On 4/20/05, Tommi Airikka
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