[Haskell-cafe] Bencoding in Haskell

[Sebastian Sylvan]

beDictionary is wrong, though. It will only find dictionaries with a
single entry.

This next parser should do the trick (again, untested!).
It basically reads a "d" and then a list of (key,value) pairs (which
is now a separate parser) and then an "e", and then it returns a "Map
String Bencode".

Should be something like this

-- parses an association list of the contents
-- of a dictionary
beDicContents :: Parser (String, Bencode)
beDicContents = 
 do (BEString key) <- beString
     val <- beParse
     return (String, Bencode)

beDictionary :: Parser Bencode
beDictionary =
   do char 'd'
        xs <- many beDicContents      
        char 'e'
        return (BEDictionary (Map.fromAscList xs))

On 4/20/05, Tommi Airikka <haskell-cafe at airikka.net> wrote:
> 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
> >
> 


-- 
Sebastian Sylvan
+46(0)736-818655
UIN: 44640862