再次閱讀haskell體會

from

	  
newtype DiffList a = DiffList{ getDiffList :: [a] -> [a] }


toDiffList :: [a] -> DiffList a
toDiffList xs = DiffList (xs++)

fromDifList :: DiffList a -> [a]
fromDifList (DiffList f) = f [] 


instance Monoid (DiffList a) where
	mempty = DiffList (\xs -> [] ++ xs)
	(DiffList f) `mappend` (DiffList g) =  DiffList (\xs -> f (g xs))
	
	
	
finalCountDown :: Int -> Writer (DiffList String) ()
finalCountDown 0 = do
	tell (toDiffList ["0"])
finalCountDown x = do
	finalCountDown (x - 1)
	tell (toDiffList [show x])