From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: Kevin Rodgers Newsgroups: gmane.emacs.devel Subject: Re: Documentation for car and cdr Date: Thu, 26 Jan 2006 15:17:25 -0700 Message-ID: References: <10943.1138305306@brains.moreideas.ca> NNTP-Posting-Host: main.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Trace: sea.gmane.org 1138314074 14999 80.91.229.2 (26 Jan 2006 22:21:14 GMT) X-Complaints-To: usenet@sea.gmane.org NNTP-Posting-Date: Thu, 26 Jan 2006 22:21:14 +0000 (UTC) Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Thu Jan 26 23:21:13 2006 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([199.232.76.165]) by ciao.gmane.org with esmtp (Exim 4.43) id 1F2FTw-0007Dn-Kb for ged-emacs-devel@m.gmane.org; Thu, 26 Jan 2006 23:20:44 +0100 Original-Received: from localhost ([127.0.0.1] helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1F2FWl-0001Pn-0w for ged-emacs-devel@m.gmane.org; Thu, 26 Jan 2006 17:23:39 -0500 Original-Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1F2FUy-0000dz-1J for emacs-devel@gnu.org; Thu, 26 Jan 2006 17:21:48 -0500 Original-Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1F2FUw-0000dR-Rg for emacs-devel@gnu.org; Thu, 26 Jan 2006 17:21:47 -0500 Original-Received: from [199.232.76.173] (helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1F2FUw-0000dG-Gi for emacs-devel@gnu.org; Thu, 26 Jan 2006 17:21:46 -0500 Original-Received: from [80.91.229.2] (helo=ciao.gmane.org) by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA:32) (Exim 4.52) id 1F2FSZ-0004Gu-NW for emacs-devel@gnu.org; Thu, 26 Jan 2006 17:19:20 -0500 Original-Received: from list by ciao.gmane.org with local (Exim 4.43) id 1F2FRR-0006Rn-6L for emacs-devel@gnu.org; Thu, 26 Jan 2006 23:18:09 +0100 Original-Received: from 207.167.42.60 ([207.167.42.60]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 26 Jan 2006 23:18:09 +0100 Original-Received: from ihs_4664 by 207.167.42.60 with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 26 Jan 2006 23:18:09 +0100 X-Injected-Via-Gmane: http://gmane.org/ Original-To: emacs-devel@gnu.org Original-Lines: 34 Original-X-Complaints-To: usenet@sea.gmane.org X-Gmane-NNTP-Posting-Host: 207.167.42.60 User-Agent: Mozilla Thunderbird 0.9 (X11/20041105) X-Accept-Language: en-us, en In-Reply-To: <10943.1138305306@brains.moreideas.ca> X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Original-Sender: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Xref: news.gmane.org gmane.emacs.devel:49600 Archived-At: Peter Whaite wrote: > Luc Teirlinck wrote: >>The only list that is not a cons cell is nil. > > Actually I seem to remember learning that nil _was_ a cons cell where > both the car and cdr pointed to itself. That makes it unnecessary to > treat nil as a special case. > > But that was a long time ago. I learned Lisp 20+ years ago, and nil has always been an atom (in fact, a symbol whose value is itself, just like t and now keywords). A cons cell like you describe would be a circular structure: +----+ +----+ | | | | | v v | | +---+---+ | | | | +--+ | +-+-+---+ | | +----+ (let ((print-circle t) (cons-cell (cons nil nil))) (setcar cons-cell cons-cell) (setcdr cons-cell cons-cell) cons-cell) ==> (#0 . #0) If the symbol nil had such a value, (eq nil 'nil) would not be true. -- Kevin Rodgers