clojure - clojure有merge-if-exists函数吗？

Question

clojure 是否有一个 merge-if-exists 函数来合并两个映射，就像

(defn merge-if-exists [map1 map2])

返回一个包含map1所有key的新map，如果某个key出现在多个map中，则使用map2该key的值作为返回map值，否则为 map1 的值。

例如

(merge-if-exists {:a 1 :b 2} {:b 3})
;=> {:a 1, :b 3}

(merge-if-exists {:a 1 :b 2} {:c 3})
;=> {:a 1, :b 2}

(merge-if-exists{:b 3} {:a 1 :b 2})
;=> {:b 2}

这个函数怎么写？

Answer 1

您可以使用merge和select-keys来定义它:

(defn merge-if-exists [m1 m2]
  (merge m1 (select-keys m2 (keys m1))))

(merge-if-exists {:a 1 :b 2} {:b 3})
=> {:a 1, :b 3}
(merge-if-exists {:a 1 :b 2} {:c 3})
=> {:a 1, :b 2}
(merge-if-exists {:b 3} {:a 1 :b 2})
=> {:b 2}

或者使用reduce-kv更快/更高效的版本:

(defn merge-if-exists [m1 m2]
  (reduce-kv
   (fn [m k v]
     (assoc m k (if-let [r (find m2 k)]
                  (val r)
                  v)))
   {}
   m1))