D const design rationale

[Reiner Pope]

Walter Bright wrote:
> 
>> I can appreciate that 'invariant' may be of tremendous use to the 
>> compiler, but I balk at the notion of adding language features that 
>> seem largely intended as compiler "hints."
> 
> It's a lot more than that. First, there's the self-documenting aspect of 
> it. Second, it opens the way for functional programming, which can be of 
> huge importance.
> 
You mention functional programming a fair bit with respect to const, 
which is nice to hear. But nothing in the current const system allows 
you to declare a verifiably 'pure' function; can we expect some 
annotation for functions which says 'this function doesn't read/write 
any global variables?'

     int b

     int foo()
     {
         b++;
         return b * 2;
     }

     pure int square(int x)
     {
         return x * x;
     }

     pure int baz()
     {
         return foo(); // fails: foo is not pure
     }