> What is wrong with implementing the states with polymorphic classes providing the API or an interface to it?

Yeah that's pretty much the basic approach to this problem: use a strategy pattern in all states of the state machine, and just provide definitions for each method in the state that the method makes sense.

That's pretty much how HTTP-based protocols are implemented when status codes drive a state machine.

I wonder why the author decided to ignore the most basic and simple solutions to a frequent problem in favour of a convoluted solution that doesn't add anything and is harder to debug.

I was thinking that using Visitors would be a solution, which is a form of implementing a polymorphic API.

Can you give an example?

Apple's GameplayKit has several examples

https://developer.apple.com/library/archive/documentation/Ge...