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From: eachus@linus.mitre.org (Robert I. Eachus)
Newsgroups: comp.lang.ada
Subject: Re: "silly" (?) Ada semantics
Message-ID: <EACHUS.90Jun6141228@aries.linus.mitre.org>
Date: 6 Jun 90 18:12:28 GMT
References: <1919@sparko.gwu.edu> <644579683.25696@minster.york.ac.uk>
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In-reply-to: ken@minster.york.ac.uk's message of 5 Jun 90 09:54:43 GMT
List-Id: <comp.lang.ada>

In article <644579683.25696@minster.york.ac.uk> ken@minster.york.ac.uk writes:

> I thought Ada was targetted at high integrity mission critical
> software? Is it a good idea to defer errors to run-time? 

     No, but it is an even worse idea to allow SOME compilers to
reject a program because it thinks the program is silly, or because
as, "a matter of taste" some vendor thinks that a particular construct
is not a good idea.

     In this particular case, it is obvious at compile time that the
aggregate must be invalid, but in general it is not possible to decide
whether an aggregate is "legal" until run time:

     A: String(1..X) := "foo";

     Given that in many cases the decision must be deferred until
run-time, the chosen policy was that mismatch is never a compile-time
error, but a compiler is expected to flag a known mismatch.  A
similar, but less surprising case is:

   declare
     X: Integer;
   begin
     X := Integer'LAST + Integer'LAST;
   end;

     A compiler can flag this, and in fact it can replace the
assignment statement with "raise CONSTRAINT_ERROR;", but it is
required to generate code...

--

					Robert I. Eachus

with STANDARD_DISCLAIMER;
use  STANDARD_DISCLAIMER;
function MESSAGE (TEXT: in CLEVER_IDEAS) return BETTER_IDEAS is...