From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: Eric Abrahamsen Newsgroups: gmane.emacs.help Subject: Re: Two questions about generalized variables Date: Tue, 23 Feb 2016 11:19:02 +0800 Message-ID: <87si0kktax.fsf@ericabrahamsen.net> References: <87ziuu4p8k.fsf@ericabrahamsen.net> <877fhydw6q.fsf@web.de> <877fhyayzn.fsf@ericabrahamsen.net> <878u2e9bsn.fsf@web.de> NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain X-Trace: ger.gmane.org 1456197575 19142 80.91.229.3 (23 Feb 2016 03:19:35 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 23 Feb 2016 03:19:35 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Tue Feb 23 04:19:26 2016 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 1aY3VV-0000AS-CP for geh-help-gnu-emacs@m.gmane.org; Tue, 23 Feb 2016 04:19:25 +0100 Original-Received: from localhost ([::1]:53508 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aY3VU-00028j-RO for geh-help-gnu-emacs@m.gmane.org; Mon, 22 Feb 2016 22:19:24 -0500 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:53713) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aY3VK-00028Q-OT for help-gnu-emacs@gnu.org; Mon, 22 Feb 2016 22:19:15 -0500 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aY3VH-00043Z-Fu for help-gnu-emacs@gnu.org; Mon, 22 Feb 2016 22:19:14 -0500 Original-Received: from plane.gmane.org ([80.91.229.3]:49611) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aY3VH-00043V-8o for help-gnu-emacs@gnu.org; Mon, 22 Feb 2016 22:19:11 -0500 Original-Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1aY3VE-0008Oe-P9 for help-gnu-emacs@gnu.org; Tue, 23 Feb 2016 04:19:09 +0100 Original-Received: from 123.122.44.213 ([123.122.44.213]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Tue, 23 Feb 2016 04:19:08 +0100 Original-Received: from eric by 123.122.44.213 with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Tue, 23 Feb 2016 04:19:08 +0100 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 40 Original-X-Complaints-To: usenet@ger.gmane.org X-Gmane-NNTP-Posting-Host: 123.122.44.213 User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1.50 (gnu/linux) Cancel-Lock: sha1:wRBFZbcM3hqARhImfdgxJwcldxQ= X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 80.91.229.3 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:109265 Archived-At: Michael Heerdegen writes: > Eric Abrahamsen writes: > >> I didn't mean symbol property lists! Just the plain old '(:key1 "value1" >> :key2 "value2") kind. They're useful for a few different things -- in >> this case, initializing class instances. > > Oh, I see. Hmm, one could define a place expander quite similarly to > the one of `alist-get', I think, but it doesn't seem one to be > predefined, indeed. Right, that was my question, I guess -- I would do this with `gv-define-setter', right? Its docstring is a bit impenetrable, but if that's the right place to start I can figure it out. >> >> The second question is setting hashtable entries. Right now I have code >> >> like this: >> >> >> >> (setf (gethash uuid my-hashtable) >> >> (append (list (list 'thingone 'thingtwo)) >> >> (gethash uuid my-hashtable))) >> > >> > Isn't that more or less `push'? >> >> I started off with `push', but the code in question doesn't know if the >> key exists or not, and you can't push to nil. > > The gv setter is calculated using the (unevaluated) place expression, so > "pushing to nil" doesn't happen. See the gv-expander of `alist-get' for > example, which of course also works for nonexistent keys. Ah, of course! It's pushing to a place, not a value, that's the whole point. Thank you for turning the light on there. Last question: How do I actually see the gv-expander of `alist-get'? Where is that? Thanks again, Eric