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From: Goswin von Brederlow <goswin-v-b@web.de>
To: Dan Bensen <danbensen@att.net>
Cc: caml-list@inria.fr
References: <1336383633.2612.YahooMailRC@web180006.mail.gq1.yahoo.com>
Date: Mon, 07 May 2012 18:47:42 +0200
In-Reply-To: <1336383633.2612.YahooMailRC@web180006.mail.gq1.yahoo.com> (Dan
	Bensen's message of "Mon, 7 May 2012 02:40:33 -0700 (PDT)")
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Subject: Re: [Caml-list] extending user-defined polymorphic variant types

Dan Bensen <danbensen@att.net> writes:

> I'm trying to write a functor that extends a user-supplied polymorphic
> variant type (PVT).  How do you declare the user's type in the signature
> for the argument to the functor without locking in the individual variant
> definitions? 
> The code below (in revised syntax) generates an error message that says
> the type isn't a PVT.
>
> code:
>
>> module type Reader = sig type ast; end;
>>
>> module Make (Read: Reader) = struct
>>   type ast = [= Read.ast | `Lid of string];
>> end;
>
> message:
>
>> Error: The type Read.ast is not a polymorphic variant type
>
> How do you make ast a PVT while allowing the user to
> specify the variants?

Even if that did work how usefull would that be? You couldn't write

let foo = function
  | `Lid s -> ()
  | x -> Read.foo x

unless Read.foo allows [> Read.ast]. In which case you don't need a
functor but can just implement foo straight up.

What is your use case? What do you want to do?

MfG
        Goswin