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