On Thu, 18 Jun 2009 18:16:36 -0400, div0 <[email protected]>
wrote:
Pure functions are not allowed to alter global state.
That's what you doing when you create a new object.
http://en.wikipedia.org/wiki/Functional_programming
I once thought as you do. You are wrong. Memory allocation is a
necessary function, and although it is inherently unpure, it must be
allowed in pure functions.
The problem is that the function boundary is where purity is enforced, not
inside. For instance, basic programming 101, split common tasks into
functions:
int[] f()
{
int[] x = new int[5];
foreach(i, ref n; x) n = i;
return x;
}
int[] g()
{
int[] x = new int[6];
foreach(i, ref n; x) n = i;
return x;
}
f and g can be pure, because they do not have side effects. However,
consider:
void fill(int[] x)
{
foreach(i, ref n; x) n = i;
}
int[] f()
{
int[] x = new int[5];
fill(x);
return x;
}
Note that f is still pure, and fill is contextually pure, since in f's
context, it is not altering external state.
However, we cannot mark fill as pure, because when called from a non-pure
function, it could alter external state. Therefore, fill cannot be called
from f if f is pure.
This is similar to calling a method on an object:
C c = new C;
c.method();
The method call translates to:
C.method(c);
The compiler cannot tell whether c is external state or not, just like it
cannot tell that fill() can be contextually pure. So it is not allowed.
I'm not sure what the solution is. Something like 'unique' which would
ensure that exclusive ownership of an object passes to the method would
possibly allow passing mutable state into a pure function. I'm unsure if
there are any plans for something like this.
-Steve
