From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on yquem.inria.fr X-Spam-Level: X-Spam-Status: No, score=0.0 required=5.0 tests=AWL autolearn=disabled version=3.1.3 X-Original-To: caml-list@yquem.inria.fr Delivered-To: caml-list@yquem.inria.fr Received: from mail3-relais-sop.national.inria.fr (mail3-relais-sop.national.inria.fr [192.134.164.104]) by yquem.inria.fr (Postfix) with ESMTP id BDC25BBAF for ; Mon, 26 May 2008 14:38:41 +0200 (CEST) X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AqgCAH9MOkjUnw7Vb2dsb2JhbACCM5AIAQwFAgQHEwOZMg X-IronPort-AV: E=Sophos;i="4.27,542,1204498800"; d="scan'208";a="13043488" Received: from ptb-relay02.plus.net ([212.159.14.213]) by mail3-smtp-sop.national.inria.fr with ESMTP; 26 May 2008 14:38:41 +0200 Received: from [80.229.56.224] (helo=beast.local) by ptb-relay02.plus.net with esmtp (Exim) id 1K0byK-0001UL-Dz for caml-list@yquem.inria.fr; Mon, 26 May 2008 13:38:40 +0100 From: Jon Harrop Organization: Flying Frog Consultancy Ltd. To: caml-list@yquem.inria.fr Subject: Re: [Caml-list] Question about type unification Date: Mon, 26 May 2008 13:33:30 +0100 User-Agent: KMail/1.9.9 References: <20080526122204.GA6727@annexia.org> In-Reply-To: <20080526122204.GA6727@annexia.org> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200805261333.30858.jon@ffconsultancy.com> X-Plusnet-Relay: bfac17180f1dfd5e5c7acc0c21fb85ea X-Spam: no; 0.00; unification:01 unification:01 unify:01 3.0:98 frog:98 wrote:01 opaque:01 caml-list:01 string:02 string:02 module:03 float:03 float:03 unit:03 types:05 On Monday 26 May 2008 13:22:04 Richard Jones wrote: > With this definition: > > type 'a t = float > > Why is this allowed? > > # ((3.0 : unit t) : string t) ;; > - : string t = 3. > > Note that unification is prevented if t is opaque, eg, hidden behind a > module signature. Because your accessible definition allows you to replace 'a t with float for any 'a, which allows you to replace both of your types with float and, therefore, they must unify. -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/products/?e