I think getRemainingLazyByteString expects at least one byte No, it works with an empty bytestring. Or, my tests do with binary 0.5.0.1.
The specific error means you are requiring more data than providing. First check the length of the bytestring you pass in to the to level decode (or 'get') routine and walk though that to figure out how much it should be consuming. I notice you have a guard on the 'getSpecific' function, hopefully you're sure the case you gave us is the branch being taken. I think the issue isn't with the code provided. I cleaned up the code (which did change behavior due to the guard and data declarations that weren't in the mailling) and it works fine all the way down to the expected minimum of 13 bytes.
import Data.ByteString.Lazy import Data.Binary import Data.Binary.Get
data RV = Rversion { size :: Word32, mtype :: Word8, tag :: Word16, msize :: Word32, ssize :: Word16, version :: ByteString} deriving (Eq, Ord, Show)
instance Binary RV where get = do s <- getWord32le mtype <- getWord8 getSpecific s mtype where getSpecific s mt = do t <- getWord16le ms <- getWord32le ss <- getWord16le v <- getRemainingLazyByteString return $ Rversion {size=s, mtype=mt, tag=t, msize=ms, ssize=ss, version=v } put _ = undefined