I'm not sure what's going on BELOW. Was working with mapAccumL earlier and decided to move over to State monad for more control. :m + Data.Map :m + Data.List Prelude Data.List Data.Map> let f key new old = new ++ old Prelude Data.List Data.Map> let moby2 = "Moby Dick is a great book. Moby Dick was a white whale." Prelude Data.List Data.Map> fst $ mapAccumL (\ (m,p) w -> ((insertWithKey' f p [w] m, (snd p, w)),"")) (singleton ("\n", "\n") [], ("\n","\n")) (words moby2) (fromList [(("\n","\n"),["Moby"]),(("\n","Moby"),["Dick"]),(("Dick","is"),["a"]),(("Dick","was"),["a"]),(("Moby","Dick"),["was","is"]),(("a","great"),["book."]),(("a","white"),["whale."]),(("book.","Moby"),["Dick"]),(("great","book."),["Moby"]),(("is","a"),["great"]),(("was","a"),["white"])],("white","whale.")) Michael BELOW =========================== import Control.Monad.State import Data.Map type Prefix = (String,String) type GeneratorState = (Map Prefix [String],Prefix,[String]) non_word = "\n" f key new old = new ++ old  buildMap :: GeneratorState (Map Prefix [String])   buildMap = do (mp,(pfx1,pfx2),all@(w1:words)) <- get               if (Prelude.null all)                 then  {- No more words. Return final map (adding non_word for prefix). -}                   return (insertWithKey' f (pfx1,pfx2) [non_word] mp)                 else  {- Add word to map at prefix. Continue. -}                   put (insertWithKey' f (pfx1,pfx2) [w1] mp, (pfx2,w1), words)                   buildMap ========================== Prelude> :l markov3.hs [1 of 1] Compiling Main             ( markov3.hs, interpreted ) markov3.hs:11:12:     `GeneratorState' is applied to too many type arguments     In the type signature for `buildMap':       buildMap :: GeneratorState (Map Prefix [String]) Failed, modules loaded: none.