Mock or be mocked… :)

To address your second claim - doing IO in tests does not mean testing IO.

I just got the sense that the author was saying that the your tests have to cover every possible failure mode of IO and that’s just silly. IO should have its own tests. This goes back to your “expectation of a contract”.

Not sure if the author doesn’t understand the purpose of mocking or if they’re just being disingenuous.

• Unit tests and mocks are only as good as one makes them.
• The code using the mock isn’t supposed to test all of the edge cases of ‘IO’. IO is supposed to have its own tests.

What? They’d never!