Hi, My application has to manage a data set. I assume the state monad is designed for this. The state changes in functions that: a. perform IO actions and b. return execution status and execution trace (right now I'm using WriteT for this). Is the best solution: 1. to build a monad stack (for example State -> Writer -> IO) or 2. to use IORef for the data set or 3. something else? Are monad stacks with 3 and more monads common? How could an example implementation look like? What I have for now is: -- Status data Status = OK | FAILED deriving (Show, Read, Enum) -- Example data set manages by state type Config = [String] -- WriterT transformer type OutputWriter = WriterT [String] IO Status -- example execute function execute :: [String] -> OutputWriter execute fs = do rs <- liftIO loadData fs tell $ map show rs return OK -- run it inside e.g. main (s, os) <- runWriterT $ execute files How do I bring a state into this, for example for: execute fs = do ?? conf <- get ?? -- get Config from state rs <- liftIO loadData conf fs ?? set conf ?? -- change Config and set to state tell "new state:" tell $ show conf return OK Do I have to use and how do I use StateT in this context: data DataState = StateT Config OutputWriter ?? and how do I run it runStateT . runWriterT? Thanks for help, Adam