parsing commandline arguments using parsec

[Marc Weber]

Hello.

I've rewritten the source position handing of parsec so that you can now
use different position markers such as commandline arguments.

You can find it with a demo application here: (Still very untested)
http://www.mawercer.de/marcweber/haskell/fparsec/

Do you think this would be useful merging back (after fixing some small bugs
like missing spaces in error message and adding some more documentation) ?

Example application:
----------------------------------------------------
module Main where
import Data.List
import Text.ParserCombinators.Parsec.Prim
import Text.ParserCombinators.Parsec.Combinator
import Text.ParserCombinators.Parsec.Token
import Text.ParserCombinators.Parsec.Char
import Text.ParserCombinators.Parsec.Argument

usage = unlines [ "cat <file> <file> (- means stdin/out)"
		, "tac <file> <file> (- means stdin/out)"
		, "calc 3 + 7 \\* 8 ... <- shell escape"
		]

cat :: ( String  ->  String ) -> String -> ArgParser () (IO ())
cat f s = do expectToken s
	     input <- inputFile
	     output <- outputFile
	     return $ input >>= output . f

calc :: ArgParser () (IO ())
calc = expectToken "calc" >> sum >>= return . print
  where sum = fmap (foldr1 (+)) (sepBy product (expectToken "+") )
	product =fmap (foldr1 (*)) (sepBy value (expectToken "*")) 
	value = intArg
	 


parser = choice  [ cat id "cat" -- cat
		 , cat (unlines . map reverse . lines) "tac" -- cat but reverse lines
		 , calc -- simple calculator, only knows how to do + and * operations
		 ]

main = do
  action <- handleArgs parser () usage
  action

----------------------------------------------------

Marc Weber