main.dart

sealed class Result<T, E> {
  const Result();
}

class Ok<T, E> extends Result<T, E> {
  const Ok(this.value);

  final T value;
}

class Err<T, E> extends Result<T, E> {
  const Err(this.error);

  final E error;
}

extension Foo<T> on T? {
  Result<T, E> okOr<E>(E err) {
    if (this != null) {
      return Ok<T, E>(this);
    }
    return Err<T, E>(err);
  }
}

void main() {
  int? a = null;
  Result<int, String> result = a.okOr("Value is null");
  print(result);
}

open in dartpad