xmonad: switch to the previous layout (experimental)

PrevLayout.hs

module PrevLayout (prevLayout) where

import XMonad
import qualified XMonad.StackSet as W

prevLayout :: X ()
prevLayout = do
  ss@W.StackSet{ W.current = c@W.Screen{ W.workspace = ws }} <- gets windowset
  mp <- findPrev (description (W.layout ws)) (W.layout ws)
  case mp of
    Nothing -> pure ()
    Just p  -> windows
             $ const ss{ W.current = c{ W.workspace = ws{ W.layout = p } } }
 where
  findPrev cur l = do
    ml' <- handleMessage l (SomeMessage NextLayout)
    case ml' of
      Nothing -> pure Nothing
      Just l' -> if description l' == cur
        then pure (Just l)
        else findPrev cur l'

BTW, findPrev (description (W.layout ws)) (W.layout ws) is the same as description >>= findPrev $ W.layout, in the name of the function monad 🤣