Memory allocation purity

[via Digitalmars-d]

On Thursday, 15 May 2014 at 05:51:16 UTC, Ola Fosheim Grøstad 
wrote:
> On Thursday, 15 May 2014 at 02:49:28 UTC, Adam Sakareassen via 
> Digitalmars-d wrote:
>> The more D allows 'pure' functions to diverge from functional 
>> purity the less relevant the flag is for compiler 
>> optimisations.
> ...
>> By the same reasoning cacheability is important.  A pure 
>> function might be called within a loop with a parameter that 
>> is not altered during the loop.  If the compiler knows the 
>> function is pure, it can perform the calculation before the 
>> loop and simply reuse the cached result for each iteration.
>
> Yep, purity implies memoing. Malloc and new are not pure 
> because they return objects that can be differentiated by 
> address.

There's an important difference between malloc and new: malloc 
returns a pointer, but new returns a typed object. This is 
crucial IMO, because the returned objects are equal to each 
other. They aren't identical, but then different int variables 
with the same value aren't identical either, and a function 
returning int is still considered pure. So it's not identity (~ 
address) that matters.

>
> Pure in D seems pointless to me.

Not at all: Don't think of it in terms of low-level optimization 
opportunities, but in terms of semantics. For example, you get 
the concept of uniqueness. And the optimizations can still be 
done, because strongly pure functions can be recognized by their 
signatures.