forcing weak purity

[Steven Schveighoffer]

On Wed, 23 May 2012 08:21:42 -0400, deadalnix <deadalnix at gmail.com> 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.

Yes.  Memory allocation and deallocation from a global heap is by  
definition an impure operation (it affects global state).  However, we  
must make exceptions because without being able to allocate memory, pure  
functions become quite trivial and useless.

In functional languages, if such exceptions were not granted, a program  
would not be able to do much of anything.

> I think what is need here is to break the type system to allow call of  
> impure function into a pure one.

We already are breaking the type system by marking an impure function (a  
function whose implementation is not marked pure) pure.  This only works  
for extern(C) fucntions, which are not mangled with the pure attribute.

But we need to be more specific about how the compiler should treat these  
functions.  They need to undergo no optimization based on purity.  They  
simply need to be able to be called from a pure function.

-Steve