ENH test for battery before displaying discharge thing in xmobar

bin/xmobar.hs

@@ -32,6 +32,7 @@ import           Xmobar.Plugins.VPN
 
 import           XMonad                              (getXMonadDir)
 import           XMonad.Hooks.DynamicLog             (wrap, xmobarColor)
+import           XMonad.Internal.Command.Power       (hasBattery)
 import           XMonad.Internal.DBus.Common         (xmonadBus)
 import           XMonad.Internal.DBus.Control        (pathExists)
 import           XMonad.Internal.DBus.IntelBacklight (blPath)
@@ -138,6 +139,31 @@ getEthernet = do
     [n] -> Just $ ethernetCmd n
     _   -> Nothing
 
+batteryCmd :: CmdSpec
+batteryCmd =  CmdSpec
+  { csAlias = "battery"
+  , csDepends = Nothing
+  , csRunnable = Run
+    $ Battery
+    [ "--template", "<acstatus><left>"
+    , "--Low", "10"
+    , "--High", "80"
+    , "--low", "red"
+    , "--normal", T.fgColor
+    , "--high", T.fgColor
+    , "--"
+    , "-P"
+    , "-o" , "<fn=1>\xf0e7</fn>"
+    , "-O" , "<fn=1>\xf1e6</fn>"
+    , "-i" , "<fn=1>\xf1e6</fn>"
+    ] 50
+  }
+
+getBattery :: IO (Maybe CmdSpec)
+getBattery = do
+  b <- hasBattery
+  return $ if b then Just batteryCmd else Nothing
+
 vpnCmd :: CmdSpec
 vpnCmd = CmdSpec
   { csAlias = vpnAlias
@@ -161,6 +187,7 @@ myCommands = do
   wirelessSpec <- getWireless
   ethernetSpec <- getEthernet
   vpnSpec <- getVPN
+  batterySpec <- getBattery
   let left =
         [ CmdSpec
           { csAlias = "UnsafeStdinReader"
@@ -196,24 +223,7 @@ myCommands = do
             ]
           }
 
-        , Just $ CmdSpec
-          { csAlias = "battery"
-          , csDepends = Nothing
-          , csRunnable = Run
-            $ Battery
-            [ "--template", "<acstatus><left>"
-            , "--Low", "10"
-            , "--High", "80"
-            , "--low", "red"
-            , "--normal", T.fgColor
-            , "--high", T.fgColor
-            , "--"
-            , "-P"
-            , "-o" , "<fn=1>\xf0e7</fn>"
-            , "-O" , "<fn=1>\xf1e6</fn>"
-            , "-i" , "<fn=1>\xf1e6</fn>"
-            ] 50
-          }
+        , batterySpec
 
         , Just $ CmdSpec
           { csAlias = "intelbacklight"

lib/XMonad/Internal/Command/Power.hs

@@ -11,16 +11,20 @@ module XMonad.Internal.Command.Power
   , runSuspend
   , runSuspendPrompt
   , runQuitPrompt
+  , hasBattery
   ) where
 
 import           Control.Arrow               (first)
 
+import           Data.Either
 import qualified Data.Map                    as M
 
 import           Graphics.X11.Types
 
 import           System.Directory
 import           System.Exit
+import           System.FilePath.Posix
+import           System.IO.Error
 import           System.Process
 
 import           XMonad.Core
@@ -98,17 +102,21 @@ hasSwitchableGPU = withShellOutput cmd hasIntelAndOther
           ivendors = filter (== "Intel Corporation") vendors in
       length vendors > length ivendors && not (null ivendors)
 
--- this is hacky but so much easier than the "pure haskell" solution
-hasBattery :: IO (Maybe Bool)
-hasBattery = withShellOutput (fmtCmd "cat" ["/sys/class/power_supply/*/type"])
-  $ elem "Battery" . lines
+hasBattery :: IO Bool
+hasBattery = do
+  ps <- fromRight [] <$> tryIOError (listDirectory syspath)
+  ts <- mapM readType ps
+  return $ "Battery\n" `elem` ts
+  where
+    readType p = fromRight [] <$> tryIOError (readFile $ syspath </> p </> "type")
+    syspath = "/sys/class/power_supply"
 
 requireOptimus :: IO Bool
 requireOptimus = do
   s <- hasSwitchableGPU
   b <- hasBattery
   case (s, b) of
-    (Just True, Just True) -> return True
+    (Just True, True) -> return True
     _                 -> warn >> return False
   where
     warn = putStrLn