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.<p>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.