print version information on --version

If --version is given, `walint` prints version information, then exists
with code 0.

src/Main.hs

@@ -12,17 +12,19 @@ import           Data.Aeson               (eitherDecode, encode)
 import           Data.Aeson.Encode.Pretty (encodePretty)
 import           Data.Aeson.KeyMap        (coercionToHashMap)
 import qualified Data.ByteString.Lazy     as LB
-import qualified Data.Text.Encoding as T
-import qualified Data.Text.IO as T
 import           Data.Maybe               (fromMaybe)
+import qualified Data.Text.Encoding       as T
+import qualified Data.Text.IO             as T
 import           System.Exit              (ExitCode (..), exitWith)
 import           WithCli
 
 import           CheckDir                 (recursiveCheckDir, resultIsFatal)
+import           Control.Monad            (when)
 import           LintConfig               (LintConfig (..), patchConfig)
 import           System.IO                (hPutStrLn, stderr)
 import           Types                    (Level (..))
 import           Util                     (printPretty)
+import qualified Version                  as V (version)
 import           WriteRepo                (writeAdjustedRepository)
 
 -- | the options this cli tool can take
@@ -43,6 +45,7 @@ data Options = Options
   -- ^ path to a config file. Currently required.
   , config     :: Maybe (LintConfig Maybe)
   -- ^ a "patch" for the configuration file
+  , version    :: Bool
   } deriving (Show, Generic, HasArguments)
 
 
@@ -53,6 +56,10 @@ run :: Options -> IO ()
 run options = do
   aesonWarning
 
+  when (version options) $ do
+    putStrLn V.version
+    exitWith ExitSuccess
+
   let repo = fromMaybe "." (repository options)
   let entry = fromMaybe "main.json" (entrypoint options)
   let level = fromMaybe Suggestion (lintlevel options)

src/Version.hs

+{-# LANGUAGE TemplateHaskell #-}
+
+
+module Version ( version ) where
+
+import           Control.Monad.Trans (liftIO)
+import qualified Language.Haskell.TH as TH
+import           System.Process      (readProcess)
+
+version :: String
+version = "walint rc3 2021 (" <>
+    $(do
+        hash <- liftIO $ readProcess "git" ["rev-parse", "HEAD"] ""
+        pure . TH.LitE . TH.StringL $ take 40 hash) ++
+    ")"

walint.cabal

@@ -68,6 +68,9 @@ executable walint
                       aeson-pretty,
                       bytestring,
                       mtl,
-                      text
+                      text,
+                      template-haskell,
+                      process
     hs-source-dirs:   src
     default-language: Haskell2010
+    other-modules: Version