“It has a field of type X” has never been a useful test for me, my tests are always more like:
“if I send message X I get return value or action Y”
… with my admittedly limited experience of types I don’t see how they replicate this.
Therefore it looks like I’d only be “replacing” tests that I’d never write in the first place.
What am I missing?
Unit tests typically test for behaviours. This could be both positive and negative tests. But we often test only a subset of possibilities just because how people generally think (more positive cases than negative cases). Theoretically we can do all those tests with unit testing. But we need to ask ourselves honestly, do we have that kind of test coverage as SQLLite? If yes, do we have that for very large codebases?
For these tests we don’t care about the content only that something didn’t get incorrectly set or the mailer interface changed.
Now if the developer changes the Mailer to require a user object the compiler tells us there is an error. Sorbet will error and say “hey you need to update your code here and here by adding a User object”
Before we would have had test coverage for that - or maybe not and missed the error.