Bind ghc-modi executable to right ghc-mod exe

ghc-mod.cabal

@@ -184,15 +184,18 @@ Executable ghc-modi
   Main-Is:              GHCModi.hs
   Other-Modules:        Paths_ghc_mod
                         Misc
+                        Utils
   GHC-Options:          -Wall -threaded -fno-warn-deprecations
   if os(windows)
       Cpp-Options:      -DWINDOWS
   Default-Extensions:   ConstraintKinds, FlexibleContexts
-  HS-Source-Dirs:       src
+  HS-Source-Dirs:       src, .
   Build-Depends:        base >= 4.0 && < 5
                       , directory
                       , filepath
                       , process
+                      , time
+                      , old-time
 
 Test-Suite doctest
   Type:                 exitcode-stdio-1.0

src/GHCModi.hs

@@ -5,16 +5,51 @@
 
 module Main where
 
+import Control.Applicative
+import Control.Monad
+import Control.Exception
+import Data.Version
+import Data.Maybe
+import System.IO
 import System.Exit
 import System.Process
 import System.FilePath
 import System.Environment
 import Paths_ghc_mod
+import Utils
+import Prelude
 
 main :: IO ()
 main = do
+  hPutStrLn stderr $
+    "Warning: ghc-modi is deprecated please use 'ghc-mod legacy-interactive' instead"
+
   args <- getArgs
   bindir <- getBinDir
-  (_, _, _, h) <-
-    createProcess $ proc (bindir </> "ghc-mod") $ ["legacy-interactive"] ++ args
-  exitWith =<< waitForProcess h
+  let installedExe = bindir </> "ghc-mod"
+  mexe <- mplus <$> mightExist installedExe <*> pathExe
+  case mexe of
+    Nothing -> do
+      hPutStrLn stderr $
+        "ghc-modi: Could not find '"++installedExe++"', check your installation!"
+      exitWith $ ExitFailure 1
+
+    Just exe -> do
+      (_, _, _, h) <-
+          createProcess $ proc exe $ ["legacy-interactive"] ++ args
+      exitWith =<< waitForProcess h
+
+pathExe :: IO (Maybe String)
+pathExe = do
+  ev <- try $ words <$> readProcess "ghc-mod" ["--version"] ""
+  let mexe = case ev of
+               Left (SomeException _) -> Nothing
+               Right ["ghc-mod", "version", ver
+                     , "compiled", "by", "GHC", _]
+                   | showVersion version == ver -> do
+                       Just "ghc-mod"
+               Right _ -> Nothing
+
+  when (isNothing mexe) $
+      hPutStrLn stderr "ghc-modi: ghc-mod executable on PATH has different version, check your installation!"
+  return mexe