference of (expr * expr)
>    | Product of (expr * expr)
> 
> What is the best way to pass an expr object to C/C++?
> How can C/C++ code visit such a structure?
> Many thanks for your time and patience,

This section in the manual:

http://caml.inria.fr/ocaml/htmlman/manual032.html#htoc217

contains a rather unclear explanation.

Rich.

-- 
Richard Jones. http://www.annexia.org/ http://www.j-london.com/
Merjis Ltd. http://www.merjis.com/ - improving website return on investment
'There is a joke about American engineers and French engineers. The
American team brings a prototype to the French team. The French team's
response is: "Well, it works fine in practice; but how will it hold up
in theory?"'

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Re: [Caml-list] How OCaml objects of sum types can be passed to a C/C++ functions?

[Basile Starynkevitch [local]]

On Wed, Jul 07, 2004 at 10:56:04AM +0200, Claudio Trento wrote:
> 
> on the subject of interfacing OCaml with C/C++, it is not clear
> to me whether and how OCaml objects of sum types can be passed
> to a C/C++ function.  For example, suppose I have the declaration
> 
> type expr =

(* I Basile added the 2 cases below for explanations *)
       Nonsense
     | Bizarre

>    | Variable of int
>    | Coefficient of int
>    | UPlus of expr
>    | UMinus of expr
>    | Sum of (expr * expr)
>    | Difference of (expr * expr)
>    | Product of (expr * expr)
> 
> What is the best way to pass an expr object to C/C++?

It is documented in the reference manual:
http://caml.inria.fr/ocaml/htmlman/manual032.html sections 18.2.2 and
18.3.4

The two cases Nonsense and Bizarre I added (for the example) have no
argument (i.e. no "of" keyword - for representation issues, Foo of unit
is not like just Foo). So they are represented by small non-negative
integer, so Nonsense is 0 and Bizarre is 1.

The other cases have an "of" keyword, so are represented as tagged
blocks. The first case Variable has tag 0, the next Coefficient has
tag 1, etc

> How can C/C++ code visit such a structure?

Test if a value is a tagged integer, and otherwise it is a block, test
its tag:

in Ocaml: 
   external c_fun : expr -> unit = "c_fun_ml"

in C // untested code

value c_fun_ml(value ex) {
   CAMLparam1(ex);
   switch (ex) {
   case Val_int(0): /* Nonsense */
     break;
   case Val_int(1): /* Bizarre */
     break;
   default: {
     int tag = Tag_val(ex);
     switch (tag) {
       case 0: /* Variable varnum */
         { int varnum = Int_val(Field(ex,0); 
           break; }
       case 1: /* Coefficient */ 
       default: 
           break;
      } /* end switch tag */
     break;
   };
   } /* end switch ex */
   CAMLreturn(Val_unit);
}


I hope you got the idea. I advise you to avoid coding in C, and to
call C functions only inside your Ocaml wrappers (which do appropriate
checks, raise exceptions, etc...)

Regards.

-- 
Basile STARYNKEVITCH -- basile dot starynkevitch at inria dot fr
Project cristal.inria.fr - phone +33 1 3963 5197 - mobile 6 8501 2359
http://cristal.inria.fr/~starynke --- all opinions are only mine 

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