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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Contract checking in Ada
Newsgroups: comp.lang.ada
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Date: Fri, 1 Apr 2005 10:55:22 +0200
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Date: 2005-04-01T10:55:18+02:00
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On Fri, 01 Apr 2005 08:02:54 GMT, Tapio Kelloniemi wrote:

> Randy Brukardt <randy@rrsoftware.com> wrote:

>>For instance, Postconditions required a mechanism to get at the original
>>values of parameters. Which meant that those values had to be saved
>>somewhere. That would be a huge performance hit unless it is possible to
>>tell in advance whether or not the original value would be required. We
>>never had a proposal with that property (it really needs to be visible on a
>>purely syntax basis; otherwise it can be too complex to figure out, as it
>>would depend on name resolution and visibility).
> 
> Programming by contract features are IMHO disabled (speaking in free
> software terminology) when a stable version is released, or in other
> words, when a final product goes out. Xconditions are certainly a huge
> performance hit, but not as much as inserting a break point at the
> beginning and end of every subprogram in a debugger and then manually
> examining parameter and result values, if program behaves oddly. All other
> run-time checks are also expensive and that is why Ada provides a way to
> disable them. Xconditions could actually speed up code that is considered
> to be stable. This is because subprograms' parameters' validity checking
> can be written as a precondition and does not need to be executed, when
> the caller knows that a bad value cannot be passed in any situation.
> For example subprograms of Ada.Strings's child packages have many checks for
> their parameters' validity and as some of the subprograms are implemented
> (in GNAT) in terms of others, the checks are doubled.

That is the problem. Basically the question is: can a contract violation be
handled at run-time? I would say no. Theoretically, a contract violation
should kill the program, reboot the computer (and all other computers of
world (:-)).

If so, then the validity checks cannot be turned into contract checks.
Compare:

begin
   Do_One_Thing (X);
exception
   when Constraint_Error =>
      Do_Other_Thing (X); -- This is OK
end;

Compare with:

begin
   Do_One_Thing (X);
exception
   when Contract_Error =>
      Do_Other_Thing (X); -- Nonsense
end;

Validity, constraint checks /= contract checks.

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