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: How to mapcar or across a list? Date: Thu, 16 Jul 2015 00:28:56 +0200 Organization: Informatimago Message-ID: <87oajdkqc7.fsf@kuiper.lan.informatimago.com> References: <87io9lmb4z.fsf@mbork.pl> 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 1436999424 32525 80.91.229.3 (15 Jul 2015 22:30:24 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 15 Jul 2015 22:30:24 +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 Jul 16 00:30:17 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 1ZFVBx-0003Ny-IR for geh-help-gnu-emacs@m.gmane.org; Thu, 16 Jul 2015 00:30:17 +0200 Original-Received: from localhost ([::1]:37699 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZFVBw-0001Mi-P5 for geh-help-gnu-emacs@m.gmane.org; Wed, 15 Jul 2015 18:30:16 -0400 Original-Path: usenet.stanford.edu!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 53 Original-X-Trace: individual.net 7t8im5oM7r+91cQZ4pSYaAVsKMxT5Hltwl6WHCL6U8rijB1cvJ Cancel-Lock: sha1:MTIxNjYyMjJmZjFlNmE4ZTI1NWRiZWVlNjRlZTQyNjRlYWVhN2Y5ZA== sha1:RZr/hx9Qcw+xCOkYgsowMeuS+7s= 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:213459 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:105745 Archived-At: Emanuel Berg writes: > Marcin Borkowski writes: > >> so here's my problem: I have a list of Boolean >> values, and I want to `mapcar' an `or' across it >> (IOW, I want to test whether at least one of them is >> true). Of course, (apply #'or my-list) does not >> work. Of course, I can (cl-reduce (lambda (x y) (or >> x y)) my-list) -- but is there a better method? > > "Better" I don't know, but how about using your > new-found skills with the backquote? (`or' itself > should be used, of course.) > > (setq boolean-list-1 '(t nil nil nil t nil nil)) > (setq boolean-list-2 '(nil nil nil nil nil nil nil)) > > (eval `(or ,@boolean-list-1)) ; t > (eval `(or ,@boolean-list-2)) ; nil Nope. It's always a bad idea to use eval, because eval evalutes in the nil environment, not in the local lexical envrionment. Your example works because those lists are literal, but then you don't need or to know the answer. In pratice, translating: (flet ((some-complex-predicate (x) …)) (let ((some-data (generate-some-data))) (some (function some-complex-predicate) some-data))) to: (flet ((some-complex-predicate (x) …)) (let ((some-data (generate-some-data))) (eval `(or ,@(mapcar (lambda (data) `(some-complex-predicate ',data)) some-data))))) fails lamentably. It is also very slow, since it makes you build a new list, and it makes you interpret, or worse, compile, some new code! -- __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