From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: Stefan Monnier Newsgroups: gmane.emacs.help Subject: Re: Replacement for Common Lisp's GENSYM in Emacs Lisp Date: Sun, 07 Mar 2010 14:06:14 -0500 Message-ID: References: <87tysssj78.fsf@rapttech.com.au> NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: dough.gmane.org 1272990364 5922 80.91.229.12 (4 May 2010 16:26:04 GMT) X-Complaints-To: usenet@dough.gmane.org NNTP-Posting-Date: Tue, 4 May 2010 16:26:04 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Tue May 04 18:26:00 2010 connect(): No such file or directory 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 4.69) (envelope-from ) id 1O9Kwa-0007KK-BU for geh-help-gnu-emacs@m.gmane.org; Tue, 04 May 2010 18:26:00 +0200 Original-Received: from localhost ([127.0.0.1]:37614 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1O9KwZ-0003Zw-IZ for geh-help-gnu-emacs@m.gmane.org; Tue, 04 May 2010 12:25:59 -0400 Original-Path: usenet.stanford.edu!news.glorb.com!news2.glorb.com!npeer01.iad.highwinds-media.com!news.highwinds-media.com!feed-me.highwinds-media.com!post02.iad.highwinds-media.com!newsfe05.iad.POSTED!7564ea0f!not-for-mail Original-Newsgroups: gnu.emacs.help User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1.93 (gnu/linux) Cancel-Lock: sha1:0HWu+YfwChWM5r2IhCxjrVuX16c= Original-Lines: 26 Original-X-Complaints-To: abuse@UsenetServer.com Original-NNTP-Posting-Date: Sun, 07 Mar 2010 19:06:14 UTC Original-Xref: usenet.stanford.edu gnu.emacs.help:177391 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:72926 Archived-At: > This is actually a non-trivial problem. Best approach would probably be > to do something similar to how they generate universally unique IDs. The best approach often is to not use "unique symbols" at all. A common technique is to rely on lexical scoping to precisely control which variables are visible to each part of a macro's argument during its evaluation. E.g.: (defmacro dotimes (xl e) (let ((x (car xl)) (l (cadr xl))) `(let ((body (lambda (,x) ,e)) (list ,l)) (while (consp list) (funcall body (car list)) (setq list (cdr list)))))) Notice how the expressions `l' and `e' are evaluated in a context where neither of `body', nor `list' exist. No need for gensym. Of course Elisp's lack of good support for lexical scoping makes such a technique impractical. But `make-symbol' can be used instead of `gensym' and works very well for that purpose. Stefan