Basic example of Nix modules

flake.nix

{
  description = "A very basic example of modules";

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

  outputs = { self, nixpkgs }: {
    # nix eval .#.lib.boo.config
    lib = {
        boo = nixpkgs.lib.evalModules {
            modules = [
                ({lib, ...} : {
                    options =
                    {
                     a = lib.mkOption { type = lib.types.str; };
                     b = lib.mkOption { type = lib.types.str; };
                     c = lib.mkOption { type = lib.types.str; };
                    };
                })

                ({lib, ...} : {
                    config = { a = "aaa"; };
                })

                ({lib, ...} : {
                    # https://nix.dev/tutorials/module-system/index.html
                    b = "bbb";
                })

                ({
                    # https://nix.dev/tutorials/module-system/index.html
                    c = "ccc";
                })
            ];
        };
        # nix eval .#.lib.ty
        ty = nixpkgs.lib.attrNames self.lib.boo.type;
    };

  };
}