k8s 源码分析，语法提问

``` var _ generic.StorageDecorator = StorageWithCacher // Creates a cacher based given storageConfig. func StorageWithCacher( copier runtime.ObjectCopier, // 配置 etcd api version, server addr 等参数. storageConfig *storagebackend.Config, capacity int, objectType runtime.Object, resourcePrefix string, keyFunc func(obj runtime.Object) (string, error), newListFunc func() runtime.Object, getAttrsFunc storage.AttrFunc, triggerFunc storage.TriggerPublisherFunc) (storage.Interface, factory.DestroyFunc) { // 读取配置 etcd 的配置，如 API 版本，tls 认证等信息，创建 etcd 连接以及 Storage Interface s, d := generic.NewRawStorage(storageConfig) // TODO: we would change this later to make storage always have cacher and hide low level KV layer inside. // Currently it has two layers of same storage interface -- cacher and low level kv. cacherConfig := storage.CacherConfig{ CacheCapacity: capacity, Storage: s, Versioner: etcdstorage.APIObjectVersioner{}, Copier: copier, Type: objectType, ResourcePrefix: resourcePrefix, KeyFunc: keyFunc, NewListFunc: newListFunc, GetAttrsFunc: getAttrsFunc, TriggerPublisherFunc: triggerFunc, Codec: storageConfig.Codec, } cacher := storage.NewCacherFromConfig(cacherConfig) destroyFunc := func() { cacher.Stop() d() } return cacher, destroyFunc } ... package generic type StorageDecorator func( copier runtime.ObjectCopier, config *storagebackend.Config, capacity int, objectType runtime.Object, resourcePrefix string, keyFunc func(obj runtime.Object) (string, error), newListFunc func() runtime.Object, getAttrsFunc storage.AttrFunc, trigger storage.TriggerPublisherFunc) (storage.Interface, factory.DestroyFunc) ``` 提问： StorageWithCacher 这个函数，返回两个值，这个 ```var _ generic.StorageDecorator``` 中下划线```_```是什么意思， 它并不是``` _ , xxx := ```