Simplify quickcheck-classes tests for Stack

We don't need the compat hacks for GHC 8.4.

tests/Properties/Stack.hs

@@ -12,11 +12,11 @@ import qualified XMonad.StackSet as S (filter)
 import Data.Maybe
 
 #ifdef VERSION_quickcheck_classes
+import Data.Proxy
 import Test.QuickCheck.Classes (
     Laws (lawsTypeclass, lawsProperties), Proxy1 (Proxy1),
     foldableLaws, traversableLaws,
     )
-import Data.Functor.Classes (Eq1 (liftEq), Show1 (liftShowsPrec))
 #endif
 
 
@@ -66,47 +66,18 @@ prop_differentiate xs =
 newtype TestStack a = TestStack (Stack a)
     deriving (Eq, Read, Show, Foldable, Functor)
 
-instance Arbitrary1 TestStack where
-   liftArbitrary gen =
-       (\ x xu xd -> TestStack (Stack x xu xd))
-       <$> gen
-       <*> liftArbitrary gen
-       <*> liftArbitrary gen
-
 instance (Arbitrary a) => Arbitrary (TestStack a) where
-   arbitrary = arbitrary1
-   shrink = shrink1
+   arbitrary = TestStack <$> (Stack <$> arbitrary <*> arbitrary <*> arbitrary)
+   shrink = traverse shrink
 
 instance Traversable TestStack where
    traverse f (TestStack sx) = fmap TestStack (traverse f sx)
 
-instance Eq1 TestStack where
-    liftEq f (TestStack (Stack x xu xd)) (TestStack (Stack y yu yd)) =
-        f x y && liftEq f xu yu && liftEq f xd yd
-
-instance Show1 TestStack where
-    liftShowsPrec shwP shwL p (TestStack (Stack x xu xd)) =
-        showString "TestStack (Stack {focus = "
-                <> shwP p x
-                <> showString ", up = "
-                <> shwL xu
-                <> showString ", down ="
-                <> shwL xd
-                <> showString "})"
-
-proxy_TestStack :: Proxy1 TestStack
-proxy_TestStack = Proxy1
-laws_Stack_Traversable, laws_Stack_Foldable :: Laws
-laws_Stack_Traversable = traversableLaws proxy_TestStack
-laws_Stack_Foldable = foldableLaws proxy_TestStack
-prop_laws_Stack =
-    format laws_Stack_Foldable <> format laws_Stack_Traversable
+prop_laws_Stack = format (foldableLaws p) <> format (traversableLaws p)
   where
-    format laws =
-        fmap
-        (\(name, prop) ->
-            ("Stack: " <> lawsTypeclass laws <> ": " <> name, prop))
-        (lawsProperties laws)
+    p = Proxy :: Proxy TestStack
+    format laws = [ ("Stack: " <> lawsTypeclass laws <> ": " <> name, prop)
+                  | (name, prop) <- lawsProperties laws ]
 #else
 prop_laws_Stack = []
 #endif

xmonad.cabal

@@ -143,8 +143,9 @@ test-suite properties
                 , containers
                 , xmonad
 
-  if flag(quickcheck-classes)
+  if flag(quickcheck-classes) && impl(ghc > 8.5)
     -- no quickcheck-classes in LTS-12
+    -- GHC 8.4 and lower needs too much boilerplate (Eq1, Show1, …)
     build-depends: quickcheck-classes >= 0.4.3
 
   if flag(pedantic)