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: Real-life examples of lexical binding in Emacs Lisp Date: Fri, 29 May 2015 14:28:13 +0200 Organization: Informatimago Message-ID: <87twuv7ele.fsf@kuiper.lan.informatimago.com> References: 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 1432902633 19277 80.91.229.3 (29 May 2015 12:30:33 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 29 May 2015 12:30:33 +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 May 29 14:30:29 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 1YyJQf-0007yc-In for geh-help-gnu-emacs@m.gmane.org; Fri, 29 May 2015 14:30:25 +0200 Original-Received: from localhost ([::1]:35539 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YyJQe-0002e7-Ne for geh-help-gnu-emacs@m.gmane.org; Fri, 29 May 2015 08:30:24 -0400 Original-Path: usenet.stanford.edu!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 104 Original-X-Trace: individual.net OZhi80y1fpC8ortZ2RP+zAYF8FbjR0+p9wxU8BD3ExmKmYpXI/ Cancel-Lock: sha1:MjllYzRhNTc5YzQ0MzlhMzhiZGNjZjUyZDlmNjc2NjQwZWNjZWNhOA== sha1:MT8MgDvQgmsk/tqQGz914CQiND0= 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:212380 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:104664 Archived-At: Marcin Borkowski writes: > Hi all, > > I googled a bit, and could not find /real-world/ examples of using > lexical binding and its advantages /in Emacs Lisp/. I understand that > it's a nice thing to be able to create closures, and that lexical > binding is in general faster than dynamic binding (which is a bonus in > itself), but could anyone show me a real /text editing/ problem that > lexical binding solves, like something that is easier done with > l.b. than with d.b.? (Examples of general-purpose programming problems > made easier with l.b. are more or less obvious/easy to find, but Emacs > is a text editor, after all, and this is its primary area.) Lexical binding matters for two things: - it allows the creation of closures. - it prevents the clobbering of variables. Closures: A typical example, is visible in the thread "~`symbol-function' to get code as list even when byte-compiled?": ;;;; -*- mode:emacs-lisp;lexical-binding:t;coding:utf-8 -*- (defun add-one-shot-meat (hook fun) (let ((name (gensym))) (setf (symbol-function name) (lambda () (remove-hook hook name) (funcall fun))) (add-hook hook name))) Without lexical binding, fun and hook would be dynamic, and therefore their bindings would disappear when add-one-shot-meat returns. Therefore they would be undefined variable when the function is called, or worse, they may be bound at that time by some other function to something different. Compare: (setf lexical-binding t) (defun e (f) (let ((v 42)) (funcall f))) (let ((v 33)) (e (lambda () v))) --> 33 (setf lexical-binding nil) (defun e (f) (let ((v 42)) (funcall f))) (let ((v 33)) (e (lambda () v))) --> 42 Clobering variables: For example, if we have a package such as: (setf lexical-binding nil) (defun d () v) (defun e (f) (let ((v 42)) (funcall f))) and we used it with a function in another package such as: (defun h () (let ((v 33)) (d))) we obtain: (e (function h)) --> 33 instead of the expected 42. Hence the workaround of prefixing all variables by the package name, but this is often insufficient (because package names being often generic, a different package may name its internal variables similarly) and not always applied, notably for internal variables. To be 100% safe without lexical binding, you would have to prefix ALL your variables with very long package and function name prefixes. -- __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