Hi, I'm relatively new to Haskell so please bear with me. I'm trying to parse Java class files with Data.Binary and I'm having a few problems: (The class file format is described here: http://java.sun.com/docs/books/jvms/second_edition/html/ClassFile.doc.html and the bytecode instructions are described here: http://java.sun.com/docs/books/jvms/second_edition/html/Instructions.doc.htm... ) 1. The class file format contains a number of tables. The table definitions start with the length of the list and carry on with that many table entries. Lists would be a good representation for them in Haskell, because there is not need to index them directly (except with the constants table). I've created my own list type so that I can redefine the serialisation functions for it so that the serialisation matches the format defined in the class file format: newtype MyList e = MkList ([e]) deriving Show instance (Binary e) => Binary (MyList e) where put (MkList es) = do put (fromIntegral (length es) :: Word16) mapM_ put es get = do n <- get :: Get Word16 xs <- replicateM (fromIntegral n) get return (MkList xs) The problem is that one of the tables, namely the attribute_info structures, use a u32 length field whereas all the other tables use a u16 length field. My implementation uses u16, but it would be nice to be able to use the same data type for both types of tables. I think I can do it by adding a lenght field to MyList and specifying the type when I use MyList in some other data structure, but that would also mean that I have to keep track of the length of the list manually? I'm basically copy-pasting the same code just to use a u32 length field in the serialised form: data Info = MkInfo [Word8] deriving Show instance Binary Info where put (MkInfo xs) = do put (fromIntegral (length xs) :: Word32) mapM_ put xs get = do n <- get :: Get Word32 xs <- replicateM (fromIntegral n) get return (MkInfo xs) 2. This is the bigger problem. The Java class file contains a constants table in the beginning of the file. The other fields later on in the class file contain indexes that reference entries in that constants table. So in order to be able to replace an index in a data structure with the actual string, I need to be able to look up the string from the constants table while I'm deserialising the field. My guess is that I should be able to put the constants table into a state monad. On the other hand Data.Binary already uses the state monad for holding onto the binary data being deserialised. So it's not clear to me if I can use StateT with Data.Binary.Get? And if not, can I implement my own state monad and do it that way? I'm not very comfortable with Monads yet, so I might be missing something very obvious. This is what the get function looks like in my top-level Data.Binary instance: get = do magic <- get :: Get Word32 case magic == magicNumber of -- class files start with 0xCAFEBABE True -> do min <- get -- minor version number u16 maj <- get -- major version number u16 c <- get -- constants table a <- get -- access flags (public, abstract, ...) u16 t <- get -- a u16 index pointing to the name of this class in the constants table s <- get -- a u16 index pointing to the name of the super class in the constants table i <- get -- a table of interfaces f <- get -- a table of fields m <- get -- a table of methods attrs <- get -- a table of class level attributes return (ClassFile (min, maj) c a t s i f m attrs) False -> error "Invalid magic number" Thanks for all the help! -- ! Lauri