forcing weak purity

[Alex Rønne Petersen]

On 23-05-2012 15:55, deadalnix wrote:
> Le 23/05/2012 15:47, Alex Rønne Petersen a écrit :
>> On 23-05-2012 15:46, deadalnix wrote:
>>> 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!
>>>> }
>>>>
>
> Rethinking about this, it show something interesting. To make sense,
> allocation in pure function should either be scoped or immutable.
>
> Otherwise we can't ensure any « strong purity » on a function that
> return anything that can reference something else.
>
>>>
>>> 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.
>>
>> Sure there'll be a direct call to that. It's effectively what the GC
>> does when collecting.
>>
>
> At this point, you are in the allocation mecanism in druntime. You are
> not in pure code anymore.
>
> If it is the case, then the type system is broken at the wrong place.

The runtime is a bit of a special case in the first place. It's an 
extremely low-level component of the language.

Mind you, I'm not saying there isn't room for improvement in our type 
system!

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