I don t understand why kotlin is so different to others

Question

ghedeon · Answer

your `vv` is just a local variable

ghedeon · Answer

hope you don t expect local variables to be accesable from outside of the class Otherwisie I m afraid to ask about others

Ifvwm · Answer

instance variable I don t know if there s this concept in kotlin

ghedeon · Answer

probably you re looking for ``` class C lt T gt v T val vv = v ```

Ifvwm · Answer

functions in class they need to visit variables in the class

Ifvwm · Answer

in python we call self var

ghedeon · Answer

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

ghedeon · Answer

< C0B8MA7FA|getting started>

Ifvwm · Answer

T here is Int

marstran · Answer

Not in your add function There it s just `T` You probably want this ``` fun add x C lt Int gt y C lt Int gt Int return x vv + y vv ```

Ifvwm · Answer

yes that is what I want

Ifvwm · Answer

but I don t know why T doesn t work here

cbruegg · Answer

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 · Answer

`T` doesn t work there because it could be literally any type You could invoke `add C false C false `

Ifvwm · Answer

yeah but T is made for polymorphic

Ifvwm · Answer

pass string sure it may be error

cbruegg · Answer

Sorry had a bad example Boolean is a better example here

Ifvwm · Answer

sneaky string + string is ok

cbruegg · Answer

You mean it should be an error for strings at runtime

cbruegg · Answer

We don t want that we want to prevent this at compile time

cbruegg · Answer

We want to avoid functions that only work for selected inputs

cbruegg · Answer

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 · Answer

ok what if I don t return x vv+y vv I only return x vv that s ok

cbruegg · Answer

That s okay exactly

Ifvwm · Answer

but the editor pops up Unit was expected

cbruegg · Answer

`fun lt T gt justReturn C lt T gt c T return c vv `

cbruegg · Answer

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

Ifvwm · Answer

I don t understand why function should return only Unit type

cbruegg · Answer

Using ` T`

cbruegg · Answer

If you omit that it defaults to ` Unit`

cbruegg · Answer

Which is comparable to not returning anything

Ifvwm · Answer

return

cbruegg · Answer

Sorry

Ifvwm · Answer

how return x vv

cbruegg · Answer

`fun lt T gt justReturn C lt T gt c T return c vv `

Ifvwm · Answer

ok one more thing why I should put a lt T gt before the function s name

Ifvwm · Answer

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

cbruegg · Answer

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 · Answer

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 · Answer

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

Ifvwm · Answer

ok

gildor · Answer

Have you checked guide Kotlin for Python developers by Khan Academy It should be helpful for you < C0B8MA7FA|getting started> also a good channel for such questions

Ifvwm · Answer

ok