Great, thanks again!
On Tuesday, September 15, 2015 at 4:10:54 PM UTC-4, Gabriel Gonzalez wrote:
>
> 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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