{
  description = "Generate documentation for your own projects using the NixOS module system";

  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-21.05";

  outputs = { self, nixpkgs }:
    let pkgs = nixpkgs.legacyPackages.x86_64-linux;
    in
    {

      lib.modules = {
        doc-options-md = import ./doc-options-md.nix;
        mdbook = import ./mdbook.nix;
        manpage = import ./manpage.nix;
      };

      checks.x86_64-linux =
        let
          evalModules = modules: pkgs.lib.evalModules {
            modules = [
              ({ config._module.args = { inherit pkgs; }; })
            ] ++ modules;
          };
          simpleModule = with pkgs.lib; {
            options.my.simple.module.outputs = mkOption {
              type = with types; attrsOf package;
              default = { };
              description = ''
                Output products of my simple module system.
              '';
            };
          };
          params = {
            outputAttrPath = [ "my" "simple" "module" "outputs" ];
            optionsAttrPath = [ "my" "simple" "module" "doc" ];
            optionsInternal = false;
          };

          simple-manpage = {
            name = "my simple module system";
            shortDescription = "A sample module system";
          };

          advanced-manpage = {
            name = "my simple module system";
            section = 5;
            shortDescription = "A sample module system";
            description = ''
              This is a very advanced module system, for advanced people.
            '';

            textBefore = ''
              # A SECTION BEFORE

              This is a section before the options.
            '';

            textAfter = ''
              # A SECTION AFTER

              This is a section after the options.
            '';
          };
        in
        {
          simple-doc-options-md = (evalModules [
            simpleModule
            (self.lib.modules.doc-options-md params)
          ]).config.my.simple.module.outputs.doc-options-md;

          simple-manpage = (evalModules [
            simpleModule
            (self.lib.modules.doc-options-md params)
            (self.lib.modules.manpage params)
            {
              my.simple.module.doc.manpage = simple-manpage;
            }
          ]).config.my.simple.module.outputs.manpage;

          advanced-manpage = (evalModules [
            simpleModule
            (self.lib.modules.doc-options-md params)
            (self.lib.modules.manpage params)
            {
              my.simple.module.doc.manpage = advanced-manpage;
            }
          ]).config.my.simple.module.outputs.manpage;

          simple-mdbook = (evalModules [
            simpleModule
            (self.lib.modules.doc-options-md params)
            (self.lib.modules.mdbook params)
            {
              my.simple.module.doc.mdbook.src = ./checks/simple-mdbook;
            }
          ]).config.my.simple.module.outputs.mdbook;

        };

      devShell.x86_64-linux = pkgs.mkShell {
        nativeBuildInputs = with pkgs; [ mdbook pandoc ];
      };
    };
}