{
  description = "An over-engineered Hello World in C";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";

    treefmt-nix = {
      url = "github:numtide/treefmt-nix";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs =
    {
      self,
      nixpkgs,
      treefmt-nix,
    }:
    let
      # to work with older version of flakes
      lastModifiedDate = self.lastModifiedDate or self.lastModified or "19700101";

      # Generate a user-friendly version number.
      version = builtins.substring 0 8 lastModifiedDate;

      # System types to support.
      supportedSystems = [
        "x86_64-linux"
        "x86_64-darwin"
        "aarch64-linux"
        "aarch64-darwin"
      ];

      forEachSupportedSystem =
        f:
        nixpkgs.lib.genAttrs supportedSystems (
          system:
          f ({
            pkgs = import nixpkgs {
              inherit system;
              overlays = [ self.overlays.default ];
            };
          })
        );
    in
    {
      formatter = forEachSupportedSystem (
        { pkgs, ... }: (treefmt-nix.lib.evalModule pkgs ./treefmt.nix).config.build.wrapper
      );
      devShells = forEachSupportedSystem (
        { pkgs, ... }:
        {
          default = pkgs.mkShell {
            nativeBuildInputs = with pkgs; [
              gcc14
            ];
          };
        }
      );
      packages = forEachSupportedSystem (
        { pkgs, ... }:
        {
          default = pkgs.hello;
        }
      );
      overlays.default = final: prev: {
        hello =
          with final;
          gcc14Stdenv.mkDerivation {
            pname = "main";
            inherit version;

            src = ./.;

            buildPhase = ''
              gcc gcc14 --version
              gcc --std=gnu23 main.c -o main
            '';
            installPhase = ''
              mkdir -p $out/bin
              install ./main $out/bin
            '';
          };
      };

      nixosModules.hello =
        { pkgs, ... }:
        {
          nixpkgs.overlays = [ self.overlay ];

          environment.systemPackages = [ pkgs.hello ];
        };

      # Tests run by 'nix flake check' and by Hydra.
      checks = forEachSupportedSystem (
        { pkgs, ... }:
        with pkgs;
        {
          inherit (self.packages.${system}) hello;

          # Additional tests, if applicable.
          test = stdenv.mkDerivation {
            pname = "hello-test";
            inherit version;

            buildInputs = [ hello ];

            dontUnpack = true;

            buildPhase = ''
              echo 'running some integration tests'
              [[ $(hello) = 'Hello Nixers!' ]]
            '';

            installPhase = "mkdir -p $out";
          };
        }
        // lib.optionalAttrs gcc14stdenv.isLinux {
          # A VM test of the NixOS module.
          vmTest =
            with import (nixpkgs + "/nixos/lib/testing-python.nix") {
              inherit system;
            };

            makeTest {
              nodes = {
                client =
                  { ... }:
                  {
                    imports = [ self.nixosModules.hello ];
                  };
              };

              testScript = ''
                start_all()
                client.wait_for_unit("multi-user.target")
                client.succeed("hello")
              '';
            };
        }
      );
    };
}