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Feels similar to Kotlin's sealed classes

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.

Adding to this, I see two other elements that make this tough to replicate in Kotlin:

1. Type erasure

2. Using sealed classes requires instantiation, while the Rust version is zero overhead.

> 1. Type erasure

There is no need to access type information at runtime here.

also Monad in Haskell

Huh? A monad isn't a sealed class, it's a generic type equipped with the functions bind and return...

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