diff --git a/HScope.hs b/HScope.hs
--- a/HScope.hs
+++ b/HScope.hs
@@ -20,11 +20,14 @@
 import Data.Generics.Uniplate.Data (transformBiM)
 import System.Directory (doesFileExist)
 import Data.Data
+import Language.Preprocessor.Cpphs (runCpphs, defaultCpphsOptions,  CpphsOptions(..))
 
-data Flag = Build Bool | File FilePath | Line | Query String | CPPInclude String deriving (Show)
+data Flag = Build Bool | File FilePath | Line | Query String
+                | CPPInclude String | OExtension String deriving (Show)
 
 data Config = Config { cBuild :: Bool, cFile :: FilePath, cLine :: Bool
-                        , cQuery :: Maybe String, cCPPIncludes :: [String] } deriving Show
+                        , cQuery :: Maybe String, cCPPIncludes :: [String]
+                        , cExtensions :: [String] } deriving Show
 
 data IType = Definition | Call deriving (Enum, Show, Eq)
 data Info = Info IType String Int B.ByteString deriving Show
@@ -53,25 +56,31 @@
             , Option ['f'] [ "file" ] (ReqArg File "REFFILE") $
                 "Use REFFILE as the cross-reference file name instead of the default"
                     ++ " \"hscope.out\""
-            , Option ['1'] [ "definition" ] (ReqArg (Query . ('1':)) "SYMBOL") $ "Find the definition of the SYMBOL"
-            , Option ['3'] [ "callers" ] (ReqArg (Query . ('3':)) "SYMBOL") $ "Find SYMBOLs calling this SYMBOL"
+            , Option ['1'] [ "definition" ] (ReqArg (Query . ('1':)) "SYMBOL")
+                    $ "Find the definition of the SYMBOL"
+            , Option ['3'] [ "callers" ] (ReqArg (Query . ('3':)) "SYMBOL")
+                    $ "Find SYMBOLs calling this SYMBOL"
             , Option ['4'] [ "text" ] (ReqArg (Query . ('4':)) "TEXT") $ "Find TEXT in files"
-            , Option ['7'] [ "file" ] (ReqArg (Query . ('7':)) "FILE") $ "Find files matching FILE"
-            , Option ['I'] [ "cpp-include" ] (ReqArg CPPInclude "DIRECTORY") $ "Include path for CPP preprocessor"
+            , Option ['7'] [ "file" ] (ReqArg (Query . ('7':)) "FILE")
+                    $ "Find files matching FILE"
+            , Option ['I'] [ "cpp-include" ] (ReqArg CPPInclude "DIRECTORY")
+                    $ "Include path for CPP preprocessor"
+            , Option ['X'] [ "extension" ] (ReqArg OExtension "EXTENSION") $ "Add GHC extension"
           ]
 
 parseFlags :: [Flag] -> Config
-parseFlags = foldl' go $ Config False "hscope.out" False Nothing [] where
+parseFlags = foldl' go $ Config False "hscope.out" False Nothing [] [] where
     go c (Build b) = c { cBuild = b }
     go c (File f) = c { cFile = f }
     go c Line = c { cLine = True }
     go c (Query q) = c { cQuery = Just q }
     go c (CPPInclude i) = c { cCPPIncludes = i:(cCPPIncludes c) }
+    go c (OExtension i) = c { cExtensions = i:(cExtensions c) }
 
 addInfo :: Lines -> IType -> Name SrcSpanInfo -> CDBMake
 addInfo vec ity n = cdbAdd f $ encode $ Info ity (fileName src) (fst lp) (snd lp)
     where l = startLine src
-          lp = maybe (error $ "Bad line: " ++ show n ++ ", " ++ show vec) id $ vec V.!? (l - 1)
+          lp = maybe (error $ "Bad line: " ++ show n ++ ", " ++ show vec) id $ vec V.!? (l - 2)
           (src, f) = case n of
             (Ident src' f') -> (src', f')
             (Symbol src' f') -> (src', f')
@@ -104,31 +113,30 @@
 
 mapLines :: Show a => FilePath -> [a] -> [String] -> [a]
 mapLines f to = reverse . snd . foldl' go ((to, True), []) where
-    go ((xs@(x:_), _), res) ('#':str) = ((xs, isInfixOf f str), x:res)
-    go (((x:xs), True), res) _ = ((xs, True), x:res)
-    go ((xs@(x:_), False), res) _ = ((xs, False), x:res)
-    go t l = error $ "mapLines: " ++ show t ++ " at " ++ l
+    go t ('#':str) = let ((xs, _), res) = ret t in ((xs, isInfixOf f str), res)
+    go t _ = ret t
+    ret ((a@(x:xs), b), res) = ((if b then xs else a, b), x:res)
+    ret t = t
 
