dagger VS clara-rules

I learned a ton about Java and pragmatic algorithm development from reading Dagger (https://github.com/square/dagger) and porting it to C#. It's small enough that you can grok it in a reasonable amount of time, but sophisticated enough that there's a lot to learn. (Yes it's deprecated in favor of Dagger 2, but the latter is a tougher slog IMO)

Actually most of the big Square OSS libraries are great to read - okio, okhttp, picasso.

I've met a few young programmers who heard somewhere that object-oriented programming was bad and they want to get the enlightenment of functional programming that they've heard about. Frequently they travel from job to job like itinerant martial artists always looking for somewhere where they practice the true technique but they always seem disappointed as it is just as easy if not easier to screw up handling errors with monads than it is with exceptions and they find analogies like "a monad is like a burrito" just get them more confused.

As for something profound I'd point you to

https://github.com/cerner/clara-rules

which many people will struggle with because like many other production rules engines in LISP (and many other examples of simple compilers), there is hardly any code! Contrast that to the orders of magnitude larger rules engine Drools

https://github.com/kiegroup/drools

which is so crazy-complicated primarily because the Drools language is Java-based so you need all sorts of things that Clara or CLIPS don't need.