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

-- Monada

{- Con la monada identidad, antes de evaluar Div
type T a = a

t :: (a -> b) -> T a -> T b
t f = f

eta :: a -> T a
eta a = a

mu :: T (T a) -> T a
mu tta = tta

eval :: Exp -> T Int
eval (Val i) = eta i
eval (Mas a b) = mu $ t (\x -> mu $ t (\y -> eta (x+y)) (eval b)) (eval a)
eval (Menos a b) = mu $ t (\x -> mu $ t (\y -> eta (x-y)) (eval b)) (eval a)
eval (Por a b) = mu $ t (\x -> mu $ t (\y -> eta (x*y)) (eval b)) (eval a)
-}

type T a = Maybe a

t :: (a -> b) -> T a -> T b
t f = \ta -> case ta of
               Just a -> Just (f a)
               Nothing -> Nothing

eta :: a -> T a
eta = Just

mu :: T (T a) -> T a
mu tta = case tta of
           Just ta -> ta
           Nothing -> Nothing

div0 :: T a
div0 = Nothing

eval :: Exp -> T Int
eval (Val i) = eta i
eval (Mas a b) = mu $ t (\x -> mu $ t (\y -> eta (x+y)) (eval b)) (eval a)
eval (Menos a b) = mu $ t (\x -> mu $ t (\y -> eta (x-y)) (eval b)) (eval a)
eval (Por a b) = mu $ t (\x -> mu $ t (\y -> eta (x*y)) (eval b)) (eval a)
eval (Div a b) = mu $ t (\x -> mu $ t (\y -> if y == 0 then div0 else eta (x `div` y)) (eval b)) (eval a)