From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <colanderman@gmail.com>
X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on yquem.inria.fr
X-Spam-Level: *
X-Spam-Status: No, score=1.9 required=5.0 tests=DNS_FROM_RFC_ABUSE,
	SPF_NEUTRAL autolearn=disabled version=3.1.3
X-Original-To: caml-list@yquem.inria.fr
Delivered-To: caml-list@yquem.inria.fr
Received: from nez-perce.inria.fr (nez-perce.inria.fr [192.93.2.78])
	by yquem.inria.fr (Postfix) with ESMTP id EBEE1BB83
	for <caml-list@yquem.inria.fr>; Fri,  1 Sep 2006 20:33:54 +0200 (CEST)
Received: from nf-out-0910.google.com (nf-out-0910.google.com [64.233.182.189])
	by nez-perce.inria.fr (8.13.6/8.13.6) with ESMTP id k81IXsSL027835
	for <caml-list@yquem.inria.fr>; Fri, 1 Sep 2006 20:33:54 +0200
Received: by nf-out-0910.google.com with SMTP id l37so757253nfc
        for <caml-list@yquem.inria.fr>; Fri, 01 Sep 2006 11:33:54 -0700 (PDT)
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
        s=beta; d=gmail.com;
        h=received:message-id:date:from:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references;
        b=rnxlcht865/ucwB9ouYdSs4iRiyR7jMaqGRH/LvTpx+qCrj+xD6XaAnvj7o5KRGjLSUn0BddDRLp3EC6/F0489vvH865sEmEqTvERLps/R4oXSOs84aWBji9Bh7LVudnCWZLnFdKVSijCzckWq6sogq306sn3HZHeDiNdi0FTe8=
Received: by 10.48.48.15 with SMTP id v15mr3401101nfv;
        Fri, 01 Sep 2006 11:33:54 -0700 (PDT)
Received: by 10.49.49.16 with HTTP; Fri, 1 Sep 2006 11:33:53 -0700 (PDT)
Message-ID: <875c7e070609011133udd506c7v223d8ece952b95ae@mail.gmail.com>
Date: Fri, 1 Sep 2006 14:33:53 -0400
From: "Chris King" <colanderman@gmail.com>
To: "David Allsopp" <dra-news@metastack.com>
Subject: Re: Polymorphic variants question
Cc: "OCaml List" <caml-list@yquem.inria.fr>
In-Reply-To: <012901c6cdec$64edf490$6a7ba8c0@treble>
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
References: <012901c6cdec$64edf490$6a7ba8c0@treble>
X-j-chkmail-Score: MSGID : 44F87D12.001 on nez-perce : j-chkmail score : X : 0/20 1
X-Miltered: at nez-perce with ID 44F87D12.001 by Joe's j-chkmail (http://j-chkmail.ensmp.fr)!
X-Spam: no; 0.00; variants:01 bool:01 compiler:01 val:01 bool:01 wrote:01 polymorphic:01 coercion:01 clarify:02 let:03 let:03 perhaps:04 somewhat:05 chris:05 chris:05 

On 9/1/06, David Allsopp <dra-news@metastack.com> wrote:
> Now, if I try to constrain it to what I'm after with
> 
> let (f : [`A | `C] -> bool * [`A | `B | `C]) = fun x -> ...
> 
> then I get a type error unless I change
> 	(false, x)
> to
> 	(false, id x)
> with 
> 	let id = function `A -> `A | `C -> `C
> 
> Is there a better way of writing this?

Yes, you can use the coercion operator to tell the compiler to expand the type of x to include `B:

# let f (x: [`A | `C]) = if x = `A then (true, `B) else (false, (x :> [`A | `B | `C]))
val f : [ `A | `C ] -> bool * [ `A | `B | `C ] = <fun>

Perhaps someone else can clarify how this accomplishes the same thing as your id function, as my understanding of type theory is somewhat rudimentary.

- Chris King