[corethree]

I've used it to optimize SQL. The principles I used in SQL can be used in other places as well.

I think functors are easier to understand and more fundamental to category theory and programming than monads.

[contravariant]

Monads are fairly simple as well if you use C#'s brilliantly chosen names SELECT and SELECTMANY for the two relevant maps.

[agumonkey]

> SELECTMANY

is that a form of flatmap ?

[mormegil]

That's exactly it, SelectMany is what in .NET corresponds to flatMap in e.g. Java. (And, unlike the parent, I don't like the narrow scope of .NET naming very much, focusing on just the list monad (a.k.a. IEnumerable).)