Optimize date plugin

We avoid calling getTimeZone for each of the time the date has to be
updated. Instead, it's computed once at the start and re-used for each
invocation.

Looking at the implementation of 'getTimeZone', we can see that it's
very expensive:
https://www.stackage.org/haddock/lts-15.15/time-1.9.3/src/Data-Time-LocalTime-Internal-TimeZone.html#getTimeZone

It calls a C FFI each time to get the time
zone (getTimeZoneCTime). This is something which we can avoid and the
MR implements that.

I have been using my xmobar with this patch and the result has been
quite good. My xmobar CPU usage has used to hit 3~7%
intermittently. With this MR, It hits only 0.7% intermittently which
is nice. :-)

1 files changed, 11 insertions, 3 deletions

diff --git a/src/Xmobar/Plugins/Date.hs b/src/Xmobar/Plugins/Date.hs
index 1cb0596..83ca1c9 100644
--- a/src/Xmobar/Plugins/Date.hs
+++ b/src/Xmobar/Plugins/Date.hs
@@ -31,8 +31,16 @@ data Date = Date String String Int
 
 instance Exec Date where
     alias (Date _ a _) = a
-    run   (Date f _ _) = date f
     rate  (Date _ _ r) = r
+    start (Date f _ r) cb = do
+      t <- getCurrentTime
+      zone <- getTimeZone t
+      go zone
+     where
+      go zone = doEveryTenthSeconds r $ date zone f >>= cb
 
-date :: String -> IO String
-date format = fmap (formatTime defaultTimeLocale format) getZonedTime
+date :: TimeZone -> String -> IO String
+date timezone format = do
+  time <- getCurrentTime
+  let zonedTime = utcToZonedTime timezone time
+  pure $ formatTime defaultTimeLocale format zonedTime