Re: [Caml-list] Create a constraint between variant type and data list

[Martin Jambon <martin.jambon@ens-lyon.org>]

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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