    -----------------------------------------------------------------------------
    -- |
    -- Module      :  XMonad.Layout.IndependentScreens
    -- Copyright   :  (c) 2009 Daniel Wagner
    -- License     :  BSD3
    --
    -- Maintainer  :  <daniel@wagner-home.com>
    -- Stability   :  unstable
    -- Portability :  unportable
    --
    -- Utility functions for simulating independent sets of workspaces on
    -- each screen (like dwm's workspace model), using internal tags to
    -- distinguish workspaces associated with each screen.
    -----------------------------------------------------------------------------

    module XMonad.Layout.IndependentScreens (
        -- * Usage
        -- $usage
        VirtualWorkspace, PhysicalWorkspace,
        workspaces',
        withScreens, onCurrentScreen,
        marshallPP,
        whenCurrentOn,
        countScreens,
        -- * Converting between virtual and physical workspaces
        -- $converting
        marshall, unmarshall, unmarshallS, unmarshallW,
        marshallWindowSpace, unmarshallWindowSpace, marshallSort
    ) where

    -- for the screen stuff
    import Control.Applicative((<*), liftA2)
    import Control.Arrow hiding ((|||))
    import Control.Monad
    import Data.List (nub, genericLength)
    import Graphics.X11.Xinerama
    import XMonad
    import XMonad.StackSet hiding (filter, workspaces)
    import XMonad.Hooks.DynamicLog

    -- $usage
    -- You can use this module with the following in your @~\/.xmonad\/xmonad.hs@:
    --
    -- > import XMonad.Layout.IndependentScreens
    --
    -- You can define your workspaces by calling @withScreens@:
    --
    -- > myConfig = def { workspaces = withScreens 2 ["web", "email", "irc"] }
    --
    -- This will create \"physical\" workspaces with distinct internal names for
    -- each (screen, virtual workspace) pair.
    --
    -- Then edit any keybindings that use the list of workspaces or refer
    -- to specific workspace names.  In the default configuration, only
    -- the keybindings for changing workspace do this:
    --
    -- > keyBindings conf = let m = modMask conf in fromList $
    -- >     {- lots of other keybindings -}
    -- >     [((m .|. modm, k), windows $ f i)
    -- >         | (i, k) <- zip (XMonad.workspaces conf) [xK_1 .. xK_9]
    -- >         , (f, m) <- [(W.greedyView, 0), (W.shift, shiftMask)]]
    --
    -- This should change to
    --
    -- > keyBindings conf = let m = modMask conf in fromList $
    -- >     {- lots of other keybindings -}
    -- >     [((m .|. modm, k), windows $ onCurrentScreen f i)
    -- >         | (i, k) <- zip (workspaces' conf) [xK_1 .. xK_9]
    -- >         , (f, m) <- [(W.greedyView, 0), (W.shift, shiftMask)]]
    --
    -- In particular, the analogue of @XMonad.workspaces@ is
    -- @workspaces'@, and you can use @onCurrentScreen@ to convert functions
    -- of virtual workspaces to functions of physical workspaces, which work
    -- by marshalling the virtual workspace name and the currently focused
    -- screen into a physical workspace name.
    --
    -- A complete example abusing many of the functions below is available in the
    -- "XMonad.Config.Dmwit" module.

    type VirtualWorkspace  = WorkspaceId
    type PhysicalWorkspace = WorkspaceId

    -- $converting
    -- You shouldn't need to use the functions below very much. They are used
    -- internally. However, in some cases, they may be useful, and so are exported
    -- just in case. In general, the \"marshall\" functions convert the convenient
    -- form (like \"web\") you would like to use in your configuration file to the
    -- inconvenient form (like \"2_web\") that xmonad uses internally. Similarly,
    -- the \"unmarshall\" functions convert in the other direction.

    marshall :: ScreenId -> VirtualWorkspace -> PhysicalWorkspace
    marshall (S sc) vws = show sc ++ '_':vws

    unmarshall  :: PhysicalWorkspace -> (ScreenId, VirtualWorkspace)
    unmarshallS :: PhysicalWorkspace -> ScreenId
    unmarshallW :: PhysicalWorkspace -> VirtualWorkspace

    unmarshall  = ((S . read) *** drop 1) . break (=='_')
    unmarshallS = fst . unmarshall
    unmarshallW = snd . unmarshall

    workspaces' :: XConfig l -> [VirtualWorkspace]
    workspaces' = nub . map (snd . unmarshall) . workspaces

    withScreens :: ScreenId            -- ^ The number of screens to make workspaces for
                -> [VirtualWorkspace]  -- ^ The desired virtual workspace names
                -> [PhysicalWorkspace] -- ^ A list of all internal physical workspace names
    withScreens n vws = [marshall sc pws | pws <- vws, sc <- [0..n-1]]

    onCurrentScreen :: (VirtualWorkspace -> WindowSet -> a) -> (PhysicalWorkspace -> WindowSet -> a)
    onCurrentScreen f vws = screen . current >>= f . flip marshall vws

