There are two solutions to this.
The first solution is to enable the `{-# LANGUAGE RankNTypes #-}`
extension like the compiler suggests.
The reason why is that the `Parser` type synonym is:
type Parser a m r = forall x . StateT (Producer a m x) m r
... which means that this type:
Monad m => Int -> Parser ByteString m ()
... expands out to this type:
Monad m => Int -> (forall x . StateT (Producer a m x) m r)
... which is a higher-rank type (because of the `forall` after the
`(->)`). This is normal in the case of `pipes-parse` and is used to
simplify type signatures.
There's a second solution, which is to just use the type inferred by the
compiler, which will be a little bit more verbose:
Monad m => Int -> StateT (Producer a m x) m r
... which is almost the same as the previous type, except now that the
`forall` has been implicitly been pulled out of the type like this:
forall x . Monad m => Int -> StateT (Producer a m x) m r
This type doesn't require the `RankNTypes` language extension, but you
can no longer simplify the type using the `Parser` type synonym.
It's up to you which solution you prefer.
On 9/15/15 1:03 PM, Dylan Tisdall wrote:
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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