Simple parser in haskell

Question

I try to parse benncoded torrent file (part of my toy project to learn Haskell) into a dedicated structure in Haskell:

import qualified Data.ByteString.Char8 as BC

data BEncode =  BString BC.ByteString
         | BInt Integer
         | BList [BEncode]
         | BDic [(BEncode, BEncode)]  <-- here is the problem I want to make it BDic [(BString, BEncode)]
         deriving (Eq,Show)

Everything works fine but I would like to make small improvement.
BDic data constructor takes (BEncode <- this is key, BEncode ,<- this is value) list. It's too general I would like to limit keys to be only BStrings, is there a way to do this?

Here is rest of the parser:

num::P.Parser String             
num = many1 digit

bInt::P.Parser BEncode
bInt = (BInt . read) <$> (char 'i' *> num <* char 'e') 

bString :: P.Parser BEncode
bString = do n <- num
         _ <- char ':'
         BString <$> (P.take (read n))

bList :: P.Parser BEncode
bList = (BList) <$> (char 'l' *> (many1 (bInt <|> bString <|> bList)) <* char 'e')                  


dicEntry :: P.Parser (BEncode, BEncode)
dicEntry = ((,)<$>bString <*> bencodeParser)

bDic :: P.Parser BEncode
bDic = BDic<$>((char 'd' *> many1 dicEntry <* char 'e'))               

bencodeParser :: P.Parser BEncode
bencodeParser = bInt <|> bString <|> bList <|> bDic

Sibi · Accepted Answer · 2015-02-07 19:34:39Z

2

BDic data constructor takes (BEncode <- this is key, BEncode ,<- this is value) list. It's too general I would like to limit keys to be only BStrings, is there a way to do this?

One way to achieve this is by changing the structure:

data BStringT = BString BC.ByteString deriving (Eq, Show)

data BEncode =  BStringT
             | BInt Integer
             | BList [BEncode]
             | BDic [(BStringT, BEncode)]  
               deriving (Eq,Show)

This way you can enforce that it will be BStringT in it's first parameter.

answered Feb 7, 2015 at 19:34

Sibi

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3 Comments

Blezz Over a year ago

Thx! I was looking for something like that, but I got another problem: bList :: P.Parser BEncode function doesn't compile: Couldn't match expected type BEncode' with actual type BStringT' Expected type: attoparsec-0.11.1.0:Data.Attoparsec.Internal.Types.Parser BC.ByteString BEncode Actual type: Parser BStringT In the second argument of (<|>)', namely bString' In the first argument of (<|>)', namely bInt <|> bString', I don't understand why...

Blezz Over a year ago

Its the <|> function, it seems bInt<|>bString doesn't work anymore (inside bList, the same for bencodeParser function)

chi Over a year ago

The first case of BEncode lacks a constructor. It should be e.g. data BEncode = BString BStringT | ...

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