With great power comes great responsibility. It is developer responsibility to ensure that you will not receive cat during designing stage of your classes. And

With great power comes great responsibility. It is developer responsibility to ensure that you will not receive cat during designing stage of your classes. And inheritance is bad, this is also sharp knife. But annotating everywhere sig { params(cat: Cat) } does not improve your design, it just makes noisy and clumsy. I would think that if your code need type annotations, it smells like bad design and should be considered for refactoring.