ENH use log file instead of stderr

bin/xmobar.hs

@@ -65,7 +65,7 @@ parseTest =
     (long "test" <> short 't' <> help "test dependencies without running")
 
 xio :: XOpts -> IO ()
-xio o = runXIO $
+xio o = runXIO "xmobar.log" $
   case o of
     XDeps -> printDeps
     XTest -> withDBus_ evalConfig

bin/xmonad.hs

@@ -91,7 +91,7 @@ parseTest =
     (long "test" <> short 't' <> help "test dependencies without running")
 
 xio :: XOpts -> IO ()
-xio o = runXIO $
+xio o = runXIO "xmonad.log" $
   case o of
     XDeps -> printDeps
     XTest -> undefined

lib/Data/Internal/XIO.hs

@@ -118,7 +118,7 @@ import qualified RIO.Text as T
 import System.Posix.Files
 import System.Process.Typed (nullStream)
 import UnliftIO.Environment
-import XMonad.Core (X, io)
+import XMonad.Core (X, dataDir, getDirectories, io)
 import XMonad.Internal.IO
 import XMonad.Internal.Shell hiding (proc, runProcess)
 import XMonad.Internal.Theme
@@ -131,9 +131,18 @@ import XMonad.Internal.Theme
 
 -- | Run feature evaluation(s) with the cache
 -- Currently there is no easy way to not use this (oh well)
-runXIO :: XIO a -> IO a
-runXIO x = do
-  logOpts <- setLogVerboseFormat True . setLogUseTime True <$> logOptionsHandle stderr False
+runXIO :: FilePath -> XIO a -> IO a
+runXIO logfile x = do
+  -- TODO this directory will not exist on a fresh system
+  p <- (</> logfile) . dataDir <$> getDirectories
+  catchIO (withFile p AppendMode $ runXIOInner x) $ \e -> do
+    print e
+    putStrLn "could not open log file, falling back to stderr"
+    runXIOInner x stderr
+
+runXIOInner :: XIO a -> Handle -> IO a
+runXIOInner x h = do
+  logOpts <- setLogVerboseFormat True . setLogUseTime True <$> logOptionsHandle h False
   pc <- mkDefaultProcessContext
   withLogFunc logOpts $ \f -> do
     p <- getParams