Re: [Haskell-cafe] Bencoding in Haskell

20 Apr 2005


      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  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  wrote:
...
On 4/20/05, Tommi Airikka  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