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!mx02.eternal-september.org!.POSTED!not-for-mail
From: Bob Duff <bobduff@theworld.com>
Newsgroups: comp.lang.ada
Subject: Re: SPARK: missing case value
Date: Fri, 09 Oct 2015 10:39:01 -0400
Organization: A noiseless patient Spider
Message-ID: <87twq09ke2.fsf@theworld.com>
References: <ad7b2827-9111-4d9b-b065-1ebd26795365@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain
Injection-Info: mx02.eternal-september.org;
 posting-host="ac11a35aadf635436bedb61baed87c73";
	logging-data="32708"; mail-complaints-to="abuse@eternal-september.org";
	posting-account="U2FsdGVkX1/i5a19xmUhkhNtx688Zl62"
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)
Cancel-Lock: sha1:i8tPN2Uy6aXCKv7gvRyvgiR4Mok=
	sha1:phwOhwa5qDNiAUT/qlheRsskkus=
Xref: news.eternal-september.org comp.lang.ada:27947
Date: 2015-10-09T10:39:01-04:00
List-Id: <comp.lang.ada>

Maciej Sobczak <see.my.homepage@gmail.com> writes:

> Consider:
>
>    type Enum is (A, B, C);
>
>    procedure Test (E : in Enum)
>       with Pre => E /= C
>    is
>    begin
>       case E is
>          when A => null;
>          when B => null;
>       end case;
>    end Test;

That's illegal Ada, as you noted.  And illegal SPARK.

But this works:

   type Enum is (A, B, C);
   subtype A_C is Enum with Predicate => A_C /= B;

   procedure Test (E : in A_C) is
   begin
      case E is
         when A => null;
         -- "when B" is not needed.
         when C => null;
      end case;
   end Test;

And that has the advantage that A_C need not be a subrange;
it can have holes.

I find that predicates are often better than preconditions,
because the same precondition often applies to many parameters,
and also to local variables.  The predicate allows you to avoid
repetition.

("Predicate =>" is a GNAT-specific extension.  In Ada, you need to
say "Static_Predicate =>".  IMHO the "Static_" part is just noise,
but I couldn't convince the rest of ARG.)

- Bob