What about when you are handling external data? Don't you test what will happen when unexpected types are received? How do you test the handling when a JSON key

What about when you are handling external data? Don't you test what will happen when unexpected types are received? How do you test the handling when a JSON key you were expecting to be an array turns out to be a string or an integer or vice versa? Do you handle nil's correctly in every place they might occur. These are all things that can be covered by a powerful type system (such as Swift's unless you overuse "!"[0]). [0] Anything other than use for a member that is initialised during the init method is a code smell in my view.