Haskell: Reading from FS to "dynamically" create widgets in taffybar - haskell

Question

I'm trying to use taffybar as my status bar (uses Dyre framework so configuration is code). It has a widget that can show network interface statistics. In the default configuration this widget requires a String at compile time. I want it to create a widget per (non-loopback) interface dynamically instead.
This is what I have so far:
listNetworkDevices :: IO [String]
listNetworkDevices = fmap (map takeBaseName) $ getDirectoryContents "/sys/class/net/"
filterOutLoopback :: [String] -> [String]
filterOutLoopback = filter (\y -> not (elem y ["", ".", "lo"]))
netDevList :: IO [String]
netDevList = fmap filterOutLoopback listNetworkDevices
...
let nets = fmap (fmap (netMonitorNew 1)) netDevList
...
defaultTaffybar defaultTaffybarConfig {
...
endWidgets = [ tray, clock, mem, cpu] ++ nets ++ [ bat ]
}
At this point I'm down to the following compiler error:
Couldn't match expected type ‘[IO gtk-0.14.2:Graphics.UI.Gtk.Types.Widget]’
with actual type ‘IO [IO gtk-0.14.2:Graphics.UI.Gtk.Types.Widget]’
Overall this the error makes sense since I'm making an IO Widget out of every String. So IO [String] turns into IO [IO Widget].
What I don't understand is how to avoid doing this. Clearly I must have gone wrong at some point but I can't see where. I'm not even sure how to put the issue in words. I would be glad for any pointers!
Relevant material:
endWidgets docs: https://hackage.haskell.org/package/taffybar-0.4.6/docs/System-Taffybar.html#v:endWidgets
netMonitorNew docs: https://hackage.haskell.org/package/taffybar-0.4.6/docs/System-Taffybar-NetMonitor.html#v:netMonitorNew
Source on github: https://github.com/travitch/taffybar/tree/master/src/System/Taffybar
Edit: complete listing:
import System.Taffybar
import System.Taffybar.Systray
import System.Taffybar.Pager
import System.Taffybar.TaffyPager
import System.Taffybar.SimpleClock
import System.Taffybar.Battery
import System.Taffybar.NetMonitor
import System.Taffybar.Weather
import System.Taffybar.Widgets.PollingBar
import System.Taffybar.Widgets.PollingGraph
import System.Information.Memory
import System.Information.CPU
import System.Directory ( getDirectoryContents )
import System.FilePath ( takeBaseName )
import Control.Monad
memCallback = do
mi <- parseMeminfo
return [memoryUsedRatio mi]
cpuCallback = do
(userLoad, systemLoad, totalLoad) <- cpuLoad
return [totalLoad, systemLoad]
listNetworkDevices :: IO [String]
listNetworkDevices = fmap (map takeBaseName) $ getDirectoryContents "/sys/class/net/"
filterOutLoopback :: [String] -> [String]
filterOutLoopback = filter (\y -> not (elem y ["", ".", "lo"]))
netDevList :: IO [String]
netDevList = fmap filterOutLoopback listNetworkDevices
myPagerConfig = defaultPagerConfig {}
main = do
let memCfg = defaultGraphConfig { graphDataColors = [(1, 0, 0, 1)] }
cpuCfg = defaultGraphConfig { graphDataColors = [ (0, 1, 0, 1)
, (1, 0, 1, 0.5)
]
}
clock = textClockNew Nothing "<span fgcolor='orange'>%a %b %d %H:%M</span>" 1
pager = taffyPagerNew myPagerConfig
mem = pollingGraphNew memCfg 1 memCallback
cpu = pollingGraphNew cpuCfg 1 cpuCallback
bat = textBatteryNew "$percentage$% $time$" 1
tray = systrayNew
--nets = [ netMonitorNew 1 "wlp0s2" ]
nets = fmap (fmap (netMonitorNew 1)) netDevList
defaultTaffybar defaultTaffybarConfig { startWidgets = [ pager ]
, endWidgets = [ tray, clock, mem, cpu] ++ nets ++ [ bat ]
}
Edit: Error message:
.config/taffybar/taffybar.hs:
Couldn't match expected type ‘[IO
gtk-0.14.2:Graphics.UI.Gtk.Types.Widget]’
with actual type ‘IO [IO gtk-0.14.2:Graphics.UI.Gtk.Types.Widget]’
In the first argument of ‘(++)’, namely ‘nets’
In the second argument of ‘(++)’, namely ‘nets ++ [bat]’

Answer 1

I haven't tested this as I usually do, since I don't have taffybar installed on this machine, but I suspect the following small change should help you make progress:
main = do
let memCfg = defaultGraphConfig { graphDataColors = [(1, 0, 0, 1)] }
cpuCfg = defaultGraphConfig { graphDataColors = [ (0, 1, 0, 1)
, (1, 0, 1, 0.5)
]
}
clock = textClockNew Nothing "<span fgcolor='orange'>%a %b %d %H:%M</span>" 1
pager = taffyPagerNew myPagerConfig
mem = pollingGraphNew memCfg 1 memCallback
cpu = pollingGraphNew cpuCfg 1 cpuCallback
bat = textBatteryNew "$percentage$% $time$" 1
tray = systrayNew
-- this line is the only one that changed
nets <- fmap (fmap (netMonitorNew 1)) netDevList
defaultTaffybar defaultTaffybarConfig { startWidgets = [ pager ]
, endWidgets = [ tray, clock, mem, cpu] ++ nets ++ [ bat ]
}
There may be other problems, but this should address the one described in the question.
There are plenty of stylistic changes that could/should be made as well, of course; for example, I think I would probably write the last two lines this way instead:
nets <- netDevList
defaultTaffybar ... { ..., endWidgets = ... ++ map netMonitorNew nets ++ ... }