add Read instance to Layout.

darcs-hash:20070920174529-72aca-b1cf11419d5cccbf67edbc3a7bbb356502e9186d

1 files changed, 3 insertions, 1 deletions

diff --git a/XMonad.hs b/XMonad.hs
index 56d7a00..a9253c9 100644
--- a/XMonad.hs
+++ b/XMonad.hs
@@ -134,8 +134,10 @@ atom_WM_STATE           = getAtom "WM_STATE"
 data SomeLayout a = forall l. Layout l a => SomeLayout (l a)
 instance Show (SomeLayout a) where
     show (SomeLayout l) = show l
+instance Read (SomeLayout a) where
+    readsPrec _ _ = [] -- We can't read an existential type!!!
 
-class Show (layout a) => Layout layout a where
+class (Show (layout a), Read (layout a)) => Layout layout a where
     doLayout :: layout a -> Rectangle -> Stack a -> X ([(a, Rectangle)], Maybe (layout a))
     modifyLayout :: layout a -> SomeMessage -> X (Maybe (layout a))