class Either<L, R> {
  final L? leftValue;
  final R? rightValue;

  Either.left(this.leftValue) : rightValue = null;

  Either.right(this.rightValue) : leftValue = null;

  bool get isLeft => leftValue != null;

  bool get isRight => rightValue != null;

  Either<L, R2> flatMap<R2>(Either<L, R2> Function(R r) f) {
    if (isRight) {
      return f(rightValue as R);
    }

    return Either.left(leftValue as L);
  }
}

class Failure {}

class FooFailure implements Failure {}

Either<Failure, int> query() => Either.left(FooFailure());

Either<FooFailure, int> query2() => Either.left(FooFailure());

Either<Failure, int> getFooQuery() => query();

Either<Failure, int> getFooQuery2() => query2();

final Either<Failure, int> straight = Either.left(FooFailure());

void main() {
  final fooQuery = getFooQuery();

  final fooQuery2 = getFooQuery2();

  fooQuery.flatMap(
    (a) => Either.right(a),
  );
  print('fooQuery flatMap works fine');

  /// works just fine

  straight.flatMap(
    (a) => Either.right(a),
  );
  print('straight flatMap works fine');

  /// works just fine

  fooQuery2.flatMap(
    (a) => Either.right(a),
  );
  print('fooQuery2 flatMap works fine');

  /// type '(int) => Either<Failure, int>' is not a subtype of type '(int) => Either<FooFailure, int>' of 'f'
}