On 2013-04-08, 17:07, Jacob Carlborg wrote:
> On 2013-04-08 14:52, Iain Buclaw wrote:
>> On 8 April 2013 13:25, Jacob Carlborg <doob at me.com <mailto:doob at me.com>>
>> wrote:
>>
>> On 2013-04-08 10:29, Iain Buclaw wrote:
>>
>> This information could possibly be helpful. Though given that
>> most of
>> (gdc) codegen is on par with g++, there's probably not much on
>> the list
>> that isn't already detected by the backend optimisation passes.
>>
>>
>> Multiple calls to pure functions could be cached.
>>
>> --
>> /Jacob Carlborg
>>
>>
>> Not always, but in some circumstances, yes.
>>
>> ---
>> struct Foo
>> {
>> int a = 0;
>> pure int bar (immutable int x)
>> {
>> ++a;
>> return x * 2;
>> }
>> }
>>
>>
>> void main()
>> {
>> Foo f;
>> int i = f.bar(2) + f.bar(2);
>>
>> assert (i == 8);
>> assert (f.a == 2);
>> }
>
> I though that wasn't possible. What's the point of pure if that's
> possible?
Like others have stated, it's so you can do this:
struct Foo {
int a = 0;
pure int bar( int n ) { // Weakly pure
a += n;
return a;
}
}
pure int Baz( int n ) { // Strongly pure
Foo foo;
return foo.bar( n );
}
Foo.bar has only local mutability, so if the Foo originated in a
(strongly) pure function, Foo.bar can be called inside that pure
function without violating the purity requirement.
The three levels (weakly pure/strongly pure/impure) are needed,
but a redesign would perhaps use a different keyword for weakly
pure.
--
Simen