    -----------------------------------------------------------------------------
    -- |
    -- Module      :  XMonad.Actions.Submap
    -- Copyright   :  (c) Jason Creighton <jcreigh@gmail.com>
    -- License     :  BSD3-style (see LICENSE)
    --
    -- Maintainer  :  Jason Creighton <jcreigh@gmail.com>
    -- Stability   :  unstable
    -- Portability :  unportable
    --
    -- A module that allows the user to create a sub-mapping of key bindings.
    --
    -----------------------------------------------------------------------------

    module XMonad.Actions.Submap (
                                 -- * Usage
                                 -- $usage
                                 submap,
                                 submapDefault,
                                 submapDefaultWithKey
                                ) where
    import Data.Bits
    import Data.Maybe (fromMaybe)
    import XMonad hiding (keys)
    import qualified Data.Map as M
    import Control.Monad.Fix (fix)

    {- $usage

    First, import this module into your @~\/.xmonad\/xmonad.hs@:

    > import XMonad.Actions.Submap

    Allows you to create a sub-mapping of keys. Example:

    >    , ((modm, xK_a), submap . M.fromList $
    >        [ ((0, xK_n),     spawn "mpc next")
    >        , ((0, xK_p),     spawn "mpc prev")
    >        , ((0, xK_z),     spawn "mpc random")
    >        , ((0, xK_space), spawn "mpc toggle")
    >        ])

    So, for example, to run 'spawn \"mpc next\"', you would hit mod-a (to
    trigger the submapping) and then 'n' to run that action. (0 means \"no
    modifier\"). You are, of course, free to use any combination of
    modifiers in the submapping. However, anyModifier will not work,
    because that is a special value passed to XGrabKey() and not an actual
    modifier.

    For detailed instructions on editing your key bindings, see
    "XMonad.Doc.Extending#Editing_key_bindings".

    -}

    -- | Given a 'Data.Map.Map' from key bindings to X () actions, return
    --   an action which waits for a user keypress and executes the
    --   corresponding action, or does nothing if the key is not found in
    --   the map.
    submap :: M.Map (KeyMask, KeySym) (X ()) -> X ()
    submap = submapDefault (return ())

    -- | Like 'submap', but executes a default action if the key did not match.
    submapDefault :: X () -> M.Map (KeyMask, KeySym) (X ()) -> X ()
    submapDefault = submapDefaultWithKey . const

    -- | Like 'submapDefault', but sends the unmatched key to the default
    -- action as argument.
    submapDefaultWithKey :: ((KeyMask, KeySym) -> X ())
                         -> M.Map (KeyMask, KeySym) (X ())
                         -> X ()
    submapDefaultWithKey defAction keys = do
        XConf { theRoot = root, display = d } <- ask

        io $ grabKeyboard d root False grabModeAsync grabModeAsync currentTime
        io $ grabPointer d root False buttonPressMask grabModeAsync grabModeAsync
                         none none currentTime

        (m, s) <- io $ allocaXEvent $ \p -> fix $ \nextkey -> do
            maskEvent d (keyPressMask .|. buttonPressMask) p
            ev <- getEvent p
            case ev of
              KeyEvent { ev_keycode = code, ev_state = m } -> do
                keysym <- keycodeToKeysym d code 0
                if isModifierKey keysym
                    then nextkey
                    else return (m, keysym)
              _ -> return (0, 0)
        -- Remove num lock mask and Xkb group state bits
        m' <- cleanMask $ m .&. ((1 `shiftL` 12) - 1)

        io $ ungrabPointer d currentTime
        io $ ungrabKeyboard d currentTime

        fromMaybe (defAction (m', s)) (M.lookup (m', s) keys)