From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: Michael Heerdegen Newsgroups: gmane.emacs.help Subject: Re: How does letf work? Date: Thu, 30 Jan 2014 00:53:43 +0100 Message-ID: <87mwie2vlk.fsf@web.de> References: <52E838C8.5020101@miszellen.de> NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain X-Trace: ger.gmane.org 1391039665 12355 80.91.229.3 (29 Jan 2014 23:54:25 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 29 Jan 2014 23:54:25 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Thu Jan 30 00:54:32 2014 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W8exk-0004xG-5g for geh-help-gnu-emacs@m.gmane.org; Thu, 30 Jan 2014 00:54:32 +0100 Original-Received: from localhost ([::1]:45679 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1W8exj-0002ZR-Nk for geh-help-gnu-emacs@m.gmane.org; Wed, 29 Jan 2014 18:54:31 -0500 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:45844) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1W8exU-0002ZF-1L for help-gnu-emacs@gnu.org; Wed, 29 Jan 2014 18:54:23 -0500 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1W8exI-0003Ap-J2 for help-gnu-emacs@gnu.org; Wed, 29 Jan 2014 18:54:15 -0500 Original-Received: from plane.gmane.org ([80.91.229.3]:50080) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1W8exI-0003AX-Bm for help-gnu-emacs@gnu.org; Wed, 29 Jan 2014 18:54:04 -0500 Original-Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1W8exC-0004ep-Tr for help-gnu-emacs@gnu.org; Thu, 30 Jan 2014 00:53:58 +0100 Original-Received: from ip-90-186-253-188.web.vodafone.de ([90.186.253.188]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 30 Jan 2014 00:53:58 +0100 Original-Received: from michael_heerdegen by ip-90-186-253-188.web.vodafone.de with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 30 Jan 2014 00:53:58 +0100 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 19 Original-X-Complaints-To: usenet@ger.gmane.org X-Gmane-NNTP-Posting-Host: ip-90-186-253-188.web.vodafone.de User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux) Cancel-Lock: sha1:DHh7fRy3fOhtApnPVDwKXM0pblI= X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 80.91.229.3 X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Original-Sender: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Xref: news.gmane.org gmane.emacs.help:95772 Archived-At: Joost Kremers writes: > (letf (((cdr test-x) '(a b c d))) > (cdr test-x)) > > = > (KEY a b c d) > > Taking what you say at face value, one might expect that at the moment > the value of (cdr test-x) is printed, it's already been changed back to > the original value, so it should say (KEY 1 2 3 4), but it doesn't. (cdr test-x) here references an object being a list equal to '(a b c d). But after the letf has been left, that object is not anymore the cdr of test-x, because its cdr has been restored. But that doesn't make the return value "switch" to a different object. The analogy with boxes is very helpful here. Michael.