From mboxrd@z Thu Jan 1 00:00:00 1970 Path: eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: "Randy Brukardt" Newsgroups: comp.lang.ada Subject: Re: Limited with too restrictive? Date: Sat, 13 Jan 2024 22:31:12 -0600 Organization: A noiseless patient Spider Message-ID: References: Injection-Date: Sun, 14 Jan 2024 04:30:39 -0000 (UTC) Injection-Info: dont-email.me; posting-host="b4104de19fa1ce2211373a2c928c3d16"; logging-data="348350"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+m74T9ajLP+ZAGlSo1X5uSyb+9MhPabdA=" Cancel-Lock: sha1:pLY6Lo2/6MwBOV/e4PTzJBdk8IU= X-MSMail-Priority: Normal X-Priority: 3 X-Newsreader: Microsoft Outlook Express 6.00.2900.5931 X-RFC2646: Format=Flowed; Response X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.7246 Xref: news.eternal-september.org comp.lang.ada:65993 List-Id: "Blady" wrote in message news:unucno$89u$1@dont-email.me... > Hello, > > I want to break some unit circularity definitions with access types as for > instance with record: > > type R1; > type AR1 is access R1; > type R1 is record > Data : Natural; > Next : AR1; > end record; > > In my case, I have a unit: > > package test_20240113_modr is > type R2 is record > Data : Natural; > end record; > type AR2 is access R2; > end test_20240113_modr; > > "limited withed" in: > > limited with test_20240113_modr; > package test_20240113_mods is > procedure PS1 (V : test_20240113_modr.R2); > procedure PS2 (V : test_20240113_modr.AR2); > end; > > Let's imagine the circularity, thus PS1 and PS2 definition are legal. > > Of course the following isn't legal: > > type AS1 is array (1..2) of test_20240113_modr.R2; -- illegal > > However why not with access type: > > type AS2 is array (1..2) of test_20240113_modr.AR2; -- illegal For a limited with, one only knows the syntactic declarations (we cannot assume any analysis). Therefore, we cannot know the representation of any type, including access types. Specifically, compilers may support multiple representations for access types, for a variety of reasons (the underlying machine has different representations, as on the 8086 and U2200 that we did compilers for; because additional data needs to be carried along to implement Ada semantics - GNAT did that for access to unconstrained arrays, and so on). The representation can depend upon aspect specifications, the designated subtype, and more, none of which is known at the point of a limited with. We couldn't restrict implementations to a single representation for access types, and thus limited with has to treat them the same as other types. It's necessary to declare local access types for entities that are accessed from a limited view. The reason that anonymous access types were expanded was to make that less clunky -- but I don't think it succeeded. ... > Well, I could make some code transferts from unit to another or access > conversions, that's I actually do but at heavy cost. Yup, but the alternative is worse - requiring all access types to be the most general representation (which can have a heavy performance cost). Randy.