You can make a finite state machine to model your expectations on random input, but then your test code is almost as complex as the code :) I've found quickchec

You can make a finite state machine to model your expectations on random input, but then your test code is almost as complex as the code :) I've found quickcheck super useful as a way to fuzz user input or find edge cases in unit tests I didn't see, but I still write all my integration tests by hand because they are so hard to formalize.