forcing weak purity

[Alex Rønne Petersen]

On 23-05-2012 16:42, deadalnix wrote:
> Le 23/05/2012 16:03, deadalnix a écrit :
>> 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 ?
>
> You'll find a different between :
>
> pure void foo() {
> new Stuff(); // May collect
> }
>
> and
>
> pure void foo() {
> gc.collect();
> }
>
> The second one is obviously not pure. And the first one need a hook to
> an allocating function in druntime, function which isn't pure, but made
> look like a pure one, and that is able to call impure gc.collect .
>
> gc.collect is a system wide procedure that involve all thread runniong
> in your application and every single piece of memory in it. This is
> probable the most far away from pure function I can think of.

You're still just lying to yourself by thinking that a GC allocation is 
strictly pure. I understand that there is a difference between implicit 
and explicit collection, but it boils down to the same thing. D is a 
systems language, and a pragmatic language. We're not Haskell. Let's be 
practical.

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