[gergoerdi]

As always, prefer prebuilt recursion scheme combinators instead of making your own. The following, using `forever`, is IMO much more readable because you don't have to mentally resolve the recursion:

  main :: IO ()
  main = withFile ledFile AppendMode $ \h -> do
      let put s = hPutStrLn h s >> threadDelay 2000000                          
      forever $ do
         put "0" 
         put "1"
    where
      ledFile = "/sys/class/leds/beaglebone:green:usr0/brightness"