How to use the stone.backends.swift_helpers.fmt_var function in stone

if data_type.doc:
            doc = self.process_doc(data_type.doc, self._docf)
        else:
            doc = 'The {} struct'.format(fmt_class(data_type.name))
        self.emit_wrapped_text(doc, prefix='/// ', width=120)
        protocols = []
        if not data_type.parent_type:
            protocols.append('CustomStringConvertible')

        with self.class_block(data_type, protocols=protocols):
            for field in data_type.fields:
                fdoc = self.process_doc(field.doc,
                    self._docf) if field.doc else undocumented
                self.emit_wrapped_text(fdoc, prefix='/// ', width=120)
                self.emit('public let {}: {}'.format(
                    fmt_var(field.name),
                    fmt_type(field.data_type),
                ))
            self._generate_struct_init(namespace, data_type)

            decl = 'open var' if not data_type.parent_type else 'open override var'

            with self.block('{} description: String'.format(decl)):
                cls = fmt_class(data_type.name) + 'Serializer'
                self.emit('return "\\(SerializeUtil.prepareJSONForSerialization' +
                          '({}().serialize(self)))"'.format(cls))

        self._generate_struct_class_serializer(namespace, data_type)

rtype = fmt_serial_type(route.result_data_type)
        etype = fmt_serial_type(route.error_data_type)

        self._maybe_generate_deprecation_warning(route)

        with self.function_block('@discardableResult open func {}'.format(func_name),
                args=self._func_args(arg_list, force_first=False),
                return_type='{}<{}, {}>'.format(req_obj_name, rtype, etype)):
            self.emit('let route = {}.{}'.format(fmt_class(namespace.name), func_name))
            if is_struct_type(route.arg_data_type):
                args = [(name, name) for name, _ in self._struct_init_args(route.arg_data_type)]
                func_args += [('serverArgs', '{}({})'.format(arg_type, self._func_args(args)))]
                self.emit('let serverArgs = {}({})'.format(arg_type, self._func_args(args)))
            elif is_union_type(route.arg_data_type):
                self.emit('let serverArgs = {}'.format(fmt_var(route.arg_data_type.name)))

            if not is_void_type(route.arg_data_type):
                return_args += [('serverArgs', 'serverArgs')]

            return_args += client_args

            txt = 'return client.request({})'.format(
                self._func_args(return_args, not_init=True)
            )

            self.emit(txt)
        self.emit()

def _generate_enumerated_subtype_serializer(self, namespace,  # pylint: disable=unused-argument
            data_type):
        with self.block('switch value'):
            for tags, subtype in data_type.get_all_subtypes_with_tags():
                assert len(tags) == 1, tags
                tag = tags[0]
                tagvar = fmt_var(tag)
                self.emit('case let {} as {}:'.format(
                    tagvar,
                    fmt_type(subtype)
                ))

                with self.indent():
                    block_txt = 'for (k, v) in Serialization.getFields({}.serialize({}))'.format(
                        fmt_serial_obj(subtype),
                        tagvar,
                    )
                    with self.block(block_txt):
                        self.emit('output[k] = v')
                    self.emit('output[".tag"] = .str("{}")'.format(tag))
            self.emit('default: fatalError("Tried to serialize unexpected subtype")')

def _generate_struct_class_serializer(self, namespace, data_type):
        with self.serializer_block(data_type):
            with self.serializer_func(data_type):
                if not data_type.all_fields:
                    self.emit('let output = [String: JSON]()')
                else:
                    intro = 'var' if data_type.has_enumerated_subtypes() else 'let'
                    self.emit("{} output = [ ".format(intro))
                    for field in data_type.all_fields:
                        self.emit('"{}": {}.serialize(value.{}),'.format(
                            field.name,
                            fmt_serial_obj(field.data_type),
                            fmt_var(field.name)
                        ))
                    self.emit(']')

