add getMany

[imalsogreg]

Hi Renzo,
Gabriel gave me this function yesterday for recreating the old behaviour of decodeGetMany (from before the days of pipes-parse). He said it might be useful to have it in the library. What do you think?

[kvanbere]

I've actually put together something like this for personal usage, because I prefer exceptions to Either (from decoded) -- it's easier for me to compose the smaller types.

data PacketParseException = PacketParseException DecodingError
    deriving (Typeable)

instance Show PacketParseException where
    show _ = "PacketParseException"

instance Exception PacketParseException


decoder :: Monad m
    => Get a
    -> Producer ByteString m r
    -> Producer a m r
decoder getter = decoder_
  where
    decoder_ p0 = do
        x <- lift (next p0)
        case x of
            Left r  -> return r
            Right (bs, p1) -> do
                (ea, p2) <- lift $ S.runStateT (decodeGet getter) (yield bs >> p1)
                case ea of
                    Left e  -> throw (PacketParseException e)
                    Right a -> yield a >> decoder_ p2