shared - i need it to be useful

[Manu]

On Thu, Oct 18, 2018 at 6:50 AM Steven Schveighoffer via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
>
> On 10/18/18 9:35 AM, Steven Schveighoffer wrote:
> >
> > struct NotThreadsafe
> > {
> >    private int x;
> >    void local()
> >    {
> >      ++x; // <- invalidates the method below, you violate the other
> > function's `shared` promise
> >    }
> >    void notThreadsafe() shared
> >    {
> >      atomicIncrement(&x);
> >    }
> > }
> >
>
> [snip]
>
> > But on top of that, if I can't implicitly cast mutable to shared, then
> > this ACTUALLY IS thread safe, as long as all the casting in the module
> > is sound (easy to search and verify), and hopefully all the casting is
> > encapsulated in primitives like you have written. Because someone on the
> > outside would have to cast a mutable item into a shared item, and this
> > puts the responsibility on them to make sure it works.
> >
>
> Another thing to point out -- I can make x public (not private), and
> it's STILL THREAD SAFE.

I'm not sure that's an interesting design goal though.
Most things don't have shared methods. If you're writing a thing with
shared methods, you're very in the business of implementing
threadsafety... you're going to want to make it the tightest,
most-unlikely-to-have-threading-bugs thing you can write.
I predict that you're not going to be upset about the restriction.

And if you are, AND you're confident in your application to maintain a
mutually-exclusive shared/TL separation, then you can do this to your
hearts content! Nobody will stop you, and it will be fine.
But I don't think it should be default, because the rules as designed
that way, enforce *users* to perform unsafe casts when they're, on
average, not qualified to make those decisions.