StateMachine-TransitionFunction.swift

struct StateMachine<State, Action> {
  private(set) var state: State
  private let transition: (State, Action) -> State?

  init(startState: State, transition: @escaping (State, Action) -> State?) {
    self.state = startState
    self.transition = transition
  }

  mutating func apply(action: Action) -> Bool {
    if let nextState = transition(state, action) {
      state = nextState
      return true
    }
    return false
  }
}

struct Light {
  enum State {
    case on, off, broken
  }

  enum Action {
    case turn, cut
  }

  private var sm = StateMachine<State, Action>(startState: .off) { state, action -> State? in
    switch (state, action) {
    case (.off, .turn): return .on
    case (.on, .turn): return .off
    case (_, .cut): return .broken
    default: return nil
    }
  }

  var state: State {
    return sm.state
  }

  @discardableResult
  mutating func apply(action: Action) -> Bool {
    return sm.apply(action: action)
  }
}

var light = Light()

light.apply(action: .turn)
light.state
light.apply(action: .turn)
light.state
light.apply(action: .turn)
light.state
light.apply(action: .cut)
light.state
light.apply(action: .cut)
light.state
light.apply(action: .turn)
light.state