                    if data_type.has_enumerated_subtypes():
                        self._generate_enumerated_subtype_serializer(namespace, data_type)
                self.emit('return .dictionary(output)')
            with self.deserializer_func(data_type):
                with self.block("switch json"):
                    dict_name = "let dict" if data_type.all_fields else "_"
                    self.emit("case .dictionary({}):".format(dict_name))
                    with self.indent():
                        if data_type.has_enumerated_subtypes():
                            self._generate_enumerated_subtype_deserializer(namespace, data_type)
                        else:
                            self._generate_struct_base_class_deserializer(namespace, data_type)
                    self.emit("default:")

def _generate_struct_init(self, namespace, data_type):  # pylint: disable=unused-argument
        # init method
        args = self._struct_init_args(data_type)
        if data_type.parent_type and not data_type.fields:
            return
        with self.function_block('public init', self._func_args(args)):
            for field in data_type.fields:
                v = fmt_var(field.name)
                validator = self._determine_validator_type(field.data_type, v)
                if validator:
                    self.emit('{}({})'.format(validator, v))
                self.emit('self.{} = {}'.format(v, v))
            if data_type.parent_type:
                func_args = [(fmt_var(f.name),
                              fmt_var(f.name))
                             for f in data_type.parent_type.all_fields]
                self.emit('super.init({})'.format(self._func_args(func_args)))

def _generate_struct_base_class_deserializer(self, namespace, data_type):
        args = []
        for field in data_type.all_fields:
            var = fmt_var(field.name)
            value = 'dict["{}"]'.format(field.name)
            self.emit('let {} = {}.deserialize({} ?? {})'.format(
                var,
                fmt_serial_obj(field.data_type),
                value,
                fmt_default_value(namespace, field) if field.has_default else '.null'
            ))

            args.append((var, var))
        self.emit('return {}({})'.format(
            fmt_class(data_type.name),
            self._func_args(args)
        ))

def _docf(self, tag, val):
        if tag == 'route':
            if ':' in val:
                val, version = val.split(':', 1)
                version = int(version)
            else:
                version = 1
            return fmt_func(val, version)
        elif tag == 'field':
            if '.' in val:
                cls, field = val.split('.')
                return ('{} in {}'.format(fmt_var(field),
                        fmt_class(cls)))
            else:
                return fmt_var(val)
        elif tag in ('type', 'val', 'link'):
            return val
        else:
            import pdb
            pdb.set_trace()
            return val

def _get_route_args(self, namespace, route):
        data_type = route.arg_data_type
        arg_type = fmt_type(data_type)
        if is_struct_type(data_type):
            arg_list = self._struct_init_args(data_type, namespace=namespace)

            doc_list = [(fmt_var(f.name), self.process_doc(f.doc, self._docf)
                if f.doc else undocumented) for f in data_type.fields if f.doc]
        elif is_union_type(data_type):
            arg_list = [(fmt_var(data_type.name), '{}.{}'.format(
                fmt_class(namespace.name), fmt_class(data_type.name)))]
            doc_list = [(fmt_var(data_type.name),
                self.process_doc(data_type.doc, self._docf)
                if data_type.doc else 'The {} union'.format(fmt_class(data_type.name)))]
        else:
            arg_list = [] if is_void_type(data_type) else [('request', arg_type)]
            doc_list = []
        return arg_list, doc_list

def _tag_type(self, data_type, field):
        return "{}.{}".format(
            fmt_class(data_type.name),
            fmt_var(field.name)
        )

def _struct_init_args(self, data_type, namespace=None):  # pylint: disable=unused-argument
        args = []
        for field in data_type.all_fields:
            name = fmt_var(field.name)
            value = fmt_type(field.data_type)
            data_type, nullable = unwrap_nullable(field.data_type)

            if field.has_default:
                if is_union_type(data_type):
                    default = '.{}'.format(fmt_var(field.default.tag_name))
                else:
                    default = fmt_obj(field.default)
                value += ' = {}'.format(default)
            elif nullable:
                value += ' = nil'
            arg = (name, value)
            args.append(arg)
        return args