gistfile1.hs

-- ch14/Logger.hs
type Log = [String]
newtype Logger a = Logger { execLogger :: (a, Log) }

instance Monad Logger where
    return a = Logger (a, [])
    m >>= k = let (a, s) = execLogger m
                      mb = k a
                  (b, s') = execLogger mb
               in Logger (b, s ++ s')

record s = Logger ((), [s])