Hi guys. I’m studying kotlin and I’m on type proje...

# kotlin-in-action

[robinho]

Hi guys. I’m studying kotlin and I’m on type projection section.
What I miss?

open class Noun(val n: String)
open class City(val c: String) : Noun(c)
open class Cabrobro(val j: String) : City(j)

fun <T> copyData(source: MutableList<T>, destination: MutableList<in T>) {
    for (item in source) {
        destination.add(item)
    }
}

fun main(args: Array<String>) {
    val source= mutableListOf(City("c1"), City("c2"), City("c3"))
    val destination= mutableListOf<Cabrobro>()
    copyData(source, destination)
}

[robinho]

Why the compiler are not accepting MutableList<Cabrobro> as destination?

[bdawg.io]

This seems to be a LSP issue. All Cabrobro instances are also City, but not all City instances are Cabrobro. So there’s not really a way to convert City into all potential sub types implicitly.

Trying to do the inverse and qualifying a second generic type as a subtype of the first would work instead

fun <T, R : T> copyData(source: List<R>, destination: MutableList<T>) {
    for (item in source) {
        destination.add(item)
    }

    // can also be written as just:
    // source.map(destination::add)
}

[bdawg.io]

This is also a great use case for an extension function

fun <T, R : T> List<R>.copyTo(destination: MutableList<T>): MutableList<T> {
    this.map(destination::add)
    return destination
}

//inside main
val destination = source.copyTo(mutableListOf<City>())

[robinho]

Thanks @bdawg.io