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=unavailable autolearn_force=no version=3.4.4 Path: eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!news.eternal-september.org!feeder.eternal-september.org!aioe.org!.POSTED!not-for-mail From: "Dmitry A. Kazakov" Newsgroups: comp.lang.ada Subject: Re: Why doesn't Ada allow user-defined attributes? Date: Sat, 22 Jul 2017 22:12:51 +0200 Organization: Aioe.org NNTP Server Message-ID: References: NNTP-Posting-Host: MajGvm9MbNtGBKE7r8NgYA.user.gioia.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Complaints-To: abuse@aioe.org User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 X-Notice: Filtered by postfilter v. 0.8.2 Content-Language: en-US Xref: news.eternal-september.org comp.lang.ada:47495 Date: 2017-07-22T22:12:51+02:00 List-Id: On 2017-07-22 18:33, G.B. wrote: > On 21.07.17 21:39, Dmitry A. Kazakov wrote: >> On 2017-07-21 19:11, Georg Bauhaus wrote: >>> Dmitry A. Kazakov wrote: >>>> On 21/07/2017 06:46, Georg Bauhaus wrote: >>>> >>>>> Do you really need 'Length? >>>> >>>> Sure, when for example the result depends on the number of elements. >>> >>> Count is computed easily, albeit inefficiently, or even efficiently >>> whenever a collection type caches its number of elements. >>> So, is 'Length, if not a static property, more than a prim op? >>> >>>> 'Length is a general property of any container (set). >>>> >>>>> It seems an implementation detail whenever the language doesn't >>>>> have forall >>>>> or exists. >>>> >>>> Enumeration requires some additional assumptions, stronger than length. >>> >>> What real thing can be counted yet not one by one? >> >> Unordered set cannot be enumerated, > > https://en.wikipedia.org/wiki/Enumeration The sixth word there. >> Lots of external things cannot be, e.g. a file you have no permission >> to read. > > A file needs to have been _counted_ byte by byte, > or allocated using a known _count_ of bytes, etc. > in order to have this length. No. File length is an attribute of a file. You don't need to read file or to allocate it. You can try it yourself, take an uncompressed movie file. Reading 4GB takes a *lot* of time, yet file manager shows its size instantly. > Permissions don't change this state of affairs. They prevent reading the file. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de