2010/9/3 Sylvain Le Gall <sylvain@le-gall.net>

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.

As said by other guys, you can't enforce statically such an invariant without a code generator.
In pure ocaml, the below solution use a "social check" : it works only if the developer who adds a new constructor is not too bad and takes care of context when solving an error ;-).

type license = GPL | LGPL

let data =
[ GPL, "GNU Public license";
   LGPL, "GNU Lesser General Public license" ]

let () = match GPL with
| GPL | LGPL ->
     assert
       (let s = "Do not forget to add new constructors to the data list above." in
        Format.ifprintf Format.std_formatter "%s" s;
        true)

Hope this helps,
Julien