On Fri, Sep 3, 2010 at 2:51 PM, Martin Jambon
<martin.jambon@ens-lyon.org> wrote:
Sylvain Le Gall wrote:
> Hello all,
>
> I would like to somehow enforce that a variant type is associated with
> an entry in a data list.
>
> For example,
>
> I would like to define:
>
> type license = GPL | LGPL
>
> and
>
> let data = [ GPL, "GNU Public license";
> LGPL, "GNU Lesser General Public license" ]
>
>
> I would like to enforce that all variants of license are in the
> association list.
>
> I have tried to use polymorphic variants, but don't see how to enforce
> this constraint.
>
> The point, is that if I add a new variant to license (e.g. BSD3), the
> compiler output an error because this new variant is not in data list.
>
> Any ideas ? If you need to use another type expression rather than
> variant, please do so, as long as I am able to link the license type
> and data list.
I don't see a solution other than meta-programming or runtime checks.
Here is a simple code generator that would do the job:
(* license_gen.ml *)
open Printf
let print_licenses l =
printf "type license =";
List.iter (fun (k, v) -> printf " | %s" k) l;
printf "\n";
printf "let licences = [\n";
List.iter (fun (k, v) -> printf " %s, %S;\n" k v) l;
printf "]\n"
let () =
print_licenses [
"GPL", "GNU Public license";
"LGPL", "GNU Lesser General Public license";
]
(* end *)
$ ocaml license_gen.ml > license.ml
Martin
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