interface Result<T, E> {
  [Symbol.iterator](): Generator<E, T>
}

type NotResultOf<T> = T extends Result<any, any> ? never : T;
type ErrTypeOf<T> = T extends Result<unknown, infer E> ? E : never;
type OkTypeOf<T> = T extends Result<infer R, unknown> ? R : never;
type AsyncJob<T, E> = () => AsyncGenerator<E, T>;

declare const asyncDo: <T, E>(job: AsyncJob<T, E>) => Promise<Result<
  OkTypeOf<T> | NotResultOf<T>,
  E | ErrTypeOf<T>
>>;
declare const mapErr: <E, F>(mapper: (error: E) => F) => <T>(result: Result<T, E>) => Result<T, F>;

type Book = { id: string; title: string; authorId: string };
type Author = { id: string; name: string };
type BookWithAuthor = Book & { author: Author };

declare const fetchBook: (id: string) => Promise<Result<Book, "NOT_FOUND">>;

declare const fetchAuthor: (id: string) => Promise<Result<Author, "NOT_FOUND">>;

export const fetchBookWithAuthor1 =
  (bookId: string) =>
    asyncDo(async function* () {
      const book: Book = yield* await fetchBook(bookId)
        .then(mapErr(() => "NOT_FOUND_BOOK" as const))

      const author: Author = yield* await fetchAuthor(book.authorId)
        .then(mapErr(() => "NOT_FOUND_AUTHOR" as const))

      return { ...book, author } as BookWithAuthor;
    });

export const fetchBookWithAuthor2 =
  (bookId: string): Promise<Result<BookWithAuthor, "NOT_FOUND_BOOK" | "NOT_FOUND_AUTHOR">> =>
    asyncDo(async function* () {
      const book: Book = yield* await fetchBook(bookId)
        .then(mapErr(() => "NOT_FOUND_BOOK" as const))

      const author: Author = yield* await fetchAuthor(book.authorId)
        .then(mapErr(() => "NOT_FOUND_AUTHOR" as const))

      return { ...book, author } as BookWithAuthor;
    });

interface Result<T, E> {
    [Symbol.iterator](): Generator<E, T>;
}
type Book = {
    id: string;
    title: string;
    authorId: string;
};
type Author = {
    id: string;
    name: string;
};
type BookWithAuthor = Book & {
    author: Author;
};
export declare const fetchBookWithAuthor1: (bookId: string) => Promise<Result<BookWithAuthor, "NOT_FOUND_BOOK" | "NOT_FOUND_AUTHOR">>;
export declare const fetchBookWithAuthor2: (bookId: string) => Promise<Result<BookWithAuthor, "NOT_FOUND_BOOK" | "NOT_FOUND_AUTHOR">>;
export {};