I was using the example of income tax as an entirely theoretical example of where types won't save you in response to the GP. That said, I don't follow what yo

I was using the example of income tax as an entirely theoretical example of where types won't save you in response to the GP. That said, I don't follow what you mean by "to test is business logic, not code logic." To me, that is a distinction without difference. An income tax calculator in a (sane) tax system is essentially a set of pure of functions or classes. This is a perfect place for a unit test. I might have additional unit tests for sub components of the hairy bits, but it's still unit tests all the way.