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!feeder.eternal-september.org!aioe.org!.POSTED!not-for-mail From: "Dmitry A. Kazakov" Newsgroups: comp.lang.ada Subject: Re: How to get Ada to ?cross the chasm?? Date: Tue, 8 May 2018 23:34:33 +0200 Organization: Aioe.org NNTP Server Message-ID: References: <1c73f159-eae4-4ae7-a348-03964b007197@googlegroups.com> <87k1su7nag.fsf@nightsong.com> <87po2la2qt.fsf@nightsong.com> <87in8buttb.fsf@jacob-sparre.dk> <87wowqpowu.fsf@nightsong.com> <16406268-83df-4564-8855-9bd0fe9caac0@googlegroups.com> <87o9i2pkcr.fsf@nightsong.com> <87in88m43h.fsf@nightsong.com> <87efiuope8.fsf@nightsong.com> <87lgd1heva.fsf@nightsong.com> <87zi1gz3kl.fsf@nightsong.com> <878t8x7k1j.fsf@nightsong.com> NNTP-Posting-Host: kQkuQcRDy1QFvWpyB1foYw.user.gioia.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; 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/52.7.0 X-Notice: Filtered by postfilter v. 0.8.3 Content-Language: en-US Xref: reader02.eternal-september.org comp.lang.ada:52134 Date: 2018-05-08T23:34:33+02:00 List-Id: On 2018-05-08 23:02, Niklas Holsti wrote: > On 18-05-07 00:02 , Dmitry A. Kazakov wrote: >> On 2018-05-06 21:27, Niklas Holsti wrote: >> >>>> "Applicative" is not a substantial property that hints any concrete >>>> implementation of publisher/subscribers bound through the data >>>> structure. If Ada should do anything, then provide much better support >>>> for user ADTs than it has now. >>> >>> An applicative data structure is a data structure that can be mutated >>> only by constructing what seems to be a modified copy of the entire >>> original data structure, but without actually copying most of the >>> data, and leaving the original data structure available unmodified. >> >> That is not a visible property the client may or should see. > > The client naturally sees if the data structure provides an interface > with value semantics or with reference semantics. How? Usually Ada does not advertise the method of parameter passing. > Value semantics are > nicer for the client, but expensive if the implementation uses full, > deep, copies. Applicative data structures provide value semantics > without needing deep copies, by using hidden references. Maybe you mean mutability here. By-value can be mutable if there is an identity of the object keeping the value. > A client with infinite time and space resources would not care if the > value semantics are provided expensively or cheaply. A client with > finite resources does care. > >>> In other words, a mutated version of the structure can be constructed >>> only by applying a function to the original version, with the function >>> returning the mutated version without harm to the original version. >> >> This is easily done in present Ada using handles to a reference-counted >> objects. > > Yes, it surely can be implemented in Ada, for some value of "easily". > The GC-based languages, especially the lazy functional ones, provide > value semantics by default, and systematic ways to make value semantics > efficient by means of applicative data structures. I don't see how X := new T'(...); is much simpler than Handle := Ref (new T'(...)); You could have the sugar above if you convinced ARG to finally introduce user-defined access types. With more luck you could convince them to have user-defined assignment, to be able to write: Handle := T'(...); Or even user-defined aggregates and constructors to write just: Handle := (...); >> As I said it is quite easy and I used the described approach many times, >> e.g. for dealing with decision trees. > > I too have implemented such things in Ada. I would not call it "quite > easy", however. You need to do it only once, actually. Handles are easily reused. The only problem is generics. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de