TypesafeStateMachineWithPhantomType.swift

// Type-safe State Machine with Phantom Type  May 2022 @AtarayoSD
import Foundation

protocol State {}

// Transitions
protocol TransferableToB {}
protocol TransferableToC {}
protocol TransferableToD {}

// States
enum StateA: State, TransferableToB, TransferableToC {}
enum StateB: State, TransferableToC, TransferableToD {}
enum StateC: State {}
enum StateD: State {}

// State Machine
final class StateMachine<T: State> {
    init() {}
}

extension StateMachine where T == StateA {
    var transitionX: StateMachine<StateB> {
        print("=> State B")
        return StateMachine<StateB>()
    }
    
    var transitionY: StateMachine<StateC> {
        print("=> State C")
        return StateMachine<StateC>()
    }
}

extension StateMachine where T == StateB {
    var transitionZ: StateMachine<StateC> {
        print("=> State C")
        return StateMachine<StateC>()
    }
    
    var transitionW: StateMachine<StateD> {
        print("=> State D")
        return StateMachine<StateD>()
    }
    
    var transitionQ: StateMachine<StateB> {
        print("=> State B")
        return StateMachine<StateB>()
    }
}

extension StateMachine where T == StateC {
}

extension StateMachine where T == StateD {
}

final class Something {
    func start() -> StateMachine<StateA> {
        return StateMachine<StateA>()
    }
}

let something = Something()
print("State A")
let stateMachine = something.start().transitionX.transitionQ.transitionW