Can enum and immutable be unified?

[Don]

Ary Borenszweig wrote:
> Michiel Helvensteijn escribió:
>  > Walter Bright wrote:
>  >
>  >> immutable - data that cannot change or be changed (imagine it is stored
>  >> in ROM)
>  >>
>  >> const - read only view of data, you cannot change it but others can
>  >>
>  >> enum - compile time constant, has no storage
>  >>
>  >> The only place these overlap is in the declaration of symbolic
>  >> constants. C++ has all three, but in a way that is context dependent
>  >> that very few people are aware of.
>  >
>  > Aren't immutable and enum the same thing? At least to the programmer?
> 
> Yesterday I was thiking the same thing. I think the only difference to 
> the programmer between those is that you can take the address of 
> something immutable, but can't take the address of a compile time constant.
> 
> Now I wonder why would you like to take the address of something that's 
> immutable. The times I remember passing something's address is to change 
> it, something like:
> 
> int x = 10;
> foo(&x);
> // now x might have changed
> 
> But immutable variables cannot change, so no need to pass it by reference.
> 
> The other usage is performance. If it's an immutable big struct you 
> might want to pass it by reference instead of copying the whole thing. 
> If you can't pass a reference to something immutable you couldn't do 
> with this optimization. But in this case, the compiler could choose to 
> rewrite mentions to big immutable variables as references to those in a 
> transparent way to the programmer. Of course this is harder for the 
> compiler writer, but it's much easier for the user.
> 
> (as a starting point the compiler could do without this optimization and 
> see if the users are having problems without it)
> 
> I can't come up with other cases where you'd want to pass a reference to 
> something immutable.
> 
> So let's suppose we don't need to take an immutable varaible's address. 
> We get rid of "enum" then and unify "immutable" and "enum" into the 
> single "immutable" keyword. The compiler still has to determine what to 
> put in ROM and what not. But I think this one is easy: primitive types 
> can go in ROM, also maybe small immutable structs, maybe primitive 
> arrays, but not classes or references to classes.
> 
> Is there something fundamentally wrong with this reasoning?

AFAIK, the only reason 'enum' manifest constants exist in D2 is because 
of linking limitations. OPTLINK isn't smart enough to be able to discard 
immutable references. There's no need to use them, unless you have 
something like the Windows headers, where they're an enormous bloat if 
they aren't discarded. Most people should forget they exist.