Hello, All! I need some help to solve the following task: ================================================================================================= import Data.Map as M import Data.List as L data Pair = Pair { name, value :: String } type IxPair = (Int, String, String) type IxPairParser = Pair -> Maybe IxPair type RecordUpdateF r = String -> String -> r -> r convertPairsToRecords :: RecordUpdateF r -> IxPairParser -> r -> [Pair] -> [r] convertPairsToRecords updateRecF parsePairF initRec pairs = M.elems $ L.foldl' processWithPairs initMap pairs where initMap = M.empty processWithPairs resMap = maybe resMap (updateMapF resMap) . parsePairF updateMapF res (idx, key, value) | idx `M.member` res = M.adjust (updateRecF key value) idx res | otherwise = M.insert idx (updateRecF key value initRec) res ================================================================================================= In short, input looks like name=value pairs: pair_1_username=user pair_1_password=pass pair_1_fullname=vasya poopking The parser IxPairParser takes the pair of name=value, and extracts ordering number of the current pair, making it a triple like below (1, username, user) then convertPairsToRecords takes the triple and performs lookup in the Map for a record with given ID. Then it updates the record and places it back into the map. If no record is found - then new one is created, updated and inserted into the Map. Now, I've got the case when input contains pairs like below pair_1_lastlogin=2010-05-24 12:23 ... pair_2_lastlogin=2009-12-20 10:30 ... pair_2_lastlogin=2010-04-06 18:20 At this point I need to update the record and keep the latest date somewhere, so it will be possible to extract the latest login date among all such pairs. I can think about post-processing list [r], extract lastlogin field and simply use fold to get the oldest one. But I'd like to calculate this information during parsing of the input. And I am not allowed to change the interface of convertPairsToRecords. Can somebody please suggest how to solve the task? Probably with Writer monad, or may be State? Thank you in advance! -- Eugene Dzhurinsky