get screen resolution in X (shells out to xrandr(1))

PrintXResolution.hs

import Data.List
import Data.Maybe
import System.Process
import Text.Regex.Posix

main :: IO ()
main = do
  (w, h) <- getScreenResolution
  putStrLn $ show w ++ "×" ++ show h

getScreenResolution :: IO (Int, Int)
getScreenResolution = do
  rawOutput <- readProcess "xrandr" [] []
  let currentMode = fromJust . find (elem '*') $ lines rawOutput
      [[_, w, h]] = currentMode =~ "([0-9]+)x([0-9]+)"
  return (read w, read h)