In Java I can have unbound wildcard like

List<List<T> slideBy(Function<T, ?> classifier) {...

How do I do this in Kotlin. I tried

fun <T> slideBy(list: List<T>, function: (T) -> *): List<List<T>> = TODO()

but apparently that’s not legal. Should I do

fun <T> slideBy(list: List<T>, function: (T) -> Any?): List<List<T>> = TODO()

I would probably be tempted to go with Unit. Because it seems to me it reflects more your use case (don't do anything with the return value). Yet, since this way you can only pass functions with return type Unit I would use

Any?

in the end