main.dart

void main() {
  final result = SuccessFailure<int, void>.success(null);
  if (result.isSuccess) { // надо будет result.isSuccess && result.success != null. Вообще весь смысл этого класса теряется
    print(result.success + 5);
  }
}

typedef Callback<T> = void Function(T value);

class SuccessFailure<S, F> {
  factory SuccessFailure.success([S result = null]) {
    return SuccessFailure._success(result, null);
  }

  factory SuccessFailure.failure([F failure = null]) {
    return SuccessFailure._failure(null, failure);
  }

  final bool _isSuccess;

  SuccessFailure._success(this._success, this._failure) : _isSuccess = true;
  SuccessFailure._failure(this._success, this._failure) : _isSuccess = false;

  final S _success;
  final F _failure;

  bool get isSuccess => _isSuccess;

  bool get isFailure => !isSuccess;

  S get success => _success;

  F get failure => _failure;
}