monadExample.hs

data Expr = Val Int | Div Expr Expr deriving Show

eval :: Expr -> Int
eval (Val x) = x
eval (Div x y) = eval x `div` eval y

safeDiv :: Int -> Int -> Maybe Int
safeDiv n m = if m == 0 then Nothing else Just (n `div` m)

safeEval :: Expr -> Maybe Int
safeEval (Val x) = Just x
safeEval (Div x y) = 
    case safeEval x of
        Nothing -> Nothing
        Just x' -> case safeEval y of
            Nothing -> Nothing
            Just y' -> safeDiv x' y'

-- (>>=) :: Monad m => m a -> (a -> m b) -> m b
safeEval' :: Expr -> Maybe Int
safeEval' (Val x) = return x
safeEval' (Div x y) = do x' <- safeEval' x
                         y' <- safeEval' y
                         safeDiv x' y'
-- safeEval' (Div x y) = 
--    safeEval' x >>= (\x' -> safeEval' y >>= (\y' -> safeDiv x' y'))