From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: "Chris K. Jester-Young" Newsgroups: gmane.lisp.guile.devel Subject: Re: when and unless Date: Wed, 7 Dec 2011 11:10:20 -0500 Message-ID: <20111207161020.GB15815@yarrow.destinee.acro.gen.nz> References: <878vsjd2fh.fsf@pobox.com> <87r50ircng.fsf@pobox.com> <4EDDC8B1.3000509@gentoo.org> <87mxb6kkzx.fsf@fencepost.gnu.org> NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: dough.gmane.org 1323274251 9067 80.91.229.12 (7 Dec 2011 16:10:51 GMT) X-Complaints-To: usenet@dough.gmane.org NNTP-Posting-Date: Wed, 7 Dec 2011 16:10:51 +0000 (UTC) To: guile-devel@gnu.org Original-X-From: guile-devel-bounces+guile-devel=m.gmane.org@gnu.org Wed Dec 07 17:10:40 2011 Return-path: Envelope-to: guile-devel@m.gmane.org Original-Received: from lists.gnu.org ([140.186.70.17]) by lo.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1RYK4o-00050r-Hd for guile-devel@m.gmane.org; Wed, 07 Dec 2011 17:10:35 +0100 Original-Received: from localhost ([::1]:44254 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RYK4n-0001CA-Oz for guile-devel@m.gmane.org; Wed, 07 Dec 2011 11:10:33 -0500 Original-Received: from eggs.gnu.org ([140.186.70.92]:38564) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RYK4k-000194-Uw for guile-devel@gnu.org; Wed, 07 Dec 2011 11:10:31 -0500 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1RYK4h-0003j8-6M for guile-devel@gnu.org; Wed, 07 Dec 2011 11:10:30 -0500 Original-Received: from mail-gy0-f169.google.com ([209.85.160.169]:33444) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RYK4g-0003iy-Rj for guile-devel@gnu.org; Wed, 07 Dec 2011 11:10:27 -0500 Original-Received: by ghbg19 with SMTP id g19so759154ghb.0 for ; Wed, 07 Dec 2011 08:10:26 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=date:from:to:subject:message-id:mail-followup-to:references :mime-version:content-type:content-disposition:in-reply-to :user-agent; bh=B9bnkSqEGfv1TyfWtLB741xAtqi2stiAM2tLkzbVG3g=; b=OP7uz9IbdSjU/Uv28FmIDFJ+/tqIGXpd1zAyOnKwWLM8+wGSqMpgF+v9ibt2+fYsNC 9inWAr78/rVnMv3/cuZeNQQ8PJ+YkXrImVBuSZdMXFIEBezh4KYdYFpquj3luO4H1NqP BRZb8tq0Jopf7Gy4ZjY8sznteW9ftrHkaHaTE= Original-Received: by 10.236.78.72 with SMTP id f48mr28276861yhe.121.1323274226166; Wed, 07 Dec 2011 08:10:26 -0800 (PST) Original-Received: from yarrow.destinee.acro.gen.nz (me42c36d0.tmodns.net. [208.54.44.228]) by mx.google.com with ESMTPS id f17sm6067808ang.20.2011.12.07.08.10.24 (version=SSLv3 cipher=OTHER); Wed, 07 Dec 2011 08:10:25 -0800 (PST) Mail-Followup-To: guile-devel@gnu.org Content-Disposition: inline In-Reply-To: <87mxb6kkzx.fsf@fencepost.gnu.org> User-Agent: Mutt/1.5.20 (2009-06-14) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.6 (newer, 2) X-Received-From: 209.85.160.169 X-BeenThere: guile-devel@gnu.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: "Developers list for Guile, the GNU extensibility library" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: guile-devel-bounces+guile-devel=m.gmane.org@gnu.org Original-Sender: guile-devel-bounces+guile-devel=m.gmane.org@gnu.org Xref: news.gmane.org gmane.lisp.guile.devel:13010 Archived-At: On Tue, Dec 06, 2011 at 12:17:06PM +0100, David Kastrup wrote: > I've actually wondered if it would not make sense to return > *unspecified* in the case of the plain else-less if even if the > condition is true, namely when you write (if #t #t). This cannot be done without breaking the tail position guarantee that "if" has. i.e., Scheme specifies that for expressions of the form (if TEST THEN ELSE) (if TEST THEN) that THEN and ELSE are both in tail position. Tail position means that the evaluation of THEN or ELSE returns to the caller directly, and there is no chance for the system to intervene, such as by replacing the return value with unspecified. Cheers, Chris.