As a beginner, I'm not directly following the usefulness of these alternative implementations. I thought I would give some example code. Here I am trying to handle errors with Either String. You can read it here or in hpaste.org: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=9393#a9393 import Data.Ratio import qualified Data.Map as Map -- An elemental music object such as note, rest, dynamic mark, etc. data MusicObject = MusicObject ... -- A composition has several "streams". A stream could be a continuous -- melody that appears on a single staff, or other types of data that -- are arranged serially in time. data Time = Rational data StreamId = StreamId ... data MusicStream = (StreadId, Map.Map Time MusicObject) data Comp = [MusicStream] -- A cursor is a concept used to "point to" a note or generalized location -- in the composition so that editing can be done at that point. For now, -- all we need is to point to the stream and time. data Cursor = Cursor { getCurId :: StreamId , getCurTime :: Time } -- Utility to make it easier to annotate an Either monad with a function -- that catches an error message, prepends a context message, and rethrows. ce :: String -> Either String a -> Either String a ce c = (flip catchError) (\s -> throwError (c ++ "\n" ++ s)) -- Utility to replace an item in an assoc list, inside the Either String -- monad. replaceAlist :: Eq a => a -> b -> [(a,b)] -> Either String [(a,b)] replaceAlist _ _ [] = throwError "Item not found in alist." replaceAlist iid obj (x:xs) = if fst x == iid then return $ (iid,obj) : xs else do rem <- replaceAlist iid obj xs return $ x : rem ... -- Delete a note from a composition. Deleting the last note in a stream is -- an error condition. -- -- Conditions that will cause an error: -- - cursor stream id doesn't exist in the composition -- - there is no note at the given cursor -- - there is only one note in the stream (so deleting it would delete -- the last note) compDeleteNote :: Cursor -> Comp -> Either String Comp compDeleteNote cur comp = ce "In compeDeleteNote:" $ do let Cursor { getCurId=iid, getCurTime=t } = cur -- First internal error might occur if no stream with the cursor's id -- occurs in the Comp. oldMap <- maybe (Left "no such stream") Right (lookup iid comp) -- Second internal error: no music object is found at the cursor's time. moAtCur <- maybe (Left "no m.o. at cursor") Right (Map.lookup t oldMap) let durAtCur = getDur moAtCur (l,r) = Map.split t oldMap r' = Map.mapKeys (\k -> k - durAtCur) r joined = Map.union l r' -- Third error condition: this action deleted the last note. if Map.null joined then (Left "deleted last note") else Right () replaceAlist iid joined comp