Add support for extensible config in contrib modules

It's often difficult to make contrib modules work together. When one
depends on a functionality of another, it is often necessary to expose
lots of low-level functions and hooks and have the user combine these
into a complex configuration that works. This is error-prone, and
arguably a bad UX in general.

This commit presents a simple solution to that problem inspired by
"extensible state": extensible config. It allows contrib modules to
store custom configuration values inside XConfig. This lets them create
custom hooks, ensure they hook into xmonad core only once, and possibly
other use cases I haven't thought of yet.

For more, see the related pull request to xmonad-contrib.

Related: https://github.com/xmonad/xmonad-contrib/pull/547

CHANGES.md

@@ -41,6 +41,9 @@
     Instead, the manpage source is regenerated and manpage rebuilt
     automatically in CI.
 
+  * Added the `extensibleConf` field to `XConfig` which makes it easier for
+    contrib modules to have composable configuration (custom hooks, …).
+
 ## 0.15 (September 30, 2018)
 
   * Reimplement `sendMessage` to deal properly with windowset changes made

src/XMonad/Config.hs

@@ -277,6 +277,7 @@ instance (a ~ Choose Tall (Choose (Mirror Tall) Full)) => Default (XConfig a) wh
     , XMonad.handleExtraArgs = \ xs theConf -> case xs of
                 [] -> return theConf
                 _ -> fail ("unrecognized flags:" ++ show xs)
+    , XMonad.extensibleConf     = M.empty
     }
 
 -- | The default set of configuration values itself

src/XMonad/Core.hs

@@ -23,7 +23,7 @@ module XMonad.Core (
     XConf(..), XConfig(..), LayoutClass(..),
     Layout(..), readsLayout, Typeable, Message,
     SomeMessage(..), fromMessage, LayoutMessages(..),
-    StateExtension(..), ExtensionClass(..),
+    StateExtension(..), ExtensionClass(..), ConfExtension(..),
     runX, catchX, userCode, userCodeDef, io, catchIO, installSignalHandlers, uninstallSignalHandlers,
     withDisplay, withWindowSet, isRoot, runOnWorkspaces,
     getAtom, spawn, spawnPID, xfork, recompile, trace, whenJust, whenX,
@@ -122,6 +122,11 @@ data XConfig l = XConfig
     , rootMask           :: !EventMask           -- ^ The root events that xmonad is interested in
     , handleExtraArgs    :: !([String] -> XConfig Layout -> IO (XConfig Layout))
                                                  -- ^ Modify the configuration, complain about extra arguments etc. with arguments that are not handled by default
+    , extensibleConf     :: !(M.Map TypeRep ConfExtension)
+                                                 -- ^ Stores custom config information.
+                                                 --
+                                                 -- The module "XMonad.Util.ExtensibleConf" in xmonad-contrib
+                                                 -- provides additional information and a simple interface for using this.
     }
 
 
@@ -383,7 +388,7 @@ data LayoutMessages = Hide              -- ^ sent when a layout becomes non-visi
 instance Message LayoutMessages
 
 -- ---------------------------------------------------------------------
--- Extensible state
+-- Extensible state/config
 --
 
 -- | Every module must make the data it wants to store
@@ -410,6 +415,9 @@ data StateExtension =
   | forall a. (Read a, Show a, ExtensionClass a) => PersistentExtension a
     -- ^ Persistent extension
 
+-- | Existential type to store a config extension.
+data ConfExtension = forall a. Typeable a => ConfExtension a
+
 -- ---------------------------------------------------------------------
 -- | General utilities
 --