{-# LANGUAGE FlexibleContexts #-}

import Control.Applicative (Applicative((<*>), (*>)), (<$>))
import Text.Parsec

expandRange :: String -> Maybe [Int]
expandRange = either (const Nothing) Just . parse rangeParser ""

rangeParser
  :: (Enum a, Read a, Stream s m Char)
  => ParsecT s u m [a]
rangeParser = concat <$> (item `sepBy` char ',')
  where
    item = do
      n1 <- num
      n2 <- option n1 (char '-' *> num)
      return [n1 .. n2]
    num = read `dot` (++) <$> option "" (string "-") <*> many1 digit
    dot = (.) . (.)

main :: IO ()
main = print $ expandRange "-6,-3--1,3-5,7-11,14,15,17-20"