Clojure - 使用自定义实现混合协议默认实现

时间:2013-02-23 10:23:31

标签: clojure protocols

在Clojure中,我希望有一个协议,其中一些方法有默认实现,有些方法有自定义方法。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"))

然而,它不会以这种方式工作。即使在collection-nameUser实例上调用NewsItem返回正确的集合字符串,在其上调用save也会导致AbstractMethodError。我怎么能用Clojure来实现这个琐碎的OO形目标呢?

1 个答案:

答案 0 :(得分:12)

使保存功能成为正常功能:

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

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。

请记住:OO风格经常隐藏明显简单的事情:)