This module contains fuctions and templates for building up and breaking down packed bit structures. It's something like Erlang's bitsyntax (or, actually, more like Python's struct module).
This code uses Data.ByteString which is included in GHC 6.5 and you can get it for 6.4 at http://www.cse.unsw.edu.au/~dons/fps.html
 data BitBlock
 makeBits :: [BitBlock] > ByteString
 data ReadType
 bitSyn :: [ReadType] > Q Exp
 decodeU8 :: ByteString > Word8
 decodeU16 :: ByteString > Word16
 decodeU32 :: ByteString > Word32
 decodeU16LE :: ByteString > Word16
 decodeU32LE :: ByteString > Word32
Building bit structures
The core function here is makeBits, which is a perfectly normal function.
Here's an example which makes a SOCKS4a request header:
makeBits [U8 4, U8 1, U16 80, U32 10, NullTerminated "username",
NullTerminated "www.haskell.org"]
U8 Int  Unsigned 8bit int 
U16 Int  Unsigned 16bit int 
U32 Int  Unsigned 32bit int 
U16LE Int  Littleendian, unsigned 16bit int 
U32LE Int  Littleendian, unsigned 32bit int 
NullTerminated String  Appends the string with a trailing NUL byte 
RawString String  Appends the string without any terminator 
RawByteString ByteString  Appends a ByteString 
PackBits [(Int, Int)]  Packs a series of bit fields together. The argument is a list of pairs where the first element is the size (in bits) and the second is the value. The sum of the sizes for a given PackBits must be a multiple of 8 
makeBits :: [BitBlock] > ByteStringSource
Make a binary string from the list of elements given
Breaking up bit structures
The main function for this is bitSyn, which is a template function and
so you'll need to run with fth
to enable template haskell
http://www.haskell.org/th/.
To expand the function you use the splice command:
$(bitSyn [...])
The expanded function has type ByteString > (...)
where the elements of
the tuple depend of the argument to bitSyn (that's why it has to be a template
function).
Heres an example, translated from the Erlang manual, which parses an IP header:
decodeOptions bs ([_, hlen], _, _, _, _, _, _, _, _, _)  hlen > 5 = return $ BS.splitAt (fromIntegral ((hlen  5) * 4)) bs  otherwise = return (BS.empty, bs)
ipDecode = $(bitSyn [PackedBits [4, 4], Unsigned 1, Unsigned 2, Unsigned 2, PackedBits [3, 13], Unsigned 1, Unsigned 1, Unsigned 2, Fixed 4, Fixed 4, Context 'decodeOptions, Rest])
ipPacket = BS.pack [0x45, 0, 0, 0x34, 0xd8, 0xd2, 0x40, 0, 0x40, 0x06, 0xa0, 0xca, 0xac, 0x12, 0x68, 0x4d, 0xac, 0x18, 0x00, 0xaf]
This function has several weaknesses compared to the Erlang version: The elements of the bit structure are not named in place, instead you have to do a pattern match on the resulting tuple and match up the indexes. The type system helps in this, but it's still not quite as nice.
Unsigned Integer  An unsigned number of some number of bytes. Valid arguments are 1, 2 and 4 
UnsignedLE Integer  An unsigned, littleendian integer of some number of bytes. Valid arguments are 2 and 4 
Variable Name  A variable length element to be decoded by a custom
function. The function's name is given as the single
argument and should have type

Skip Integer  Skip some number of bytes 
Fixed Integer  A fixed size field, the result of which is a ByteString of that length. 
Ignore ReadType  Decode a value and ignore it (the result will not be part of the returned tuple) 
Context Name  Like variable, but the decoding function is passed the
entire result tuple so far. Thus the function whose name
passed has type

LengthPrefixed  Takes the most recent element of the result tuple and interprets it as the length of this field. Results in a ByteString 
PackedBits [Integer]  Decode a series of bit fields, results in a list of Integers. Each element of the argument is the length of the bit field. The sums of the lengths must be a multiple of 8 
Rest  Results in a ByteString containing the undecoded bytes so far. Generally used at the end to return the trailing body of a structure, it can actually be used at any point in the decoding to return the trailing part at that point. 
decodeU8 :: ByteString > Word8Source
decodeU16 :: ByteString > Word16Source
decodeU32 :: ByteString > Word32Source