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,FREEMAIL_FROM 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!feeder.eternal-september.org!aioe.org!.POSTED!not-for-mail From: Victor Porton Newsgroups: comp.lang.ada Subject: Re: Type invariants and private extensions? Date: Thu, 14 Sep 2017 16:52:45 +0300 Organization: Aioe.org NNTP Server Message-ID: References: <364ff8e0-c7dd-4980-b19f-5d438edd8353@googlegroups.com> <7df81b3c-1fde-4395-8b9b-1a945820a7f7@googlegroups.com> <4565e696-3294-4c27-a761-d18eccb1e1e1@googlegroups.com> NNTP-Posting-Host: qUTqmJlZKcITHxg9FVl9Fw.user.gioia.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7Bit X-Complaints-To: abuse@aioe.org User-Agent: KNode/4.14.10 X-Notice: Filtered by postfilter v. 0.8.2 Xref: news.eternal-september.org comp.lang.ada:48131 Date: 2017-09-14T16:52:45+03:00 List-Id: Jere wrote: > On Tuesday, September 12, 2017 at 6:59:17 PM UTC-4, Victor Porton wrote: >> Shark8 wrote: >> >> > On Tuesday, September 12, 2017 at 2:09:08 PM UTC-6, Victor Porton >> > wrote: >> >> Jeffrey R. Carter wrote: >> >> >> >> > >> >> > "I want to" is not the same as "there is no way to solve the problem >> >> > in current Ada". Ada has a feature that provides exactly what you >> >> > need. It's called a variant record. >> >> >> >> It is a tagged type. AFAIK, a type cannot be both a variant record and >> >> tagged. >> > >> > You certainly can: >> > >> > Type Type_Enumeration is ( TInteger, TReal, TBoolean, Nothing ); >> > >> > Type Example( Item_Type : Type_Enumeration ) is tagged record >> > case Item_Type is >> > when TInteger => I : Integer; >> > when TReal => R : Float range Float'Range; >> > when TBoolean => B : Boolean; >> > when Nothing => Null; >> > end case; >> > end record; >> > >> >> Moreover, the object in consideration is a wrapper over a certain C >> >> API. It surely cannot be described in variant record terms. >> > >> > Sure it can; there's a reason that there's a separation between >> > specification and body, part of which is so you can hide something like >> > an interface to C and present something sensible to your >> > program/clients. >> >> Thanks. >> >> But this seems not to solve my problem: >> >> The base type for Example cannot be defined as a discriminated type for >> certain Type_Enumeration, because it is possible that when creating the >> object it may be yet unknown what the value of the discriminant should >> have (my main problem is to invent somethings if this value is known at >> object creation, but we must support the unknown case too). AFAIK, it is >> not possible to change the discriminant later. >> > > I may be misunderstanding what you are looking for, but you can > make discriminant types unconstrained to let you choose the type > at run time: > > Type Type_Enumeration is ( TInteger, TReal, TBoolean, Nothing ); > > Type Example( Item_Type : Type_Enumeration := Nothing) is tagged > record > case Item_Type is > when TInteger => I : Integer; > when TReal => R : Float range Float'Range; > when TBoolean => B : Boolean; > when Nothing => Null; > end case; > end record; > > Then all you have to do is declare an object with no > specified discriminant: > > Some_Object : Example; --notice no discriminant > > In Ada, when you do this, you are able to assign new versions > of the type (with different discriminants: > > Some_Integer : Example := (Item_Type => TInteger, I => 100); > Some_Float : Example := (Item_Type => TFloat, F => 20.2); > > Some_Object := Some_Integer; > Some_Object := Some_Float; > > That's just a play example, you can handle building them however > you want at run time. It does not suffice for me, because at the point of actual object creation, the discriminant value may be yet unknown. Sometimes, I need first create the object and LATER (when the object was already created and I did some operations with it) assign it the type. -- Victor Porton - http://portonvictor.org