[xmonad] darcs patch: hide existential Layout (mostly) from user API.

[Devin Mullins]

On Sat, Nov 10, 2007 at 04:34:21PM -0800, David Roundy wrote:
> This allows us to eliminate the wart of requiring a Layout wrapper around
> our layoutHook.  Besides being ugly, this wrapper hampers the composability
> of layouts:  we can't simply write something like:
> 
> myconfig = defaultConfig { layoutHook = Layout $ Mirror $ layoutHook defaultConfig }

I think this is the huge win here. Not only does it allow the above; it
also allows us to write more config "combinators". In particular (until
we have eventHooks), I could move withUrgencyHook to the the outer
level:
  main = xmonad $ withUrgencyHook defaultConfig { yada yada }
and it could modify the layoutHook and logHook in one go.

Another might be WindowNavigation, which could add the layout hook and
default keybindings at the same time.

Devin