I don't understand why kotlin is so different to others

# announcements

[Ifvwm]

I don't understand why kotlin is so different to others

[ghedeon]

your vv is just a local variable

[ghedeon]

hope you don't expect local variables to be accesable from outside of the class? Otherwisie I'm afraid to ask about "others".

[Ifvwm]

instance variable, I don't know if there's this concept in kotlin

[ghedeon]

probably you're looking for

class C<T>(v:T) {
    val vv = v
}

[Ifvwm]

functions in class, they need to visit variables in the class

[Ifvwm]

in python, we call self.var

[ghedeon]

also, the + operation in your code is not defined for T, so vv + vv won't work.

[ghedeon]

#C0B8MA7FA

[Ifvwm]

T here is Int

[marstran]

Not in your add function. There it's just T.
You probably want this:

fun add(x: C<Int>, y: C<Int>): Int {
    return x.vv + y.vv
}

[Ifvwm]

yes, that is what I want

[Ifvwm]

but I don't know why T doesn't work here

[cbruegg]

Python is very different from Kotlin when it comes to type-safety. You can do lots of stuff in Python that's not guaranteed to work in all cases and there's no compiler to scream at you that you're doing something unsafe

[cbruegg]

T doesn't work there because it could be literally any type. You could invoke add(C(false), C(false))

[Ifvwm]

yeah, but T is made for polymorphic

[Ifvwm]

pass string, sure, it may be error,

[cbruegg]

Sorry, had a bad example, Boolean is a better example here

[Ifvwm]

sneaky, string + string is ok

[cbruegg]

You mean it should be an error for strings at runtime?

[cbruegg]

We don't want that, we want to prevent this at compile-time

[cbruegg]

We want to avoid functions that only work for selected inputs

[cbruegg]

It's actually easier this way: If you declare the function properly, you know what types of values are safe to call it with. In Python for instance it's not always obvious what inputs a function actually accepts.

[Ifvwm]

ok, what if I don't return x.vv+y.vv, I only return x.vv that's ok?

[cbruegg]

That's okay, exactly

[Ifvwm]

but the editor pops up Unit was expected

[cbruegg]

fun <T> justReturn(C<T> c): T { return c.vv }

[cbruegg]

You need to declare that you want to return a value of type T

[Ifvwm]

I don't understand why function should return only Unit type

[cbruegg]

Using : T

[cbruegg]

If you omit that, it defaults to : Unit

[cbruegg]

Which is comparable to not returning anything

[Ifvwm]

return ?

[cbruegg]

Sorry?

[Ifvwm]

how return x.vv

[cbruegg]

fun <T> justReturn(C<T> c): T { return c.vv }

[Ifvwm]

ok, one more thing, why I should put a <T> before the function's name

[Ifvwm]

and I must define T in class or interface before I use it in function?

[cbruegg]

It's a type parameter, basically you're saying "this function works for all sorts of types, not only a particular one". Like a template.

[cbruegg]

Maybe you can have a look at this article to understand generic classes and functions better: https://www.oracle.com/technetwork/articles/java/juneau-generics-2255374.html

[cbruegg]

It's for Java, but many concepts explained there apply for Kotlin as well

[Ifvwm]

ok

[gildor]

Have you checked guide Kotlin for Python developers by Khan Academy?
It should be helpful for you

https://kotlinlang.org/docs/tutorials/kotlin-for-py/introduction.html

#C0B8MA7FA also a good channel for such questions

[Ifvwm]

ok