Optimise any focus-independent layout by caching rectangles?

Cached.hs

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}

module Cached
  ( Cached
  , cached
  ) where

import XMonad
import qualified XMonad.StackSet as W
import Data.Maybe (fromMaybe)

data Cache a = Cache !(Maybe [(a, Rectangle)]) ![a]
  deriving (Show, Read)

data Cached l a = Cached !(Cache a) !(l a)
  deriving (Show, Read)

instance (Show a, Eq a, LayoutClass l a) => LayoutClass (Cached l) a where

  runLayout (W.Workspace i (Cached (Cache mwrs ois) cl) ms) sr = case mwrs of
    Just wrs | ois == nis -> return (wrs, Nothing)
    _                     -> do
      (nwrs, mncl) <- runLayout (W.Workspace i cl ms) sr
      return (nwrs, Just $ Cached (Cache (Just nwrs) nis) (fromMaybe cl mncl))
    where nis = W.integrate' ms

  handleMessage (Cached (Cache _ ois) cl) =
    (fmap . fmap) (Cached $ Cache Nothing ois) . handleMessage cl

cached :: l a -> Cached l a
cached = Cached (Cache Nothing [])