Until recently, the choice of whether or not to use monads was easy. I didn't understand them, so I avoided them! But now that I think I understand enough to begin to use them, I have a different problem: I don't have the experience to know when using them is a good idea, and when it's overkill. This is my first serious Haskell project, and I would appreciate any advice. Here's the scenario: I'm developing a virtual world with alife creatures. At one extreme, I guess I could use the State monad for everything that has state, right down to the neurons in their brains. (I assume that would be overkill.) At the other extreme, I could do without the State monad altogether, and just have functions that take a creature, let it have an experience, and return a new creature. I guess somewhere in-between there's a happy medium. Perhaps I should us the State monad for the creatures, but not for anything lower-level than that. Or maybe use it for the creatures and their brains. One issue that complicates things slightly is that a *lot* of the processing I'm doing requires random numbers. The neurons don't require any randomness, but the brain as a whole does. And reproduction requires randomness down at the DNA level. The code works, but I feel it's a bit messy because of needing randomness in so many places, and I'd like to clean up the design a bit. Generally speaking, is it best to go for the "bon-bon" approach, with a pure functional core, and a monadic layer on the outside?