Relaxed assignment conversions for sealed types

[Brian Goetz]

No one bit on this, but let me just point out a connection that may help 
motivate this: sealed types are union types.

If we say

     sealed interface A permits X, Y { }

then this is like:

     A = X | Y

A structural interpretation of union types says that

     X <: I  &  Y <: I   -->    A <: I

Essentially, this idea says we can borrow from the union nature of 
sealed types when convenient, to provide better type checking.

As mentioned below, the real value of this is not avoiding the cast, but 
letting the type system do more of the work, so that if the implicit 
assumption is later invalidated, the compiler can catch save us from 
ourselves.  A cast would push assumption failures to runtime, where they 
are harder to detect and potentially more costly.


On 10/9/2020 11:16 AM, Brian Goetz 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.
>
>
>
>
>
>

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