At Thu, 22 Dec 2005 11:26:51 +0000, Joel Reymont wrote:
Folks,
I'm trying to monadify the pickler code. sequ below positively looks like >>= but you can't really join both pickle and unpickle into a single monad. I would like to keep the ops together, though, as this allows me a single specification for both pickling and unpickling.
Last weekend, I hacked up a pickling/unpickling library of my own. The code is currently a little confusing because I decided to change the naming scheme half way through. So, don't assume to much from the names of things. darcs get http://www.n-heptane.com/nhlab/repos/BerkeleyDB The file you are interested in is Binary.hs. -== Short Summary ==- My library splits the pickling into two parts you can mix and match. (1) the part that turns a value into a byte stream (2) the part that reads/writes values from/to the byte stream -== Core of pickler ==- My pickling/unpickling code is based around the data type: data BinParser state m a = BinParser { runBinParser :: state -> m (a, state) } This type is used for both pickling and unpickling (the type needs a better name). It is abstracted over three types: state - for the pickler, state will hold the data that has currently been pickled. for the unpickler, state will hold the data to unpickle. m - a monad of your choice a - the value being pickled/unpickled The reason for abstracting over state is to allow you to pickle directly to [Word8], Ptr Word8, or whatever else you wish to implement. Some times a monad my be needed for reading/writing the state. For example, Ptr Word8 requires the IO monad. If you don't really need a monad, then the Identity monad can be used. -== BinParser Monad Instance ==- The monad instance for BinParser is pretty straight-forward: -- A monad instance for BinParser instance (Monad m) => Monad (BinParser state m) where return a = BinParser (\s -> return (a, s)) bp >>= f = BinParser ( \state -> do (a, state') <- runBinParser bp state runBinParser (f a) state' ) As I mentioned before, BinParser is used for both pickling *and* unpickling. Normally we think of Parsers as consuming the state, but there is no reason the 'parser' can not instead produce the state. Also, note that this monad instance is not very specific to pickling/unpickling at all. It is pretty much just a state monad. As a matter of fact, I hope to be able to switch to Control.Monad.State when I have time to work on this again. -== Adding new 'states' to pickle/unpickle ==- To add a new type of state to pickle/unpickle, you just add a new instance to this class (once again, needs a better name): class (Monad m) => BinState s m where getWord8 :: BinParser s m Word8 putWord8 :: Word8 -> BinParser s m () For example: instance BinState (Ptr Word8) IO where getWord8 = BinParser $ \p -> do v <- peek p return (v, advancePtr p 1) putWord8 w = BinParser $ \p -> do poke p w return ((), advancePtr p 1) -== pickle vs. unpickle ==- Here is where we actually combine the above to do pickling (once again, naming should be updated): class ToBin state m a where binary :: a -> BinParser state m () unbinary :: BinParser state m a Here is a simple pickler for 'Char' -- May want to store as 4 bytes to support Unicode later. instance (BinState state m) => ToBin state m Char where binary c = putWord8 (fromIntegral (ord c)) unbinary = do w <- getWord8 return $! (chr (fromIntegral w)) Here is a pickler for lists that shows the monad usage a bit better: instance (BinState state m, ToBin state m a) => ToBin state m [a] where binary l = do binary (length l) mapM_ binary l unbinary = getList getList :: (BinState state m, ToBin state m a) => BinParser state m [a] getList = do len <- getInt replicateM len unbinary -== User Friendly Interface ==- binary/unbinary are not very user friendly interfaces, so we also define some user friendly interefaces. If I switched to Control.Monad.State, I could just use the similar interfaces defined there... pickleM/unpickleM is useful if your state requires a monad. -- NOTE: you may need to force the type to get this to work -- eg. pickleM "hi" :: IO [Word8] pickleM :: (Monad m, ToBin state m a) => state -> a -> m state pickleM initState a = do (_,finalState) <- (runBinParser (binary a) initState) return finalState -- NOTE: you may need to force the type to get this to work -- eg. fromBin (unPickleM "hi" :: [Word8]) :: IO String unpickleM :: (Monad m, ToBin state m a) => state -> m a unpickleM state = do (a,_) <- runBinParser unbinary state return a pickle/unpickle are useful if your state does not need a monad. -- Some pickler's may not need to run inside a monad, in which case we -- can use these varients to avoid the monad pickle :: (ToBin state Identity a) => state -> a -> state pickle initState value = runIdentity (pickleM initState value) unpickle :: (ToBin state Identity a) => state -> a unpickle state = runIdentity (unpickleM state) -== Example Usage ==- Here is an example of using the picklers: -- First define some data types to pickle data Foo = Bar String | Baz Int Char deriving Show data FooBar a = FooBar a deriving Show -- Use TH to derive some piclkers $( mkBinInstance ''Foo ) $( mkBinInstance ''FooBar) -- try them out -- NOTE: not sure if I am using the terms monomorphic/polymorphic -- correctly main = -- first pickle/unpickle a monomorphic (?) type do print (unpickle (pickle [] (Bar "hello") :: [Word8]) :: Foo) -- then pickle/unpickle a polymorphic (?) type print (unpickle (pickle [] (FooBar (Bar "hello")) :: [Word8]) :: (FooBar Foo)) -- use a pickler that outputs to (Ptr Word) instead of [Word8] allocaBytes 512 $ \ (p :: Ptr Word8) -> do encoded <- pickleM p (Baz 4 'd') decoded <- unpickleM p :: IO Foo print decoded -== Summary ==- As I mentioned, the current implementation is a bit of hack-job, but I think the design is somewhat compelling because of the flexibility gained by seperating the pickling/unpickling from the mechanism used to write/read the bytes. I hope to clean to code up and submit a TMR article eventually. j. ps. DStore.hs contains some code for deriving new instances of ToBin (the pickler/unpickler). I highly recommend you do not look at that code -- I am not sure I even understand how it works anymore :p