Monadic IO, concurrency, exceptions, and foreign-language calls in Haskell

That is a pretty good walkthrough of the "harder" parts of functional programming.

Although, I always found it telling that the language designers needed the "do" imperative notation for monadic state computations.

While the benefits of monadic-state are pretty well known at this point, it does seem interesting that for widespread adoption it has to be "wrapped" in an abstraction more familiar to developers.