``` abstract class A abstract val value String abstract fun kotlinlang #announcements

04/10/2019, 1:45 PM

abstract class A {
    abstract val value: String
    abstract fun <T: A> test(): T
}

data class B(
    override val value: String,
    val value2: String
): A() {
    override fun test() 
        = this.copy(value = "HelloWorld")
}

How can I achieve something like this?

04/10/2019, 1:47 PM

abstract class A<T : A<T>> {
    abstract val value: String
    abstract fun test(): T
}

data class B(
    override val value: String,
    val value2: String
): A<B>() {
    override fun test()
            = this.copy(value = "HelloWorld")
}

04/10/2019, 1:48 PM

Recursive templates are about the only way you can approximate self types on the JVM unfortunately

04/10/2019, 1:51 PM

I tried this way, this results in getting A back. I have some other function that takes B in, so I want to be able to call

funTakesB(b.test())

, but this fails as b.test() returns

A<*>

04/10/2019, 1:52 PM

04/10/2019, 1:52 PM

an instance of

I'll just cast it, I know it will always be the right one

04/10/2019, 1:54 PM

I can't reproduce what you're reporting:

abstract class A<T : A<T>> {
    abstract val value: String
    abstract fun test(): T
}

data class B(
    override val value: String,
    val value2: String
): A<B>() {
    override fun test()
            = this.copy(value = "HelloWorld")
}

fun funTakesB(b: B) {

}

fun foo() {
    val b = B("foo", "bar")
    funTakesB(b.test())
}

This works fine

04/10/2019, 1:54 PM

If it's an instance of

it will work fine. Are you sure it's not inferred as an instance of

04/10/2019, 1:55 PM

probably, I will write a better example. Wait a min

I don't find any easy way of showing the use case for this, as I'm working in a bigger project. But this is it: https://pl.kotl.in/B1SSwdiF4

as T

there, which works fine. Was just wondering if I somehow can get rid of it

streetsofboston

04/10/2019, 2:04 PM

abstract class Service<T: A<T>> {
    abstract fun extra(any: T)
    fun someWork(any: T) {
        extra(any.test())
    }
}

04/10/2019, 2:05 PM

Ah, sorry. That was obvious xD

Thanks for the help