Modify screen brightness from Haskell (for use from XMonad)

Brightness.hs

module Brightness (inc, dec) where

import System.IO
import Control.Monad.IO.Class (MonadIO, liftIO)

modifyFile :: (String -> String) -> Handle -> IO ()
modifyFile f h = f <$> hGetLine h >>= hPutStr h

modifyBrightness :: MonadIO m => (Int -> Int) -> m ()
modifyBrightness f = liftIO $ withFile "/sys/class/backlight/intel_backlight/brightness" ReadWriteMode $ modifyFile (show . f . read)

inc, dec :: MonadIO m => Int -> m ()
inc i = modifyBrightness (+ i)
dec i = modifyBrightness (subtract i)