[Haskell-cafe] Bencoding in Haskell

[Tommi Airikka]

Thank you very much! I really appreciate your help!
I have to read a little bit more about Parsec to fully understand what
your code does, but it seems to be what I was looking for.

Regards,
Tommi

On Wed, Apr 20, 2005 at 08:58:41PM +0200, Sebastian Sylvan wrote:
> I was bored so I ran it through ghci and fixed the small errors I
> found, here's the "working" version, I don't really have much of test
> data to play with, but it seems to be working with the small examples
> I copy-n-pasted from the wiki and the bittorrent website:
> 
> import qualified Data.Map as Map
> import Data.Map(Map)
> import Text.ParserCombinators.Parsec
> 
> data Bencode = BEInteger Integer 
>              | BEString String 
>              | BEList [Bencode] 
>              | BEDictionary (Map String Bencode)
>              deriving (Show, Eq)
> 
> number :: Parser Integer
> number = 
>     do n_str <- many1 digit 
>        let n = read n_str
>        return n
> 
> beString :: Parser Bencode
> beString =
>     do n <- number 
>        char ':'
>        str <- count (fromInteger n) anyChar
>        return (BEString str) 
> 
> 
> beInt :: Parser Bencode
> beInt =
>     do char 'i'
>        n <- number
>        char 'e'
>        return (BEInteger n)
> 
> -- parse any Bencoded value
> beParse :: Parser Bencode
> beParse = beInt <|> beString <|> beDictionary <|> beList
> 
> beList :: Parser Bencode
> beList =
>     do char 'l'
>        xs <- many beParse -- parse many bencoded values
>        char 'e'
>        return (BEList xs)
> 
> beDictionary :: Parser Bencode
> beDictionary =
>     do char 'd'
>        (BEString key) <- beString
>        val <- beParse
>        (BEDictionary m) <- beDictionary 
>                            <|> do char 'e'
>                                   return (BEDictionary Map.empty)
> 
>        return (BEDictionary (Map.insert key val m))
> 
> -- main parser function
> parseBencoded :: String -> Maybe [Bencode]
> parseBencoded str = case parse (many beParse) "" str of
>                                 Left err -> Nothing
>                                 Right val -> Just val
> 
> 
> 
> /S
> 
> On 4/20/05, Sebastian Sylvan <sebastian.sylvan at gmail.com> wrote:
> > Yeah, you probably want the main parser to be "many beParser" and not
> > just beParser:
> > 
> > -- main parser function
> > parseBencoded :: String -> Maybe [Bencode]
> > parseBencode str = case parse (many beParse) "" str of
> >                                 Left err -> Nothing
> >                                 Right val -> Just val
> > 
> > On 4/20/05, Sebastian Sylvan <sebastian.sylvan at gmail.com> wrote:
> > > On 4/20/05, Tommi Airikka <haskell-cafe at airikka.net> wrote:
> > > > Hi!
> > > >
> > > > I was just wondering if there are any good ways to represent a bencoded
> > > > (http://en.wikipedia.org/wiki/Bencoding) message in Haskell? Any
> > > > suggestions?
> > > >
> > >
> > > Not that I know of, but it should be very easy to write a parser using
> > > the parser library Parsec.
> > >
> > > You'll need a datatype, something like this:
> > >
> > > data Bencode = BEInteger Integer |
> > >                         BEString String |
> > >                         BEList [Bencode] |
> > >                         BEDictionary (Data.Map String Bencode)
> > >                         deriving (Show, Eq)
> > >
> > > Which should be sufficient to represent any Bencoded message (if I
> > > didn't make a misstake).
> > > Then you could probably use the standard char-parser in parsec to
> > > parse it quite easily. Read the docs, they're quite straightforward.
> > >
> > > I'm a bit rusty but something like this:
> > >
> > > -- just parse an integer, parsec might have one of these already
> > > number :: Parser Integer
> > > number =
> > >   do n_str <- many1 digit -- parse a number
> > >        let n = read n_str      -- convert to an Int
> > >        return n                   -- return the number
> > >
> > > beString :: Parser Bencode
> > > beString =
> > >   do n <- number                 -- the length prefix
> > >       char ':'                          -- now a ':'
> > >       str <- count n anyChar   -- and now n number of letters
> > >       return (BEString str)       -- return the string wrapped up as a
> > > BEString
> > >
> > > beInt :: Parser Bencode
> > > beInt =
> > >   do char 'i'
> > >        n <- number
> > >        char 'e'
> > >        return n
> > >
> > > -- parse any Bencoded value
> > > beParse :: Parser Bencode
> > > beParse =
> > >   do beInt <|> beString <|> beDictionary <|> beList
> > >
> > > beList :: Parser Bencode
> > > beList =
> > >   do char 'l'
> > >        xs <- many beParse -- parse many bencoded values
> > >        char 'e'
> > >        return (BEList xs)
> > >
> > > beDictionary :: Parser Bencode
> > > beDictionary =
> > >   do char 'd'
> > >        key <- beString
> > >        val <- beParse
> > >        m <- beDictionary <|> char 'e' >> return Data.Map.empty
> > >        return (Data.Map.insert key val m)
> > >
> > > -- main parser function
> > > parseBencoded :: String -> Maybe Bencode
> > > parseBencode str = case parse beParse "" str of
> > >                                  Left err -> Nothing
> > >                                  Right val -> Just val
> > >
> > > Note: This is all untested code that I just scribbled down real quick.
> > > There's probably tons of misstakes, but you should get the picture.
> > > Read the Parsec docs and then write your own.
> > >
> > > /S
> > > --
> > > Sebastian Sylvan
> > > +46(0)736-818655
> > > UIN: 44640862
> > >
> > 
> > --
> > Sebastian Sylvan
> > +46(0)736-818655
> > UIN: 44640862
> > 
> 
> 
> -- 
> Sebastian Sylvan
> +46(0)736-818655
> UIN: 44640862
>