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Date: Sun, 4 Mar 2007 17:24:43 +0100
From: "Lukasz Stafiniak" <lukstafi@gmail.com>
To: "Zheng Li" <li@pps.jussieu.fr>
Subject: Re: [Caml-list] Re: records with polymorphic variants?
Cc: caml-list@inria.fr
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References: <45EA613F.3030104@generation.net> <87k5xxjdgv.fsf@gmail.com>
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X-Spam: no; 0.00; lukasz:01 variants:01 polymorphism:01 polymorphic:01 polymorphic:01 wrote:01 caml-list:01 writes:01 pps:01 jussieu:01 constraint:01 constraint:01 int:01 int:01 expression:02 

On 3/4/07, Zheng Li <li@pps.jussieu.fr> wrote:
>
> Hi,
>
> Eliot Handelman <eliot@generation.net> writes:
> > type f = [ `A of int ]
> >
> > type r = { x : f }
> >
> > but then I can't do this:
> >
> > { x = `B "test" }
> Here you want type f polymorphic, however as the type of a record field, its
> polymorphism has to be reflected (bound) as type parameter in the declaration
> of r.

Yes, but the type f itself is not polymorphic, it is not [> f].
>
> I guess you want the follows
>
> # type 'a r = { x : 'a } constraint 'a = [> f]
> type 'a r = { x : 'a; } constraint 'a = [> f ]
> # {x = "test"}
> Characters 6-12:
>   { x = "test"};;
>         ^^^^^^
> This expression has type string but is here used with type [> f ]
> # {x = `B "test"}
> - : [> `A of int | `B of string ] r = {x = `B "test"}
>
Nice! I didn't realize this is possible. Thanks.