function combines<TArgs, TReturn, TFns>(_sources, fn): DeclaredFn<TArgs, TReturn, MergeErrors<TFns>>;

const getUser = declares([NotFoundError, DbError], ...);
const getOrder = declares([OrderError, DbError], ...);

const getUserOrder = combines(
  [getUser, getOrder],
  async (userId: string, orderId: string) => {
    const user = await getUser(userId);
    const order = await getOrder(orderId);
    return { user, order };
  },
);
// getUserOrder can throw NotFoundError | DbError | OrderError

const getUser = declares([UserNotFoundError, DbError], ...);
const getBilling = declares([BillingError, DbError], ...);
const getPermissions = declares([AuthError], ...);

const getDashboard = combines(
  [getUser, getBilling, getPermissions],
  async (userId: string) => {
    const [user, billing, perms] = await Promise.all([
      getUser(userId),
      getBilling(userId),
      getPermissions(userId),
    ]);
    return { user, billing, perms };
  },
);

// Error type is UserNotFoundError | DbError | BillingError | AuthError
const result = await tryAsync(getDashboard, userId);

const findUser = declares([DbError], ...);
const validateToken = declares([AuthError, TokenExpiredError], ...);

const authenticateUser = combines(
  [findUser, validateToken],
  async (token: string) => {
    const claims = validateToken(token);
    return findUser(claims.userId);
  },
);
// authenticateUser throws DbError | AuthError | TokenExpiredError