只在所有键都存在的情况下才更新

Question

是否存在类似于update-in的现有clojure函数，但只有在所有键都存在时才会进行更改？

会表现得像：

(def e1 {"one" "two"})
(def e2 {"one" "two" "three" "four"})

(update-in-if-present e1 ["three"] (fn [x] (str x x)))
;; => {"one" "two"}
(update-in e1            ["three"] (fn [x] (str x x)))
;; => {"one" "two", "three" ""}
(update-in-if-present e2 ["three"] (fn [x] (str x x)))
;; => {"one" "two", "three" "fourfour"}
(update-in e2            ["three"] (fn [x] (str x x)))
;; => {"one" "two", "three" "fourfour"}

(defn update-in-if-present [m [k] f]

下面是我所说的一个简化版本(只支持一个键)：

(if-not (get m k)
    m
    (update-in m [k] f)))

Answer 1

据我所知，没有这样的函数，但是它很容易用一个简单的reduce来定义，例如

(defn update-in-if-present
  "Apply f to every k from ks in m if the key is present in m."
  [m ks f]
  (reduce (fn [acc k]
            (if (contains? acc k)
              (update-in acc [k] f)
              acc)) m ks))

Update发现我误解了这个问题，但是here's a link to the correct answer。