supporting "lang" command.

GHCMod.hs

@@ -5,6 +5,7 @@ import Check
 import Control.Exception hiding (try)
 import List
 import Param
+import Lang
 import Prelude hiding (catch)
 import System.Console.GetOpt
 import System.Environment (getArgs)
@@ -63,6 +64,7 @@ main = flip catch handler $ do
       "browse" -> browseModule opt (cmdArg !! 1)
       "list"   -> listModules opt
       "check"  -> checkSyntax opt (cmdArg !! 1)
+      "lang"   -> listLanguages opt
       _        -> error usage
     putStr res
   where

Lang.hs

@@ -0,0 +1,14 @@
+module Lang where
+
+import Control.Applicative
+import Param
+import System.IO
+import System.Process
+
+listLanguages :: Options -> IO String
+listLanguages opt = convert opt . lines <$> getLangs opt
+
+getLangs :: Options -> IO String
+getLangs opt = do
+  (_,hout,_,_) <- runInteractiveProcess (ghc opt) ["--supported-languages"] Nothing Nothing
+  hGetContents hout