Редукс с $ObjMap

const ACTION1: "ACTION1" = "ACTION1";
const ACTION2: "ACTION2" = "ACTION2";

const ACTIONS = {
  a: () => ({ type: ACTION1, payload: { num: 1 } }),
  b: () => ({ type: ACTION2, payload: { str: "A" } })
};

function reducer(action: $Values<$ObjMap<typeof ACTIONS, <R>(() => R) => R>>): string {
  switch (action.type) { // any | "ACTION1" | "ACTION2"
    case ACTION1:
      return "" + action.payload.num; // any | number
    case ACTION2:
      return "" + action.payload.str; // any | string
    default:
      ;(action.type: empty)
      return "";
  }
}