PhantomType.swift

import Foundation

// The life of a developer using Phantom Types

enum Eating {}
enum Coding {}
enum Sleeping {}

struct Transition<From, To> { }

struct Life<State> {
    func transition<To>(with transition: Transition<State, To>) -> Life<To> {
        .init()
    }
}

let start = Transition<Eating, Coding>()
let process = Transition<Coding, Sleeping>()
let reset = Transition<Sleeping, Eating>()

let l1 = Life<Eating>()
let l2 = l1.transition(with: start)
let l3 = l2.transition(with: process)
let l4 = l3.transition(with: reset)