Syntaxfree is hacking on Martin Erwig's probability monad. This is one of the coolest monads out there---it allows you to trivially solve all kinds of probability problems.

Mikael Johansson has a good example.

I hope to write a bit more about probability monads soon. There's already a long post sitting on my hard drive, and some more ideas that I'm still trying to puzzle out.

In the meantime, I'd like to recommend The Haskell Road to Logic, Maths and Programming. There's an excellent review available.