forcing weak purity

[deadalnix]

Le 23/05/2012 14:32, Alex Rønne Petersen a écrit :
> On 23-05-2012 14:21, deadalnix wrote:
>> Le 23/05/2012 05:22, Steven Schveighoffer a écrit :
>>> I have come across a dilemma.
>>>
>>> Alex Rønne Petersen has a pull request changing some things in the GC to
>>> pure. I think gc_collect() should be weak-pure, because it could
>>> technically run on any memory allocation (which is already allowed in
>>> pure functions), and it runs in a context that doesn't really affect
>>> execution of the pure function.
>>>
>>> So I think it should be able to be run inside a strong pure function.
>>> But because it has no parameters and no return, marking it as pure makes
>>> it strong pure, and an optimizing compiler can effectively remove the
>>> call completely!
>>>
>>> So how do we force something to be weak-pure? What I want is:
>>>
>>> 1. it can be called from a pure function
>>> 2. it will not be optimized out in any way.
>>>
>>> This solution looks crappy to me:
>>>
>>> void gc_collect(void *unused = null);
>>>
>>> any other ideas?
>>>
>>> -Steve
>>
>> Why a pure function can call a collection cycle ???? This is an impure
>> operation by essence.
>>
>> I think what is need here is to break the type system to allow call of
>> impure function into a pure one.
>
> I think you're missing an amusing point:
>
> class C { this() pure {} }
>
> C foo() pure
> {
> return new C(); // can trigger a collection!
> }
>

Ok, but no direct call to GC collect will be done, so the function don't 
need to be pure, it need to be somehow hacked into the allocation 
mecanism, probably using compiler magic.

So basically the allocation mecanism have to break the type system to 
act like it is pure, even if it isn't.