Let's not make invariants const

[Alex Rønne Petersen]

On 03-08-2012 22:32, Davidson Corry wrote:
> On 8/3/2012 1:14 PM, Paulo Pinto wrote:
>> Personally I feel D's contracts are still a bit off of what Eiffel,
>    .NET and Ada 2012 offer.
>
> On 8/3/2012 1:01 PM, Alex Rønne Petersen wrote:
>> So what if, for whatever reason, the invariant needs to track and
>> maintain some state in order to catch some kind of ugly bug?
>>
>> I agree that ideally an invariant should not ever need to change state.
>> But I think in some cases it can be a useful debugging tool.
>
> The "track and maintain some state" bit usually means capturing some
> state at entry to the class (via a public method), so that the invariant
> can at exit from the class compare the new, possibly modified, state
> to the previous one.
>
> This is the purpose of Eiffel's 'old' construct, which D does not have.
> (Yet.) (One hopes.) (And it would be nice to be able to capture an old
> *expression* and not just an old variable.)
>
> 'old' is used more often with postconditions than with invariants, but
> it can be useful in both.
>
>> But even ignoring that, making invariants const at this point in time is
>> just not practical. We seem to be doing things in the completely wrong
>> order. The library should be made const-friendly and *then* invariants
>> could be made const (not that I want them to be anyway).
>
> Certainly the libraries should be made as const-correct as possible. But
> it's not like we are forced to do only the one or the other (except for
> the insufficient supply of Walters, perhaps...). IMO, invariants should
> "be made const" (i.e. statically check for side effects) as discussed
> elsewhere. In fact, I believe that having more rigorous contract checks
> could *help* improve library code, by making it harder for loose code
> to get into the library.

The problem here is that making invariants const *now* means that using 
them in conjunction with Phobos code becomes next to impossible. Casting 
away const will become common practice, and that ain't exactly the way 
we want to go. :/

-- 
Alex Rønne Petersen
alex at lycus.org
http://lycus.org