From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Helmut Eller Newsgroups: gmane.emacs.devel Subject: Re: Make peg.el a built-in library? Date: Fri, 27 Aug 2021 08:41:19 +0200 Message-ID: References: <875yvtbbn3.fsf@ericabrahamsen.net> <87bl5k87hq.fsf@alphapapa.net> <87fsuvpod4.fsf@ericabrahamsen.net> 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="6479"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) To: emacs-devel@gnu.org Cancel-Lock: sha1:BDm9knKVOmUmGqXCX1B80urevoQ= Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Fri Aug 27 08:43:04 2021 Return-path: Envelope-to: ged-emacs-devel@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 1mJVZw-0001Vv-95 for ged-emacs-devel@m.gmane-mx.org; Fri, 27 Aug 2021 08:43:04 +0200 Original-Received: from localhost ([::1]:43506 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mJVZv-00062t-3W for ged-emacs-devel@m.gmane-mx.org; Fri, 27 Aug 2021 02:43:03 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:46340) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mJVYT-0004o2-Qb for emacs-devel@gnu.org; Fri, 27 Aug 2021 02:41:33 -0400 Original-Received: from ciao.gmane.io ([116.202.254.214]:34218) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mJVYR-0008Bz-EA for emacs-devel@gnu.org; Fri, 27 Aug 2021 02:41:33 -0400 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1mJVYP-000AAl-St for emacs-devel@gnu.org; Fri, 27 Aug 2021 08:41:29 +0200 X-Injected-Via-Gmane: http://gmane.org/ Received-SPF: pass client-ip=116.202.254.214; envelope-from=ged-emacs-devel@m.gmane-mx.org; helo=ciao.gmane.io X-Spam_score_int: 5 X-Spam_score: 0.5 X-Spam_bar: / X-Spam_report: (0.5 / 5.0 requ) BAYES_00=-1.9, DKIM_ADSP_CUSTOM_MED=0.001, FORGED_GMAIL_RCVD=1, FREEMAIL_FORGED_FROMDOMAIN=0.248, FREEMAIL_FROM=0.001, HEADER_FROM_DIFFERENT_DOMAINS=0.249, NML_ADSP_CUSTOM_MED=0.9, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.23 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-mx.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.io gmane.emacs.devel:273124 Archived-At: On Thu, Aug 26 2021, Eric Abrahamsen wrote: > Whoo, I've been trying to get enough of a handle on the parsing actions > to write a documentation patch for them -- now I'm seeing what Helmut > meant by "semantically unintuitive". What I actually meant with "semantically unintuitive" are issues described in Roman Redziejowski's "Trying to understand PEG" paper[*]. He writes: The problem with limited backtracking is that by not trying hard it may miss some inputs that it should accept. A notorious example is the rule A = aAa | aa that defines the set of strings of a’s of even length. Implemented with limited backtracking, this rule accepts only strings of length 2^n. > The sum total of docs regarding > actions is: > > A "stack action" takes VARs from the "value stack" and pushes the result > of evaluating FORMs to that stack. Using an "open stack" for actions was my rather idiosyncratic choice and I'm sure that many people will not like it. The syntax ( a b -- b a ) should be familiar to Forth programmers, where it's used to describe the stack-effect of commands. The example would be the SWAP operator. If you have never, or not recently, written some Forth or Postscript, then mentally keeping track of the stack state can be challenging. As for "documentation" of actions: there are also some examples. I think that the s-exp parsing example turned out quite elegant. Helmut [*] http://www.romanredz.se/papers/FI2017.pdf