state-machines.swift

protocol Cancellable {}
protocol Billable {}

struct Initial: Cancellable {}
struct Placed: Cancellable, Billable {}
struct Billed: Cancellable {}
struct Packed: Cancellable, Billable {}
struct Shipped {}
struct LostInTransit {}
struct DamagedInTransit {}
struct Cancelled {}

struct Order<T> {

}

extension Order where T == Initial {
	var place: Order<Placed> {
		return Order<Placed>()
	}
}

extension Order where T: Cancellable {
	var cancel: Order<Cancelled> {
		return Order<Cancelled>()
	}
}

extension Order where T: Billable {
	var bill: Order<Billed> {
		return Order<Billed>()
	}
}

extension Order where T == Billed {
	var pack: Order<Packed> {
		return Order<Packed>()
	}
}

extension Order where T == Packed {
	var ship: Order<Shipped> {
		return Order<Shipped>()
	}
}

let o1 = Order<Initial>()
let o2 = o1.place.bill.pack.ship
print(o2)