Merge pull request #337 from slotThe/lazy-lifted-boolean-ops

Make `(<&&>)` and `(<||>)` lazy in their second argument

CHANGES.md

@@ -68,6 +68,11 @@
   * Added `withUnfocused` function to `XMonad.Operations`, allowing for
     `X` operations to be applied to unfocused windows.
 
+  * Made `(<&&>)` and `(<||>)` non-strict in their right operand; i.e.,
+    these operators now implement short-circuit evaluation so the right
+    operand is evaluated only if the left operand does not suffice to
+    determine the result.
+
 ## 0.15 (September 30, 2018)
 
   * Reimplement `sendMessage` to deal properly with windowset changes made

src/XMonad/ManageHook.hs

@@ -61,11 +61,15 @@ infixr 3 <&&>, <||>
 
 -- | '&&' lifted to a 'Monad'.
 (<&&>) :: Monad m => m Bool -> m Bool -> m Bool
-(<&&>) = liftM2 (&&)
+(<&&>) x y = ifM x y (pure False)
 
 -- | '||' lifted to a 'Monad'.
 (<||>) :: Monad m => m Bool -> m Bool -> m Bool
-(<||>) = liftM2 (||)
+(<||>) x y = ifM x (pure True) y
+
+-- | If-then-else lifted to a 'Monad'.
+ifM :: Monad m => m Bool -> m a -> m a -> m a
+ifM mb t f = mb >>= \b -> if b then t else f
 
 -- | Return the window title.
 title :: Query String