David, thank you for your helpful explanations. So I need to define a new datatype encapsulating the two I want to parse and then create a new Stream, is that correct? Reading your comments and more of the haddock documentation of Parsec, this came to my mind: say I want to skip over some binary parts of the input (which is a PDF, containing ascii and deflated parts as well as images). I found the functions setPosition/getPosition, which seem to build on the notion of line and column number. Is there a way to say that I want to skip N bytes along the stream? I didn't find an obvious one myself when browsing through the docs. Thanks again, Christian 2010/8/6 David Virebayre

On Fri, Aug 6, 2010 at 11:03 AM, C Gosch wrote:

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You're right, I probably used the wrong words .. I meant that apparently the tokens Parsec uses are of type Char, and I would actually at some point like to continue parsing, but using different tokens. Sorry if I still got it wrong, I'm new :) I can post some code later, as I don't have it here right now.

Parsec (at least version 3) uses any type of token you want.

Quick example off the top of my head, I didn't check if it compiles:

-- you have to make lists of your token type an instance of Stream :

instance Stream [ MyTokenType ] Identity MyTokenType where uncons [] = return Nothing uncons (x:xs) = return $ Just (x,xs)

-- your parser type is going to look like this :

type MyParser a = ParsecT [MyTokenType] () Identity a

-- assuming your toke type looks like this

Data MyTokenType = A Char | B Word8 deriving (Show)

-- you need a basic parser from which you can make more complicated ones

satisChar :: ( Char -> Bool ) -> MyParser Char satisChar f = tokenPrim prt pos match where prt = show pos p _l _cs = incSourceLine p 1 match (A c) = if f c then Just c else Nothing match _ = Nothing

satisBin :: ( Word8 -> Bool ) -> MyParser Word8 satisBin f = tokenPrim prt pos match where prt = show pos p _l _cs = incSourceLine p 1 match (B w) = if f w then Just w else Nothing match _ = Nothing

-- You can define basic parsers like this

-- parse any letter letter = satisChar (const True)

-- parse a specific char, it will return char c = satisChar (==c)

-- parse any binary word binary = satisBin (const True)

-- parse a specific binary word w = satisBin (==w)

-- now you can combine this to make more complicated parsers.

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Ah wait ... I saw that I can get the input stream with getParserState, as it is part of the state. If that is the currently remaining stream, I could use any functions from ByteString to skip over some parts or do whatever with them, and update the state to the last position after the part I want to skip. Am I correct there, or am I then getting something in Parsec out of sync? Guess I'll just try :) Thanks! Christian 2010/8/6 Patrick LeBoutillier

Hi,

I started using Data.Binary yesterday for a project and it's nice. I'm not sure it's possible, but perhaps you could switch tools along the way:

- Parse the ASCII bit with Parsec ; - Give the remaining ByteString to Data.Binary for the binary part ; - Switch back and forth as required.

Patrick

On Fri, Aug 6, 2010 at 5:03 AM, C Gosch wrote:

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2010/8/6 Dean Herington

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At 11:42 PM +0200 8/5/10, C Gosch wrote:

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Hi, does anyone here know their way around in Parsec? I'm trying to parse a file which contains some binary parts too. I have been using Parsec 3.0.1 to parse the first ASCII part, but all

...

...

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parsers are returning Char type tokens, so it would not work with the binary parts .. is there any way to do this? I am using Text.Parsec.ByteString.Lazy.

Note that I'm new to Haskell and Parsec, and doing this within a small project that I basically do to get a grip on Haskell (and Parsec, because it appears to be really nifty and I want to learn more about it).

Thanks for any hints, Christian

Parsing binary parts of a file is not inherently a problem. A parser can return any type; different parsers in your application will likely return different types.

If you include some code we'll be able to help you more specifically.

Dean

You're right, I probably used the wrong words .. I meant that apparently

the the

...

tokens Parsec uses are of type Char, and I would actually at some point like to continue parsing, but using different tokens. Sorry if I still got it wrong, I'm new :) I can post some code later, as I don't have it here right now.

Cheers, Christian

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