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Re: [wg14/wg21 liaison] P2961R1 syntax for Contracts: viable for C?

From: Martin Uecker <ma.uecker_at_[hidden]>
Date: Fri, 06 Oct 2023 16:30:17 +0200
Am Freitag, dem 06.10.2023 um 09:40 +0300 schrieb Ville Voutilainen:
> On Fri, 6 Oct 2023 at 08:31, Martin Uecker <ma.uecker_at_[hidden]> wrote:
> > To be honest, I think it is a mistake also for C++. I believe
> > that even for C++ people will end up trying to introduce this into
> > existing code and then are forced to make it ignorable by wrapping it
> > into #ifdef or macros. The end result would neither be nice
> > nor ignorable.
> Sure, but that's no different from conditionally adopting any other
> new language facility that
> the older implementations don't recognize. Which for C++ is almost
> every new language facility.
> We have standardized feature-detection macros, even, to help with
> preprocessor-based conditional
> migration.

The difference is that this seems to be a feature which could be
usefully be added to an existing codebase incrementally. That
does not make too much sense for every new feature.

> > > It wouldn't cause me loss of sleep if C compilers were allowed to
> > > completely ignore a contract annotation.
> > > Fixing syntax errors when using a compiler that doesn't ignore them is
> > > compatible with compilers that would
> > > ignore the annotations.
> >
> > Right. And especially in a context where headers are shared, the
> > scenario that for some transition time (probably decades) the code
> > needs be processed also by compilers not supporting contracts seems
> > very likely.
> >
> > For C I think it would be important to have a syntax that is ignorable.
> Just to make sure I understand.. in effect, to make the feature
> optional? So as to not hoist
> its burden on implementations that don't want it?

No, I would make it mandatory for new C versions. But a useful goal for
C could be to make the code acceptable to older compilers that do
not have full support for it during a transition time without needing
preprocessor wrappers.

> > A syntax as proposed here (it seems, correct me if I am wrong) such
> > as:
> >
> > void f(int x)
> > pre <audit> (x > 0);
> >
> > would be a pretty bad choice for C.
> The <audit> part is at this point hypothetical. I'm curious why you
> think that's a bad choice.

One problem is introducing a three letter keyword. Yes, _Pre would
be an option, but if we want to transition to "pre" this would still
be a problem later.

The next problem is that this syntax can not be hidden behind a macro.
This would work for:

_Pre (<audit>, (x > 0))

Also some attribute-like mechanism would work

[[ __pre__ (x > 0) ]]

where the parsing context could allow a compiler to treat the
keywords specially.

So there is no fundamental problem with this syntax. It is just that
it is not really designed for a feature that can be phased in gradually
into existing code. But this is exactly how contracts would be most
useful in C: By enhancing existing code step by step.

> If I understand things correctly, it's novel for C to have function
> parameters be in scope
> after leaving a function declarator and before a function definition
> begins. In C++, that hasn't
> been novel for ages, with trailing return types,
> noexcept-specifications, and constraints all
> using function parameters.

This is also true, but I think this is not a major issue.


Received on 2023-10-06 14:30:20