On Wed, Jul 7, 2010 at 8:48 PM, John Meacham
<john@repetae.net> wrote:
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)
--