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 > -- > 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-pipe...@googlegroups.com <javascript:>. > To post to this group, send email to haskel...@googlegroups.com > <javascript:>. > > > -- 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.
