{
  description = "Example darwin system flake";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs";
    darwin.url = "github:lnl7/nix-darwin";
    darwin.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = { self, darwin, nixpkgs }:
  let
    configuration = { pkgs, ... }: {
      # List packages installed in system profile. To search by name, run:
      # $ nix-env -qaP | grep wget
      environment.systemPackages =
        [ pkgs.vim
        ];

      # Auto upgrade nix package and the daemon service.
      services.nix-daemon.enable = true;
      # nix.package = pkgs.nix;

      # Necessary for using flakes on this system.
      nix.settings.experimental-features = "nix-command flakes";

      # Create /etc/zshrc that loads the nix-darwin environment.
      programs.zsh.enable = true;  # default shell on catalina
      # programs.fish.enable = true;

      # Used for backwards compatibility, please read the changelog before changing.
      # $ darwin-rebuild changelog
      system.stateVersion = 4;
    };
  in
  {
    # Build darwin flake using:
    # $ darwin-rebuild build --flake .#simple
    darwinConfigurations."simple" = darwin.lib.darwinSystem {
      modules = [ configuration ];
      system = "x86_64-darwin";
    };

    # Expose the package set, including overlays, for convenience.
    darwinPackages = self.darwinConfigurations."simple".pkgs;
  };
}