Hi,
I'm new to pipes (and pretty new to Haskell), and so have a question that's
probably quite simple, but has managed to stump me for the last day.
Basically, I'm trying to parse a file that consists of the two "sub-files"
(call them first and second), with the length of first prefixed. My goal is
to write a parser that reads just the length of first, then skips over
first without read it all into memory, and then reads in and prints out the
contents of the second file. So far I can read the length, but I'm stumped
at how to do the skip, and handle the error case where even reading the
length failed. Basically, I'm looking for what should go into "parseRest"
in the following code, or any suggestions for how to refactor this to make
it more consistent with the design of pipes.
import Prelude hiding (length, concat, splitAt)
import Data.ByteString.Lazy.Char8 (pack)
import Data.ByteString.Lazy (ByteString, length, copy, concat)
import Data.Binary (Word32)
import Data.Int (Int32)
import Data.Binary.Put (runPut, putWord32le)
import Data.Binary.Get (getWord32le)
import Pipes
import Pipes.ByteString (fromLazy, stdout, splitAt)
import qualified Pipes.ByteString as P (ByteString)
import Pipes.Parse (runStateT, evalStateT, drawAll, Parser)
import Pipes.Binary (decodeGet, DecodingError, decoded, decode)
import Control.Lens (view, zoom)
first :: ByteString
first = pack "foo"
second :: ByteString
second = pack "bar"
merge :: ByteString -> ByteString -> ByteString
merge a b = concat [lengthBS a, a, b]
where
lengthBS = runPut . putWord32le . fromIntegral . length
split :: Monad m => Producer P.ByteString m r ->
m (Either DecodingError
(Producer P.ByteString m (Producer P.ByteString m r)))
split p = do
(headerLen, p') <- runStateT (decodeGet getWord32le) p
return $ case headerLen of
Left err -> Left err
Right headerLen' -> Right $
view (splitAt $ fromIntegral $ (headerLen' :: Word32)) p'
decoder :: Parser P.ByteString IO ()
decoder = do
headerLen <- decode
lift $ print (headerLen :: Either DecodingError Word32)
case headerLen of
Left err -> lift $ print err
Right hLen -> parseRest
main :: IO ()
main = do
evalStateT decoder $ fromLazy (merge first second)
Thanks,
Dylan
--
You received this message because you are subscribed to the Google Groups
"Haskell Pipes" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to haskell-pipes+unsubscr...@googlegroups.com.
To post to this group, send email to haskell-pipes@googlegroups.com.
