What purpose to const besides functional programming?

[Walter Bright]

Jason House wrote:
>>> #5 is not true.  Invariant functions require synchronization because
>>> they can manipulate global state.
>> Not so. The only invariant functions in D are member functions which
>> cannot change the state referenced by 'this'. They can certainly change
>> other state. Pure functions, on the other hand, may not manipulate
>> global state.
> 
> Maybe an example will help:
> 
> int a;
> int b;
> 
> struct bar{
>   void write(int x) invariant
>   out{ assert(a==b); }
>   body{
>     a = x;
>     b = x;
>   }
> }
> 
> The state of bar is never modified, but bar should not be used without
> synchronization.  The function's contract can be violated due to a race
> condition.  This may be an artificial example, but I hope it shows that #5
> requires qualification.

You're imputing meaning to an "invariant function" that isn't there. An 
invariant function is a function that does not change the state of the 
object it is a member of. It can change other state.