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

# announcements

[jeggy]

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?

[diesieben07]

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")
}

[Shawn]

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

[jeggy]

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<*> instead of B

[Ruckus]

What is b?

[jeggy]

an instance of B

[jeggy]

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

[diesieben07]

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())
}

[diesieben07]

This works fine

[Ruckus]

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

[jeggy]

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

[jeggy]

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

[jeggy]

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

[streetsofboston]

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

[jeggy]

Ah, sorry. That was obvious xD

[jeggy]

Thanks for the help