My favourite introduction to monads

It is said that the most difficult concept to grasp in Haskell is that of monads. I'm not trying to tell what monads are because it is already been done so many times and I'm not even expert enough to try that. Instead, I'm going to talk about my personal monad-learning experience.

A highly heterogenous set of monad tutorials have been written. Brent Yorgey recently posted a blog entry titled Abstraction, intuition, and the "monad tutorial fallacy" where he pointed out that the numerous metaphors given to monads, each of them purpoted to be _the_ methaphor that should make monads clear for everybody, are not helping but actually harming the learning process. The metaphors are actually hiding essential properties of the concept and thus making learning harder.

I didn't understand monads when I read that they are just like burritos either. I'm still uncomfortable with monads, but after reading several tutorials on the subject, I think I'm now starting to understand what they are useful for in pure, functional programming.

No single monad tutorial can be given the glory of having positively affected most my learning process. I think that after reading several examples of monad usage, I'm gradually grasping the idea of the abstraction they provide. The text that I feel is describing the subject in a very clear way, and that does not hide essential concepts, is Wadler's 1995 paper Monads for functional programming. The paper first describes several examples of tasks that are implemented unwieldy in a pure functional language without side effects. Then it proceeds to show how the same problems can be solved better using monads. The three monadic laws are given, as they should be, and in the end a parser implemented with monads is described.

My point is that people shouldn't be afraid of the original papers. Sometimes they really are useful in _learning_ the subject although occasionally they get lost in their excessively rigorous handling (for being educational for laymen) of the subject.