From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: "Lennart Borgman (gmail)" Newsgroups: gmane.emacs.devel Subject: Re: Why is not end-of-defun-function buffer local? Date: Sun, 09 Dec 2007 04:04:30 +0100 Message-ID: <475B5B3E.3010708@gmail.com> References: <4759ED09.7060601@gmail.com> <475ADCA4.1020506@gmail.com> <475B48A7.2030509@gmail.com> NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Trace: ger.gmane.org 1197169489 30257 80.91.229.12 (9 Dec 2007 03:04:49 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 9 Dec 2007 03:04:49 +0000 (UTC) Cc: Emacs Devel To: Stefan Monnier Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Sun Dec 09 04:04:58 2007 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([199.232.76.165]) by lo.gmane.org with esmtp (Exim 4.50) id 1J1CTR-0003Uw-6Y for ged-emacs-devel@m.gmane.org; Sun, 09 Dec 2007 04:04:57 +0100 Original-Received: from localhost ([127.0.0.1] helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1J1CT9-0007VU-Tm for ged-emacs-devel@m.gmane.org; Sat, 08 Dec 2007 22:04:39 -0500 Original-Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1J1CT7-0007Tb-1G for emacs-devel@gnu.org; Sat, 08 Dec 2007 22:04:37 -0500 Original-Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1J1CT5-0007TL-Bv for emacs-devel@gnu.org; Sat, 08 Dec 2007 22:04:36 -0500 Original-Received: from [199.232.76.173] (helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1J1CT5-0007TG-8B for emacs-devel@gnu.org; Sat, 08 Dec 2007 22:04:35 -0500 Original-Received: from ch-smtp02.sth.basefarm.net ([80.76.149.213]) by monty-python.gnu.org with esmtps (TLS-1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1J1CT4-0001B0-HY for emacs-devel@gnu.org; Sat, 08 Dec 2007 22:04:34 -0500 Original-Received: from c83-254-148-228.bredband.comhem.se ([83.254.148.228]:64375 helo=[127.0.0.1]) by ch-smtp02.sth.basefarm.net with esmtp (Exim 4.68) (envelope-from ) id 1J1CT2-0008Ix-7n; Sun, 09 Dec 2007 04:04:32 +0100 User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.9) Gecko/20071031 Thunderbird/2.0.0.9 Mnenhy/0.7.5.666 In-Reply-To: X-Antivirus: avast! (VPS 071208-0, 2007-12-08), Outbound message X-Antivirus-Status: Clean X-Originating-IP: 83.254.148.228 X-Scan-Result: No virus found in message 1J1CT2-0008Ix-7n. X-Scan-Signature: ch-smtp02.sth.basefarm.net 1J1CT2-0008Ix-7n eb852c51cd205762ff52bb40b7e9fd58 X-detected-kernel: by monty-python.gnu.org: Linux 2.6? (barebone, rare!) X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Original-Sender: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Xref: news.gmane.org gmane.emacs.devel:84910 Archived-At: Stefan Monnier wrote: >>>> Looking at some code that is a bit older it looks like some of it uses >>>> make-local-variable where it is not needed since the variables in question >>>> are always buffer local. From that I draw the conclusion that the code in >>>> Emacs uses make-variable-buffer-local more often now. Is not that the case? >>> make-variable-buffer-local has the following downsides: >>> 1 - it cannot be reverted. >>> 2 - it may be done too late. >>> 3 - when you see `setq' it's not obvious that the setting is buffer-local >>> unless you remember seeing the call to make-variable-buffer-local. >>> The second problem may also explain what you're seeing: some code may >>> set a variable before the make-variable-buffer-local gets run. >>> It's actually "common" to introduce bugs this way, because people see >>> "this is automatically buffer-local" in the C-h v info, so they just use >>> `setq' without realizing that the setq may occur before the package >>> gets loaded. >>> make-variable-buffer-local is not evil, but make-local-variable is much >>> tamer and more explicit, and it works just as well in most cases. > > >> Thanks, that was a good explanation. Why not add this to the doc string of >> make-variable-buffer-local? > > Oh, and since I've been looking at the low-level code that handles > variable lookup and things like that, there's another reason: > make-variable-buffer-local has a very subtle semantics which requires > pretty ugly and debatable C code. > More specifically, the problem is to decide *when* to make a variable > buffer-local. I.e. Setting the variable via `setq' should make it > buffer-local, but setting it with `let' shouldn't. But > > (let ((var 1)) > (setq var 2)) > > should not make `var' buffer-local either, because the `setq' is > "protected" within a let. OTOH > > (let ((var 1)) > (with-current-buffer > (setq var 2))) > > should make `var' buffer-local in unless the code is itself > run within a `let' which was itself done in . Yuck! > > So every `setq' on a variable that has been make-variable-buffer-local > may require walking up the current list of `let' bindings to decide > whether to make the variable buffer-local. Yup, that's right: > the (setq var 2) will take time proportional to the stack depth :-( > > And in order to be able to walk up the stack and decide which let > binding might be relevant, the runtime representation of some > let-bindings requires an extra cons-cell, which is not used for > anything else. Perhaps make-variable-buffer-local var could be treated like this: 1) When entering (let ((var 1)) make a buffer local copy of the variable just as if (make-local-variable 'var) was called before let. 2) When leaving (let ((var 1))...) delete the buffer local copy of the variable if it has the default value. That is of course a slightly different semantic, but I wonder if it matters. The advantage is that var could be treated just as if it was made buffer local with make-local-variable. I might be misunderstanding something, of course, since I do not know this code.