Why string alias is invariant ?

[Janice Caron]

On 1/31/08, Sergey Gromov <snake.scaly at gmail.com> wrote:
> Maybe a contract should be added for standard library that if a function
> returns a mutable array, it guarantees that this array can be modified
> without side effects and therefore can be safely assumed unique.

Yes, D has a problem here, in that it has no way to express
uniqueness. (That's true of other languages too, of course). If a
string is immutable, then it doesn't /have/ to be unique (but if it
is, it's safe to cast it to invariant), but if a string is mutable
then it /must/ be. That's a problem, because one simply cannot declare

    unique(char)[] array;

(...although it would be interesting to try...) So D just makes it your problem.

One problem area where this sort of thing arises is concatenation. If
you concatenate char arrays, then /regardless/ of the constancy of the
inputs, the result is guaranteed to be unique (because, by
specification, concatenation always makes a copy). If we had a
"unique" type-constructor, one could make the result type unique(T)[].
But we don't, so D makes the (arbitrary) choice of invariant(T)[].
Also, it is currently a syntax error to concatenate a char[] with an
invariant(char)[], even though both will implicitly cast to
const(char)[].

So it's not a perfect system, but it is better than what we had
before, and (I would argue) better than C++. But remember that D2.x is
experimental. We're here to play with it. If we don't like it, things
could change. Our experiences are important here in shaping the
future.