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=-1.9 required=5.0 tests=BAYES_00 autolearn=ham autolearn_force=no version=3.4.4 X-Google-Thread: 103376,99e73f65ea2533b9 X-Google-Attributes: gid103376,domainid0,public,usenet X-Google-Language: ENGLISH,UTF8 Path: g2news1.google.com!news4.google.com!feeder3.cambrium.nl!feed.tweaknews.nl!83.80.28.12.MISMATCH!multikabel.net!newsfeed10.multikabel.net!xlned.com!feeder1.xlned.com!ecngs!feeder.ecngs.de!130.59.10.21.MISMATCH!kanaga.switch.ch!news-zh.switch.ch!switch.ch!news.ip-plus.net!newsfeed.ip-plus.net!news.post.ch!not-for-mail From: Martin Krischik Newsgroups: comp.lang.ada Subject: Re: and then... (a curiosity) Date: Tue, 02 Sep 2008 10:57:39 +0200 Organization: Swisscom IP+ (post doesn't reflect views of Swisscom) Message-ID: <48bd0003$1@news.post.ch> References: <18b41828-bda4-4484-8884-ad62ce1c831d@f36g2000hsa.googlegroups.com> <57qdnfULQ9tzKCHVnZ2dnUVZ_tHinZ2d@comcast.com> NNTP-Posting-Host: 194.41.146.1 Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Trace: atlas.ip-plus.net 1220345863 18898 194.41.146.1 (2 Sep 2008 08:57:43 GMT) X-Complaints-To: abuse@ip-plus.net NNTP-Posting-Date: Tue, 2 Sep 2008 08:57:43 +0000 (UTC) User-Agent: Thunderbird 2.0.0.16 (Windows/20080708) In-Reply-To: X-Original-NNTP-Posting-Host: w03duo.pnet.ch X-Original-Trace: 2 Sep 2008 10:57:39 +0100, w03duo.pnet.ch Xref: g2news1.google.com comp.lang.ada:1860 Date: 2008-09-02T10:57:39+02:00 List-Id: stefan-lucks@see-the.signature schrieb: > True. But the case that you want both expressions to be evaluated is a > rare exception. On a *logic* level, "A and B" implies "if A is false, the > result is false, regardless of B". So short-circuit evaluation is a > natural and good thing for a programming language, and it is regrettable > that Ada needs a more complex syntax for this. That's not what I learned in my boolean arithmetic classes. > (Well, "A and B" also > implies "if B is false, don't care about A", but in a programming language > you can't have it both ways.) That is what I learned in my boolean arithmetic classes. It's either way. There is no preference that the left parameter if more important then the right. A Human seeing "A∧0" won't evaluate A. And if you see "f(x)∧g" you would probably evaluate g first as a variable is usually easier to evaluate then a function. And it is what you have in Ada - with plain old "and" and "or" the optimizer is free to do out of order evaluation and dead code elimination which could well lead to B being evaluated first and A being optimized away if B already determines the final result. This - of course - only if the compiler can also determine that A and B have no side effect. Side effect are that little difference between arithmetic and programming. The CONSTRAINT_ERROR in: if X /= null and x.all = 5 then Do_Something; end if; or the global variable which f(x) might change: function f(x:Integer) is begin if x > 10 then g = false; end if; return x > 5; end f; It's all about those side effect which make order of evaluation so important. Otherwise it should not matter. Note that once you add: pragma Inline (f); to the example above things become very interesting for the optimizer ;-) (do remember that the optimizer will use goto without shame). > But, as others pointed it: IF the default "and" supported > short-circuit-evaluation, THEN this "and" could not be an ordinary Ada > operator. At the language semantic level, it is much more reasonable to > treat "and" like any other operator or function, such as "+" and "-". Well again, in my arithmetic classes I learned that a+b is the same and b+a - which includes that the order of evaluation is of no importance. If you want that ∧ operates the same way as + then "a∧b" must be the same as "b∧a". Martin -- mailto://krischik@users.sourceforge.net Ada programming at: http://ada.krischik.com