anchor.diff:  in stdenv.mkDerivation (rec {
     inherit version;
     name = "ghc-${version}";
  -  rev = "2689a1692636521777f007861a484e7064b2d696";
  +  rev = "b4f2afe70ddbd0576b4eba3f82ba1ddc52e9b3bd";
   
     src = fetchgit {
       url = "git://git.haskell.org/ghc.git";
       inherit rev;
  -    sha256 = "0rk6xy7kgxx849nprq1ji459p88nyy93236g841m5p6mdh7mmrcr";
  +    sha256 = "1h064nikx5srsd7qvz19f6dxvnpfjp0b3b94xs1f4nar18hzf4j0";
     };
   
     postPatch = "patchShebangs .";
  
positive.code: 
    commonBuildInputs = [ ghc perl autoconf automake happy alex python3 ];
  
    version = "8.1.20161224";
  
    commonPreConfigure =  ''
      sed -i -e 's|-isysroot /Developer/SDKs/MacOSX10.5.sdk||' configure
  
negative.code: } // stdenv.lib.optionalAttrs (cross != null) {
    name = "${cross.config}-ghc-${version}";
  
    preConfigure = commonPreConfigure + ''
      sed 's|#BuildFlavour  = quick-cross|BuildFlavour  = perf-cross|' mk/build.mk.sample > mk/build.mk
    '';
  