OK, thanks for the references. I think I've got a better handle on what's what. The library documentation mentions the ability to convert the ST monad into an IO monad. What are the reasons/pluses/minuses of doing this? (I can see one reason being that you won't have to use monad transformers to try and thread two different monads through your code). Also, I didn't see any mention of a mutable List - I suppose you have to use Data.Array.ST? cheers