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 13:51:50.073070123 +0100
@@ -4,10 +4,24 @@
         ghc, ghc_pkg, libdir, docdir
   ) where
 
+import Data.Maybe
+import System.Environment
+import System.IO.Unsafe
+
+nixLibdir, nixDocdir, nixGhc, nixGhcPkg :: Maybe FilePath
+nixLibdir = unsafePerformIO (lookupEnv "NIX_GHC_LIBDIR")
+nixDocdir = unsafePerformIO (lookupEnv "NIX_GHC_DOCDIR")
+nixGhc    = unsafePerformIO (lookupEnv "NIX_GHC")
+nixGhcPkg = unsafePerformIO (lookupEnv "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