On Sun, 18 May 2014 09:58:25 -0400, H. S. Teoh via Digitalmars-d
<[email protected]> wrote:
On Sat, May 17, 2014 at 11:51:44AM -0700, Jonathan M Davis via
Digitalmars-d wrote:
On Thu, 15 May 2014 08:43:11 -0700
Andrei Alexandrescu via Digitalmars-d <[email protected]>
wrote:
> On 5/15/14, 6:28 AM, Dicebot wrote:
> > This is not true. Because of such code you can't ever
> > automatically memoize strongly pure function results by compiler.
> > A very practical concern.
>
> I think code that doesn't return pointers should be memoizable.
> Playing tricks with pointer comparisons would be appropriately
> penalized. -- Andrei
Agreed. The fact that a pure function can return newly allocated
memory pretty much kills the idea of being able to memoize pure
functions that return pointers or references, because the program's
behavior would change if it were to memoize the result and reuse it.
Memoizing reference returns that are immutable should be fine.
However, that should have no effect on pure functions that return
value types - even if the function took pointers or references as
arguments or allocated memory internally. They should but perfectly
memoizable.
[...]
bool func(int x) /* pure? */ {
int[] a, b;
a = new int[x];
b = new int[x];
return (a.ptr < b.ptr);
}
Where do you draw the line?
Definitely this is fine being pure, and can be memoized. It just won't do
what you thought it would. So? :)
This is what Andrei meant as being "penalized."
-Steve
