Is it possible to reference a specific inheritor f...

# announcements

[Seri]

Is it possible to reference a specific inheritor from an abstract parent, rather than any possible inheritor?

For example, if I have a abstract class State with some helper functions, can I have them return the child type, without it getting generalized to just “anything that inherits from `State`“?

[karelpeeters]

Yes, you just have the inheritor as the return type. Just like everything inherits Any and you can still say you return a String.

[Seri]

But that’d only work in a case where I’ve got a single inheritor to refer to. What could I do if I wanted to make it reusable?

[Seri]

The best thing I have so far is to describe the parent with the following signature:
abstract class State<S: State<S>>
and each child like so:
sealed class TestState: State<TestState>

[Seri]

but this looks pretty ugly

[Shawn]

ohh

[Shawn]

this sounds like you kinda want bootleg self types

[Shawn]

via the curiously recurring template pattern

[Shawn]

or, rather, that’s what you’re doing

[Shawn]

and unfortunately there isn’t a better way of doing it in Kotlin or on the JVM in general 😞

[Shawn]

dunno if you already have something like this but, this is how I’d do it:

abstract class State<S : State<S>> {
  protected abstract fun self(): S

  fun helperFunc(): S {
    doHelperStuff()
    return self()
  }
}

[Shawn]

and have children implement the self() method by just returning this

[Shawn]

assuming you want to return the child itself - otherwise just having the return type be S and keeping things abstract works fine

[Seri]

Ah, gotcha, thanks @Shawn! I think I’ll just have to keep things abstract and always work with S in the parent class.

[karelpeeters]

I'm sorry, I completely misunderstood. Thanks @Shawn for actually being helpful simple smile

[Seri]

No worries, @karelpeeters! It’s a tough problem to articulate