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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Structured exception information
Newsgroups: comp.lang.ada
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Date: Fri, 19 Jan 2007 14:52:35 +0100
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Date: 2007-01-19T14:52:35+01:00
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On Fri, 19 Jan 2007 01:34:35 -0600, Randy Brukardt wrote:

> The other issue was what to do with preconditions for inherited routines.
> They ought to compose somehow, but that got ugly in a hurry.
[...]
> The proposal we were working on had a way to get the original value of a
> scalar parameter. Which meant they all had to copied on entrance - causing
> extra overhead. And the inheritance, visibility, and side-effect issues are
> still around.

Inherited subprograms should take them as-is. When overridden,
preconditions are weakened, postoconditions are strengthened. An override
should add its precondition as a disjunctive term to the precondition of
the subprogram being overridden one, and its postcondition as a conjunctive
term.

There shall be no side-effects for the same reasons why there shall be no
exceptions. (:-)) For the same reason visibility is not an issue,
everything is visible...

OK, what about an alternative. Let's forget about preconditions, let's call
it *constraint*. This is what Ada has since its beginning. What about
allowing more generous constraint expressions put on the subtypes? It could
have the desired effect, IMO. The "precondition" could be a constraint on
the subtype of an in-parameter. The "postcondition" could be one on the
subtype of an out-parameter. We could allow ad-hoc subtypes in subprogram
signatures, for lazy guys...

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de