[Haskell-beginners] Parsec

[C Gosch]

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
<dav.vire+haskell at gmail.com<dav.vire%2Bhaskell at gmail.com>
>

> On Fri, Aug 6, 2010 at 11:03 AM, C Gosch <ch.gosch at googlemail.com> wrote:
>
> > 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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