Copy of the function mountedDevices from xmobar's Disk.hs to reproduce problem

Disk.sh

import qualified Data.ByteString.Lazy.Char8 as B
import Data.Maybe (catMaybes)
import Data.List (isPrefixOf)
import System.Directory (doesFileExist, canonicalizePath)


main :: IO ()
main = mountedDevices ["mapper/home"] >>= print

mountedDevices :: [String] -> IO [(String, String)]
mountedDevices req = do
  s <- B.readFile "/etc/mtab"
  parse `fmap` mapM mbcanon (devs s)
 where
   mbcanon (d, p) = doesFileExist d >>= \e ->
                      if e
                        then Just `fmap` canon (d,p)
                        else return Nothing
   canon (d, p) = do {d' <- canonicalizePath d; return (d', p)}
   devs = filter isDev . map (firstTwo . B.words) . B.lines
   parse = map undev . filter isReq . catMaybes
   firstTwo (a:b:_) = (B.unpack a, B.unpack b)
   firstTwo _ = ("", "")
   isDev (d, _) = "/dev/" `isPrefixOf` d
   isReq (d, p) = p `elem` req || drop 5 d `elem` req
   undev (d, f) = (drop 5 d, f)