Why string alias is invariant ?

[Jari-Matti Mäkelä]

Janice Caron Wrote:

> 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).

That's not true. Linear types and their derivatives solve this problem
elegantly and have been known at least since 1990. They have interesting
properties. The data can be simultaneously "constant" (referentially
transparent) and unique, and still destructive updates are possible in
situ. The Clean language is an example of this, it has uniqueness
types[1].

But I'm afraid the current implementations of D aren't really
well suited for these kinds of "advanced" type systems in the near
future...

[1] http://www.cs.ru.nl/~clean/download/papers/1996/bare96-uniclosed.pdf


> 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.