Rust `andThen` implementation for Dart lang

main.dart

// To Avoid Result callback hell manipulate Rust lang andThen method simple way.
sealed class Result<T> {
  const Result();

  R match<R>({
    required R Function(Success<T> success) success,
    required R Function(Failure<T> failure) failure,
  }) {
    if (this is Success<T>) return success(this as Success<T>);
    if (this is Failure<T>) return failure(this as Failure<T>);
    throw StateError('Unexpected type: $this');
  }
}

class Success<T> extends Result<T> {
  final T value;
  const Success(this.value);
}

class Failure<T> extends Result<T> {
  final Object error;
  const Failure(this.error);
}

class Auth {
  final String token;
  final String email;
  final String userName;
  Auth({required this.token, required this.email, required this.userName});
}

Future<Result<Auth>> getUser(String email) async {
  return Success(Auth(token: "aaa", email: email, userName: "aaa bbb"));
}

Future<Result<String>> login(String user) async {
  if (user == "user123") {
    return const Success("[email protected]");
  }
  return Failure("Invalid user");
}

extension ResultExtension<T> on Future<Result<T>> {
  Future<Result<U>> andThen<U>(Future<Result<U>> Function(T) next) async {
    final current = await this;
    return current.match(
      success: (s) => next(s.value),
      failure: (f) => Future.value(Failure<U>(f.error)),
    );
  }
}

void main() async {
  final result = await login("user123").andThen((user) => getUser(user));

  result.match(
    success: (auth) => print("Success login for ${auth.value.userName}"),
    failure: (err) => print("Login error: $err"),
  );
}