Got lost in generics a bit. Is there any way to avoid unchecked cast here?

you do have an unchecked cast

Classic recursive generics:

abstract class FSM<STEP : FSM.Step<DATA, STEP>, DATA>(firstStep: STEP) {

    var currentStep: STEP = firstStep
        private set

    fun next(data: DATA) {
        currentStep = currentStep.next(data)
    }

    fun prev(data: DATA) {
        currentStep = currentStep.prev(data)
    }

    interface Step<DATA, S: Step<DATA, S>> {
        fun next(data: DATA): S
        fun prev(data: DATA): S
    }
}

works wonders! Probably even covariant?:

Step<DATA, out S : Step<DATA, S>>

?

Depending on the intent, sure.

You could also declare the type of

currentStep

as

Step<DATA>

I think complex type relations should be avoided.

True! A second working solution. The downside is,

currentStep

is public, so the usage is going to be strange. You feed your subclassed steps to FSM, but it gives you the superclass current.

Ah yeah, fair enough. Then Karel's solution is better :)