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A state machine where invalid transitions can't compile
protocol State {}
struct Transition<From: State, To: State> {
let transition: () -> To
struct Machine<CurrentState: State> {
var state: CurrentState
func transition<To>(with transition: Transition<CurrentState, To>) -> Machine<To> where To: State {
return Machine<To>(state: transition.transition())
struct Pending: State {}
struct Loading: State {}
struct Loaded: State {}
struct Failed: State {}
let kickoff = Transition<Pending, Loading>(transition: { return Loading() })
let finishLoading = Transition<Loading, Loaded>(transition: { return Loaded() })
let machine = Machine<Pending>(state: Pending())
let newMachine = machine.transition(with: kickoff) // compiles
newMachine.transition(with: finishLoading)
machine.transition(with: finishLoading) // Cannot convert value of type 'Transition<Loading, Loaded>' to expected argument type 'Transition<Pending, _>'
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