clojure - 如何为clojure编写dissoc-in命令？

Question

我希望编写一个类似于 assoc-in 的函数，但删除键而不是添加它:

(dissoc-in {:a {:b 0}} [:a :b])
;;=> {:a {}}

我起身到这里:

(def m {:a {:b {:c 1}}})

(assoc  m :a (assoc (:a m) :b (dissoc (:b (:a m)) :c)))
;;=> {:a {:b {}}}

但整个嵌套的事情搞乱了我的头

Answer 1

怎么样:

(defn dissoc-in
  "Dissociates an entry from a nested associative structure returning a new
  nested structure. keys is a sequence of keys. Any empty maps that result
  will not be present in the new structure."
  [m [k & ks :as keys]]
  (if ks
    (if-let [nextmap (get m k)]
      (let [newmap (dissoc-in nextmap ks)]
        (if (seq newmap)
          (assoc m k newmap)
          (dissoc m k)))
      m)
    (dissoc m k)))

例子:

(dissoc-in {:a {:b 0 :c 1}} [:a :b])

结果:

{:a {:c 1}}

dissoc-in曾经是 clojure.contrib.core 的一部分，现在是 core.incubator 的一部分.

如果要保留空 map ，可以稍微更改代码:

(defn dissoc-in
  [m [k & ks :as keys]]
  (if ks
    (if-let [nextmap (get m k)]
      (let [newmap (dissoc-in nextmap ks)]
        (assoc m k newmap))
      m)
    (dissoc m k)))

例子:

(dissoc-in {:a {:b {:c 0}}} [:a :b])

结果:

{:a {}}