Here are the snippets of my code currently hurting my coding ulcer... routes :: [Record] -> IO () routes legodata = do let all = foldl forwardRoutes M.empty legodata -- [Record] return () forwardRoutes :: M.Map String [String] -> Record -> M.Map String [String] forwardRoutes map row = let key = row !! 0 in let val = row !! 1 in case lookup key map of Nothing -> M.insert key [val] map Just routes -> M.insert key val:routes map -- some 'deductions' by me, probably erroneous, to help me try to figure it out -- --if legodata is [Record] -- then row is Record => [Field] -- -- if map is "String [String]" -- then key is Field because row is [Field] -- then val is Field because row is [Field] -- Field is "String" -- -- lookup key map => lookup String M.Map String [String] -- If I am fold-ing over [Record] then I assume that the data that is passed to the callback function is of the type Record, i.e. a single thing from the list of things. Record is defined as [Field] so presumably I could use [Field] or Record in my function 'forwardRoutes'. When I compile the above I get: scread.hs:92:18: Couldn't match expected type `[(Field, b)]' against inferred type `M.Map String [String]' In the second argument of `lookup', namely `map' In the expression: lookup key map In the expression: case lookup key map of { Nothing -> M.insert key [val] map Just routes -> M.insert key val : routes map } I have spent a long time searching and reading and trying to fathom it out from first principles but I cannot for the life of me figure out where '[(Field, b)]' is coming from! I can smell Field in there because Record is typed as [Field], but a tuple? And what is 'b' ? The 'inferred type' matches what I thought it should be in the error message and I am right (I think) in stating that the 'expected type' is what it actually got from the code at compile time?! Help! :)