From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: "Pascal J. Bourguignon" Newsgroups: gmane.emacs.help Subject: Re: predicates Date: Fri, 27 Nov 2015 05:28:54 +0100 Organization: Informatimago Message-ID: <8737vst69l.fsf@kuiper.lan.informatimago.com> References: <87r3js7e8l.fsf@linux-qg7d.fritz.box> <874mgnystc.fsf@point.pointsman.de> <87d1v67nwo.fsf@linux-qg7d.fritz.box> <87two8bqe2.fsf@linaro.org> NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Trace: ger.gmane.org 1448598626 4742 80.91.229.3 (27 Nov 2015 04:30:26 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 27 Nov 2015 04:30:26 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Fri Nov 27 05:30:20 2015 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 1a2Afq-0002uw-QU for geh-help-gnu-emacs@m.gmane.org; Fri, 27 Nov 2015 05:30:18 +0100 Original-Received: from localhost ([::1]:54594 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1a2Aft-0001bS-5V for geh-help-gnu-emacs@m.gmane.org; Thu, 26 Nov 2015 23:30:21 -0500 Original-Path: usenet.stanford.edu!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 34 Original-X-Trace: individual.net ZxenlIQUivs00jnkKwEucgRRh4JnXGPHesYrSBu0OqCQLx2w1T Cancel-Lock: sha1:Njc3NTE1M2QxZmFlZjEyMDZhNGY1OTJmMTQ5NzU1OTQ4MGJlYjE4OA== sha1:bcJ/EALmNXpGGin3lBh25+Myi5U= Face: iVBORw0KGgoAAAANSUhEUgAAADAAAAAwAQMAAABtzGvEAAAABlBMVEUAAAD///+l2Z/dAAAA oElEQVR4nK3OsRHCMAwF0O8YQufUNIQRGIAja9CxSA55AxZgFO4coMgYrEDDQZWPIlNAjwq9 033pbOBPtbXuB6PKNBn5gZkhGa86Z4x2wE67O+06WxGD/HCOGR0deY3f9Ijwwt7rNGNf6Oac l/GuZTF1wFGKiYYHKSFAkjIo1b6sCYS1sVmFhhhahKQssRjRT90ITWUk6vvK3RsPGs+M1RuR mV+hO/VvFAAAAABJRU5ErkJggg== X-Accept-Language: fr, es, en User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) Original-Xref: usenet.stanford.edu gnu.emacs.help:215943 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:108233 Archived-At: Emanuel Berg writes: > Alex Bennée writes: > >> It's a predicate, giving a true/false response. > > This is how I understand it: > > In practice predicates can return anything. > > nil, and the empty list, should be interpreted as *false*. They return a generalized boolean, which can indeed be anything. > Computers are deterministic but that doesn't mean they > are consistent. One notable predicate that returns something useful not obtainable otherwise, is: (digit-char-p ?4) --> 4 You could use: (ignore-errors (parse-integer (string ?4))) --> 4 but parse-integer uses digit-char-p which is the more fundamental operation. -- __Pascal Bourguignon__ http://www.informatimago.com/ “The factory of the future will have only two employees, a man and a dog. The man will be there to feed the dog. The dog will be there to keep the man from touching the equipment.” -- Carl Bass CEO Autodesk