From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED!not-for-mail From: Paul Eggert Newsgroups: gmane.emacs.devel Subject: Re: Predicate for true lists Date: Mon, 4 Jun 2018 18:23:39 -0700 Organization: UCLA Computer Science Department Message-ID: References: <87fu3vdjjk.fsf@tcd.ie> <87bmcqhhsf.fsf@tcd.ie> NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1528161710 20510 195.159.176.226 (5 Jun 2018 01:21:50 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 5 Jun 2018 01:21:50 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 To: "Basil L. Contovounesios" , emacs-devel@gnu.org Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Tue Jun 05 03:21:46 2018 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fQ0fN-00056y-P1 for ged-emacs-devel@m.gmane.org; Tue, 05 Jun 2018 03:21:41 +0200 Original-Received: from localhost ([::1]:42641 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fQ0hV-0000kJ-1f for ged-emacs-devel@m.gmane.org; Mon, 04 Jun 2018 21:23:53 -0400 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:52710) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fQ0hP-0000kC-KT for emacs-devel@gnu.org; Mon, 04 Jun 2018 21:23:48 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fQ0hM-000799-Fq for emacs-devel@gnu.org; Mon, 04 Jun 2018 21:23:47 -0400 Original-Received: from zimbra.cs.ucla.edu ([131.179.128.68]:41070) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fQ0hM-00078d-67 for emacs-devel@gnu.org; Mon, 04 Jun 2018 21:23:44 -0400 Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 373A9160607; Mon, 4 Jun 2018 18:23:41 -0700 (PDT) Original-Received: from zimbra.cs.ucla.edu ([127.0.0.1]) by localhost (zimbra.cs.ucla.edu [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id e2wKdy20xUxV; Mon, 4 Jun 2018 18:23:40 -0700 (PDT) Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id F285C160632; Mon, 4 Jun 2018 18:23:39 -0700 (PDT) X-Virus-Scanned: amavisd-new at zimbra.cs.ucla.edu Original-Received: from zimbra.cs.ucla.edu ([127.0.0.1]) by localhost (zimbra.cs.ucla.edu [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id EPayOn7onBFw; Mon, 4 Jun 2018 18:23:39 -0700 (PDT) Original-Received: from Penguin.CS.UCLA.EDU (Penguin.CS.UCLA.EDU [131.179.64.200]) by zimbra.cs.ucla.edu (Postfix) with ESMTPSA id D9C22160607; Mon, 4 Jun 2018 18:23:39 -0700 (PDT) Openpgp: preference=signencrypt Autocrypt: addr=eggert@cs.ucla.edu; prefer-encrypt=mutual; keydata= xsFNBEyAcmQBEADAAyH2xoTu7ppG5D3a8FMZEon74dCvc4+q1XA2J2tBy2pwaTqfhpxxdGA9 Jj50UJ3PD4bSUEgN8tLZ0san47l5XTAFLi2456ciSl5m8sKaHlGdt9XmAAtmXqeZVIYX/UFS 96fDzf4xhEmm/y7LbYEPQdUdxu47xA5KhTYp5bltF3WYDz1Ygd7gx07Auwp7iw7eNvnoDTAl KAl8KYDZzbDNCQGEbpY3efZIvPdeI+FWQN4W+kghy+P6au6PrIIhYraeua7XDdb2LS1en3Ss mE3QjqfRqI/A2ue8JMwsvXe/WK38Ezs6x74iTaqI3AFH6ilAhDqpMnd/msSESNFt76DiO1ZK QMr9amVPknjfPmJISqdhgB1DlEdw34sROf6V8mZw0xfqT6PKE46LcFefzs0kbg4GORf8vjG2 Sf1tk5eU8MBiyN/bZ03bKNjNYMpODDQQwuP84kYLkX2wBxxMAhBxwbDVZudzxDZJ1C2VXujC OJVxq2kljBM9ETYuUGqd75AW2LXrLw6+MuIsHFAYAgRr7+KcwDgBAfwhPBYX34nSSiHlmLC+ KaHLeCLF5ZI2vKm3HEeCTtlOg7xZEONgwzL+fdKo+D6SoC8RRxJKs8a3sVfI4t6CnrQzvJbB n6gxdgCu5i29J1QCYrCYvql2UyFPAK+do99/1jOXT4m2836j1wARAQABzSBQYXVsIEVnZ2Vy dCA8ZWdnZXJ0QGNzLnVjbGEuZWR1PsLBfgQTAQIAKAUCTIByZAIbAwUJEswDAAYLCQgHAwIG FQgCCQoLBBYCAwECH In-Reply-To: <87bmcqhhsf.fsf@tcd.ie> Content-Language: en-US X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x [fuzzy] X-Received-From: 131.179.128.68 X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.org gmane.emacs.devel:226018 Archived-At: On 06/04/2018 05:12 AM, Basil L. Contovounesios wrote: > +(defun list-true-p (object) > + "Return t if OBJECT is a true list. > +A true list is neither circular nor dotted (i.e., its last `cdr' > +is nil)." > + (null (nthcdr (safe-length object) object))) This traverses the list twice. Wouldn't it be better to traverse it just=20 once? Also, why not return the length of the list when it is proper?=20 That would not cost anything to compute, and would yield more=20 information than just returning t. Like others, I prefer "proper" to "tru= e". Something like the following, perhaps: (defun proper-list-length (obj) =C2=A0 "Return OBJ's length if OBJ is a proper list, nil otherwise." =C2=A0 (and (listp obj) (ignore-errors (length obj))))