Guard clause

main.dart

/// You can try running this file to see what to expect.
///
/// Then, go refactor the function `describeEating()` below.
void main() {
  final filip = Human();
  final someoneElse = Human(isFull: true);

  final shoe = Item();
  final banana = Food(isDelicious: true);
  final kale = Food();

  describeEating(shoe, filip);
  describeEating(banana, filip);
  describeEating(kale, filip);

  describeEating(banana, someoneElse);

  // This is still running Dart without null safety.
  // Uncomment the line below to get a runtime error.
  // describeEating(null, filip);
}

/// This is the function you'll want to refactor.
///
/// Hint: I'm not making it too easy for you.
///       You'll learn better that way.
void describeEating(Item item, Human eater) {
  if (item != null) {
    if (!item.isFood) {
      print("Wait, that's not even food!");
    } else {
      if (!eater.isFull && item.isDelicious) {
        print("Hmm, that was good.");
      } else if (eater.isFull) {
        print("I can't! Too full.");
      } else {
        print("Not bad. Nutritious.");
      }
    }
  } else {
    throw ArgumentError.notNull("item");
  }
}

// ---- Helper classes. No need to touch.

class Item {
  bool get isFood => false;
  bool get isDelicious => false;
}

class Food extends Item {
  bool get isFood => true;
  bool isDelicious;
  Food({this.isDelicious = false});
}

class Human {
  bool isFull;
  Human({this.isFull = false});
}