diff --git a/hindent.cabal b/hindent.cabal
--- a/hindent.cabal
+++ b/hindent.cabal
@@ -1,5 +1,5 @@
 name:                hindent
-version:             4.2.0
+version:             4.2.1
 synopsis:            Extensible Haskell pretty printer
 description:         Extensible Haskell pretty printer. Both a library and an executable.
                      .
@@ -41,6 +41,7 @@
                    , hindent
                    , text
                    , descriptive >= 0.0.2 && < 0.1
+                   , haskell-src-exts
 
 executable hindent-generate-tests
   hs-source-dirs:    src/main
diff --git a/src/HIndent.hs b/src/HIndent.hs
--- a/src/HIndent.hs
+++ b/src/HIndent.hs
@@ -43,9 +43,11 @@
 import           Data.Maybe (fromMaybe)
 
 -- | Format the given source.
-reformat :: Style -> Text -> Either String Builder
-reformat style x =
-  case parseModuleWithComments parseMode
+reformat :: Style -> Maybe [Extension] -> Text -> Either String Builder
+reformat style mexts x =
+  case parseModuleWithComments (case mexts of
+                                  Just exts -> parseMode {extensions = exts}
+                                  Nothing -> parseMode)
                                (T.unpack x) of
     ParseOk (m,comments) ->
       let (cs,ast) =
@@ -81,7 +83,7 @@
 test :: Style -> Text -> IO ()
 test style =
   either error (T.putStrLn . T.toLazyText) .
-  reformat style
+  reformat style Nothing
 
 -- | Test with all styles, prints to stdout.
 testAll :: Text -> IO ()
diff --git a/src/main/Main.hs b/src/main/Main.hs
--- a/src/main/Main.hs
+++ b/src/main/Main.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE PatternGuards #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ViewPatterns #-}
 
@@ -10,6 +11,7 @@
 import           HIndent
 
 import           Control.Applicative
+import           Data.List
 import           Data.Text (Text)
 import qualified Data.Text as T
 import qualified Data.Text.Lazy.Builder as T
@@ -17,6 +19,7 @@
 import           Data.Version (showVersion)
 import           Descriptive
 import           Descriptive.Options
+import           Language.Haskell.Exts.Annotated hiding (Style)
 import           Paths_hindent (version)
 import           System.Environment
 
@@ -29,19 +32,63 @@
          error (T.unpack (textDescription (describe options [])))
        Right result ->
          case result of
-           Left{} -> putStrLn ("hindent " ++ showVersion version)
-           Right style -> T.interact
-                            (either error T.toLazyText .
-                             reformat style)
+           Left{} ->
+             putStrLn ("hindent " ++ showVersion version)
+           Right (style,exts) ->
+             T.interact
+               (either error T.toLazyText .
+                reformat style (Just exts))
 
 -- | Program options.
-options :: Consumer [Text] Option (Either Text (Style))
+options :: Consumer [Text] Option (Either Text (Style,[Extension]))
 options =
   fmap Left (constant "--version") <|>
-  (fmap Right
-        (constant "--style" *>
-         foldr1 (<|>)
-                (map (\style ->
-                        fmap (const style)
-                             (constant (styleName style)))
-                     styles)))
+  (fmap Right ((,) <$> style <*> exts))
+  where style =
+          constant "--style" *>
+          foldr1 (<|>)
+                 (map (\style ->
+                         fmap (const style)
+                              (constant (styleName style)))
+                      styles)
+        exts =
+          fmap getExtensions (many (prefix "X" "Language extension"))
+
+--------------------------------------------------------------------------------
+-- Extensions stuff stolen from hlint
+
+-- | Consume an extensions list from arguments.
+getExtensions :: [Text] -> [Extension]
+getExtensions = foldl f defaultExtensions . map T.unpack
+  where f _ "Haskell98" = []
+        f a ('N':'o':x)
+          | Just x' <- readExtension x =
+            delete x' a
+        f a x
+          | Just x' <- readExtension x =
+            x' :
+            delete x' a
+        f _ x = error $ "Unknown extension: " ++ x
+
+-- | Parse an extension.
+readExtension :: String -> Maybe Extension
+readExtension x =
+  case classifyExtension x of
+    UnknownExtension _ -> Nothing
+    x' -> Just x'
+
+-- | Default extensions.
+defaultExtensions :: [Extension]
+defaultExtensions =
+  [e | e@EnableExtension{} <- knownExtensions] \\
+  map EnableExtension badExtensions
+
+-- | Extensions which steal too much syntax.
+badExtensions :: [KnownExtension]
+badExtensions =
+    [Arrows -- steals proc
+    ,TransformListComp -- steals the group keyword
+    ,XmlSyntax, RegularPatterns -- steals a-b
+    ,UnboxedTuples -- breaks (#) lens operator
+    ,QuasiQuotes -- breaks [x| ...], making whitespace free list comps break
+    ]
diff --git a/src/main/TestGenerate.hs b/src/main/TestGenerate.hs
--- a/src/main/TestGenerate.hs
+++ b/src/main/TestGenerate.hs
@@ -58,7 +58,7 @@
 expectationFileContents :: HIndent.Style -> String -> String
 expectationFileContents style contents =
   let testDecls = parsePieces contents
-      fmt input = L.unpack $ L.toLazyText $ case HIndent.reformat style $ L.pack input of
+      fmt input = L.unpack $ L.toLazyText $ case HIndent.reformat style Nothing $ L.pack input of
                                               Left err      -> error err
                                               Right builder -> builder
       outputs = map (replaceEmptyNewlines . fmt) testDecls