Типизация ответа сервера

serverResponseTyping.ts

type SuccessResult<T> = { data: T, error: void };
type ErrorResult<E> = { data: void, error: E };
type Result<T, E = Error> = SuccessResult<T> | ErrorResult<E>

function isSuccess<T, E>(x: Result<T, E>): x is SuccessResult<T> {
  return Boolean(x.data);
}

function isError<T, E>(x: Result<T, E>): x is ErrorResult<E> {
  return Boolean(x.error);
}

function map<T, E, NewSuccessResult>
(result: Result<T, E>, processor: (arg: T) => NewSuccessResult): Result<NewSuccessResult, E> {
  if(isError(result)) {
    return result;
  }
  isSuccessResult() {
    return { data: processor(result.data), error: undefined }
  }
  trow new Error('error');
}

declare function loadUser(id: number): Result<{ id: number, balance: number}, Error>;

function run() {
  const user = swait loadUser(5);
  const userBalance = map(userResult, user => user.balance);
}