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diff --git a/CabalHelper/Shared/Common.hs b/CabalHelper/Shared/Common.hs
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+-- cabal-helper: Simple interface to Cabal's configuration state
+-- Copyright (C) 2015  Daniel Gröber <dxld ÄT darkboxed DOT org>
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as published by
+-- the Free Software Foundation, either version 3 of the License, or
+-- (at your option) any later version.
+--
+-- This program is distributed in the hope that it will be useful,
+-- but WITHOUT ANY WARRANTY; without even the implied warranty of
+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+-- GNU Affero General Public License for more details.
+--
+-- You should have received a copy of the GNU Affero General Public License
+-- along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+{-# LANGUAGE DeriveDataTypeable, OverloadedStrings #-}
+module CabalHelper.Shared.Common where
+
+import Control.Applicative
+import Control.Exception as E
+import Control.Monad
+import Data.List
+import Data.Maybe
+import Data.Version
+import Data.Typeable
+import Data.ByteString (ByteString)
+import qualified Data.ByteString as BS
+import qualified Data.ByteString.Char8 as BS8
+import System.Environment
+import System.IO
+import System.Exit
+import System.Directory
+import System.FilePath
+import Text.ParserCombinators.ReadP
+import Prelude
+
+data Panic = Panic String deriving (Typeable, Show)
+instance Exception Panic
+
+panic :: String -> a
+panic msg = throw $ Panic msg
+
+handlePanic :: IO a -> IO a
+handlePanic action =
+    action `E.catch` \(Panic msg) -> errMsg msg >> exitFailure
+
+errMsg :: String -> IO ()
+errMsg str = do
+  prog <- getProgName
+  hPutStrLn stderr $ prog ++ ": " ++ str
+
+-- | @getCabalConfigHeader "dist/setup-config"@ returns the cabal version and
+-- compiler version
+getCabalConfigHeader :: FilePath -> IO (Maybe (Version, (ByteString, Version)))
+getCabalConfigHeader file = bracket (openFile file ReadMode) hClose $ \h -> do
+  parseHeader <$> BS.hGetLine h
+
+parseHeader :: ByteString -> Maybe (Version, (ByteString, Version))
+parseHeader header = case BS8.words header of
+  ["Saved", "package", "config", "for", _pkgId ,
+   "written", "by", cabalId,
+   "using", compId]
+    -> liftM2 (,) (snd <$> parsePkgId cabalId) (parsePkgId compId)
+  _ -> Nothing
+
+parsePkgId :: ByteString -> Maybe (ByteString, Version)
+parsePkgId bs =
+    case BS8.split '-' bs of
+      [pkg, vers] -> Just (pkg, parseVer $ BS8.unpack vers)
+      _ -> Nothing
+
+parseVer :: String -> Version
+parseVer vers = runReadP parseVersion vers
+
+majorVer :: Version -> Version
+majorVer (Version b _) = Version (take 2 b) []
+
+sameMajorVersionAs :: Version -> Version -> Bool
+sameMajorVersionAs a b = majorVer a == majorVer b
+
+runReadP :: ReadP t -> String -> t
+runReadP p i = case filter ((=="") . snd) $ readP_to_S p i of
+                 (a,""):[] -> a
+                 _ -> error $ "Error parsing: " ++ show i
+
+appDataDir :: IO FilePath
+appDataDir = (</> "cabal-helper") <$> getAppUserDataDirectory "ghc-mod"
+
+isCabalFile :: FilePath -> Bool
+isCabalFile f = takeExtension' f == ".cabal"
+
+takeExtension' :: FilePath -> String
+takeExtension' p =
+    if takeFileName p == takeExtension p
+      then "" -- just ".cabal" is not a valid cabal file
+      else takeExtension p
+
+replace :: String -> String -> String -> String
+replace n r hs' = go "" hs'
+ where
+   go acc h
+       | take (length n) h == n =
+           reverse acc ++ r ++ drop (length n) h
+   go acc (h:hs) = go (h:acc) hs
+   go acc [] = reverse acc