Monads and monadic comprehension and recognizing when to use such. The fact that so many tricky things are simplified when boiled down to map/flatmap/join/ (al

Monads and monadic comprehension and recognizing when to use such. The fact that so many tricky things are simplified when boiled down to map/flatmap/join/ (along with laws) is so fucking powerful it's astounding. That you can model state transitions, concurrent computations, callbacks, dealing with 'happy path' short circuiting all with the same 'pattern' (and syntactic sugar!) seems like a cheat code.