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diff --git a/src/CabalHelper/Compiletime/GuessGhc.hs b/src/CabalHelper/Compiletime/GuessGhc.hs
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+-- Copyright (c) 2003-2014, Isaac Jones, Simon Marlow, Martin Sjögren,
+--                          Bjorn Bringert, Krasimir Angelov,
+--                          Malcolm Wallace, Ross Patterson, Ian Lynagh,
+--                          Duncan Coutts, Thomas Schilling,
+--                          Johan Tibell, Mikhail Glushenkov
+-- All rights reserved.
+
+-- Redistribution and use in source and binary forms, with or without
+-- modification, are permitted provided that the following conditions are
+-- met:
+
+--     * Redistributions of source code must retain the above copyright
+--       notice, this list of conditions and the following disclaimer.
+
+--     * Redistributions in binary form must reproduce the above
+--       copyright notice, this list of conditions and the following
+--       disclaimer in the documentation and/or other materials provided
+--       with the distribution.
+
+--     * Neither the name of Isaac Jones nor the names of other
+--       contributors may be used to endorse or promote products derived
+--       from this software without specific prior written permission.
+
+-- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+-- "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+-- LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+-- A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+-- OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+-- SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+-- LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+-- DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+-- THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+-- (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+-- OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+{-|
+Module      : CabalHelper.Compiletime.GuessGhc
+Description : Logic for finding @ghc-pkg@ based on path to @ghc@
+License     : BSD3
+-}
+
+module CabalHelper.Compiletime.GuessGhc (guessToolFromGhcPath) where
+
+import Data.Maybe
+import Data.Char
+import Distribution.Simple.BuildPaths
+import System.Directory
+import System.FilePath
+
+guessToolFromGhcPath :: FilePath -- ^ Tool name
+                     -> FilePath -- ^ GHC exe path
+                     -> IO (Maybe FilePath)
+guessToolFromGhcPath toolname ghcPath
+  = do let
+           path              = ghcPath
+           dir               = takeDirectory path
+           versionSuffix     = takeVersionSuffix (dropExeExtension path)
+           guessNormal       = dir </> toolname <.> exeExtension'
+           guessGhcVersioned = dir </> (toolname ++ "-ghc" ++ versionSuffix)
+                               <.> exeExtension'
+           guessVersioned    = dir </> (toolname ++ versionSuffix)
+                               <.> exeExtension'
+           guesses | null versionSuffix = [guessNormal]
+                   | otherwise          = [guessGhcVersioned,
+                                           guessVersioned,
+                                           guessNormal]
+       exists <- mapM doesFileExist guesses
+       return $ listToMaybe [ file | (file, True) <- zip guesses exists ]
+
+  where takeVersionSuffix :: FilePath -> String
+        takeVersionSuffix = takeWhileEndLE isSuffixChar
+
+        isSuffixChar :: Char -> Bool
+        isSuffixChar c = isDigit c || c == '.' || c == '-'
+
+        dropExeExtension :: FilePath -> FilePath
+        dropExeExtension filepath =
+          case splitExtension filepath of
+            (filepath', extension) | extension == exeExtension' -> filepath'
+                                   | otherwise                 -> filepath
+
+-- | @takeWhileEndLE p@ is equivalent to @reverse . takeWhile p . reverse@, but
+-- is usually faster (as well as being easier to read).
+takeWhileEndLE :: (a -> Bool) -> [a] -> [a]
+takeWhileEndLE p = fst . foldr go ([], False)
+  where
+    go x (rest, done)
+      | not done && p x = (x:rest, False)
+      | otherwise = (rest, True)
+
+exeExtension' :: FilePath
+exeExtension' = Distribution.Simple.BuildPaths.exeExtension