group: Added --list option

Seemed to make sense to exclude groups used only by dead repositories.

Command/Group.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2012 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2024 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -9,18 +9,38 @@ module Command.Group where
 
 import Command
 import qualified Remote
-import Logs.Group
 import Types.Group
+import Logs.Group
+import Logs.UUID
+import Logs.Trust
 import Utility.SafeOutput
 
 import qualified Data.Set as S
+import qualified Data.Map as M
 
 cmd :: Command
 cmd = noMessages $ command "group" SectionSetup "add a repository to a group"
-	(paramPair paramRemote paramDesc) (withParams seek)
+	(paramPair paramRemote paramDesc) (seek <$$> optParser)
 
-seek :: CmdParams -> CommandSeek
-seek = withWords (commandAction . start)
+data GroupOptions = GroupOptions
+	{ cmdparams :: CmdParams
+	, listOption :: Bool
+	}
+
+optParser :: CmdParamsDesc -> Parser GroupOptions
+optParser desc = GroupOptions
+	<$> cmdParams desc
+	<*> switch
+		( long "list"
+		<> help "list all currently defined groups"
+		)
+
+seek :: GroupOptions -> CommandSeek
+seek o
+	| listOption o = if null (cmdparams o)
+		then commandAction startList
+		else giveup "Cannot combine --list with other options"
+	| otherwise = commandAction $ start (cmdparams o)
 
 start :: [String] -> CommandStart
 start ps@(name:g:[]) = do
@@ -33,12 +53,21 @@ start ps@(name:g:[]) = do
 start (name:[]) = do
 	u <- Remote.nameToUUID name
 	startingCustomOutput (ActionItemOther Nothing) $ do
-		liftIO . putStrLn . safeOutput . unwords . map fmt . S.toList
-			=<< lookupGroups u
+		liftIO . listGroups =<< lookupGroups u
 		next $ return True
+start _ = giveup "Specify a repository and a group."
+
+startList :: CommandStart
+startList = startingCustomOutput (ActionItemOther Nothing) $ do
+	us <- trustExclude DeadTrusted =<< M.keys <$> uuidDescMap
+	gs <- foldl' S.union mempty <$> mapM lookupGroups us
+	liftIO $ listGroups gs
+	next $ return True
+
+listGroups :: S.Set Group -> IO ()
+listGroups = liftIO . putStrLn . safeOutput . unwords . map fmt . S.toList
   where
 	fmt (Group g) = decodeBS g
-start _ = giveup "Specify a repository and a group."
 
 setGroup :: UUID -> Group -> CommandPerform
 setGroup uuid g = do