I don't quite agree... I feel like, theoretically, yes we should feed bad data to all the functions and use property-based tests to ensure the features hold. Ho

I don't quite agree... I feel like, theoretically, yes we should feed bad data to all the functions and use property-based tests to ensure the features hold. However, in the real-world if you're using a language like Clojure for day to day development your team is probably skilled enough to write basic functions without meticulously checking every little thing for errors. There has to be a trade off between 99.9999% confidence of your functions and fast development velocity. For most products the application code matters a lot more than tests. There should still be strong checks on the external interfaces to your system with bad data tests, schemas, property based testing, etc. For a normal function inside your system it's mostly overkill to be testing those things. Integration tests should validate that your functions call each other correctly and some acceptance tests could generate data from one interface and check that it flows through the system well. There could be corrupted types that you use for testing with Clojure's spec library that either return the expected type or a random corrupted type. Those combinations should adequately exercise the functions to find most of the errors.