Serializing a String with Data.Cereal or Data.Binary

Question

I am implementing a protocol that dictates that a string representing a password should be serialized to a fixed length, 10-byte field. I am using Data.Cereal to perform this task. Here's my most recent go at it:

padText :: Int -> Text -> Text
padText fieldLen = T.justifyLeft fieldLen '\NUL'

putPassword :: Putter Password
putPassword = put . TE.encodeUtf8 . padText 10

put on ByteStrings prepends an additional 8-byte chunk onto the front of what it is encoding making:

 runPut $ putPassword "Friend"

result in :

"\NUL\NUL\NUL\NUL\NUL\NUL\NUL\nFriend\NUL\NUL\NUL\NUL"

I don't want the extra chunk. Why is put behaving this way?

Does anyone know how to serialize only the 10 original bytes?

Answer 1

I assume by "the extra chunk" you mean the first bit of "\NUL\NUL\NUL\NUL\NUL\NUL\NUL\n. That is a 64 bit length field (notice it's value is 10) which is part of the Serialize definition for ByteString. Since you already have a bytestring after calling TE.encodeUtf8, I suggest you just use putByteString to avoid the length field (or putLazyByteString if you're importing a lazy text encoding module).

Answer 2

As Thomas stated put on ByteStrings prepends the encoded length. Shown below is the actual instance:

instance Serialize B.ByteString where
    put bs = do put (B.length bs :: Int)
                putByteString bs
   ...

putByteString alone does not. The solution is to use putByteString:

putPassword :: Putter Password
putPassword = putByteString . TE.encodeUtf8 . padText 10