FSM.ts

/**REMOTE DATA IMPLEMENTATION */

type Idle = { key: "idle", data: null };
type Loading = { key: "loading", data: boolean };
type Success = { key: "success", data: string };

type RemoteDataState = Idle | Loading | Success;

type LoadTransition = [Idle, Loading];
type SuccessTransition = [Loading, Success];
type IdleTransition = [Success, Idle];
type RemoteDataTransition = LoadTransition | SuccessTransition | IdleTransition;

const makeRun2 = <State, Transition>() => <From extends State, To extends State>(t: [From, To] & Transition) => t[1];

const run2 = makeRun2<RemoteDataState, RemoteDataTransition>();

let idle: RemoteDataState = { key: "idle", data: null };
let loading: RemoteDataState = { key: "loading", data: true };
let success: RemoteDataState = { key: "success", data: "boo" };

let myRequest: RemoteDataState = run2([idle, loading]);
let shouldBeBool = myRequest.data;
myRequest = run2([loading, success]);
let shouldBeString = myRequest.data;

run2([loading, idle]); //this fails