Custom Either doesn't work

Language Design
1

I am trying to use a code like this:

sealed class Either<Animal, Plants>
abstract class Farm<Animal>(val value: Animal) : Either<Animal, Nothing>()
abstract class Garden<Plants>(val value: Plants) : Either<Nothing, Plants>()

class MyFarm : Farm<Chicken>(rosita)

fun myFunction(value: Either<Chicken, Rose>) {
}

I cannot call myfunction with an instance of MySubclass, the IDE says: “Type mismatch”
Is it correct, or it’s a language bug?

2

Sorry, your post is unreadable and does not show the error you are getting. Could you fix it?

3

Sorry, edited. It is clear now?

4

Will this suffice?

sealed class Either<A, B>
abstract class Left<A>(val value: A) : Either<A, Nothing>()
abstract class Right<B>(val value: B) : Either<Nothing, B>()

class MySubclass<A>(valueA: A) : Left<A>(valueA)

fun <A,B> myfunction(value: Either<A, B>) {
    println(value)
}
5

It’s correct, not a language bug. You said “MySubclass”, and I assume you mean “MyFarm”. If you add out to the type parameter in Either, to mark the type parameter as covariant, then your code will compile.

sealed class Either<Animal, out Plants>
...
val f = MyFarm()
myFunction(f)

A Nothing is a subtype of Rose, but unless the type parameter is covariant, a Thing<Nothing> is not a subtype of Thing<Rose>. Probably you want to mark both type parameters.

Rob

6

Here is the documentation for variance in generics:
https://kotlinlang.org/docs/reference/generics.html#variance

7

it’s correct