should pure functions accept/deal with shared data?

[Artur Skawina]

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.
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.


>> 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).

artur