Add TwoPane

TwoPane.hs

@@ -0,0 +1,25 @@
+-- A layout that splits the screen horizontally and shows two windows.  The
+-- left window is always the master window, and the right is either the
+-- currently focused window or the second window in layout order.
+
+module XMonadContrib.TwoPane where
+
+import XMonad
+import Operations
+import qualified StackSet as W
+import Control.Monad.State (gets)
+
+twoPane :: Rational -> Rational -> Layout
+twoPane delta split = Layout { doLayout = arrange, modifyLayout = message }
+ where
+    arrange rect (w:x:_) = do
+        (Just f) <- gets (W.peek . workspace) -- safe because of pattern match above
+        let (left, right) = splitHorizontallyBy split rect
+        return [(w, left), (if f == w then x else f, right)]
+    -- there are one or zero windows
+    arrange rect ws       = return . map (\w -> (w, rect)) $ ws
+
+    message x = case fromMessage x of
+                    Just Shrink -> Just (twoPane delta (split - delta))
+                    Just Expand -> Just (twoPane delta (split + delta))
+                    _           -> Nothing