feat: add new PacmanUpdates plugin

the new PacmanUpdates plugin behaves similar to the ArchUpdates plugin
while additionally allowing to pass in a custom error message for
unknown pacman failures. The default error message of `pacman: Unknown
cause of failure.` of the ArchUpdates plugin is too long for my taste.

The ArchUpdates plugin was modified to delegate to the new PacmanUpdates
plugin while providing the default error message and to show a
deprecation notice in the zero updates case.

The new name better represents the Plugin's compatibility with all
pacman-based distributions, not just Arch.

The docs have been updated to reflect the existence of the new plugin
and to highlight the similarities between the ArchUpdates and
PacmanUpdates plugins. The ArchUpdates plugin has been marked has
deprecated there to.

1 files changed, 43 insertions, 0 deletions

diff --git a/src/Xmobar/Plugins/PacmanUpdates.hs b/src/Xmobar/Plugins/PacmanUpdates.hs
new file mode 100644
index 0000000..1e8a8fc
--- /dev/null
+++ b/src/Xmobar/Plugins/PacmanUpdates.hs
@@ -0,0 +1,43 @@
+{-# LANGUAGE CPP #-}
+
+-----------------------------------------------------------------------------
+
+-----------------------------------------------------------------------------
+
+{- |
+Module      :  Plugins.Monitors.PacmanUpdates
+Copyright   :  (c) 2024 Enrico Maria De Angelis
+            ,  (c) 2025 Alexander Pankoff
+License     :  BSD-style (see LICENSE)
+
+Maintainer  :  Enrico Maria De Angelis <enricomaria.dean6elis@gmail.com>
+Stability   :  unstable
+Portability :  unportable
+
+A Pacman updates availablility plugin for Xmobar
+-}
+module Xmobar.Plugins.PacmanUpdates (PacmanUpdates (..)) where
+
+import System.Exit (ExitCode (..))
+import System.Process (readProcessWithExitCode)
+import Xmobar.Plugins.Command (Rate)
+import Xmobar.Run.Exec
+
+data PacmanUpdates = PacmanUpdates (String, String, String, String) Rate
+  deriving (Read, Show)
+
+instance Exec PacmanUpdates where
+  alias = const "pacman"
+  rate (PacmanUpdates _ r) = r
+  run (PacmanUpdates (z, o, m, e) _) = do
+    (exit, stdout, _) <- readProcessWithExitCode "checkupdates" [] ""
+    return $ case exit of
+      ExitFailure 2 -> z -- ero updates
+      ExitFailure 1 -> e
+      ExitSuccess -> case length $ lines stdout of
+        0 -> impossible
+        1 -> o
+        n -> m >>= \c -> if c == '?' then show n else pure c
+      _ -> impossible
+   where
+    impossible = error "This is impossible based on pacman manpage"