> If a system misbehaves you will catch in higher level tests in the test pyramid eventually. You might catch it eventually, or you might not. That's the probl

> If a system misbehaves you will catch in higher level tests in the test pyramid eventually. You might catch it eventually, or you might not. That's the problem with hoping you write enough tests. > he can and should add a dedicated unittest to reproduce the issue when he fixes the bug His argument is that dynamic language don't 'really' require writing more exhaustive tests than in an expressive static language. In this scenario, not only would he need to write an extra test but he also needs to spend time after the fact tracking down an error thrown in a higher-level function. > In a software solution you can't prove 100% correctness This isn't about proving 100% correctness, it's about what happens when you pass a non-list into a list processing function. More expressive type systems allow you to reduce the number of nonsensical questions you have to answer.