Partially pure (Re: Fully transitive const is not necessary)

[Walter Bright]

Don Clugston wrote:
> Walter Bright wrote:
>> A function is pure or it isn't, there really isn't any room for 
>> "unstable" purity, or the point of purity is lost.
> 
> I think there is. A function which accesses a global is inherently 
> impure. But, a function which takes a non-invariant argument, can in 
> theory safely be called from inside a pure function, provided any 
> non-invariant arguments are local variables, or invariant.
> 
> eg.
> 
> class C
> {
>   int numcalls;
>   this() { numcalls=0; }
>   void foo() { ++numcalls; } // Not pure - but no global side effects.
> }
> 
> pure int bar(int x)
> {
>     C c = new C;
>     for (int i=0; i<x; ++i) c.foo();
>     return c.numcalls;
> }
> 
> Is bar() pure or not?

I think it is pure, although it might be difficult for the compiler to 
detect it. Like for CTFE, I think the compiler should start out rather 
restrictive about what it regards as pure, and over time expand it as it 
gets more capable.