Hey everyone, I'm making a Game Monad for an assignment (yes, homework) . Here's a little explanation of what I need to do (I can't use anything from Control.Monad.*, I need to do everything myself): I need to define my newtype Game and make it's Monad instance I need to make the function : runGame :: Game a -- A particular game -> Int -- Initial amount of lives -> Maybe (a , Int ) -- Result and remaining lives. I need to make the instance for this class: class Monad m = > GameMonad m where extraLife :: m () getLives :: m Int checkPoint :: m a -> m a die :: m a now what I've done (I was inspired by the State Monad) I defined my Game type as follows:
newtype Game r = Game { execGame :: Int -> Maybe (r,Int) }
My Monad instance like this:
instance Monad Game where return a = Game $ \r -> Just (a,r) m >>= k = Game $ \r -> let x = execGame m r in case x of Just (a, r') -> execGame (k a) r' Nothing -> Nothing
and my GameMonad instance:
instance GameMonad Game where extraLife = Game $ \l -> Just ((),l+1) getLives = Game $ \l -> Just (l,l) die = do n <- getLives Game $ \_ -> Just ((),n-1) -- Here's the problem
so, what's bothering me? Look at the type signature of die in the GameMonad class, it's supposed to return something of type (m a), but I don't know what to return in that case, and whatever I try to return it doesn't work, because when I try to compile it says that it couldn't match expected type 'a' against infered type 'whatever' ('whatever' being anything, from (), to string, or a number). shouldn't 'a' match with anything I put there? Note that I can't change the signatures because they were giving to me that way and I already checked with teacher if they were right. (I fixed it returning Maybe (Maybe a, Int) instead, but I can't change the signature of the function runGame) Thanks you, Hector Guilarte