An option declaration specifies the name, type and description of a NixOS configuration option. It is invalid to define an option that hasn’t been declared in any module. An option declaration generally looks like this: options = { name = mkOption { type = type specification; default = default value; example = example value; description = "Description for use in the NixOS manual."; }; }; The function mkOption accepts the following arguments. type The type of the option (see ). It may be omitted, but that’s not advisable since it may lead to errors that are hard to diagnose. default The default value used if no value is defined by any module. A default is not required; in that case, if the option value is never used, an error will be thrown. example An example value that will be shown in the NixOS manual. description A textual description of the option, in DocBook format, that will be included in the NixOS manual.