From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Gregory Heytings Newsgroups: gmane.emacs.help Subject: Re: Lexical vs. dynamic: small examples? Date: Sat, 14 Aug 2021 19:00:29 +0000 Message-ID: <4a9bddb9ec57299b3b0c@heytings.org> References: Mime-Version: 1.0 Content-Type: text/plain; format=flowed; charset=us-ascii Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="26360"; mail-complaints-to="usenet@ciao.gmane.io" Cc: help-gnu-emacs To: Eduardo Ochs Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Sat Aug 14 21:01:05 2021 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane-mx.org Original-Received: from lists.gnu.org ([209.51.188.17]) by ciao.gmane.io with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1mEyu0-0006aX-PA for geh-help-gnu-emacs@m.gmane-mx.org; Sat, 14 Aug 2021 21:01:05 +0200 Original-Received: from localhost ([::1]:51288 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mEyty-0007tM-RU for geh-help-gnu-emacs@m.gmane-mx.org; Sat, 14 Aug 2021 15:01:02 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:53138) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mEytX-0007sl-VJ for help-gnu-emacs@gnu.org; Sat, 14 Aug 2021 15:00:35 -0400 Original-Received: from heytings.org ([95.142.160.155]:51728) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mEytT-0001rj-Hb for help-gnu-emacs@gnu.org; Sat, 14 Aug 2021 15:00:35 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=heytings.org; s=20210101; t=1628967629; bh=qaNOr/6nucn9FNO+/W8Jt9ZQOkTA2sjjF3GpuASdECk=; h=Date:From:To:cc:Subject:In-Reply-To:Message-ID:References:From; b=JUzObotynOYua68sfr1Sjwt1igDaq0CXPBsBEyqGdvjfFktqBWiRCEG4TtPI1Enu+ zJq9b/4AD7xzu/SeMQGaqLvKpv3BDsYCOceGOb+vEsXOpQbG2mBLtL5qftsqAo3WT1 TFBqIVVa679d746VIB/GUxU4cZV1Vvdm4ecdAXHSPPLEM9xBBCJpMQmYKTL+CXeSBR FhjSP1h9PMjf7BkKWoHV67ybTmF+0ehumPPnI+f2ysShR8x2A9xLoXFIv7n735CdlT VpQKRpRwgger/C7VlWca13XTY4zPX0UI5A1TcElZ8xyPw6QnQRtKrUNLh6cj5iQcxP HnXGAnnNO0hlg== In-Reply-To: Received-SPF: pass client-ip=95.142.160.155; envelope-from=gregory@heytings.org; helo=heytings.org X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.23 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-mx.org@gnu.org Original-Sender: "help-gnu-emacs" Xref: news.gmane.io gmane.emacs.help:132540 Archived-At: > > I am trying to write a section on lexical vs. dynamic binding for a > tutorial on Emacs Lisp, and I am looking for very short demos that show > how things work differently in dynamic and in lexical binding... > Suppose you write a function to remove unnecessary whitespaces at the end of lines: (defun delete-whitespace-at-eol () (interactive) (save-excursion (replace-regexp " *$" "" nil (point-min) (point-max)))) Now if you M-x trim-whitespaces-at-eol in a read-only buffer, you'll get a "Buffer is read-only" error. Suppose you want to make that function work for read-only buffers. If Emacs Lisp only had lexical binding, you would have to alter the global "buffer-read-only" variable, that is, to do something like: (defvar saved-buffer-read-only) (defun delete-whitespace-at-eol () (interactive) (setq saved-buffer-read-only buffer-read-only) (setq buffer-read-only nil) (save-excursion (replace-regexp " *$" "" nil (point-min) (point-max))) (setq buffer-read-only saved-buffer-read-only)) This becomes much simpler with dynamic binding: (defun delete-whitespace-at-eol () (interactive) (let ((buffer-read-only nil)) (save-excursion (replace-regexp " *$" "" nil (point-min) (point-max))))) The "let ((buffer-read-only nil))" creates a new "buffer-read-only" variable and sets it to nil. When "replace-regexp", and the functions called by "replace-regexp", consult the value of the "buffer-read-only" variable, they will see the "buffer-read-only" variable created in "delete-whitespace-at-eol", and its value "nil", instead of the global "buffer-read-only" variable. It is easier to think that "let ((buffer-read-only nil))" creates a new "buffer-read-only" variable, but technically this is not correct. What happens instead is that the old value of "buffer-read-only" is saved, the value of "buffer-read-only" is updated, and upon returning from the "let", the saved value is restored.