Clojure - 使用自定义实现混合协议默认实现

Question

在Clojure中,我希望有一个协议,其中一些方法具有默认实现,而一些方法具有自定义实现.第一个用于配置后者.这是一个例子:

(defprotocol Saving
  (save [this] "saves to mongodb")
  (collection-name [this] "must return a string representing the associated MongoDB collection"))

;Default implementation

(extend-type Object
  Saving
  ; the `save` method is common for all, so it is actually implemened here
  (save [this] (mc/insert (collection-name [this]) this))
  ; this method is custom to every other type
  (collection-name [this] "no_collection"))

;Particular implementations

(defrecord User
  [login password]
  Saving
  (collection-name [this] "users"))

(defrecord NewsItem
  [text date]
  Saving
  (collection-name [this] "news_items"))

但是,它不会以这种方式工作.即使调用collection-name一个User或NewsItem实例返回一个正确的集合字符串,调用save它们会导致一个AbstractMethodError.我怎样才能用Clojure实现这个琐碎的OO形目标？

Answer 1

使保存功能成为正常功能:

(defn save [obj] (mc/insert (collection-name obj) obj))

该协议应该只有 collection-name

(defprotocol Saving
  (collection-name [this] "must return a string representing the associated MongoDB collection"))

然后,每个想要"保存"的对象都可以实现此协议.

记住:OO风格经常隐藏明显简单的东西:)