I have been a big proponent of generative testing / fuzzing, where rather than hard-coding inputs you simply always generate them. You can see it in Haskell’s QuickCheck or Clojure’s test.check, but there are probably many others.

It is absolutely useful outside of just a few parsers; a type system is not able to catch all bugs. Just a recent example was a scheduling system I was working on, where the input fuzzing was able to put the system in states that I did not anticipate.

Practically speaking, with fuzzing, you’re changing tests from manually crafting certain inputs and validating the results, to defining “laws” about how your system should behave.

Here’s a good resource on the types of tests you can develop using this technique: https://fsharpforfunandprofit.com/posts/property-based-testi...