hide existential Layout (mostly) from user API.

XMonad/Config.hs

@@ -173,7 +173,7 @@ terminal = "xterm"
 --
 -- (The comment formatting character is used when generating the manpage)
 --
-keys :: XConfig -> M.Map (KeyMask, KeySym) (X ())
+keys :: XConfig Layout -> M.Map (KeyMask, KeySym) (X ())
 keys conf@(XConfig {XMonad.modMask = modMask}) = M.fromList $
     -- launching and killing programs
     [ ((modMask .|. shiftMask, xK_Return), spawn $ XMonad.terminal conf) -- %! Launch terminal
@@ -234,7 +234,7 @@ keys conf@(XConfig {XMonad.modMask = modMask}) = M.fromList $
 
 -- | Mouse bindings: default actions bound to mouse events
 --
-mouseBindings :: XConfig -> M.Map (KeyMask, Button) (Window -> X ())
+mouseBindings :: XConfig Layout -> M.Map (KeyMask, Button) (Window -> X ())
 mouseBindings (XConfig {XMonad.modMask = modMask}) = M.fromList $
     -- mod-button1 %! Set the window to floating mode and move by dragging
     [ ((modMask, button1), (\w -> focus w >> mouseMoveWindow w))
@@ -250,12 +250,11 @@ mouseBindings (XConfig {XMonad.modMask = modMask}) = M.fromList $
 -- % Extension-provided definitions
 
 -- | And, finally, the default set of configuration values itself
-defaultConfig :: XConfig
 defaultConfig = XConfig
     { XMonad.borderWidth        = borderWidth
     , XMonad.workspaces         = workspaces
     , XMonad.defaultGaps        = defaultGaps
-    , XMonad.layoutHook         = Layout layout
+    , XMonad.layoutHook         = layout
     , XMonad.terminal           = terminal
     , XMonad.normalBorderColor  = normalBorderColor
     , XMonad.focusedBorderColor = focusedBorderColor

XMonad/Core.hs

@@ -51,7 +51,7 @@ data XState = XState
 
 data XConf = XConf
     { display       :: Display        -- ^ the X11 display
-    , config        :: !XConfig       -- ^ initial user configuration
+    , config        :: !(XConfig Layout)       -- ^ initial user configuration
     , theRoot       :: !Window        -- ^ the root window
     , normalBorder  :: !Pixel         -- ^ border color of unfocused windows
     , focusedBorder :: !Pixel         -- ^ border color of the focused window
@@ -62,18 +62,18 @@ data XConf = XConf
     }
 
 -- todo, better name
-data XConfig = XConfig
+data XConfig l = XConfig
     { normalBorderColor  :: !String
     , focusedBorderColor :: !String
     , terminal           :: !String
-    , layoutHook         :: !(Layout Window)
+    , layoutHook         :: !(l Window)
     , manageHook         :: Window -> X (WindowSet -> WindowSet)
     , workspaces         :: [String]
     , defaultGaps        :: [(Int,Int,Int,Int)]
     , numlockMask        :: !KeyMask
     , modMask            :: !KeyMask
-    , keys               :: XConfig -> M.Map (ButtonMask,KeySym) (X ())
-    , mouseBindings      :: XConfig -> M.Map (ButtonMask, Button) (Window -> X ())
+    , keys               :: XConfig Layout -> M.Map (ButtonMask,KeySym) (X ())
+    , mouseBindings      :: XConfig Layout -> M.Map (ButtonMask, Button) (Window -> X ())
     , borderWidth        :: !Dimension
     , logHook            :: X ()
     }

XMonad/Main.hs

@@ -38,8 +38,10 @@ import System.IO
 -- |
 -- The main entry point
 --
-xmonad :: XConfig -> IO ()
-xmonad xmc = do
+xmonad :: (LayoutClass l Window, Read (l Window)) => XConfig l -> IO ()
+xmonad initxmc = do
+    -- First, wrap the layout in an existential, to keep things pretty:
+    let xmc = initxmc { layoutHook = Layout $ layoutHook initxmc }
     dpy   <- openDisplay ""
     let dflt = defaultScreen dpy