{
  lib,
  config,
  inputs,
  ...
}:
let
  inherit (lib) mkOption types;
  inherit (lib.types)
    attrsOf
    attrTag
    deferredModuleWith
    submoduleWith
    submodule
    ;

  functionType = import ./interface.nix;
  application-resources = {
    options.resources = mkOption {
      type = attrsOf (
        attrTag (
          lib.mapAttrs' (name: resource: {
            ${name} = mkOption { type = resource.consumer; };
          }) config.resources
        )
      );
    };
  };
  nixops4Deployment = inputs.nixops4.modules.nixops4Deployment.default;
in
{
  options = {
    resources = mkOption {
      description = "Collection of deployment resources that can be configured by hosting providers and required by applications";
      type = attrsOf (
        submodule (
          { config, ... }:
          {
            _class = "fediversity-resource";
            options = {
              consumer = mkOption {
                description = "Configuration of the resource by an application, a description of how the resource is consumed";
                type = deferredModuleWith { staticModules = [ { _class = "fediversity-resource-consumer"; } ]; };
              };
              provider = mkOption {
                description = "Configuration of the resource by the hosting provider, a description of how the resource is made available";
                type = deferredModuleWith { staticModules = [ { _class = "fediversity-resource-provider"; } ]; };
              };
            };
          }
        )
      );
    };
    applications = mkOption {
      description = "Collection of Fediversity applications";
      type = attrsOf (
        submodule (application: {
          _class = "fediversity-application";
          options = {
            module = mkOption {
              description = "Operator-facing configuration options for the application";
              type = deferredModuleWith { staticModules = [ { _class = "fediversity-application-config"; } ]; };
            };
            config-mapping = mkOption {
              description = "Mapping of application configuration to deployment resources, a description of what an application needs to run";
              type = application.config.function.implementation;
            };
            # TODO: somewhere we still need to
            # - apply = { implementation = config-mapping; input = <application configuration value from operator>; }
            # - apply.output (applications required by resources)
            function = mkOption {
              description = "Function type for the mapping from application configuration to required resources";
              type = submodule functionType;
              readOnly = true;
              default = {
                input-type = application.config.module;
                output-type = application-resources;
              };
            };
          };
        })
      );
    };
    environments = mkOption {
      description = "Run-time environments for Fediversity applications to be deployed to";
      type = attrsOf (
        submodule (environment: {
          _class = "fediversity-environment";
          options = {
            resources = mkOption {
              description = "Resources made available by the hosting provider";
              type = attrsOf (
                attrTag (
                  lib.mapAttrs' (name: resource: {
                    ${name} = mkOption { type = resource.provider; };
                  }) config.resources
                )
              );
            };
            resource-mapping = mkOption {
              description = "Mapping of resources required by applications to available resources; the result can be deployed";
              type = environment.config.function.implementation;
            };
            # TODO: somewhere we still need to
            # - apply = { implementation = resource-mapping; input = <resource requirements of configured applications>; }
            # - apply.output (deployment)
            function = mkOption {
              description = "Function type for the mapping from resources to a (NixOps4) deployment";
              type = submodule functionType;
              readOnly = true;
              default = {
                input-type = application-resources;
                output-type = submodule nixops4Deployment;
              };
            };
          };
        })
      );
    };
    configurations = mkOption {
      description = "Application configurations set by operators";
      type = attrsOf (submodule {
        options = {
          enable = mkOption {
            description = "Whether to enable the configuration";
            type = types.bool;
          };
          applications = mapAttrs (name: application: submodule application.module) config.applications;
        };
      });
    };
  };
}