    -- | In case you don't know statically how many screens there will be, you can call this in main before starting xmonad.  For example, part of my config reads
    --
    -- > main = do
    -- >   nScreens <- countScreens
    -- >   xmonad $ def {
    -- >     ...
    -- >     workspaces = withScreens nScreens (workspaces def),
    -- >     ...
    -- >     }
    --
    countScreens :: (MonadIO m, Integral i) => m i
    countScreens = liftM genericLength . liftIO $ openDisplay "" >>= liftA2 (<*) getScreenInfo closeDisplay

    -- | This turns a naive pretty-printer into one that is aware of the
    -- independent screens. That is, you can write your pretty printer to behave
    -- the way you want on virtual workspaces; this function will convert that
    -- pretty-printer into one that first filters out physical workspaces on other
    -- screens, then converts all the physical workspaces on this screen to their
    -- virtual names.
    --
    -- For example, if you have handles @hLeft@ and @hRight@ for bars on the left and right screens, respectively, and @pp@ is a pretty-printer function that takes a handle, you could write
    --
    -- > logHook = let log screen handle = dynamicLogWithPP . marshallPP screen . pp $ handle
    -- >           in log 0 hLeft >> log 1 hRight
    marshallPP :: ScreenId -> PP -> PP
    marshallPP s pp = pp {
        ppCurrent           = ppCurrent         pp . snd . unmarshall,
        ppVisible           = ppVisible         pp . snd . unmarshall,
        ppHidden            = ppHidden          pp . snd . unmarshall,
        ppHiddenNoWindows   = ppHiddenNoWindows pp . snd . unmarshall,
        ppUrgent            = ppUrgent          pp . snd . unmarshall,
        ppSort              = fmap (marshallSort s) (ppSort pp)
        }

    -- | Take a pretty-printer and turn it into one that only runs when the current
    -- workspace is one associated with the given screen. The way this works is a
    -- bit hacky, so beware: the 'ppOutput' field of the input will not be invoked
    -- if either of the following conditions is met:
    --
    -- 1. The 'ppSort' of the input returns an empty list (when not given one).
    --
    -- 2. The 'ppOrder' of the input returns the exact string @\"\\0\"@.
    --
    -- For example, you can use this to create a pipe which tracks the title of the
    -- window currently focused on a given screen (even if the screen is not
    -- current) by doing something like this:
    --
    -- > ppFocus s = whenCurrentOn s def
    -- >     { ppOrder  = \(_:_:title:_) -> [title]
    -- >     , ppOutput = appendFile ("focus" ++ show s) . (++ "\n")
    -- >     }
    --
    -- Sequence a few of these pretty-printers to get a log hook that keeps each
    -- screen's title up-to-date.
    whenCurrentOn :: ScreenId -> PP -> PP
    whenCurrentOn s pp = pp
        { ppSort = do
            sort <- ppSort pp
            return $ \xs -> case xs of
                x:_ | unmarshallS (tag x) == s -> sort xs
                _ -> []
        , ppOrder = \i@(wss:_) -> case wss of
            "" -> ["\0"] -- we got passed no workspaces; this is the signal from ppSort that this is a boring case
            _  -> ppOrder pp i
        , ppOutput = \out -> case out of
            "\0" -> return () -- we got passed the signal from ppOrder that this is a boring case
            _ -> ppOutput pp out
        }

    -- | If @vSort@ is a function that sorts 'WindowSpace's with virtual names, then @marshallSort s vSort@ is a function which sorts 'WindowSpace's with physical names in an analogous way -- but keeps only the spaces on screen @s@.
    marshallSort :: ScreenId -> ([WindowSpace] -> [WindowSpace]) -> ([WindowSpace] -> [WindowSpace])
    marshallSort s vSort = pScreens . vSort . vScreens where
        onScreen ws = unmarshallS (tag ws) == s
        vScreens    = map unmarshallWindowSpace . filter onScreen
        pScreens    = map (marshallWindowSpace s)

    -- | Convert the tag of the 'WindowSpace' from a 'VirtualWorkspace' to a 'PhysicalWorkspace'.
    marshallWindowSpace   :: ScreenId -> WindowSpace -> WindowSpace
    -- | Convert the tag of the 'WindowSpace' from a 'PhysicalWorkspace' to a 'VirtualWorkspace'.
    unmarshallWindowSpace :: WindowSpace -> WindowSpace

    marshallWindowSpace s ws = ws { tag = marshall s  (tag ws) }
    unmarshallWindowSpace ws = ws { tag = unmarshallW (tag ws) }