From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: pjb@informatimago.com (Pascal J. Bourguignon) Newsgroups: gmane.emacs.help Subject: Re: Is it safe to modify a property list directly with PLIST-PUT? Date: Mon, 27 Jul 2009 10:31:55 +0200 Organization: Informatimago Message-ID: <87k51uh6h0.fsf@galatea.local> References: <87ab2rs8kv.fsf@iki.fi> <87zlargqd3.fsf@galatea.local> <874osy7jk9.fsf@iki.fi> NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: ger.gmane.org 1248684210 1526 80.91.229.12 (27 Jul 2009 08:43:30 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 27 Jul 2009 08:43:30 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Mon Jul 27 10:43:23 2009 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane.org Original-Received: from lists.gnu.org ([199.232.76.165]) by lo.gmane.org with esmtp (Exim 4.50) id 1MVLnl-000349-3e for geh-help-gnu-emacs@m.gmane.org; Mon, 27 Jul 2009 10:43:21 +0200 Original-Received: from localhost ([127.0.0.1]:42217 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MVLnk-0000Rh-BB for geh-help-gnu-emacs@m.gmane.org; Mon, 27 Jul 2009 04:43:20 -0400 Original-Path: news.stanford.edu!newsfeed.stanford.edu!news.tele.dk!news.tele.dk!small.news.tele.dk!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 55 Original-X-Trace: individual.net 57Vk/vKyxJ6f2npchRj0dw7m3bGYgOeJAH8rOv1H55hodizQ6O Cancel-Lock: sha1:Yjc0YTYxYWEyZGUyZjczZjI2NTRjZGNiOWU5M2FiYWFmYmQ1N2JhNg== sha1:86GPIVPChiuupHjuHsHjeR6PQ6A= Face: iVBORw0KGgoAAAANSUhEUgAAADAAAAAwAQMAAABtzGvEAAAABlBMVEUAAAD///+l2Z/dAAAA oElEQVR4nK3OsRHCMAwF0O8YQufUNIQRGIAja9CxSA55AxZgFO4coMgYrEDDQZWPIlNAjwq9 033pbOBPtbXuB6PKNBn5gZkhGa86Z4x2wE67O+06WxGD/HCOGR0deY3f9Ijwwt7rNGNf6Oac l/GuZTF1wFGKiYYHKSFAkjIo1b6sCYS1sVmFhhhahKQssRjRT90ITWUk6vvK3RsPGs+M1RuR mV+hO/VvFAAAAABJRU5ErkJggg== X-Accept-Language: fr, es, en X-Disabled: X-No-Archive: no Importance: high User-Agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/22.3 (darwin) Original-Xref: news.stanford.edu gnu.emacs.help:171236 X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Original-Sender: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Xref: news.gmane.org gmane.emacs.help:66422 Archived-At: Teemu Likonen writes: > On 2009-07-26 22:07 (+0200), Pascal J. Bourguignon wrote: > >> Teemu Likonen writes: >>> (setq my-data '((:foo "one" :bar "two") >>> (:foo "three" :bar "four"))) >> >> It is never safe to modify literal data! > > Oh yes, that was a stupid example. MY-DATA is not really created that > way. We can assume this: > >> (setq my-data (list (list :foo "one" :bar "two") >> (list :foo "three" :bar "four"))) > >> Notice also that like delete, plist-put returns the result, it cannot >> always modify the property list in place. So you have to restore the >> result: >> >> (setf (nth 1 my-data) (plist-put (nth 1 my-data) :bar "New value")) > > Thanks. Now, let's go one step further while still assuming that we are > not using Emacs CL extension (perhaps just for my education). Is this > reliable: > > (let ((item (nth 1 my-data))) > (setq item (plist-put item :bar "New value"))) > > At least it seems to be working: the change appears in MY-DATA too: > > ((:foo "one" :bar "two") > (:foo "three" :bar "New value")) No, it wont' always work. The problem occurs when the plist is nil, since nil is a constant symbol that is immutable. (let ((p '())) ; the nil symbol (plist-put p :bar 2) p) --> nil It seems that in the case of a non-null plist, emacs lisp adds the missing key to the tail of the plist (which doesn't incurs any additionnal cost, since the plist is already traversed for searching the key). It only means that plist-put is a "destructive" function when the plist is not null. So if you don't want to store back the result of plist-put, you just have to ensure that the plist is not empty. You may initialize them with an unused key/value pair. -- __Pascal Bourguignon__