[Haskell-cafe] Bencoding in Haskell

[Sebastian Sylvan]

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