A few years ago I was on a team and the dev lead had a practice of sharing Gary Bernhardt's Boundaries talk every time there was some degree of rotation or churn on the team. Almost part of the onboarding. As a functional programming aficionado, I didn't need the sales pitch, but it was the presentation that was gripping.

Keep in mind, Ruby isn't a functional language, but here was this presenter describing essentially how you write good Haskell code, but in Ruby! A language that makes it so easy to mutate in place that it's known for libraries that "monkey patch" base classes and can change the definition of operators and functions at runtime!

Fantastic talk, fantastic presentation. I now share it with my new engineering colleagues too.

Here's the talk where he talks about imperative shell, functional core, "Boundaries": https://www.youtube.com/watch?v=yTkzNHF6rMs