should pure functions accept/deal with shared data?

[Steven Schveighoffer]

On Thu, 07 Jun 2012 13:46:43 -0400, Artur Skawina <art.08.09 at gmail.com>  
wrote:

> On 06/07/12 18:45, Steven Schveighoffer wrote:
>> 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.
>
> "not properly protecting shared data from races" is as language issue,  
> it's not
> specific to pure functions or templates. (Note: this does not mean that  
> "shared"
> currently does too little; it in fact does too much, but that's a  
> completely
> different issue)
>
> This
>
>    shared Atomic!int x;
>
>    void main() { x.inc(); };
>
> will do the right thing, even with your "void inc(T)(ref T i) pure"  
> template.

Right, but if you want to handle shared too, just don't mark inc as pure.   
There isn't any advantage to marking it pure.  The compiler will infer  
pure when it should.  Since pure functions (in the proposed fixed  
compiler) cannot handle shared data, there is no need to mark inc pure, it  
can always be called from a pure function.

Note that the above, while correct, is not a condition of shared.  That it  
*can* be made to work isn't a factor of inc.

But I can specify inc is pure, and if shared data isn't allowed, inc can  
be *sure* it's not going to be abused.  It's entirely possible to look at  
the inc function, and reason that it can't be used to make a race bug  
(assuming someone doesn't deliberately sabotage the increment operator on  
a type by casting away pure).

> Yes, it's not actually pure, but that won't be problem in practice  
> because it
> takes a mutable reference as input. You are proposing to disallow this.

I'm not proposing disallowing mutable references, just shared references.

>>> 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.
>
> As far as it relates to this i don't see a problem - except for the  
> "const shared"
> case, which does not really make sense - the compiler has to assume  
> another thread
> could have mutated the data, so it cannot take advantage of the fact  
> that this
> thread did not. But I guess it could be useful for statically ensuring  
> that an
> implementation does not modify a r/o view of shared data. In every other  
> case the
> function can't be really pure.
>
> The main improvement needed is real purity for functions that not only  
> take value
> inputs but also const references (including pointers, which then need  
> similar
> treatment inside such functions).

pure functions that take immutable arguments can be optimized for purity  
already.  I agree there are optimizations that could be enabled that  
aren't, but that doesn't make those uses invalid usages of purity.  Shared  
can be also lumped in there, but my contention is that there's no  
usefulness in that construct.

-Steve