From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-0.3 required=5.0 tests=BAYES_00, REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4 Path: border1.nntp.ams3.giganews.com!border1.nntp.ams2.giganews.com!border2.nntp.ams2.giganews.com!border4.nntp.ams.giganews.com!border2.nntp.ams.giganews.com!nntp.giganews.com!weretis.net!feeder4.news.weretis.net!rt.uk.eu.org!aioe.org!.POSTED!not-for-mail From: "Dmitry A. Kazakov" Newsgroups: comp.lang.ada Subject: Re: The future of Spark . Spark 2014 : a wreckage Date: Wed, 10 Jul 2013 09:49:42 +0200 Organization: cbb software GmbH Message-ID: References: <87hag4ahu5.fsf@mid.deneb.enyo.de> <1rbbala6i0jcl$.dswyfcctu6vs$.dlg@40tude.net> Reply-To: mailbox@dmitry-kazakov.de NNTP-Posting-Host: IenaDxMXK2hi7fvYcb+MlQ.user.speranza.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit X-Complaints-To: abuse@aioe.org User-Agent: 40tude_Dialog/2.0.15.1 X-Notice: Filtered by postfilter v. 0.8.2 X-Original-Bytes: 2424 Xref: number.nntp.dca.giganews.com comp.lang.ada:182416 Date: 2013-07-10T09:49:42+02:00 List-Id: On Tue, 09 Jul 2013 18:51:36 -0400, Robert A Duff wrote: > "Dmitry A. Kazakov" writes: > >> On Mon, 08 Jul 2013 22:59:30 +0200, Florian Weimer wrote: >>> This is nothing new. Boolean implication p → q is already written as >>> P <= Q in Ada. >> >> Where? >> >> It would be plain wrong, ... > > Less-or-equal on Booleans is indeed equivalent to implication. > But that's a hack. Considering Booleans as ordered is questionable -- > I never heard of such a thing in mathematical logic. Truth values are ordered in some logics. In others they are not. >>...because logical implication (not P or Q) must be >> denoted as P => Q. > > It must be? Yes, if logical implication is indeed meant here, of which I am not sure. > In Eiffel, it is denoted by the "implies" keyword. > In Ada 2012, one writes "(if P then Q)", which is shorthand for > "(if P then Q else True)". But this is not an implication (Boolean operation). It is a rule based on implication. E.g. Modus ponens: P, P=>Q ------------ Q So "implies" and "if-then" are correct, while P->Q looks wrong even when spelled correctly as P=>Q. If there should be a symbol, I think, P |= Q or P |- Q would be more appropriate. Why had we introduced Unicode? After all, there is a code point for this: P ⊧ Q U+22A7 Logical consequence -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de