Паттерн Option та Either (Result): типобезпечна обробка помилок без throw

async function fetchUserProfile(userId: string): Promise<UserProfile> {
  // Може викинути NetworkError, ParseError, UnauthorizedError або NotFoundError
}

+-------------------------------------------------------------------------+
|                КОНВЕЄРНА ОБРОБКА ОБЧИСЛЕНЬ (EITHER)                     |
+-------------------------------------------------------------------------+
| Успішна гілка (Right):                                                  |
| [Вхідний JSON] ---> map(parseDTO) ---> flatMap(validate) ---> Right(User)|
|                           |                   |                         |
|                           v (збій)            v (збій)                  |
| Помилкова гілка (Left):   |                   |                         |
| [Виняток]     ---> Left(SyntaxError)  Left(ValidationError)             |
+-------------------------------------------------------------------------+
                                     |
                                     v
+-------------------------------------------------------------------------+
|                  ТЕРМІНАЛЬНЕ РОЗПАКУВАННЯ (MATCH / FOLD)                |
|  match({                                                                |
|    onLeft:  (error) => renderErrorBanner(error), // Сувора гілка помилки|
|    onRight: (user)  => renderUserProfile(user)  // Чистий тип User     |
|  })                                                                     |
+-------------------------------------------------------------------------+

export interface Some<A> {
  readonly _tag: 'Some';
  readonly value: A;
}

export interface None {
  readonly _tag: 'None';
}

export type Option<A> = Some<A> | None;

export const Option = {
  some: <A>(value: A): Option<A> => ({ _tag: 'Some', value }),
  none: (): Option<never> => ({ _tag: 'None' }),

  fromNullable: <A>(value: A | null | undefined): Option<NonNullable<A>> =>
    value !== null && value !== undefined
      ? Option.some(value as NonNullable<A>)
      : Option.none(),

  isSome: <A>(fa: Option<A>): fa is Some<A> => fa._tag === 'Some',
  isNone: <A>(fa: Option<A>): fa is None => fa._tag === 'None',

  map: <A, B>(fa: Option<A>, f: (a: A) => B): Option<B> =>
    fa._tag === 'Some' ? Option.some(f(fa.value)) : Option.none(),

  flatMap: <A, B>(fa: Option<A>, f: (a: A) => Option<B>): Option<B> =>
    fa._tag === 'Some' ? f(fa.value) : Option.none(),

  getOrElse: <A>(fa: Option<A>, onNone: () => A): A =>
    fa._tag === 'Some' ? fa.value : onNone(),

  match: <A, B>(fa: Option<A>, onNone: () => B, onSome: (a: A) => B): B =>
    fa._tag === 'Some' ? onSome(fa.value) : onNone(),
};

export interface Left<E> {
  readonly _tag: 'Left';
  readonly left: E;
}

export interface Right<A> {
  readonly _tag: 'Right';
  readonly right: A;
}

export type Either<E, A> = Left<E> | Right<A>;

export const Either = {
  left: <E, A = never>(error: E): Either<E, A> => ({ _tag: 'Left', left: error }),
  right: <A, E = never>(value: A): Either<E, A> => ({ _tag: 'Right', right: value }),

  isLeft: <E, A>(ma: Either<E, A>): ma is Left<E> => ma._tag === 'Left',
  isRight: <E, A>(ma: Either<E, A>): ma is Right<A> => ma._tag === 'Right',

  // Безпечний міст від імперативних функцій із throw до Either
  tryCatch: <E, A>(
    tryFn: () => A,
    onError: (error: unknown) => E
  ): Either<E, A> => {
    try {
      return Either.right(tryFn());
    } catch (err: unknown) {
      return Either.left(onError(err));
    }
  },

  map: <E, A, B>(ma: Either<E, A>, f: (a: A) => B): Either<E, B> =>
    ma._tag === 'Right' ? Either.right(f(ma.right)) : Either.left(ma.left),

  flatMap: <E, A, B>(ma: Either<E, A>, f: (a: A) => Either<E, B>): Either<E, B> =>
    ma._tag === 'Right' ? f(ma.right) : Either.left(ma.left),

  match: <E, A, B>(
    ma: Either<E, A>,
    onLeft: (error: E) => B,
    onRight: (value: A) => B
  ): B => (ma._tag === 'Left' ? onLeft(ma.left) : onRight(ma.right)),
};

// Визначення суворого об'єднання доменних помилок
export type DomainError =
  | { readonly code: 'INVALID_JSON'; readonly raw: string }
  | { readonly code: 'VALIDATION_FAILED'; readonly field: string; readonly reason: string }
  | { readonly code: 'UNAUTHORIZED'; readonly userId: string };

export interface UserAccount {
  readonly id: string;
  readonly email: string;
  readonly balanceCents: number;
}

// 1. Безпечний парсинг рядка
export function parseRawJson(rawText: string): Either<DomainError, unknown> {
  return Either.tryCatch(
    () => JSON.parse(rawText) as unknown,
    () => ({ code: 'INVALID_JSON', raw: rawText })
  );
}

// 2. Валідація структури даних
export function validateAccountDto(input: unknown): Either<DomainError, UserAccount> {
  if (typeof input !== 'object' || input === null) {
    return Either.left({
      code: 'VALIDATION_FAILED',
      field: 'root',
      reason: 'Payload must be an object',
    });
  }

  const candidate = input as Record<string, unknown>;

  if (typeof candidate.id !== 'string' || candidate.id.trim() === '') {
    return Either.left({
      code: 'VALIDATION_FAILED',
      field: 'id',
      reason: 'ID must be a non-empty string',
    });
  }

  if (typeof candidate.email !== 'string' || !candidate.email.includes('@')) {
    return Either.left({
      code: 'VALIDATION_FAILED',
      field: 'email',
      reason: 'Invalid email address format',
    });
  }

  if (typeof candidate.balanceCents !== 'number' || candidate.balanceCents < 0) {
    return Either.left({
      code: 'VALIDATION_FAILED',
      field: 'balanceCents',
      reason: 'Balance must be a non-negative integer',
    });
  }

  return Either.right({
    id: candidate.id,
    email: candidate.email,
    balanceCents: candidate.balanceCents,
  });
}

// 3. Композиція пайплайну
export function processRawAccount(rawJson: string): Either<DomainError, UserAccount> {
  const parsed = parseRawJson(rawJson);
  return Either.flatMap(parsed, validateAccountDto);
}

import { pipe } from '@/functional/pipe';

const result = pipe(
  '{"id":"usr_42","email":"[email protected]","balanceCents":1500}',
  parseRawJson,
  (either) => Either.flatMap(either, validateAccountDto),
  (either) =>
    Either.match(
      either,
      (err) => `Помилка обробки: ${err.code}`,
      (acc) => `Успішно завантажено профіль: ${acc.email} ($${acc.balanceCents / 100})`
    )
);