Attributes

[Alaric B. Williams]

> To confirm my understanding of purity, a function is pure if it only
> performs actions necessary to its purpose.  For example if a function is to
> calculate a result, it doesn't also create a persistent variable somewhere
> that remains when the function exits.

Hmmm... I think that skirts the crucial point somewhat. Purity is 
sometimes the topic of flamewars, but the general idea is that this 
is not a pure function:

int LastSum = 0;

int running_sum(int x) {
   return (LastSum += x);
}

since it matters what order the calls occur in:

running_sum(1) => 1
running_sum(10) => 11

but

running_sum(10) => 10
running_sum(1) => 11

IE, with impure functions, you have to be careful what is called 
where. Impure functions compute [some aspect of] their return value 
from a mutable value that is not local to the function - for example, 
this function is really pure, although it does dirty stuff inside, it 
hides that away:

int sum(int x,int y) {
   int temp = 0;
   temp += x;
   temp += y;
   return temp;
}

I know that's what you're getting at, but you said it in a very 
strange way IMHO :-)

> You seem to be speaking of Attributes as the same thing as Annotations.  Is
> it true that you use the two terms synonymously in all cases?

I'd say that annotation is a subset of attribute... annotation is 
one particular function.

> David E. Manifold
> dem@pacificrim.net

ABW
--
Alaric B. Williams Internet : alaric@abwillms.demon.co.uk
http://www.abwillms.demon.co.uk/