<@U3DE1TXKP> That's something else, there you prom...

# announcements

[karelpeeters]

@robstoll That's something else, there you promise you can return whatever the caller want you to return (as long as it extends T), which is very rarely true.

[robstoll]

I not really getting what you write. IMO a user cannot return anything as long as it extends T but has to return something which extend U which in turn has to extend T

[karelpeeters]

Consider the following code:

class Test<T: Any> {
    fun <U: T> foo(): U = TODO()
}

interface Parent
interface A: Parent
interface B: Parent

fun main(args: Array<String>) {
    val test = Test<Parent>()
    val a = test.foo<A>()
    val b = test.foo<B>()
}

[karelpeeters]

I can just demand whatever type I want from foo because it promises it can return whatever .

[karelpeeters]

But in practice that is almost never possible, even more so because foo doesn't have any generics type information at runtime.

[robstoll]

wow... I would have never expected that. What is the sense of using a type-parameter as upper bound if not the instantiated type parameter is taken into account?

[karelpeeters]

It only comes up in utility casting methods, eg. in android findViewById returns whatever type you want by casting.

[robstoll]

IMO that's a misconception in Kotlin, I'll file a bug 😄

[karelpeeters]

Haha it really isn't, that's how it works in Java too and how the code is expected to behave.

[robstoll]

seriously? what is wrong with those guys 😄

[robstoll]

please don't say it's also like that in Scala

[karelpeeters]

Its like that in every language with generics I'd think?

[karelpeeters]

That's really what the code is saying: I return the type specified by the generic parameter.

[robstoll]

man, it seems I should invent a new language 😄

[robstoll]

I agree on that one, but the type parameter has an upper bound restriction

[karelpeeters]

That doesn't change anything really, now it's just "I return the type specified by the parameter, and that parameter has to extend something".

[robstoll]

well, isn't that a fundamental change? Taking your example I would have expected that I can only call foo with subtypes of Parent

[robstoll]

oh man... I am such a bad reader

[karelpeeters]

That is true? A and B are subtypes.

[robstoll]

A and B are subtypes of parent

[karelpeeters]

Haha okay.

[robstoll]

phuuu... the faith in programmers is restored again 😄

[robstoll]

Thanks for the nice talk anyway 🙂

[karelpeeters]

No problem, I love torturing myself by thinking about generics simple smile