data Expr = Val Int | Div Expr Expr
eval :: Expr -> Int
eval (Val n) = n
eval (Div x y) = eval x `div` eval y
eval (Div (Val 1) (Val 0))

safediv :: Int -> Int -> Maybe Int safediv _ 0 = Nothing
safediv n m = Just (n ‘div‘ m)

eval :: Expr -> Maybe Int eval (Val n) = Just n
eval (Div x y) = case eval x of Nothing -> Nothing
                      Just n -> case eval y of
                      Nothing -> Nothing Just m -> safediv n m

eval :: Expr -> Maybe Int
eval (Val n) = pure n
eval (Div x y) = pure safediv <*> eval x <*> eval y

(>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b
mx >>= f = case mx of
                Nothing -> Nothing
                Just x -> f x

eval :: Expr -> Maybe Int
eval (Val n) = Just n
eval (Div x y) = eval x >>= \n ->
              eval y >>= \m -> 
              safediv n m

m1 >>= \x1 -> m2 >>= \x2 -> 
.
.
mn >>= \xn -> f x1 x2 ... xn

do x1 <- m1 x2 <- m2
.
.
.
xn <- mn
f x1 x2 ... xn

eval :: Expr -> Maybe Int
eval (Val n) = Just n
eval (Div x y) = do n <- eval x
              m <- eval y
              safediv n m

class Applicative m => Monad m where 
  return :: a -> m a
  (>>=) :: m a -> (a -> m b) -> m b
  return = pure

return :: Applicative f => a -> f a 
return = pure