clojure - 了解多方法调度

Question

我可以理解这是如何工作的：

(defmulti area :Shape)
(defmethod area :B [x] (println "Rect"))
(defmethod area :C [x] (println "Circle"))
(defmethod area :default [x] (do (println "Unknown") (:Shape x)))
(area {:Shape :B})
=> Rect
nil
(area {:Shape :Bb})
=> Unknown
:Bb

但是通过简单地更改:Shape为:Shap，我不再了解这是如何发送的：

(defmulti area :Shap)
(defmethod area :B [x] (println "Rect"))
(defmethod area :C [x] (println "Circle"))
(defmethod area :default [x] (do (println "Unknown") (:Shap x)))
(area {:Shap :B})
=> Unknown
:B
(area {:Shap :C})
=> Unknown
:C

Eclipse 上的 Clojure 1.5.1，带有逆时针插件

Answer 1

多方法不喜欢用新的调度函数重新定义。这是一个相当有争议的功能，但事情就是这样。为了改变多方法的调度函数，你必须首先将它定义为不是多方法的东西（例如，nil）。

(defmulti area :Shape)

(def area nil)

(defmulti area :Shap)