You seem to not understand duck typing.
Functional tests will test the correct types, too.
Edit: If you need an object that swims and quacks, you don't need to care if it is a rubber duck or an animal.
Functional tests will test the correct types, too.
Edit: If you need an object that swims and quacks, you don't need to care if it is a rubber duck or an animal.
Where did you come up with this?