All,
I've been casually developing a PacketBB (i.e. Generalized Manet Packet
Format) library in Haskell. I think I have a need to pass state
information as a phantom type - I'll step through the issue now.
With the 'AddressBlock' (S5.2.1 packetBB draft 8), network addresses are
abbreviated as sets of bytes (potentially just one byte each, with a
head or tail identical with other addresses). How many bytes are in the
set is determined, in part, by the type of address stored (ex: IPv4 or
IPv6). Thus, when serializing, I need to provide this information.
Saying this again, but in (simplified) code:
data NetworkAddress a = AddressBlock a =
AddrBlkWire {
lenHd :: Word8,
hd :: [Word8],
lenTl :: Word8,
tl :: [Word8],
nrAddrs :: Word8,
addrs :: [Word8] }
| AddrBlkAbstract [a]
data (NetworkAddress a) = SomeHigherLevelStruct a =
SHLS (AddressBlock a) Word32 Word8
-- length (addrs x) == (TotalAddressLength - lenHd - lenTl) * nrAddrs
I can think of several ways to convert between AddrBlkWire and
ByteStrings:
1) Make separate instance of 'Binary' for each data type element of
NetworkAddress.
instance Binary (AddressBlock IPv4) where
get = ...
put = ...
instance Binary (AddressBlock IPv6) where
get = ...
put = ...
This solution immediately causes problems with every higher level
structure you wish to serialize. For example, now you have to have
individual instance for SHLS, you can't do:
instance (NetworkAddress a) = Binary (SomeHigherLevelStruct a) where
...
2) You can pass another argument into a custom 'get' routine. I see
this as a hack that makes me break a good naming convention.
getNetworkAddress :: Int-- bytes per address
- Get NetworkAddress
3) If you don't worry about decoding, only encoding, then an extra field
in the data structure can fill the void of an extra argument. Also a
hack.
I'm hoping someone here has a better solution. Perhaps I am making a
mountain out of a mole hill, or perhaps this requires one of those type
system extensions I have yet to look hard at. The solution I would want
looks like this:
class NetworkAddress a where
addressByteSize :: a - Int
instance (NetworkAddress a) = Binary (AddressBlock a) where
get = do
lenH - get
h- replicateM get (fromIntegral lenH)
lenT - get
t- replicateM get (fromIntegral lenT)
nr - get
let addrSize = addressByteSize (undefined :: a)
bytes = (addrSize - lenH - lenT) * nr
addrs - replicateM get (fromIntegral bytes)
return ...
The line 'addrSize = ' is what I don't know how to write. How does one
call an instance of a type class without knowing the type at compile
time?
Thanks,
Tom
--
The philosophy behind your actions should never change, on the other
hand, the practicality of them is never constant. - Thomas Main
DuBuisson
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