Hello all, I wrote a Logger which, under certain conditions, prepends log-entries to a log and a Monoid instance of it. But as soon as I mappend two Loggers my performance drops by 50%. This even happens when I mappend mempty as shown below in --<2--. I understand that the system has to do *something*, but it seems to cost a bit much. Without the strictness annotation in --<1-- the performance degradation is even more dramatic (orders of magnitude). The profile tells me that more that 50% of the time is spent in mappend. COST CENTRE MODULE %time %alloc mappend.\ Logger 50.6 35.8 logCount'.f Logger 18.7 40.3 logCount' Logger 5.4 0.0 Why is that so, and can I do anything about it? I am willing to change the overall design if required. This is the code -- | A writer does the formatting newtype Wtr a log = Wtr {runWtr :: a -> log} -- | A looger is a writer plus an internal state data Logger a log = Lgr {runLogger :: a -> log -> (log, Logger a log)} instance Monoid (Logger a log) where mempty = Lgr (\_ l -> (l,mempty)) mappend lgr1 lgr2 = Lgr $ \a l -> let !(log1',!lgr1') = runLogger lgr1 a l --<1-- !(log2',!lgr2') = runLogger lgr2 a log1' --<1-- in (log2', mappend lgr1' lgr2') and this is how I test it -- | Count calls __s__ and write log when s has reached nxt and then every dn calls logCount' :: Monoid log => Int -> Int -> Int -> Wtr (Int,a) log -> Logger a log logCount' dn nxt s wtr = Lgr f where f a l = if s == nxt then (runWtr wtr (s,a) <> l, logCount' dn (nxt+dn) (s+1) wtr) else (l, logCount' dn nxt (s+1) wtr) -- | Count calls and write log every dn calls logCount dn = logCount' dn dn 0 -- testLogger :: Logger Int Int [String] -> [String] testLogger lgr xs = fst $ foldl' f ([],lgr) xs where f (log', lgr') x = runLogger lgr' x log' ex_wtr :: Wtr (Int,a) [String] ex_wtr = Wtr $ \(x,_) -> ["Counted to " ++ (show x)] ex_wtr2 :: Wtr Int [String] ex_wtr2 = Wtr $ \x -> ["Counted to " ++ (show x)] ex_inputs :: [Int] ex_inputs = [1..10000000] ex_logger = mempty <> logCount 300000 ex_wtr <> mempty --<2-- -- ex_logger = logCount 300000 ex_wtr ex_main = do timeIt $ putStrLn $ ppShow $ testLogger ex_logger ex_inputs main = ex_main