Either, but in Dart

either.dart

typedef Function1<A, R> = R Function(A param);

class Either<L, R> {

  bool _isRight;
  L _left;
  R _right;

  C fold<C>(Function1<L, C> ifLeft, Function1<R, C> ifRight) {
    if (_isRight) {
      return ifRight(_right);
    } else {
      return ifLeft(_left);
    }
  }
  
  static Either<L1, R1> left<L1, R1>(L1 value) {
    Either<L1, R1> e = Either();
    e._left = value;
    e._isRight = false;
    return e;
  }
  
  static Either<L1, R1> right<L1, R1>(R1 value) {
    Either<L1, R1> e = Either();
    e._right = value;
    e._isRight = true;
    return e;
  }

}