[Haskell-beginners] More De/Serialisation Woes

Tom Hobbs tvhobbs at googlemail.com
Tue Sep 7 05:23:21 EDT 2010

Hi all,

Can anyone help me with some Haskell de/serialisation woes I'm encountering?
 What I'm trying to do it implement a proprietary network protocol which
will enable me to talk to a network Java/Tuple Space.

Specifically, I'm trying to serialise a "TypeStructurePreamble" and send
that down the Handle (my network stream), I'm then expecting the Space to
send me a TypeStructurePreamble back again, but with different data.  So I
need to de/serialise this type.  I've not got all the code together yet,
because I'm trying to figure out the concepts rather than any particular
coding problem.

Here's some code I have so far;

import System.IO
import Data.Binary.Get
import qualified Data.ByteString.Lazy as L

type TypeStructurePreamble = (String,Int,[String])

-- This function I know works because it was spoon-fed to me by someone on
this list!  :-)
readStrings :: Int -> Get [String]
readStrings n = replicateM n $ do
                len <- getWord32be
                name <- getByteString $ fromIntegral len
                return $ UTF.toString name

deserialisePreamble :: Get(String, Int, [String])
deserialisePreamble = do
                      len <- getWord32be
                      typename <- getByteString $ fromIntegral len
                      c <- getWord32be
                      numFields <- getWord32be
                      fields <- readStrings (fromIntegral numfields)
                      return (typename,c,numFields)

-- not sure what this type should be
serialisePreamble :: Handle -> TypeStructurePreamble -> ??
serialisePreamble  h (n,c,fs) = do
                                L.hPut h (encode (0xFAB1000A :: Word32))
                                L.hPut h (encode (length n))
                                L.hPut h (encode n)
                                L.hPut h (encode (c :: Word32))
                                L.hPut h (encode (length fs))
                                -- not sure how I would L.hPut out every
String in fs

It is my (maybe wrong) understanding that these de/serialise functions don't
actually do the work, they basically just form a description of what must be
done in order to get the work done, so I'm slightly confused on how I should
use it.

For example;

main = do
       -- set up Handle called h here, I know how to do this
       serialisePreamble h ("MyType",0,["field1","field2"])
       --now I'm expecting to be able to read a modified
TypeStructurePreamble from h
       (n,c,fs) <- deserialisePreamble -- but according to the type of
deserialisePreamble, how does the h get into it?
       print "c was "+c -- can I just slot sout style statements into here?
       evaluate (n,c,fs)
       hClose h
       return (n,c,fs)

Does this make sense?  Can someone help me fill in the blanks of
serialisePreamble and main?  I've read the tutorials on the Get monad which
google turns up, am I missing some really good ones?  Is there some other
resource which I'm missing?  I'm been thrown a lot of the Hackage
documentation on these things but (assuming I'm looking in the right place)
they seem to work more like JavaDocs and give the "what it does", rather
than the "how it should be used".

Many thanks,

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