1 files changed, 20 insertions, 1 deletions
diff --git a/src/Rahm/Desktop/Common.hs b/src/Rahm/Desktop/Common.hs
index 6ca3513..f436239 100644
--- a/src/Rahm/Desktop/Common.hs
+++ b/src/Rahm/Desktop/Common.hs
@@ -29,6 +29,7 @@ where
 import Control.Applicative ((<*))
 import Control.Exception (SomeException (SomeException), catch)
 import Control.Monad (forM_, guard, void, when)
+import Control.Monad.Except (MonadError (catchError, throwError))
 import Control.Monad.Trans.Class
 import Control.Monad.Trans.Except (ExceptT (..), catchE, runExceptT, throwE)
 import Control.Monad.Trans.Identity (IdentityT (..))
@@ -253,8 +254,26 @@ floatAll ws = do
 
   windows endo
 
-class (Monad m) => Xish m where
+class Xish m where
   liftFromX :: X a -> m a
+  finallyX :: m a -> m () -> m a
 
 instance Xish X where
   liftFromX = id
+  finallyX mt err = do
+    maybe (fail "original action failed.") (\a -> err >> return a)
+      =<< catchX (Just <$> mt) (return Nothing)
+
+instance Xish (MaybeT X) where
+  liftFromX = lift
+  finallyX mt err = MaybeT $ do
+    maybeR <- runMaybeT mt
+    case maybeR of
+      Nothing -> runMaybeT err >> return Nothing
+      Just r -> fmap (r <$) (runMaybeT err)
+
+instance Xish (ExceptT e X) where
+  liftFromX = lift
+  finallyX mt err = do
+    either (\e -> err >> throwError e) (\a -> err >> return a)
+      =<< catchError (Right <$> mt) (return . Left)