-- strict foldl: O(n) space required
foldl' :: (a -> b -> a) -> a -> [b] -> a
foldl' f a []     = a
foldl' f a (x:xs) = (foldl' f $! f a x) xs

suml' :: (Num a) => [a] -> (a,Int)
suml' = foldl' (\(s,l) x -> (s+x,l+1)) (0,0)

mean' xs = (fst p) / fromIntegral (snd p) where p = suml' xs

suml :: (Num a) => [a] -> (a,Int)
suml = foldl (\(s,l) x -> (s+x,l+1)) (0,0)

mean xs = (fst p) / fromIntegral (snd p) where p = suml xs