[Haskell-cafe] Re: doubts about runGetState in the binary package

[ChrisK]

Manlio Perillo wrote:
> Hi.
> 
> I have some doubts about the runGetState function in the binary package.
> The signature is:
> runGetState :: Get a -> LBS -> Int64 -> (a, LBS, Int64)
> 
> 
> however the Int64 "input parameter" is not documented.
> What value should I pass?
> How will be used?
> 
> 
> Thanks  Manlio Perillo

hackage has the code at
http://hackage.haskell.org/packages/archive/binary/0.5.0.1/doc/html/src/Data-Binary-Get.html#runGetState

And I have pieced together an answer at the bottom...

> 
> -- | The parse state
> data S = S {-# UNPACK #-} !B.ByteString  -- current chunk
>            L.ByteString                  -- the rest of the input
>            {-# UNPACK #-} !Int64         -- bytes read
> 
> -- | The Get monad is just a State monad carrying around the input ByteString
> -- We treat it as a strict state monad. 
> newtype Get a = Get { unGet :: S -> (a, S) }

> mkState :: L.ByteString -> Int64 -> S
> mkState l = case l of
>     L.Empty      -> S B.empty L.empty
>     L.Chunk x xs -> S x xs

> -- | Run the Get monad applies a 'get'-based parser on the input
> -- ByteString. Additional to the result of get it returns the number of
> -- consumed bytes and the rest of the input.
> runGetState :: Get a -> L.ByteString -> Int64 -> (a, L.ByteString, Int64)
> runGetState m str off =
>     case unGet m (mkState str off) of
>       (a, ~(S s ss newOff)) -> (a, s `join` ss, newOff)

> getBytes :: Int -> Get B.ByteString
> getBytes n = do
>     S s ss bytes <- get
>     if n <= B.length s
>         then do let (consume,rest) = B.splitAt n s
>                 put $! S rest ss (bytes + fromIntegral n)
>                 return $! consume
>         else
 > ...

The Int64 passed to runGetState just initializes the running total of consumed 
bytes.  The updated total is returned by runGetState.  The absolute value of the 
Int64 is never used; it is only increased by "getBytes".

Cheers,
   Chris