Functor, Applicative, and Monad
I don't think this article is very useful. It doesn't adequately provide an introduction to OCaml code (or adequately explain what a given code snipped is doing) and yet frequently defers to just code to explain a concept. 
It's an unrealistic expectation to expect an unfamiliar reader to simultaneously infer what a particular code snippet is doing then also go a level deeper and understand the concept that is trying to be presented.This article is most readable to those who already understand OCaml code, and if you can already read OCaml code you already understand these concepts.