diff -Naur ghc-paths-0.1.0.9/GHC/Paths.hs ghc-paths-0.1.0.9-new/GHC/Paths.hs
--- ghc-paths-0.1.0.9/GHC/Paths.hs	2012-12-16 13:53:45.720148396 +0100
+++ ghc-paths-0.1.0.9-new/GHC/Paths.hs	2012-12-16 17:22:12.765576568 +0100
@@ -1,13 +1,35 @@
 {-# LANGUAGE CPP #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 
 module GHC.Paths (
         ghc, ghc_pkg, libdir, docdir
   ) where
 
+import Control.Exception as E
+import Data.Maybe
+import System.Environment
+import System.IO.Unsafe
+
+-- Yes, there's lookupEnv now, but we want to be compatible
+-- with older GHCs.
+checkEnv :: String -> IO (Maybe String)
+checkEnv var = E.catch (fmap Just (getEnv var))
+                       (\ (e :: IOException) -> return Nothing)
+
+nixLibdir, nixDocdir, nixGhc, nixGhcPkg :: Maybe FilePath
+nixLibdir = unsafePerformIO (checkEnv "NIX_GHC_LIBDIR")
+nixDocdir = unsafePerformIO (checkEnv "NIX_GHC_DOCDIR")
+nixGhc    = unsafePerformIO (checkEnv "NIX_GHC")
+nixGhcPkg = unsafePerformIO (checkEnv "NIX_GHCPKG")
+{-# NOINLINE nixLibdir #-}
+{-# NOINLINE nixDocdir #-}
+{-# NOINLINE nixGhc    #-}
+{-# NOINLINE nixGhcPkg #-}
+
 libdir, docdir, ghc, ghc_pkg :: FilePath
 
-libdir  = GHC_PATHS_LIBDIR
-docdir  = GHC_PATHS_DOCDIR
+libdir  = fromMaybe GHC_PATHS_LIBDIR  nixLibdir
+docdir  = fromMaybe GHC_PATHS_DOCDIR  nixDocdir
 
-ghc     = GHC_PATHS_GHC
-ghc_pkg = GHC_PATHS_GHC_PKG
+ghc     = fromMaybe GHC_PATHS_GHC     nixGhc
+ghc_pkg = fromMaybe GHC_PATHS_GHC_PKG nixGhcPkg