should pure functions accept/deal with shared data?

[Steven Schveighoffer]

On Thu, 07 Jun 2012 11:55:32 -0400, Artur Skawina <art.08.09 at gmail.com>  
wrote:

> On 06/07/12 16:43, Steven Schveighoffer wrote:
>> I understand the implementation is not correct for shared, and that  
>> actually is my point.  The current compiler lets you do the wrong thing  
>> without complaint.  Given that the shared version of the function needs  
>> to be written differently than the unshared version, we gain nothing  
>> but bugs by allowing pure functions that operate on shared.
>>
>> In essence, a pure-accepting-shared (PAS) function is not realistically  
>> useful from a strong-pure function.  A strong-pure function will have  
>> no ties to shared data, and while it may be able to create data that  
>> could potentially be shared, it can't actually share it!  So a PAS  
>> function being called from a strong-pure function is essentially doing  
>> extra work (even if it's not implemented, the expectation is it will be  
>> some day) for no reason.
>>
>> So since a PAS function cannot usefully be optimized (much better to  
>> write an unshared version, it's more accurate), and must be written  
>> separately from the unshared version, I see no good reason to allow  
>> shared in pure functions ever.  I think we gain a lot by not allowing  
>> it (more sanity for one thing!)
>
> While it's true that "shared" inside pure functions doesn't _look_  
> right, can you think
> of a case where it is actually wrong, given the pure model currently in  
> use?
> Would inferring templated functions as impure if they access (and not  
> just reference)
> shared data help?

I contend it would make marking a template as pure more useful -- you can  
with one keyword ban all use of shared via template parameters on a  
template function, given that it does not properly protect shared data  
 from races.  You can do the same with template constraints, but it's  
unnecessary boilerplate, only there because the compiler incorrectly  
allows PAS functions.  Unless you plan to implement a shared version, in  
which case there's no less or more code.

> IOW, a function marked as pure that deals with shared data can not be  
> "truly" pure, and
> can not be called from a "really" pure function (as that one would have  
> to handle shared,
> so it couldn't be pure either) - so does it make sense to add new  
> restrictions now, and
> not delay this until the "pure" model is improved?

To what improvements do you refer?  I think what is currently in place is  
sound design, with incomplete or buggy implementation.

-Steve