Hi! Can anyone explain why this code doesn't work?...

# announcements

[Dougrinch]

Hi! Can anyone explain why this code doesn't work?

typealias F<T> = (T) -> T

fun test() {
  val id: F<*> = { it }
  consume(id)//error here
}

fun <T> consume(v: F<T>) {
}

It looks like all should be fine, but the compiler tells me Type mismatch: inferred type is Any? but CapturedType(*) was expected. I thought that the star means "some unknown T", am I wrong?

[streetsofboston]

F<*> is like (in Nothing) -> out Any?

[Dougrinch]

Yeah, I see it in the documentation, but why?

[Dougrinch]

Ok, I can ask another question. What would go wrong if I this compiled.

[streetsofboston]

Because you won’t be able to provide an instance of F for any given type parameter that satisfies Function1<in Nothing, out Any?>

[streetsofboston]

In other words, there is no type T that is a (sub)class of Nothing and a (super)class of Any?

[streetsofboston]

Your consume function is declared in such a way that it cares about the type T of F, but you give it an instance that is declared to not care about its generic type. A function fun consume(v: F<*>) will compile fine.

[Dougrinch]

In other words, it could compile, but in this case, inside the consume, I wouldn't do anything with v except default Any things (like toString). Is this what you mean?

[streetsofboston]

Yup. consume will actually never call invoke(..) on v, it will never call any other method that deals with T , if F had other such methods. Using v: F<*> is therefore sufficient.

[Dougrinch]

Ok. Thanks a lot

[streetsofboston]

E.g. you can call size or hashCode() or isEmpty() on a MutableList<*>. None of these functions/properties use the generic parameter T.

You can even call get(Int): T on it and get an Any? back safely, but it is kind-of meaningless.

You can_not_ call add(item: T) on it (since there is no way of proving a value for item, since it is of type `Nothing`…