anchor.diff:  { stdenv, ghc, packages, buildEnv, makeWrapper, ignoreCollisions ? false }:
   
  -assert packages != [];
  +# This wrapper works only with GHC 6.12 or later.
  +assert stdenv.lib.versionOlder "6.12" ghc.version;
  +
  +# It's probably a good idea to include the library "ghc-paths" in the
  +# compiler environment, because we have a specially patched version of
  +# that package in Nix that honors these environment variables
  +#
  +#   NIX_GHC
  +#   NIX_GHCPKG
  +#   NIX_GHC_DOCDIR
  +#   NIX_GHC_LIBDIR
  +#
  +# instead of hard-coding the paths. The wrapper sets these variables
  +# appropriately to configure ghc-paths to point back to the wrapper
  +# instead of to the pristine GHC package, which doesn't know any of the
  +# additional libraries.
  +#
  +# A good way to import the environment set by the wrapper below into
  +# your shell is to add the following snippet to your ~/.bashrc:
  +#
  +#   if [ -e ~/.nix-profile/bin/ghc ]; then
  +#     eval $(grep export ~/.nix-profile/bin/ghc)
  +#   fi
   
   let
     ghc761OrLater = stdenv.lib.versionOlder "7.6.1" ghc.version;
  -  packageDBFlag = if ghc761OrLater then "--package-db" else "--package-conf";
  +  packageDBFlag = if ghc761OrLater then "--global-package-db" else "--global-conf";
     libDir        = "$out/lib/ghc-${ghc.version}";
  +  docDir        = "$out/share/doc/ghc/html";
     packageCfgDir = "${libDir}/package.conf.d";
     isHaskellPkg  = x: (x ? pname) && (x ? version);
   in
  +if packages == [] then ghc else
   buildEnv {
     name = "haskell-env-${ghc.name}";
     paths = stdenv.lib.filter isHaskellPkg (stdenv.lib.closePropagation packages) ++ [ghc];
  
positive.code:         --add-flags "-f $out/bin/ghc"                   \
          --set "NIX_GHC"        "$out/bin/ghc"           \
          --set "NIX_GHCPKG"     "$out/bin/ghc-pkg"       \
          --set "NIX_GHC_LIBDIR" "${libDir}"
      done
  
      for prg in ghc-pkg ghc-pkg-${ghc.version}; do
        rm -f $out/bin/$prg
        makeWrapper ${ghc}/bin/$prg $out/bin/$prg --add-flags "${packageDBFlag} ${packageCfgDir}"
      done
  
      $out/bin/ghc-pkg recache
  
negative.code:         --add-flags '"-B$NIX_GHC_LIBDIR"'               \
          --set "NIX_GHC"        "$out/bin/ghc"           \
          --set "NIX_GHCPKG"     "$out/bin/ghc-pkg"       \
          --set "NIX_GHC_DOCDIR" "${docDir}"              \
          --set "NIX_GHC_LIBDIR" "${libDir}"
      done
  
  