~/.nixpkgs/config.nix

config.nix

{
  haskellPackageOverrides = self: super: {
   mkDerivation = expr: super.mkDerivation (expr // { enableLibraryProfiling = true; });
  };

  packageOverrides = super: let self = super.pkgs; in
  {
    haskellEnv = self.haskellngPackages.ghcWithPackages (p: with p; [
      xmonad xmonad-contrib cabal-install cabal2nix
    ]);

    haskellngPackages = super.haskellngPackages.override {
       overrides = self: super: {
         monad-hdbc = self.callPackage ./local/monad-hdbc.nix {};
         scrumgr = self.callPackage ./local/scrumgr.nix {};
         chess = self.callPackage ./local/chess.nix {};
         lagrange = self.callPackage ./local/lagrange.nix {};
       };
    };
  };

}