Hey! I just wanted to let you know I made it. I just changed the newtype declaration to:

newtype Game r = Game { execGame :: Int -> (Maybe r,Int) } and from there everything went just fine.

Thank you for your responses, Hector Guilarte On Wed, Jul 7, 2010 at 9:24 PM, Hector Guilarte wrote:

On Wed, Jul 7, 2010 at 8:48 PM, John Meacham wrote:

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Are you sure you are interpreting what 'die' should do properly? Your code makes sense if die should decrement your life counter and continue along, however if 'die' is meant to end your whole game, then there is another implementation that does type check.

John

You're absolutely right, I sen't the wrong code, here's the "correct" one and a little bit more explanation about what checkpoint does.

The result of die makes sense for the checkPoint function since there are three cases for it: 1) The player died and has no remaining lifes. The game can't continue, I just return Noting in the die function and in checkpoint make the corresponding case. 2) The player died and has remaining lifes. The game can be retried with a life subtracted. I would need to tell checkpoint that I died and I want to retry, that's where I think the result is important, because of the next case. 3) The player didn't died, it finished the particular game and checkpoint m equals m. Here I would need to see if the result of the game was different from the result from die, and continue.

instance GameMonad Game where extraLife = Game $ \l -> Just ((),l+1) getLives = Game $ \l -> Just (l,l) die = do n <- getLives if n <= 0 then Game $ \_ -> Nothing else Game $ \_ -> Just ("player died",n-1) checkPoint a = do n <- getLives case execGame a n of Nothing -> Game $ \_ -> Nothing Just c -> gameOn $ fst c where gameOn "player died" = a >>= \_ -> (checkPoint a) gameOn _ = a

Obviously this fails to compile because I'm returning a String and it doesn't match with a either, but the idea of what I think I need to do is right there.

Ivan Miljenovic told me to use error, and actually I though something like that. in STM retry combined with atomically does something similar as what I need checkpoint and die to do, and they use exceptions to accomplish it. I really think that's the solution I want, but then I have another question, when I 'throw' the exception in die and 'catch' it in checkpoint to call it again, is the number of lives gonna be lives - 1?

Thanks for answering so quickly,

Hector Guilarte

Pd: Here's an example run of how my homework should work after is finished

printLives :: ( GameMonad m , MonadIO m ) = > String -> m () printLives = do n <- getLives liftIO $ putStrLn $ s ++ " " ++ show n test1 :: ( GameMonad m , MonadIO m ) = > m () test1 = checkPoint $ do printLives " Vidas : " die liftIO $ putStrLn " Ganamos ! "

lastChance :: GameMonad m = > m () lastChance = do n <- getLives if n == 1 then return () else die test2 :: ( GameMonad m , MonadIO m ) = > m String test2 = checkPoint $ do printLives " Inicio " n <- getLives if n == 1 then do liftIO $ putStrLn " Final " return " Victoria ! " else do checkPoint $ do printLives " Checkpoint anidado " lastChance extraLife printLives " Vida extra ! " die

AND THE OUTPUT TO SOME CALLS

ghci > runGameT test1 3 Vidas : 3 Vidas : 2 Vidas : 1 Nothing ghci > runGameT test2 3 Inicio 3 Checkpoint anidado 3 Checkpoint anidado 2 Checkpoint anidado 1 Vida extra ! 2 Inicio 1 Finish Just ( " Victoria ! " ,1)

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John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe