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
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