Hi Gabriel,
Thanks for the quick reply. I had to modify the snippet a little
bit so ghci find definitions for all the symbols (just changed
the import lines a bit), but now I'm getting an error about
illegal types. Below is the code, and the output from ghci. Any
suggestions what I've done wrong?
--- code ---
import Prelude hiding (splitAt)
import Control.Lens (zoom)
import Pipes -- from the `pipes` package
import Pipes.ByteString (ByteString, splitAt) -- from the
`pipes-bytestring` package
import Pipes.Parse (Parser, skipAll) -- from the `pipes-parse`
package
skipNBytes :: Monad m => Int -> Parser ByteString m ()
skipNBytes n = zoom (splitAt n) skipAll
--- ghci 7.10.2 output --
Illegal polymorphic or qualified type: Parser ByteString m ()
Perhaps you intended to use RankNTypes or Rank2Types
In the type signature for ‘skipNBytes’:
skipNBytes :: Monad m => Int -> Parser ByteString m ()
Thanks,
Dylan
On Tuesday, September 15, 2015 at 3:24:31 PM UTC-4, Gabriel
Gonzalez wrote:
Here's how you can skip N bytes in the context of a
`Parser`. I can add this to `Pipes.ByteString` in some form
as a useful utility in the "Parsers" section if it solves
your issue:
import Lens.Micro (zoom)-- from the `microlens` package
import Pipes-- from the `pipes` package
import Pipes.ByteString (splitAt)-- from the
`pipes-bytestring` package
import Pipes.Parse (Parser, skipAll) -- from the
`pipes-parse` package
skipNBytes :: Monad m => Int -> Parser ByteString m ()
skipNBytes n = zoom (splitAt n) skipAll
On 9/15/15 12:17 PM, Dylan Tisdall wrote:
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
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