anchor.diff:          --add-flags "-f $out/bin/ghc"                   \
           --set "NIX_GHC"        "$out/bin/ghc"           \
           --set "NIX_GHCPKG"     "$out/bin/ghc-pkg"       \
  +        --set "NIX_GHC_DOCDIR" "${docDir}"              \
           --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}"
  +      makeWrapper ${ghc}/bin/$prg $out/bin/$prg --add-flags "${packageDBFlag}=${packageCfgDir}"
       done
   
       $out/bin/ghc-pkg recache
  
positive.code:         --add-flags '"-B$NIX_GHC_LIBDIR"'               \
          --set "NIX_GHC"        "$out/bin/ghc"           \
          --set "NIX_GHCPKG"     "$out/bin/ghc-pkg"       \
          --set "NIX_GHC_LIBDIR" "${libDir}"
      done
  
  
negative.code: { stdenv, ghc, packages, buildEnv, makeWrapper, ignoreCollisions ? false }:
  
  # 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 "--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];
  