They are similar to Kotlin's sealed classes, except there's no concept in Kotlin of an object "consuming" itself, which is necessary for maintaining safety in the API.

For example, in Kotlin, if a method on state A returns state B, there's no way to invalidate all existing references to state A. Normally this invariant would be enforced on the caller's side, or perhaps by throwing an IllegalStateException if state A is called after producing state B.