Michael P Mossey wrote:
I've got one of those algorithms which "threatens to march off the right edge" (in the words of Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. So I presume I need StateT or MaybeT. However, I'm still (slowly) learning about monads from first principles. I thought I might present my code and get some pointers... maybe someone could actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything will help.
Here's a version using ErrorT from mtl. I added some missing IO bits on your types; the type error that remains is for you to fix :-) With ErrorT you can use throwError when you want to break out of the block and give back an error, which seems to fit what you were doing. The downside is you have to add all these liftIO bits wherever you do a plain IO computation. insertNote :: NoteRecord -> Connection -> IO () insertNote nr conn = either putStrLn return =<< runErrorT (do -- Check if it exists in the database already. status <- liftIO $ checkPreExistingText nr conn when status $ throwError "Skipping... text exists already." -- Find best fit for all topics and source. -- See type signatures below. bestFitTopics <- liftIO $ fitTopics nr conn bestFitSource <- liftIO $ fitSource nr conn case any isNothing bestFitTopics of True -> throwError "Error... some topic couldn't be matched." False -> case bestFitSource of Nothing -> throwError "Error.. source couldn't be matched." _ -> do b <- liftIO $ isUserOkay nr bestFitTopics bestFitSource when (not b) $ throwError "Abort due to user request." -- Create a new NoteRecord with matched -- and validated topics/source. let nrValidated = nr { recordTopics = bestFitTopics , recordSource = bestFitSource } liftIO $ insertRow nrValidated conn ) checkPreExistingText :: NoteRecord -> Connection -> IO Bool fitTopics :: NoteRecord -> Connection -> IO [Maybe String] fitSource :: NoteRecord -> Connection -> IO (Maybe String) isUserOkay :: NoteRecord -> [Maybe String] -> Maybe String -> IO Bool insertRow :: NoteRecord -> Connection -> IO () Thanks, Neil.