Thanks every, Daniel, the list at large... I have finally solved my issue and moved on... for the record my function now looks like this: forwardRoutes :: M.Map String [String] -> Record -> M.Map String [String] forwardRoutes map row = let key = row !! 0 in let val = row !! 1 in case M.lookup key map of Nothing -> M.insert key [val] map Just routes -> M.insert key (nub (val:routes)) map and does just what I want! The thing I ha failed to see was using Prelude.lookup instead of M.lookup and some errant (). I know nub is 'potentially inefficient' but the source data set is reasonably small and the script only has to run three to four times a year so "So what!" Just one thing: IF the error message had printed out the Prelude.lookup in the error message, that would have been incredibly useful! Is that a command line switch I haven't come across perhaps ? Thanks again. Sean.