Casting to Unit

[allanrenucci]

With Scala 2:

scala> "Hello".asInstanceOf[Unit]
// OK, nothing happens

With Dotty:

scala> "Hello".asInstanceOf[Unit]                                                                                                                                                                               
java.lang.ClassCastException: java.lang.String cannot be cast to scala.runtime.BoxedUnit

What is the right behavior?

[sjrd]

FWIW, the behavior of Scala 2 is, let's say, convenient for Scala.js. ;)

[smarter]

@sjrd can you elaborate ? This seems pretty wrong to me, and the scala spec doesn't mention it.

[sjrd]

Scala.js specifies that the result of any call to a JS method is explicitly cast to the declared result type of the method. We need this because JS can return anything, really, so the cast is necessary for soundness.

Now for methods that are declared as returning Unit, we want to tolerate the fact that JS might return something else than undefined, and we don't to spend code and time checking that it's actually undefined. The fact that .asInstanceOf[Unit] works as it does in Scala 2 means we don't need to special-case that (both in spec and in the implementation).

[smarter]

Sounds like Scala.js should use an ascription : Unit here really :).

[sjrd]

That wouldn't be consistent with the other types, which must be cast.

[abgruszecki]

Since Unit is a singleton type, here's more food for thought:

scala> 0.asInstanceOf["a"]
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String

scala> val notA: "a" = "b".asInstanceOf["a"]
val notA: String("a") = b

scala> "b".isInstanceOf["a"]
val res0: Boolean = false

In my opinion, this makes the semantics of asInstanceOf very confusing. As long as we're able to do it, we should ensure that the value we're casting is actually an instance of the type we're casting to.

@sjrd could you take a look at the above? It seems to me that just blindly accepting "b" as an instance of "a" is not something that we'd want for casting the values returned from JS methods in Scala.js, no?

[LPTK]

@AleksanderBG asInstanceOf is just an unsafe escape hatch from the type system. It's not meant to be sound and never will be, because of erasure. For example consider List(1).asInstanceOf[String], or def f[A](x:Any):A=x.asInstanceOf[A]; f["b"]("a").

[sjrd]

To go a bit further than what @LPTK says, x.asInstanceOf[A] is basically specified as "do something implementation-dependent that convinces the runtime to let me use x as an A". So "b".asInstanceOf["a"] succeeding is OK, because the runtime was convinced.

Similarly, in Scala.js, we are looking at soundness from the point of view of the erased type system of the IR. As such "b".asInstanceOf["a"] would also succeed and be just as sound, because the IR deals with strings, not with singleton types.

[abgruszecki]

I believe we can conclude our offline discussion with @sjrd as: x.asInstanceOf[T] is more like a coercion that ensures the runtime does not explode right now. As a side note, it really is quite surprising to me that isInstanceOf and asInstanceOf have basically unrelated semantics.

With that being said, I think this specific issue is an unintended change of semantics and should be fixed.