Hi I have a trouble to run an example of "Learn Your A Haskell.." Chap 13 below: ======== import Data.Monoid --Don't import Control.Monad.Writer newtype Writer w a = Writer { runWriter :: (a, w) } instance (Monoid w) => Monad (Writer w) where return x = Writer (x, mempty) (Writer (x,v)) >>= f = let (Writer (y, v')) = f x in Writer (y, v `mappend` v') --Define tell tell :: [String] -> Writer [String] Int tell w = Writer (0, w) -- what'sa hell "0" for ???!!! logNumber :: Int -> Writer [String] Int logNumber x = Writer (x, ["Got number: " ++ show x]) multWithLog :: Writer [String] Int multWithLog = do a <- logNumber 3 b <- logNumber 5 tell ["Gonna multiply these two"] return (a*b) main = putStrLn . show $ runWriter multWithLog ======== I changed two places to run it without error: [1] Ambiguity error of Writer, uneless I comment out "import Control.Monad.Writer", and [2] Define tell function My questions are: Why does LYAH sample fail as is? Do the changes above look reasonable? I'm not certain about my "tell". Where is the correct instantiation of "tell" included? Thanks, TJ