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From: Jon Harrop <jon@ffconsultancy.com>
Organization: Flying Frog Consultancy Ltd.
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Simple(?) subtyping problem...
Date: Tue, 24 Oct 2006 22:53:20 +0100
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On Tuesday 24 October 2006 21:45, Till Varoquaux wrote:
> I'm currently trying to split functions matching against given variant
> type and I'm running across this pb:
>
> let a= function
>
>  | `A -> ()
>  | `B -> ()
>
> doesn't split into
>
> let c=function
>
>  | `B -> ()
>
> let b =function
>
>  | `A -> ()
>  |  x -> c x
>
> since it messes up the type rules. I really want to avoid having to
> write down precise type informations (The point here is to have an
> extensible system)...

How about:

# type c = [`B];;
type c = [ `B ]
# let c = function
    | #c -> ();;
val c : [< c ] -> unit = <fun>
# let b k = function
    | `A -> ()
    | #c as x -> c x
    | x -> k x;; 
val b : (([> `A | `B ] as 'a) -> unit) -> 'a -> unit = <fun>

> I am sure this question has been asked loads of times (but I couldn't
> find the right thread in the archives) and I apologize for asking it
> yet again.

I don't think it comes up very often.

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
Objective CAML for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scientists