From: Simon Wright <simon@pushface.org>
Subject: Re: Class wide preconditions: error in GNAT implementation?
Date: Mon, 18 Feb 2013 20:42:56 +0000
Date: 2013-02-18T20:42:56+00:00 [thread overview]
Message-ID: <lyppzxpd4f.fsf@pushface.org> (raw)
In-Reply-To: 1d2fr74npgdwt$.mcfs4zhtbvg6$.dlg@40tude.net
"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> On Mon, 18 Feb 2013 18:04:53 +0000, Simon Wright wrote:
>
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>>
>>> Anyway, trivially, consider the following types:
>>>
>>> type A is ...;
>>> type B is new A with ...;
>>>
>>> The predicate P=x in A'Class is true for both A and B. The predicate
>>> Q=y in B'Class is true B and false for A. hence Q=>P, Q is stronger.
>>
>> If you allow code like this, how can you draw any conclusions about the
>> correctness of a call through a pointer to A'Class?
>
> Do you believe in correctness proofs without looking into the bodies?
Well, you've consructed a situation which is plainly wrong if we're
going to be talking substitutablity.
I would expect that if you could prove that
* all calls via A'Class meet A's precondition; and
* all derived type implementations produce valid output when given
parameters that meet A's precondition
then you'd be a long way toward a correctness proof.
>>> [Tag checks violate LSP]
>>
>> Don't understand.
>
> Because these could be used to construct predicates which would depend
> on the actual type. Absolute substitutability as required by "naive"
> interpretations of LSP is based on types being absolutely
> indistinguishable for the clients, which tags obviously defeat.
Then don't write predicates like that! (and slap the wrist of anyone you
find doing it).
next prev parent reply other threads:[~2013-02-18 20:42 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-02-15 3:55 Class wide preconditions: error in GNAT implementation? ytomino
2013-02-15 10:15 ` Georg Bauhaus
2013-02-15 13:16 ` ytomino
2013-02-16 1:01 ` Randy Brukardt
2013-02-16 2:50 ` ytomino
2013-02-16 9:28 ` Dmitry A. Kazakov
2013-02-16 13:13 ` ytomino
2013-02-16 16:35 ` Dmitry A. Kazakov
2013-02-16 19:55 ` ytomino
2013-02-16 20:34 ` ytomino
2013-02-18 8:30 ` Dmitry A. Kazakov
2013-02-18 9:17 ` Simon Wright
2013-02-18 14:25 ` Dmitry A. Kazakov
2013-02-18 18:04 ` Simon Wright
2013-02-18 19:27 ` Dmitry A. Kazakov
2013-02-18 20:42 ` Simon Wright [this message]
2013-02-19 9:07 ` Dmitry A. Kazakov
2013-02-18 19:02 ` ytomino
2013-02-18 19:44 ` Dmitry A. Kazakov
2013-02-16 15:16 ` Georg Bauhaus
2013-02-16 20:23 ` Simon Wright
2013-02-17 15:12 ` ytomino
2013-02-18 17:49 ` Simon Wright
2013-02-18 18:45 ` ytomino
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