forcing weak purity

[deadalnix]

Le 23/05/2012 15:57, Steven Schveighoffer a écrit :
> On Wed, 23 May 2012 09:52:31 -0400, deadalnix <deadalnix at gmail.com> wrote:
>
>> Le 23/05/2012 14:35, Steven Schveighoffer a écrit :
>>> 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.
>>>
>>
>> Yes, you are missing the point.
>>
>> collect is not something you should be able to call in a pure
>> function. It can be triggered by allocating, but at this point you
>> already are in an impure context called from a pure context.
>>
>> At the end, you need an unsafe way to call impure code in pure functions.
>
> I'm failing to see an argument in this response. If I can call an impure
> function for allocating memory, why is it illegal to call an impure
> function for collecting unused memory?
>
> -Steve

Allocating is a much more simpler operation than collect, and its impact 
is way more reduced.

Plus, allocating memory is something mandatory to do anything non 
trivial. GC collect isn't that important, as it can be triggered by 
allocating mecanism itself (and this mecanism is already impure).

If you do allow everything impure to be done on pure function based on 
the fact that allocating is impure, what is the point to have pure 
function at all ?