Question

我很难让Clojure中的多种方法像我期望的那样工作。我的代码的提炼如下。

(defn commandType [_ command] (:command-type command))

(defmulti testMulti commandType)

(defmethod testMulti :one [game command] (str "blah"))

(defmethod testMulti :default [& args] "Cannot understand")

(testMulti "something" {:command-type :one})

(commandType "something" {:command-type :one})

现在我希望在这个参数上调用方法commandType当然会返回：一个应该将它发送到第一个defmethod而是得到一个空指针异常。即使是最简单的多方法调用我也能得到一个空指针：

(defmulti simpleMulti :key)

(defmethod simpleMulti "basic" [params] "basic value")

(simpleMulti {:key "basic"})

然而位于here的clojure文档中的示例运行正常。有什么根本我做错了吗？

Answer 1

据我所见，它有效。

鉴于

(defmulti testMulti (fn [_ command] (:command-type command)))

(defmethod testMulti :one [game command] (str "blah"))

(defmethod testMulti :default [& args] "Cannot understand")

然后

(testMulti "something" {:command-type :one})
; "blah"

(testMulti "something" {:command-type :two})
; "Cannot understand"

(testMulti "something" 5)
; "Cannot understand"

正如所料。

我在重新运行上面之前重置了REPL。

这个简单的例子也有效。鉴于

(defmulti simpleMulti :key)

(defmethod simpleMulti "basic" [params] "basic value")

然后

(simpleMulti {:key "basic"})
; "basic value"

Clojure multimethod给出了意想不到的空指针

1 个答案: