# ~/.nixpkgs/config.nix
pkgs : {
  packageOverrides = pkgs : rec {
    myHaskellPackages =
      let
        callPackage = pkgs.lib.callPackageWith myHaskellPackages;
      in
        pkgs.recurseIntoAttrs (pkgs.haskellPackages.override {
          extension = self : _ : {
            abc = self.callPackage ../path/to/package { haskellPackages = myHaskellPackages; };
          };
        });
  };
}