data Exp = Val Int
         | Mas Exp Exp
         | Menos Exp Exp
         | Por Exp Exp
         | Div Exp Exp

{- antes de Div
eval :: Exp -> Int
eval (Val i) = i
eval (Mas a b) = eval a + eval b
eval (Menos a b) = eval a - eval b
eval (Por a b) = eval a * eval b
eval (Div a b) = eval a `div` eval b
-}

eval :: Exp -> Maybe Int
eval (Val i) = Just i
eval (Mas a b) = case eval a of
                   Just x -> case eval b of
                               Just y -> Just (x+y)
                               Nothing -> Nothing
                   Nothing -> Nothing
eval (Menos a b) = case eval a of
                   Just x -> case eval b of
                               Just y -> Just (x-y)
                               Nothing -> Nothing
                   Nothing -> Nothing
eval (Por a b) = case eval a of
                   Just x -> case eval b of
                               Just y -> Just (x*y)
                               Nothing -> Nothing
                   Nothing -> Nothing
eval (Div a b) = case eval a of
                   Just x -> case eval b of
                               Just y -> if y == 0 then Nothing
                                         else Just (x `div` y)
                               Nothing -> Nothing
                   Nothing -> Nothing