[xmonad] Haskell question: avoiding code duplication

[adam vogt]

On Sat, Feb 2, 2013 at 1:08 PM, Peter Jones <mlists at pmade.com> wrote:
> I've been working on a few customizations to xmonad but as I play with
> Haskell I keep seeing a pattern of duplication in my code that I don't
> know how to resolve.  Here's an example:
>
> -- | Enables 'focusFollowsMouse' for tiled windows only.  For this to
> -- work you need to turn off 'focusFollowsMouse' in your configuration
> -- and then add this function to your 'handleEventHook'.
> focusFollowsTiledOnly :: Event -> X All
> focusFollowsTiledOnly e@(CrossingEvent {ev_window = w, ev_event_type = t})
>   | isNormalEnter = whenX bothTiled (focus w) >> continueHooks
>   where isNormalEnter   = t == enterNotify && ev_mode e == notifyNormal
>         bothTiled       = (&&) <$> notFloating w <*> currentIsTiled
>         currentIsTiled  = currentWindow >>= maybe (return True) notFloating
>         currentWindow   = gets $ W.peek . windowset
>         notFloating w'  = gets $ not . M.member w' . W.floating . windowset
>         continueHooks   = return . mempty $ True
> focusFollowsTiledOnly _ = return . mempty $ True
>
>
> The last two lines demonstrate the pattern I've been seeing.  The only
> way I know how to remove this duplication is to move it out into a
> top-level function.  Is that correct?

Hi Peter,

Maybe you'll like this version below better. You can use mempty
instead of 'return . mempty $ True'. The latter is a bit confusing
since the True doesn't end up in value, since there's a "instance
Monoid b => Monoid (a -> b)" whose mempty ignores the argument `a'.

import qualified XMonad.StackSet as W
import XMonad
import Data.Monoid
import Data.Map as M
import Control.Applicative

focusFollowsTiledOnly :: Event -> X All
focusFollowsTiledOnly e
  | CrossingEvent {ev_window = w, ev_event_type = t} <- e,
   let  isNormalEnter   = t == enterNotify && ev_mode e == notifyNormal
        bothTiled       = notFloating w <&&> currentIsTiled
        currentIsTiled  = currentWindow >>= maybe (return True) notFloating
        currentWindow   = gets $ W.peek . windowset
        notFloating w'  = gets $ not . M.member w' . W.floating . windowset,
    isNormalEnter = whenX bothTiled (focus w) >> mempty
  | otherwise = mempty


--
Adam