    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE CPP#-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  XMonad.Util.Brightness
    -- License     :  MIT
    --
    -- Stability   :  unstable
    -- Portability :  unportable
    --
    -- Module to control the brightness of the screen in linux environments
    --
    -- [@Requirements@]
    --     This module assumes that the following files exists:
    --
    --     * __\/sys\/class\/backlight\/intel_backlight\/max_brightness__
    --
    --     * __\/sys\/class\/backlight\/intel_backlight\/brightness__
    --
    --     Also, brightness should be updatable by changing the content of
    --     __\/sys\/class\/backlight\/intel_backlight\/brightness__.
    --
    -- [@Permissions@]
    --     To use this module, the owner of the __xmonad__ process will need to
    --     have permission to write to __\/sys\/class\/backlight\/intel_backlight\/brightness__.
    --     To achieve this, you can:
    --
    --     * Create a group with your user and root and give permissions to this
    --     group to write to the file. I usually follow these steps:
    --
    --         * Create a group named xmonad
    --
    --         > $ sudo groupadd xmonad
    --
    --         * Add user root and your user name to the group xmonad.
    --
    --         > $ sudo usermod -a -G xmonad root
    --         > $ sudo usermod -a -G xmonad sibi
    --
    --         * The files under __\/sys__ are virtual. It's a RAM based filesystem through which you can access kernel data structures. The permission you give there won't persist after reboot. One of the way for persisting is creating a <https://unix.stackexchange.com/a/409780/29539 systemd script>:
    --
    --         > $ cat /etc/systemd/system/brightness.service
    --         > [Unit]
    --         > Description=Set brightness writable to everybody
    --         > Before=nodered.service
    --         >
    --         > [Service]
    --         > Type=oneshot
    --         > User=root
    --         > ExecStart=/bin/bash -c "chgrp -R -H xmonad /sys/class/backlight/intel_backlight && chmod g+w /sys/class/backlight/intel_backlight/brightness"
    --         >
    --         > [Install]
    --         > WantedBy=multi-user.target
    --         >
    --         > $ sudo systemctl enable brightness.service
    --         > $ sudo systemctl start brightness.service
    --         > $ sudo systemctl status brightness.service
    --
    --
    --     * Allow anyone to write the file through 646 permissions: __-rw-r--rw-__;
    --
    -----------------------------------------------------------------------------
    module XMonad.Util.Brightness
        ( increase
        , decrease
        , change
        , setBrightness
        ) where

    import XMonad
    #if (MIN_VERSION_base(4,10,0))
    import Data.Traversable (traverse)
    #endif
    import Prelude
    import System.IO (hPutStrLn, stderr)
    import Control.Monad (join)
    import Data.Bifunctor (first)
    import Control.Exception (try)
    import Control.Applicative (liftA2)
    import Data.ByteString.Char8 (unpack)
    import qualified Data.ByteString as BS

    maxfile :: FilePath
    maxfile = "/sys/class/backlight/intel_backlight/max_brightness"

    currentfile :: FilePath
    currentfile = "/sys/class/backlight/intel_backlight/brightness"

    -- | Update brightness by +100
    increase :: X ()
    increase = liftIO $ change (+100) *> (pure ())

    -- | Update brightness by -100
    decrease :: X ()
    decrease = liftIO $ change (+ (-100)) *> (pure ())

    -- | Change brightness to a particular level
    --
    -- @since 0.13.4
    setBrightness :: Int -> X ()
    setBrightness level = liftIO $ change (\_ -> level) *> pure ()

    -- | Perform all needed IO to update screen brightness
    change :: (Int -> Int) -> IO (Either () ())
    change f = do
      maxBright <- getFromFile maxfile readInt
      current <- getFromFile currentfile readInt
      printError =<< apply (writeToFile currentfile) (liftA2 (guard f) maxBright current)

    apply :: (Int -> IO (Either String ())) -> Either String Int -> IO (Either String ())
    apply f = fmap join . traverse f

    guard :: (Int -> Int) -> Int -> Int -> Int
    guard f limit current
      | value > limit = limit
      | value < 0   = 0
      | otherwise = value
      where value = f current

    readInt :: BS.ByteString -> Either String Int
    readInt str = case (reads (unpack str)) of
                    [(n, "\n")] -> Right n
                    [(n, "")]   -> Right n
                    _           -> Left "Could not parse string to int"

    printError :: Either String e -> IO (Either () e)
    printError es = either (\str -> hPutStrLn stderr str *> (return . Left $ ())) (\_ -> return . Left $ ()) es

    getFromFile :: FilePath -> (BS.ByteString -> Either String a) -> IO (Either String a)
    getFromFile filename fcast = fmap (fcast =<<) (try' $ BS.readFile filename)

    writeToFile :: FilePath -> Int -> IO (Either String ())
    writeToFile filename value = try' $ writeFile filename (show value)

    try' :: forall a . IO a -> IO (Either String a)
    try' x = fmap (first show) (try x :: IO (Either IOError a))