{- A multiline comment which can continue for many lines -} 

data Expr = Val Int | Div Expr Expr 
-- data Maybe a = Nothing | Just a


eval :: Expr -> Maybe Int
eval (Val n) = return n
eval (Div x y) = eval x >>=  (\n -> eval y >>= (\m -> safediv n m))
						                    
					
safediv :: Int -> Int -> Maybe Int
safediv n m = if m == 0 then Nothing else Just (n `div` m)

{-
m >>= f = case m of  
				Nothing -> Nothing
				Just x -> f x
-}				
				
-- Just 4 >>= (\x -> Just (x+1))  				
--  eval (Div (Val 5) (Val 3))      
-- eval (Div (Val 5) (Val 0))