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. Cale suggested that PU is really an arrow in that it supports both input and output. I could not find an example of such an arrow, though. I thought that it could be a "dual arrow" but then could not find a description for one. I would appreciate your suggestions! The original paper is at http:// research.microsoft.com/ ~akenn/fun/picklercombinators.pdf Thanks, Joel P.S. data PU a = PU { appP :: Ptr Word8 -> Int -> a -> IO Int, appU :: Ptr Word8 -> Int -> IO (a, Int), appS :: a -> IO Int } pickle :: PU a -> Ptr Word8 -> Int -> a -> IO Int pickle p ptr ix value = appP p ptr ix value unpickle :: PU a -> Ptr Word8 -> Int -> IO (a, Int) unpickle p ptr ix = appU p ptr ix sizeup :: PU a -> a -> IO Int sizeup p value = appS p value lift :: a -> PU a lift x = PU (\_ ix _ -> return ix) (\_ ix -> return (x, ix)) (\_ -> return 0) sequ :: (b -> a) -> PU a -> (a -> PU b) -> PU b sequ f pa k = PU (\ptr ix b -> do let a = f b pb = k a ix1 <- appP pa ptr ix a appP pb ptr ix1 b) (\ptr ix -> do (a, ix1) <- appU pa ptr ix let pb = k a appU pb ptr ix1) (\b -> do let a = f b pb = k a sz1 <- appS pa a sz2 <- appS pb b return $ sz1 + sz2) pair :: PU a -> PU b -> PU (a,b) pair pa pb = sequ fst pa (\ a -> sequ snd pb (\ b -> lift $! (a, b))) -- http://wagerlabs.com/