Dear all, I am a Haskell newbie and I try to find my way through Monad territory. Actually I am studying the non-deterministic monadic parser combinators as descibed by Hutton and Meijer. Although I find this very elegant and concise I seem to have problems extending the parser monads. I use newtype Parser a = Parser { runParser :: (PState -> [(a, PState)])} as the parsing monad with the Parser state "PState" that contains the remaining input after matching and possibly some additional user defined state elements. I want to add logging such that the application of every element parser/parser combinator gets recorded in a string. In the end I want to print out the trace of all encountered parsers for successful and for failed matches. I tried to use the WriterT transformer to add a writer monad on top of the Parser monad but for failed matches (i.e. runParser gives []) the log is also "lost" since WriterT gives a monad of "m (a,w)". What I would look for is "(m a, w)". Finally, my questions: What is the most elegant way to achieve logging parsers ? Can I use the standard transformes or do I need to write my own transformers ? How can I make the Parser monad given above an instance of MonadState ? (I always get kind errors ...) Any hints are really appreciated. Thank you Harald Rotter.