import Control.Applicative (Alternative(..)) import Control.Monad (liftM, ap, guard) import Control.Monad.Fail (MonadFail(..)) import Control.Monad.Trans.Class (MonadTrans(lift)) -- State Monad -------------- newtype State s a = State { runState :: s -> (a, s) } instance Functor (State s) where fmap = undefined instance Applicative (State s) where pure = undefined (<*>) = undefined instance Monad (State s) where return a = State $ \s -> (a, s) m >>= k = State $ \s -> let (x, s') = runState m s in runState (k x) s' get :: State s s get = State $ \s -> (s, s) put :: s -> State s () put s = State $ \_ -> ((), s) modify :: (s -> s) -> State s () modify f = State $ \s -> ((), f s) -- MaybeT MonadTransformer --------------------------- newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) } instance Monad m => Functor (MaybeT m) where fmap = undefined {- instantieer hier ook: - Applicative - Monad - Alternative - MonadFail -} instance MonadTrans MaybeT where -- "herwrappen" van het argument lift = undefined -- Stack Manipulation --------------------- type Stack = [Int] -- plaats het argument bovenop de stack push :: Int -> State Stack () push x = undefined -- geef de grootte van de stack terug size :: State Stack Int size = undefined -- neem het eerste element van de stack, als het aanwezig is -- (hint: hoogle naar `guard`) pop :: MaybeT (State Stack) Int pop = undefined -- haal de twee bovenste elementen van de stack, en plaats hun som er -- terug op. add :: MaybeT (State Stack) () add = undefined mul :: MaybeT (State Stack) () mul = undefined