Use XDG config directory as default config path

- Look for config at $XDG_CONFIG_HOME/converge/config.yaml by default
- --config still overrides to an explicit path
- Falls back to single-repo --repo mode if no config file found
- New defaultConfigPath and configFileExists helpers in Converge module

src/Converge.hs

@@ -9,6 +9,8 @@ module Converge (
     Config (..),
     loadConfig,
     configToOptions,
+    defaultConfigPath,
+    configFileExists,
 
     -- * Running
     runConverge,
@@ -31,7 +33,7 @@ import Data.Maybe (fromMaybe)
 import Data.Text (Text)
 import qualified Data.Text as T
 import Data.Yaml (FromJSON (..), decodeFileThrow, withObject, (.:), (.:?))
-import System.Directory (getCurrentDirectory)
+import System.Directory (XdgDirectory (XdgConfig), doesFileExist, getCurrentDirectory, getXdgDirectory)
 import System.Exit (ExitCode (..))
 import System.FSNotify
 import System.FilePath (isAbsolute, (</>))
@@ -114,6 +116,14 @@ configToOptions cfg =
         , optDebounceMs = fromMaybe 5000 (cfgDebounceMs cfg)
         }
 
+-- | The default XDG config file path: @$XDG_CONFIG_HOME/converge/config.yaml@.
+defaultConfigPath :: IO FilePath
+defaultConfigPath = (</> "converge/config.yaml") <$> getXdgDirectory XdgConfig ""
+
+-- | Check whether the default config file exists.
+configFileExists :: IO Bool
+configFileExists = defaultConfigPath >>= doesFileExist
+
 ----------------------------------------------------------------------
 -- Running
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