Add --version CLI option (#106)

* Add --version CLI option

* Extract ghcide version from cabal

* Extract precise GHC version from cabal preprocessor macro

Author: jacg

exe/Arguments.hs

@@ -10,6 +10,7 @@ data Arguments = Arguments
     {argLSP :: Bool
     ,argsCwd :: Maybe FilePath
     ,argFiles :: [FilePath]
+    ,argsVersion :: Bool
     }
 
 getArguments :: IO Arguments
@@ -25,3 +26,4 @@ arguments = Arguments
       <$> switch (long "lsp" <> help "Start talking to an LSP server")
       <*> optional (strOption $ long "cwd" <> metavar "DIR" <> help "Change to this directory")
       <*> many (argument str (metavar "FILES/DIRS..."))
+      <*> switch (long "version" <> help "Show ghcide and GHC versions")

exe/Main.hs

@@ -1,6 +1,7 @@
 -- Copyright (c) 2019 The DAML Authors. All rights reserved.
 -- SPDX-License-Identifier: Apache-2.0
 {-# OPTIONS_GHC -Wno-dodgy-imports #-} -- GHC no longer exports def in GHC 8.6 and above
+{-# LANGUAGE CPP #-} -- To get precise GHC version
 
 module Main(main) where
 
@@ -29,12 +30,13 @@ import qualified Data.Text as T
 import qualified Data.Text.IO as T
 import Language.Haskell.LSP.Messages
 import Linker
-import System.Info
 import Data.Version
 import Development.IDE.LSP.LanguageServer
 import System.Directory.Extra as IO
 import System.Environment
 import System.IO
+import System.Exit
+import Paths_ghcide
 import Development.Shake hiding (Env)
 import qualified Data.Set as Set
 
@@ -47,13 +49,19 @@ import HIE.Bios
 getLibdir :: IO FilePath
 getLibdir = fromMaybe GHC.Paths.libdir <$> lookupEnv "NIX_GHC_LIBDIR"
 
+ghcideVersion :: String
+ghcideVersion = "ghcide version: " <> showVersion version
+             <> " (GHC: "          <> VERSION_ghc <> ")"
+
 main :: IO ()
 main = do
     -- WARNING: If you write to stdout before runLanguageServer
     --          then the language server will not work
-    hPutStrLn stderr $ "Starting ghcide (GHC v" ++ showVersion compilerVersion ++ ")"
     Arguments{..} <- getArguments
 
+    if argsVersion then putStrLn ghcideVersion >> exitSuccess
+    else hPutStrLn stderr {- see WARNING above -} ghcideVersion
+
     -- lock to avoid overlapping output on stdout
     lock <- newLock
     let logger = Logger $ \pri msg -> withLock lock $