Add Typeable constraint to a few LayoutClass instances

This enables adding the Typeable constraint to LayoutClass itself
(https://github.com/xmonad/xmonad/pull/242) which in turn enables
querying the current layout state. That might be useful to e.g. show the
current X.L.WorkspaceDir in xmobar.

This is a preparation commit that fixes the compile failures that would
result in merging that change to xmonad. For this to be generally useful
we first need to merge (and ideally also release) that xmonad change,
and then we'll need some documentation and perhaps a type class to help
find the right LayoutModifier in the tree of ModifiedLayouts and
Choices. That will come later.

CHANGES.md

@@ -251,6 +251,11 @@
 
     - Export `Minimize` type constructor.
 
+  * Several `LayoutClass` instances now have an additional `Typeable`
+    constraint which may break some advanced configs. The upside is that we
+    can now add `Typeable` to `LayoutClass` in `XMonad.Core` and make it
+    possible to introspect the current layout and its modifiers.
+
 ## 0.16
 
 ### Breaking Changes

XMonad/Layout/LayoutBuilder.hs

@@ -259,7 +259,7 @@ relBox x y w h = SubBox (Rel x) (Rel y) (Rel w) (Rel h)
 
 --------------------------------------------------------------------------------
 instance ( LayoutClass l1 a, LayoutClass l2 a
-         , Read a, Show a, Show p, Eq a, Typeable a, Predicate p a
+         , Read a, Show a, Show p, Typeable p, Eq a, Typeable a, Predicate p a
          ) => LayoutClass (LayoutB l1 l2 p) a where
 
     -- | Update window locations.

XMonad/Layout/LayoutBuilderP.hs

@@ -78,7 +78,7 @@ layoutAll box sub =
   let a = alwaysTrue (Proxy :: Proxy a)
   in  LayoutP Nothing Nothing a box Nothing sub Nothing
 
-instance (LayoutClass l1 w, LayoutClass l2 w, Predicate p w, Show w, Read w, Eq w, Typeable w, Show p) =>
+instance (LayoutClass l1 w, LayoutClass l2 w, Predicate p w, Show w, Read w, Eq w, Typeable w, Show p, Typeable p) =>
     LayoutClass (LayoutP p l1 l2) w where
 
         -- | Update window locations.

XMonad/Layout/LayoutModifier.hs

@@ -250,7 +250,7 @@ class (Show (m a), Read (m a)) => LayoutModifier m a where
 
 -- | The 'LayoutClass' instance for a 'ModifiedLayout' defines the
 --   semantics of a 'LayoutModifier' applied to an underlying layout.
-instance (LayoutModifier m a, LayoutClass l a) => LayoutClass (ModifiedLayout m l) a where
+instance (LayoutModifier m a, LayoutClass l a, Typeable m) => LayoutClass (ModifiedLayout m l) a where
     runLayout (Workspace i (ModifiedLayout m l) ms) r =
         do ((ws, ml'),mm')  <- modifyLayoutWithUpdate m (Workspace i l ms) r
            (ws', mm'') <- redoLayout (maybe m id mm') r ms ws

XMonad/Layout/MultiToggle.hs

@@ -188,7 +188,7 @@ instance (Transformer a w, HList b w) => HList (HCons a b) w where
 geq :: (Typeable a, Eq a, Typeable b) => a -> b -> Bool
 geq a b = Just a == cast b
 
-instance (Typeable a, Show ts, HList ts a, LayoutClass l a) => LayoutClass (MultiToggle ts l) a where
+instance (Typeable a, Show ts, Typeable ts, HList ts a, LayoutClass l a) => LayoutClass (MultiToggle ts l) a where
     description mt = currLayout mt `unEL` \l -> description l
 
     runLayout (Workspace i mt s) r = case currLayout mt of

XMonad/Layout/ZoomRow.hs

@@ -185,7 +185,7 @@ zoomReset = ZoomTo 1
 
 -- * LayoutClass instance
 
-instance (EQF f a, Show a, Read a, Show (f a), Read (f a))
+instance (EQF f a, Show a, Read a, Show (f a), Read (f a), Typeable f)
     => LayoutClass (ZoomRow f) a where
     description (ZC _ Nothing) = "ZoomRow"
     description (ZC _ (Just s)) = "ZoomRow" ++ if full $ W.focus s