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: test for presence of library Date: Fri, 19 Feb 2010 02:09:32 +0100 Organization: Informatimago Message-ID: <87mxz6116b.fsf@galatea.lan.informatimago.com> References: NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: ger.gmane.org 1266543806 17978 80.91.229.12 (19 Feb 2010 01:43:26 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 19 Feb 2010 01:43:26 +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 Feb 19 02:43:23 2010 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 1NiHtq-0002pv-BM for geh-help-gnu-emacs@m.gmane.org; Fri, 19 Feb 2010 02:43:22 +0100 Original-Received: from localhost ([127.0.0.1]:40037 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1NiHtp-0002JB-Po for geh-help-gnu-emacs@m.gmane.org; Thu, 18 Feb 2010 20:43:21 -0500 Original-Path: news.stanford.edu!usenet.stanford.edu!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 43 Original-X-Trace: individual.net 0h62ox6xj4HXzjch0N3ltAL3MnJjxboflPQvi3a/l29h+YKu4M Cancel-Lock: sha1:YmYxOTljNDgwZDNjMmE3MzJhZDFjMDA5M2FmN2IwYzYwOGFjMzM4MQ== sha1:gYSyusqZE/kRrIQIeW+SBY7ScyY= 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/5.13 (Gnus v5.13) Emacs/23.1 (darwin) Original-Xref: news.stanford.edu gnu.emacs.help:176876 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:71943 Archived-At: Harry Putnam writes: > Can one test for the presence of a library before requiring it? > > I mean in the way one might test for the presence of a file in common > shell script; if [ -f /my/file ];then > blab blab > fi > > Only here we are testing for the presence of a library somewhere in > the load path rather than a specific file location. > > The lisp equivalent of: > > if some-lib > then > (require some-lib) > fi require does the test itself! C-h f require RET Now, assume that we wrote: (when (library-exists-p 'some-lib) (require 'some-lib)) and that just after your emacs executed (library-exists-p 'some-lib) and returned true, some other process would delete that some-lib.el file. What would happen to your (require 'some-lib)? That is, basically, your above shell script with if [ -f /my/file ] is just WRONG! If you see such tests in scripts, you are allowed to think poorly of their authors. -- __Pascal Bourguignon__