Hi Haskellers, in order to learn Monad Transformers, I'm trying to write a simple calculator that has an internal state of type Calc that is modified after each line the user enters. I started like this: main :: IO () handleLine :: StateT Calc IO Bool -- return's False in order to exit main uses runState in order to extract the action it needs from the transformer. Up to this point, it works fine. But in order to separate I/O and parsing, I'd prefer to have another function processInput :: String -> State Calc (Maybe String) that updates the internal state and returns either a String that should be printed or Nothing in order to exit. Trying to do that, I didn't see another possibility but to call runState again from inside handleLine. I guess there is a better solution. Can you help? Thanks in advance Tim