Fix workspaceHistoryHook memory leak

The XS.modify was leaving thunk on the history that the demand analyser
could not prove to be neccesary as they depended on the future user
interaction. This was bad as the time advance there was less and less
neccesity to force such value, so the thunk would be increasing. Since the
datatypes that the `WorkspaceHistory` are really simple, we can just
evaluate and save a good chunk of memory.

XMonad/Hooks/WorkspaceHistory.hs

@@ -28,9 +28,9 @@ module XMonad.Hooks.WorkspaceHistory (
   , workspaceHistoryModify
   ) where
 
+import Control.Seq (using, seqFoldable, seqTuple2, rseq)
 import           Control.Applicative
 import           Prelude
-
 import XMonad
 import XMonad.StackSet hiding (delete, filter, new)
 import XMonad.Prelude (delete, find, foldl', groupBy, nub, sortBy)
@@ -72,16 +72,21 @@ instance ExtensionClass WorkspaceHistory where
 -- | A 'logHook' that keeps track of the order in which workspaces have
 -- been viewed.
 workspaceHistoryHook :: X ()
-workspaceHistoryHook = gets windowset >>= (XS.modify . updateLastActiveOnEachScreen)
+workspaceHistoryHook = workspaceHistoryHookExclude []
 
 -- | Like 'workspaceHistoryHook', but with the ability to exclude
 -- certain workspaces.
 workspaceHistoryHookExclude :: [WorkspaceId] -> X ()
 workspaceHistoryHookExclude ws = do
   s <- gets windowset
-  let update' a = force (updateLastActiveOnEachScreenExclude ws s a)
+  let update' = forceHistory . updateLastActiveOnEachScreenExclude ws s
   XS.modify' update'
 
+forceHistory :: WorkspaceHistory -> WorkspaceHistory
+forceHistory (WorkspaceHistory l) =
+  let l' = l `using` seqFoldable (seqTuple2 rseq rseq)
+  in l' `seq` WorkspaceHistory l'
+
 workspaceHistoryWithScreen :: X [(ScreenId, WorkspaceId)]
 workspaceHistoryWithScreen = XS.gets history