X.A.Prefix: Add orIfPrefixed

- Generalise signature of withPrefixArgument, in order to accommodate
  this.

CHANGES.md

@@ -232,6 +232,11 @@
     - The `emacsLikeXPKeymap` and `vimLikeXPKeymap` keymaps now treat
       `C-m` the same as `Return`.
 
+  * `XMonad.Actions.Prefix`
+
+    - Added `orIfPrefixed`, a combinator to decide upon an action based
+      on whether any prefix argument was given.
+
 ### Other changes
 
 ## 0.17.1 (September 3, 2022)

XMonad/Actions/Prefix.hs

@@ -29,6 +29,7 @@ module XMonad.Actions.Prefix
        , withPrefixArgument
        , isPrefixRaw
        , isPrefixNumeric
+       , orIfPrefixed
        , ppFormatPrefix
        ) where
 
@@ -173,7 +174,7 @@ handlePrefixArg events = do
 --
 -- First, fetch the current prefix, then pass it as argument to the
 -- original function.  You should use this to "run" your commands.
-withPrefixArgument :: (PrefixArgument -> X ()) -> X ()
+withPrefixArgument :: (PrefixArgument -> X a) -> X a
 withPrefixArgument = (>>=) XS.get
 
 -- | Test if 'PrefixArgument' is 'Raw' or not.
@@ -186,6 +187,13 @@ isPrefixNumeric :: PrefixArgument -> Bool
 isPrefixNumeric (Numeric _) = True
 isPrefixNumeric _ = False
 
+-- | Execute the first action, unless any prefix argument is given,
+-- in which case the second action is chosen instead.
+--
+-- > action1 `orIfPrefixed` action2
+orIfPrefixed :: X a -> X a -> X a
+orIfPrefixed xa xb = withPrefixArgument $ bool xa xb . isPrefixRaw
+
 -- | Format the prefix using the Emacs convetion for use in a
 -- statusbar, like xmobar.
 --