Monad transformers: Implementation of a stack machine with MaybeT (State Stack)

Question

I'm trying to implement a Maybe-State monad transformer and use it to implement a simple stack machine. The definitions of state monad and maybe should be correct. Now I'm trying to implement pop:

pop :: MaybeT (State Stack) Int

So that if the stack is empty it returns nothing, otherwise it returns Just <popped stack>. This is what I have so far:

pop :: MaybeT (State Stack) Int
pop = guard True (do (r:rs) <- get
                     put rs
                     return r)

(Obviously True is just a dummy placeholder - I'll implement the condition later, for now I want to get the other part right).

What is wrong with my code? From my understanding guard takes a conditional (True) and a function f. If the conditional is true it then gives pure f.

In my case,

pure = MaybeT . return . Just

So shouldn't my function f just return a State Stack Int?

---

Here is the full code, with my implementations of MaybeT and State:

import Control.Applicative (Alternative(..))
import Control.Monad (liftM, ap, guard)
import Control.Monad.Trans.Class (MonadTrans(lift))

main :: IO()
main = return ()


-- State Monad
--------------

newtype State s a = MakeState { runState :: s -> (a, s) }

instance Functor (State s) where
    fmap  = liftM


instance Applicative (State s) where
    pure a = MakeState $ \s -> (a, s)
    (<*>)  = ap

instance Monad (State s) where
    return a = MakeState $ \s -> (a, s)
    m >>= k  = MakeState $ \s -> let (x, s') = runState m s
                              in runState (k x) s'

get :: State s s
get = MakeState $ \s -> (s, s)

put :: s -> State s ()
put s = MakeState $ \_ -> ((), s)

modify :: (s -> s) -> State s ()
modify f = MakeState $ \s -> ((), f s)

-- MaybeT MonadTransformer
---------------------------

newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }

instance Monad m => Functor (MaybeT m) where
    fmap a x = MaybeT $ do e <- runMaybeT x
                           return $ fmap a e



instance Monad m => Applicative (MaybeT m) where
    pure      = MaybeT . return . Just
    (<*>) a b = MaybeT $ do e <- runMaybeT a
                            f <- runMaybeT b
                            return $ e <*> f

instance Monad m => Monad (MaybeT m) where
    return  = pure
    a >>= b = MaybeT $ do aa <- runMaybeT a
                          maybe (return Nothing) (runMaybeT . b) aa


instance Monad m => Alternative (MaybeT m) where
  empty   = MaybeT $ return Nothing
  a <|> b = MaybeT $ do aa <- runMaybeT a
                        bb <- runMaybeT b
                        return $ aa <|> bb


instance MonadTrans MaybeT where
    -- "herwrappen" van het argument
    lift x = MaybeT $ do r <- x
                         return $ Just r


-- Stack Manipulation
---------------------

type Stack = [Int]

-- plaats het argument bovenop de stack
push :: Int -> State Stack ()
push x = do r <- get
            put (x:r)
-- geef de grootte van de stack terug
size :: State Stack Int
size = do r <- get
          return $ length r

-- neem het eerste element van de stack, als het aanwezig is
-- (hint: hoogle naar `guard`)
pop :: MaybeT (State Stack) Int
pop = guard (True) (do (r:rs) <- get
                       put rs
                       return r)

Answer 1

guard doesn't take two arguments, it only takes a Bool argument.

You also need to lift your state manipulations into MaybeT:

pop :: MaybeT (State Stack) Int
pop = do
  guard True
  (r:rs) <- lift get
  lift $ put rs
  return r

Answer 2

First of all, you should understand if your stack is empty, your pattern r:rs <- get fails. But you write it in do-block, so the fail function will be called. It is implemented for Monad m => MaybeT m like this: fail _ = MaybeT (return Nothing). This means that if the pattern fails it returns Nothing. That what you want.

So, you can do like this:

pop :: MaybeT (State Stack) Int
pop = do r:rs <- get
         put rs
         return r

Answer 3

For the sake of comparison, here is a cruder implementation which doesn't rely neither on guard nor on fail:

pop :: MaybeT (State Stack) Int
pop = do
  stk <- lift get
  case stk of
    [] -> empty
    (r:rs) -> do
      lift (put rs)
      return r

Producing empty when the stack is [] amounts to the same thing that using guard in the way you intend, or using fail to exploit a failed pattern match (as in freestyle's answer).