X.Prelude: Add safeGetWindowAttributes

Move the function from X.U.DebugWindow, where it was defined already.
This is a safe version of getWindowAttributes, returning a Maybe instead
of throwing an exception, in case the window attributes could not be
retrieved.

XMonad/Prelude.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE LambdaCase   #-}
 --------------------------------------------------------------------
 -- |
 -- Module      :  XMonad.Prelude
@@ -20,8 +21,12 @@ module XMonad.Prelude (
     (!?),
     NonEmpty((:|)),
     notEmpty,
+    safeGetWindowAttributes,
 ) where
 
+import Foreign (alloca, peek)
+import XMonad
+
 import Control.Applicative as Exports
 import Control.Monad       as Exports
 import Data.Bool           as Exports
@@ -68,3 +73,10 @@ chunksOf i xs = chunk : chunksOf i rest
 notEmpty :: HasCallStack => [a] -> NonEmpty a
 notEmpty [] = error "unexpected empty list"
 notEmpty (x:xs) = x :| xs
+
+-- | A safe version of 'Graphics.X11.Extras.getWindowAttributes'.
+safeGetWindowAttributes :: Window -> X (Maybe WindowAttributes)
+safeGetWindowAttributes w = withDisplay $ \dpy -> io . alloca $ \p ->
+  xGetWindowAttributes dpy w p >>= \case
+    0 -> pure Nothing
+    _ -> Just <$> peek p

XMonad/Util/DebugWindow.hs

@@ -153,14 +153,6 @@ wrap s =  ' ' : '"' : wrap' s ++ "\""
                   | otherwise  =        s' : wrap' ss
     wrap' ""                   =             ""
 
--- Graphics.X11.Extras.getWindowAttributes is bugggggggy
-safeGetWindowAttributes     :: Display -> Window -> IO (Maybe WindowAttributes)
-safeGetWindowAttributes d w =  alloca $ \p -> do
-  s <- xGetWindowAttributes d w p
-  case s of
-    0 -> return Nothing
-    _ -> Just <$> peek p
-
 -- and so is getCommand
 safeGetCommand     :: Display -> Window -> X [String]
 safeGetCommand d w =  do