D const design rationale

[Sean Kelly]

Walter Bright wrote:
> http://www.digitalmars.com/d/const.html

So in short, 'const' protects data and 'final' freezes references.  How 
do these two apply to an int declaration?

     const final int x = 5;

Is either a compiler error? are they synonyms in this case?

This aspect of the design seems very straightforward, aside from the 
question above.  What bothers me, however, is the use of 'invariant'. 
Adding a third keyword simply to represent data that's "really really 
const" just confuses things to me, and I haven't been able to get past 
this.  Given that adding a third keyword doubles the number of 
permutations for describing const behavior, I think the addition of 
'invariant' should be very carefully considered.  Is there any way we 
could get along without it?  I realize that 'invariant' would be rarely 
used in practice, but that doesn't change the impact an additional 
attribute has on the complexity of this design.

Frankly, I think we could almost get away with one keyword, but for the 
fact that D doesn't use a reference qualifier for class references. 
About the only workaround I could think of to describe a const reference 
to mutable data would be something like this:

     const (ref MyClass) x;

And inserting the 'ref' seems even more confusing than simply having 
'final' as in the current design.


Sean