After hearing quite a lot about monads from pretty excited folks in Haskel and javascript I’ve decided to figure out on my own what the M-word meant and everyone was so excited about it.
After reading the “You could have invented Monads” and the “Translation from Haskell to Javascript of selected portions of the best introductions to monads I’ve ever read”, it seems that monads can be well represented by a type-modifying wrapper combined with an associated composition rule that would make the compositions that were working on the non-wrapped functions work on the wrapped functions.
One of the reasons why monads might be really exciting for the people is because they are a set of mathematical objects whose implementation in code allows a very powerful control over the side-effects in the functional style programming. A second would be the fact that they avoid a vast amount of the template, boilerplate code that is there to manage the piping of the program and not its business logic. As an instance, a monad would allows a simple modification of the function so that it can manage an std.err pipe in addition to the std.out pipe, without having to manually edit every single functions that might require such a re-piping.
If you ever had to maintain any larger codebase, you are probably aware by now that your worst enemies are side-effects. You can wrap them into arguments of functions you pass them to, but that is long and tedious. You can also try to break them down into classes or namescopes, but if you are trying to do something sufficiently complex, at some point you will end up either with a superclass or an arbitrarily shattered module.
As a Pythonista, I am able to offset the complexity problems a little bit with use of wrappers, iterators and a bit of functional programming capacities in Python, and if monads are really as good as they are claimed to be in controlling the computational complexity, I am all for having more of them!
However, there are a couple of StackExchange questions and blog posts make me wonder if there is not more to it and if I just don’t have the background to understand it yet.
Update on 18 Nov 2015:
After reading more in dept about the monads in Haskell and monad transformers, it seems that monads are a way of beefing-up types by defining a modification that shifts normal types into monadic type space to add additional properties to them, making the logic more clear. All in all it seems to be a pretty neat pattern for a well-controlled type alteration (quite in the spirit of LISP macros, but with more rigidity). Monad transformes seem to be taking the combination power even further by allowing the type alterations to be mixed together, but it still seems that there is no unique and clear definition of what they are and how they work, at least outside Haskell.