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

  functionType = import ./function.nix;
  application-resources = {
    options.resources = mkOption {
      # TODO: maybe transpose, and group the resources by type instead
      type = attrsOf (
        attrTag (lib.mapAttrs (_name: resource: mkOption { type = resource.request; }) config.resources)
      );
    };
  };
in
{
  _class = "nixops4Deployment";

  options = {
    applications = mkOption {
      description = "Collection of Fediversity applications";
      type = attrsOf (
        submodule (application: {
          _class = "fediversity-application";
          options = {
            description = mkOption {
              description = "Description to be shown in the application overview";
              type = types.str;
            };
            module = mkOption {
              description = "Operator-facing configuration options for the application";
              type = deferredModuleWith { staticModules = [ { _class = "fediversity-application-config"; } ]; };
            };
            implementation = mkOption {
              description = "Mapping of application configuration to deployment resources, a description of what an application needs to run";
              type = application.config.config-mapping.function-type;
            };
            resources = mkOption {
              description = "Compute resources required by an application";
              type = functionTo application.config.config-mapping.output-type;
              readOnly = true;
              default = input: (application.config.implementation input).output;
            };
            config-mapping = 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;
              };
            };
          };
        })
      );
    };
    configuration = mkOption {
      description = "Configuration type declaring options to be set by operators";
      type = optionType;
      readOnly = true;
      default = submodule {
        options = {
          enable = lib.mkEnableOption {
            description = "your Fediversity configuration";
          };
          applications = lib.mapAttrs (
            _name: application:
            mkOption {
              description = application.description;
              type = submodule application.module;
              default = { };
            }
          ) config.applications;
        };
      };
    };
    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";
              # TODO: type = consumer-resources /* same as the output of application.config-mapping, should be in a `let` */ -> nixops4Deployment
            };
          };
        })
      );
    };
  };
}