Overriding ghc-mod and creating non-hackage nixpkgs.

gistfile1.txt

ben at Bens-MacBook-Pro in ~
$ cat ~/.nixpkgs/haskell/ghc-mod.nix
{ mkDerivation, async, base, bytestring, cabal-helper, cereal
, containers, data-default, deepseq, directory, djinn-ghc, doctest
, emacs, fetchgit, filepath, ghc, ghc-paths, ghc-syb-utils
, haskell-src-exts, hlint, hspec, makeWrapper, monad-control
, monad-journal, mtl, old-time, pretty, process, split, stdenv, syb
, temporary, text, time, transformers, transformers-base
}:
mkDerivation {
  pname = "ghc-mod";
  version = "0";
  src = fetchgit {
    url = "git://github.com/kazu-yamamoto/ghc-mod.git";
    sha256 = "b444ff17d0e97fae5f81dfd4b708ae7c79c833505f8a1b26e004e1106a2569e5";
    rev = "b52c0a5d767282369f2748c5ec070b802ed8e23c";
  };
  isLibrary = true;
  isExecutable = true;
  buildDepends = [
    async base bytestring cabal-helper cereal containers data-default
    deepseq directory djinn-ghc filepath ghc ghc-paths ghc-syb-utils
    haskell-src-exts hlint monad-control monad-journal mtl old-time
    pretty process split syb temporary text time transformers
    transformers-base
  ];
  testDepends = [ base doctest hspec ];
  buildTools = [ emacs makeWrapper ];
  doCheck = false;
  configureFlags = "--datasubdir=ghc-mod-0";
  postInstall = ''
    cd $out/share/ghc-mod-0
    make
    rm Makefile
    cd ..
    ensureDir "$out/share/emacs"
    mv ghc-mod-0 emacs/site-lisp
  '';
  homepage = "http://www.mew.org/~kazu/proj/ghc-mod/";
  description = "Happy Haskell Programming";
  license = stdenv.lib.licenses.agpl3;
}

-- Then override the old ghc-mod in your base config:
ben at Bens-MacBook-Pro in ~
$ less ~/.nixpkgs/config.nix
{ pkgs }: {
  haskellPackageOverrides = with pkgs.haskell-ng.lib; self: super: {
    zip-archive = dontCheck super.zip-archive;
    cabal-helper = dontCheck super.cabal-helper;
    ghc-mod = pkgs.haskellngPackages.callPackage ./haskell/ghc-mod.nix {};
    reflex = pkgs.haskellngPackages.callPackage ./haskell/reflex/default.nix {};
    reflex-dom = pkgs.haskellngPackages.callPackage ./haskell/reflex-dom/default.nix {};
  };
}