clojure - 如何在 Clojure REPL 中重新加载多方法

Question

我正在REPL中编写一个多方法，可以重新定义函数，但是如果我重新定义多方法的调度函数，它似乎不能使用新改进的函数:

;; simple fn to resolve defmethod to call, hardcoded to :do-it
(defn resolve-it [] :do-it)
(resolve-it) ;; :do-it, as expected

(defmulti do-something resolve-it)

(defmethod do-something :do-it [] (println "do-it"))
(defmethod do-something :oh-no [] (println "oh-no"))

(do-something) ;; "do-it", as expected

;; now change resolve-it
(defn resolve-it [] :oh-no)
(resolve-it) ;; :oh-no, as expected

(do-something) ;; "do-it", not as expected

(do-something) ;; "do-it", not expected

如何使用 mult 方法来反射(reflect)对调度函数 resolve-it 的更改？

Answer 1

有一种简单的技术可以重新定义多方法的调度函数。这个想法是将保存调度函数的 var 传递给 defmulti，而不是函数本身。请注意 defmulti 中的 #'resolve-it 而不仅仅是 resolve-it。因此，var 在运行时被取消引用，而不仅仅是在编译时。

(defn resolve-it [] :do-it)
(resolve-it) ;; :do-it, as expected

(defmulti do-something #'resolve-it)

(defmethod do-something :do-it [] (println "do-it"))
(defmethod do-something :oh-no [] (println "oh-no"))

(do-something) ;; "do-it", as expected

;; now change resolve-it
(defn resolve-it [] :oh-no)
(resolve-it) ;; :oh-no, as expected

(do-something) ;; "oh-no", expected!!