[Haskell-cafe] Success and one last issue with Data.Binary

[John Van Enk]

Perhaps there's some place in your code that's forcing the lazy read
to consume more. Perhaps you could replace it with an explict (and
strict) getBytes[1] in combination with remaining[2]?

Is there a reason you want to use lazy byte strings rather than
forcing full consumption? Do the 9P packets generally have a lot of
trailing useless data?

1. http://hackage.haskell.org/packages/archive/binary/0.5.0.1/doc/html/Data-Binary-Get.html#v%3AgetBytes
2. http://hackage.haskell.org/packages/archive/binary/0.5.0.1/doc/html/Data-Binary-Get.html#v%3Aremaining

On Tue, Jun 2, 2009 at 4:28 PM, David Leimbach <leimy2k at gmail.com> wrote:
>
>
> On Tue, Jun 2, 2009 at 10:24 AM, Thomas DuBuisson
> <thomas.dubuisson at gmail.com> wrote:
>>
>> > 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.
>
> I've shown that I am not trying to decode more than I'm providing.  I've
> asked, expliciitly, for 13 bytes, and then "remaining", and the library is
> complaining about the 20th byte.
>
>>
>> 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.
>
> The other branch is Rerror, which is a shorter message decode stream.
>  Unfortunately, I can't get Debug.Trace to show anything to prove it's
> taking this fork of the code.  I suppose I could unsafePerformIO :-)
> Perhaps I just need a new version of "binary"??  I'll give it a go and try
> your version.  But I need to decode over a dozen message types, so I will
> need a case or guard or something.
> Dave
>>
>>
>> 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
>
>



-- 
/jve