function declares<TArgs, TReturn, TErrors>(_errorClasses, fn): DeclaredFn<TArgs, TReturn, TErrors>;

const getUser = declares([NotFoundError, DatabaseError], async (id: string) => {
  const row = await db.users.findById(id);
  return ensure(row, NotFoundError, `No user: ${id}`);
});

const NotFoundError = defineError("NotFoundError");
const DbError = defineError("DbError");

const getUser = declares([NotFoundError, DbError], async (id: string) => {
  const row = await db.users.findById(id);
  return ensure(row, NotFoundError, `No user: ${id}`);
});

// Callers using tryAsync get typed errors automatically:
const result = await tryAsync(getUser, "123");
if (!result.ok) {
  result.error; // NotFoundError | DbError — not unknown
}

const DbError = defineError("DbError");

export const userRepo = {
  findById: declares([DbError], async (id: string) => {
    return await db.query("SELECT * FROM users WHERE id = $1", [id]);
  }),
  create: declares([DbError], async (data: CreateUser) => {
    return await db.query("INSERT INTO users ...", [data.name, data.email]);
  }),
};

const ApiError = defineError("ApiError");
const TimeoutError = defineError("TimeoutError");

const fetchProducts = declares([ApiError, TimeoutError], async (category: string) => {
  const res = await fetch(`/api/products?category=${category}`);
  if (!res.ok) fault(ApiError, `HTTP ${res.status}`);
  return res.json() as Promise<Product[]>;
});