XMonad: how to show the currently visible workspace on xmobar when using multiple screens? - xmonad

Question

I'm using XMonad in a setup with multiple physical screens.
I would like each of the physical screens to have an instance of xmobar which
shows which workspace is visible on that particular screen, regardless of whether
that workspace is "current"/"active" or not.
E.g.
+--------------+ +--------------+
| | | |
| | | |
| | | |
| | | |
+--------------+ +--------------+
|Workspace 3 | |Workspace 5 |
+--------------+ +--------------+
My current (minimized for clarity) xmonad.hs is below.
import XMonad
import XMonad.Layout.NoBorders
import XMonad.Hooks.ManageDocks
import XMonad.Hooks.DynamicLog
import XMonad.Util.Run
import XMonad.Layout.IndependentScreens
main = do
n <- countScreens
xmprocs <- mapM (\i -> spawnPipe $ "xmobar" ++ " -x " ++ show i) [0..n-1]
xmonad $ docks def
{ layoutHook = avoidStruts $ smartBorders $ layoutHook defaultConfig
, logHook = mapM_ (\xmobarPipe -> dynamicLogWithPP $ def
{ ppOutput = hPutStrLn xmobarPipe
, ppCurrent = \s -> s
, ppVisible = \s -> ""
, ppHidden = \s -> ""
, ppLayout = \s -> ""
, ppTitle = \s -> ""
}) xmprocs
}
That is, I managed to spawn 2 instances of xmobar, one for each screen. However it simply shows the currently active Workspace (across screens) on both screens' xmobar. E.g. it would show:
+--------------+ +--------------+
| | | |
| | | |
| | | |
| | | |
+--------------+ +--------------+
|Workspace 3 | |Workspace 3 |
+--------------+ +--------------+
Now, how do I achieve what I actually want?
I think the configuration here
https://github.com/nwf/xconfig/blob/208e6d6ce48fba45ec30bb1df1389f9ff2263edd/xmonad/lib/XMonad/Actions/XMobars.hs#L163
might contain hints to the answer but I'm not proficient enough in Haskell to
work back from that example to something minimal that I can use.

Answer 1

This works:
import GHC.IO.Handle.Types (Handle)
import XMonad (MonadIO, WorkspaceId, Layout, Window, ScreenId, ScreenDetail, WindowSet, layoutHook, logHook, X, io, ScreenId(..), gets, windowset, xmonad)
import Graphics.X11.ExtraTypes.XF86 (xF86XK_MonBrightnessUp, xF86XK_MonBrightnessDown)
import XMonad.Layout.NoBorders (smartBorders)
import XMonad.Hooks.ManageDocks (docks, avoidStruts)
import XMonad.Hooks.DynamicLog (def)
import XMonad.Util.Run (spawnPipe, hPutStrLn)
import XMonad.Layout.IndependentScreens (countScreens)
import XMonad.StackSet (current, screen, visible, Screen, workspace, tag)
spawnXMobar :: MonadIO m => Int -> m (Int, Handle)
spawnXMobar i = (spawnPipe $ "xmobar" ++ " -x " ++ show i) >>= (\handle -> return (i, handle))
spawnXMobars :: MonadIO m => Int -> m [(Int, Handle)]
spawnXMobars n = mapM spawnXMobar [0..n-1]
type ScreenFoo = Screen WorkspaceId (Layout Window) Window ScreenId ScreenDetail
visibleScreens :: WindowSet -> [ScreenFoo]
visibleScreens cws = ([current cws]) ++ (visible cws)
joinToString :: [String] -> String
joinToString workspaceIds = foldl (++) "" workspaceIds
myLogHookForPipe :: WindowSet -> (Int, Handle) -> X ()
myLogHookForPipe currentWindowSet (i, xmobarPipe) =
io $ hPutStrLn xmobarPipe $ -- write to pipe and lift to XMonad
joinToString $ map (tag . workspace) $ -- extract workspace names and join into a single string
filter ((==) (S i) . screen) $ -- filter to this xmobar's screen only
visibleScreens currentWindowSet -- get list of all visible screens
myLogHook :: [(Int, Handle)] -> X ()
myLogHook xmobarPipes = do
currentWindowSet <- gets windowset
mapM_ (myLogHookForPipe currentWindowSet) xmobarPipes
main = do
n <- countScreens
xmobarPipes <- spawnXMobars n
xmonad $ docks def
{ layoutHook = avoidStruts $ smartBorders $ layoutHook def
, logHook = myLogHook xmobarPipes
}