 preprocess :: [String] -> FilePath -> IO (String, Lines)
 preprocess idirs f = do
-    origs <- B.lines <$> (liftIO $ B.readFile f)
-    flns <- fmap lines $ readProcess "cpp" (incs ++ [ "-traditional-cpp", f ]) ""
-    return (fconts flns, V.fromList $ mapLines f (zip [1 ..] origs) flns)
+    rf <- B.readFile f
+    flns <- fmap lines $ runCpphs cpphsOpts f $ B.unpack rf
+    return (fconts flns, V.fromList $ mapLines f (zip [1 ..] $ B.lines rf) flns)
     where fconts = intercalate "\n" . map cmnt
           cmnt ('#':_) = ""
           cmnt x = x
-          incs = concatMap (\i -> [ "-I", i ]) idirs
+          cpphsOpts = defaultCpphsOptions { includes = idirs }
 
-parseCurrentFile :: FilePath -> String -> Either String (Module SrcSpanInfo)
-parseCurrentFile f fstr = go [] where
-    go exts = case parseFileContentsWithMode (pmode exts) fstr of
+parseCurrentFile :: FilePath -> String -> [Extension] -> Either String (Module SrcSpanInfo)
+parseCurrentFile f fstr exts = case parseFileContentsWithMode (pmode exts) fstr of
         ParseOk m -> Right m
         ParseFailed src str -> let (ext, rest) = break (== ' ') str
                 in if rest == " is not enabled"
-                   then go ((classifyExtension ext):exts)
-                   else Left $ str ++ " for " ++ f ++ " at " ++ show src
-    pmode exts = defaultParseMode { fixities = Just []
-                            , extensions = exts
+                   then parseCurrentFile f fstr ((classifyExtension ext):exts)
+                   else Left $ str ++ " for " ++ f ++ " at " ++ show src ++ " with " ++ show exts
+    where pmode exs = defaultParseMode { fixities = Just []
+                            , extensions = exs
                             , parseFilename = f }
 
 buildOne :: Config -> FilePath -> CDBMake
@@ -136,6 +144,7 @@
     cdbAdd "0_hs_files" f
     (fstr, lns) <- liftIO $ preprocess (cCPPIncludes cfg) f
     either (liftIO . putStrLn) (go lns) $ parseCurrentFile f fstr
+        $ map classifyExtension $ cExtensions cfg
     where go lns modul = do
                     traverseAST (handleDefinitions lns) modul
                     traverseAST (handleCalls lns) modul
diff --git a/hscope.cabal b/hscope.cabal
--- a/hscope.cabal
+++ b/hscope.cabal
@@ -1,5 +1,5 @@
 Name:           hscope
-Version:        0.1.2
+Version:        0.2
 License:        BSD3
 License-File:   COPYING
 Category:       source-tools
@@ -38,7 +38,7 @@
     Main-Is: HScope.hs
     GHC-Options: -Wall
     Build-Depends: base (< 5), hs-cdb, haskell-src-exts, mtl, uniplate, cereal
-                             , vector, bytestring, process, directory
+                             , vector, bytestring, process, directory, cpphs
     Default-Language: Haskell2010
 
 test-suite Build
diff --git a/t/Build.hs b/t/Build.hs
--- a/t/Build.hs
+++ b/t/Build.hs
@@ -23,7 +23,7 @@
 
 main :: IO ()
 main = withTemporaryDirectory "/tmp/hscope_test_XXXXXX" $ \td -> testSimpleMain $ do
-    plan 45
+    plan 46
     hpath <- liftIO $ canonicalizePath "./dist/build/hscope/hscope"
     tfile <- liftIO $ canonicalizePath "./t/files/Simple.hs"
     ec1 <- liftIO $ system $ "cd " ++ td ++ " && " ++ hpath ++ " -b " ++ tfile
@@ -136,5 +136,9 @@
     res19 <- liftIO $ readProcess hpath [ "-d", "-f", td ++ "/foo.out"
                                             , "-3", "==>" ] ""
     like res19 "0 ==>"
+
+    res20 <- liftIO $ readProcess hpath [ "--build", "-f", td ++ "/foo.out"
+                            , "-X", "MagicHash", "t/files/Unbox.hs" ] ""
+    is res20 $ ""
 
     return ()