pkgs = import <nixpkgs> { overlays = [(self: super:
  {
    haskellPackages = super.haskellPackages.extend (selfHS: superHS: {
      andromeda = superHS.callCabal2nix "andromeda" (super.fetchFromGitHub {
        owner  = "jaredloomis";
        repo   = "andromeda";
        rev    = "...";
        sha256 = "...";
      }) {};
    });
  })];
};