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Date: Fri, 3 Sep 2010 23:13:18 +0200
From: Maxence Guesdon <maxence.guesdon@inria.fr>
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Create a constraint between variant type and data
 list
Message-ID: <20100903231318.7b92af7c@haddock.home>
In-Reply-To: <slrni82bc0.skq.sylvain@gallu.homelinux.org>
References: <slrni82bc0.skq.sylvain@gallu.homelinux.org>
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X-Spam: no; 0.00; maxence:01 guesdon:01 maxence:01 guesdon:01 le-gall:01 lgpl:01 lgpl:01 variants:01 variants:01 compiler:01 compiler:01 polymorphic:01 gpl:01 gpl:01 caml-list:01 

Le Fri, 3 Sep 2010 17:16:48 +0000 (UTC),
Sylvain Le Gall <sylvain@le-gall.net> a =C3=A9crit :

> Hello all,
>=20
> I would like to somehow enforce that a variant type is associated with
> an entry in a data list.=20
>=20
> For example,=20
>=20
> I would like to define:
>=20
> type license =3D GPL | LGPL=20
>=20
> and=20
>=20
> let data =3D [ GPL, "GNU Public license";=20
>              LGPL, "GNU Lesser General Public license" ]
>=20
>=20
> I would like to enforce that all variants of license are in the
> association list.
>=20
> I have tried to use polymorphic variants, but don't see how to enforce
> this constraint.
>=20
> 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.
>=20
> 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.

A solution is to add your new license to your list of associations, then
the compiler will complain about the unknown variant :)

Regards,

--=20
Maxence Guesdon