TypeScript discriminated union state

type RequestState<T> =
  | { status: "idle" }
  | { status: "loading" }
  | { status: "success"; data: T }
  | { status: "error"; message: string };

function label<T>(state: RequestState<T>): string {
  switch (state.status) {
    case "idle": return "Ready";
    case "loading": return "Loading…";
    case "success": return "Complete";
    case "error": return state.message;
  }
}