Relaxed assignment conversions for sealed types

[Tagir Valeev]

Hello!

I'm a little bit scared of introducing this kind of implicit cast into
the assignment operation. This looks like the right operand in
MyFooImpl mfi = f; becomes a poly-expression, as the type of the f
expression now depends on the mfi type. How will it interact with
other constructs? E.g. can we write MyFooImpl getFooImpl(MyFoo f)
{return f;}? Or aMethodAcceptingFooImpl(f)? What about something that
requires more inference, like Function<MyFoo, MyFooImpl> = f -> f; ?

I believe this would be either a very special rule applicable to very
specific constructors (e.g. variable declaration only) or it will
overcomplicate the type inference, possibly introducing new constructs
where all the types cannot be unambiguously inferred. In the former
case, this could be confusing. People would ask: why it works in
declarations but doesn't work in assignments/method
calls/returns/lambdas?

If we want a compile-time check that 'this cast is safe as there's
exactly one implementor', I think this should be done in a more
explicit way. Btw we already designed such a way:
MyFooImpl mfi = switch(f) {case MyFooImpl _mfi -> _mfi;};
This would do the thing, though may look somewhat ugly. We can add
some sugar if anybody doesn't like this syntax. Probably like
MyFooImpl mfi = (safe-cast-to MyFooImpl)f;
But my point is that this kind of downcast should be explicit.

I'm also not sure that this construct will be necessary so often.
First, it's for internal use only, so the surface is quite limited:
the clients don't need it. Second, even in internal uses, it's likely
that in most of the cases the public API of interface Foo will be
enough, so the downcast won't be necessary. Third, it would not be so
hard to create a private one-liner method for this:

private class MyFooImpl {
  private static MyFooImpl asImpl(Foo f) {
    return switch (f) {case MyFooImpl mfi -> mfi;};
  }
}

This will keep a compile-time check and allow downcasting in any
context. E.g. even in qualifiers, like
asImpl(f).methodThatExistsInImplOnly().

I don't think we should overcomplicate the language here.

With best regards,
Tagir Valeev.

On Fri, Oct 9, 2020 at 10:16 PM Brian Goetz <brian.goetz at oracle.com> wrote:
>
> Here's an idea that I've been thinking about for a few days, it's not urgent to decide on now, but I think it is worth considering in the background.
>
> When we did expression switch, we had an interesting discussion about what is the point of not writing a default clause on an optimistically total enum switch (and the same reasoning applies on switches on sealed types.)  Suppose I have:
>
>     var x = switch (trafficLight) {
>         case RED -> ...
>         case YELLOW -> ...
>         case GREEN -> ...
>     }
>
> People like this because they don't have to write a silly default clause that just throws an silly exception with a silly message (and as a bonus, is hard to cover with tests.)  But Kevin pointed out that this is really the lesser benefit of the compiler reasoning about exhaustiveness; the greater benefit is that it allows you to more precisely capture assumptions in your program about totality, which the compiler can validate for you.  If later, someone adds BLUE to traffic lights, the above switch fails to recompile, and we are constructively informed about an assumption being violated, whereas if we had a default clause, the fact that our assumption went stale gets swept under the rug.
>
> I was writing some code with sealed classes the other day, and I discovered an analogue of this which we may want to consider.  I had:
>
>     public sealed interface Foo
>         permits MyFooImpl { }
>     private class MyFooImpl implements Foo { }
>
> which I think we can agree will be a common enough pattern.  And I found myself wanting to write:
>
>     void m(Foo f) {
>         MyFooImpl mfi = (MyFooImpl) f;
>         ...
>     }
>
> This line of code is based on the assumption that Foo is sealed to permit only MyFooImpl, which is a valid assumption right now, since all this code exists only on my workstation.  But some day, someone else may extend Foo to permit two private implementations, but may not be aware of the time bombs I've buried here.
>
> Suppose, though, that U were assignable to T if U is a sealed type and all permitted subtypes of U are assignable to T.  Then I'd be able to write:
>
>     MyFooImpl mfi = f;
>
> Not only do I not have to write the cast (the minor benefit), but rather than burying the assumption "all implementations of Foo are castable to MyFooImpl" in implementation code that can only fail at runtime, I can capture it in a way the compiler can verify on every recompilation, and when the underlying assumption is invalidated, so is the code that makes the assumption.  This seems less brittle (the major benefit.)
>
> This generalizes, of course.  Suppose we have:
>
>     sealed interface X permits A, B { }
>     class A extends Base implements X { }
>     class B extends Base implements X { }
>
> Then X becomes assignable to Base.
>
> I'm not quite sure yet how to feel about this, but I really do like the idea of being able to put the assumptions like "X must be a Y" -- which people _will_ make -- in a place where the compiler can typecheck it.
>
>
>
>
>
>