From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Stefan Monnier via Users list for the GNU Emacs text editor Newsgroups: gmane.emacs.help Subject: Re: Appending lists Date: Wed, 16 Jun 2021 18:35:35 -0400 Message-ID: References: <87o8c8l32h.fsf@posteo.net> <87zgvs2bup.fsf@zoho.eu> <87y2bby1kr.fsf@zoho.eu> <20210615091834.GB24886@tuxteam.de> <87im2ewr3k.fsf@zoho.eu> <20210616072819.GB17919@tuxteam.de> <87im2dvjgl.fsf@zoho.eu> <20210616184915.GA18839@tuxteam.de> Reply-To: Stefan Monnier Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="3527"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) To: help-gnu-emacs@gnu.org Cancel-Lock: sha1:BumBmuOjHzHlQLKEzWlEvlYr+Vk= Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Thu Jun 17 00:36:24 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 1lte91-0000o5-J5 for geh-help-gnu-emacs@m.gmane-mx.org; Thu, 17 Jun 2021 00:36:23 +0200 Original-Received: from localhost ([::1]:33784 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lte90-00055m-AR for geh-help-gnu-emacs@m.gmane-mx.org; Wed, 16 Jun 2021 18:36:22 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:39566) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lte8W-00055c-Ud for help-gnu-emacs@gnu.org; Wed, 16 Jun 2021 18:35:52 -0400 Original-Received: from ciao.gmane.io ([116.202.254.214]:39510) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lte8P-0006xX-EG for help-gnu-emacs@gnu.org; Wed, 16 Jun 2021 18:35:49 -0400 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1lte8M-000AWU-4U for help-gnu-emacs@gnu.org; Thu, 17 Jun 2021 00:35:42 +0200 X-Injected-Via-Gmane: http://gmane.org/ Received-SPF: pass client-ip=116.202.254.214; envelope-from=geh-help-gnu-emacs@m.gmane-mx.org; helo=ciao.gmane.io X-Spam_score_int: -16 X-Spam_score: -1.7 X-Spam_bar: - X-Spam_report: (-1.7 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.249, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=no 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:130929 Archived-At: > To put a point across it would be better to announce or make a title > that you don't talk how "variable never changes", but you talk about > the structures in memory and somewhat interesting though practically > within the spoken context useless information. What matters for > programmer is that variable A instead of holding value 1 now has value > 2, that is a change, and how that change was internally accomplished > does not change the fact that A is now not 1 but is 2. (づ｡◕‿‿◕｡)づ That's the problem with mutation. In a language like Haskell where mutation is not allowed, when you have x = [1, 2, 3] then the value held in variable `x` is really a list of 3 elements. But in a language like ELisp, when you have (let ((x (list 1 2 3))) ... the value held in variable `x` is not a list of 3 elements: it's really nothing more than a reference to a location in memory holding a cons cell (constraints in ELisp's type system ensure that this location in memory will always hold a cons cell). The values held in the `car/cdr` of that cons cell will depend on the current memory state, they are not part of "the value held in `x`". The vast majority of ELisp code never uses `setcar/setcdr`, tho, so we tend to overlook this inconvenient truth and talk (and think) about those values as if we were in the presence of immutable data. That's why `nconc`, `nreverse`, `sort`, and other similar side-effecting functions need to be used with a lot of care. Stefan