Understanding Variance in Kotlin: Type Position Issues

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1

I’m facing a compilation issue with the following Kotlin code:

fun interface Producer<out R> {
    fun produce(): R

    fun <T> T.invokeConsumers(
        vararg consumers: Consumer< 
                R /*Type parameter R is declared as 'out' but occurs in 'in' position in type User<R, Producer<R>>*/, 
                Producer<R>
            >
    ): List<R> =
        consumers.map {
            it.consume(this@Producer)
        }
}

fun interface Consumer<out R, in P : Producer<R>> {
    fun consume(p: P): R
}

I am getting compilation error Type parameter R is declared as 'out' but occurs in 'in' position in type User<R, Producer<R>>, but R is in out position in both interfaces.

Why doesn’t it compile?

Also I would like to leave invokeConsumers function declaration as it is - inside the Producer interface, as an extension for generic type T

1 Like
2

You could likely use @UnsafeVariance here.

3

This code is a simplification of what I would like to use in my validation library. Won’t this cause problems in the future?
Also thank you for your response!

4

I’ve noticed it can be simplified further:

interface ListConsumer<out T> {
    fun consume(list: List<T>) // Type parameter T is declared as 'out' but occurs in 'in' position in type List<T>
}

The compiler message is a bit unclear.
This does not work, because covariant type T is being used in in position - it is used in list function argument, which is not allowed

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5

Function arguments are on example of in position (the object will receive an instance, thus in, opposite of a return type, where an object produces an instance, thus out).

7

The compiler is right. You will meet an CCE (ClassCastException) in the following case.

interface ListConsumer<out T> {
    fun consume(list: List<@UnsafeVariance T>)
}

class ChildConsumer: ListConsumer<Child> {
    override fun consume(list: List<Child>) {
        val child = list[0]
        println(child.c)
    }
}

open class Parent
class Child: Parent() {
    val c = 1
}

fun main() {
    val c: ListConsumer<Parent> = ChildConsumer()
    c.consume(listOf(Parent()))
}
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