let
  nixpkgs = import <nixpkgs> {};
  patchRepo =
    nixpkgs.fetchFromGitHub {
      owner = "mpickering";
      repo = "head.hackage";
      rev = "767dcf11b367ccff4a9fcd55df9c2432cd485fbe";
      sha256 = "1cdrcw7grpc2dyxnb7a5bg9mi1h7qnblcibi91s348034zf7a0vj";};

  patchDir = "${patchRepo}/patches";
  patchScript = "${patchRepo}/scripts/overrides.nix";
in
self: super:
{
  patches = super.callPackage patchScript
              { patches = patchDir; };

  new-ghcHEAD =
    let ghcPackageOverrides = super.callPackage self.patches {};
        localOverrides      =
          sel: sup: { haskell-src-exts
                        = sel.callHackage "haskell-src-exts" "1.20.1" {};
                      mkDerivation
                        = drv: sup.mkDerivation (drv //
                             { jailbreak = true; doHaddock = false;});
                     };

    in super.haskell.packages.ghcHEAD.extend
         (self.lib.composeExtensions localOverrides ghcPackageOverrides);
}