Clojure Google Group 讨论了这个主题以及一些解决方案。
解构可能是最接近“核心”功能的,如果您的问题相当静态并且映射具有所有预期的键(因此避免nil
),它可能是一个很好的解决方案。它可能看起来像:
(let [person {:name {:first "john" :john "smith"} :age 40 :weight 155 :something {:a "a" :b "b" :c "c" :d "d"}}
{{:keys [first]} :name {:keys [a b]} :something} person]
{:name {:first first} :something {:a a :b b}})
;; => {:name {:first "john"}, :something {:a "a", :b "b"}}
下面是对 Clojure Google Group 线程中解决方案的调查,适用于您的示例地图。他们每个人对如何指定要选择的嵌套键都有不同的看法。
这是Christophe Grand的解决方案:
(defprotocol Selector
(-select [s m]))
(defn select [m selectors-coll]
(reduce conj {} (map #(-select % m) selectors-coll)))
(extend-protocol Selector
clojure.lang.Keyword
(-select [k m]
(find m k))
clojure.lang.APersistentMap
(-select [sm m]
(into {}
(for [[k s] sm]
[k (select (get m k) s)]))))
使用它需要稍微修改语法:
(let [person {:name {:first "john" :john "smith"} :age 40 :weight 155 :something {:a "a" :b "b" :c "c" :d "d"}}]
(select person [{:name [:first] :something [:a :b]}]))
;; => {:something {:b "b", :a "a"}, :name {:first "john"}}
这是 Moritz Ulrich 的解决方案(他警告说它不适用于以 seqs 作为键的地图):
(defn select-in [m keyseq]
(loop [acc {} [k & ks] (seq keyseq)]
(if k
(recur
(if (sequential? k)
(let [[k ks] k]
(assoc acc k
(select-in (get m k) ks)))
(assoc acc k (get m k)))
ks)
acc)))
使用它需要另一种稍微修改的语法:
(let [person {:name {:first "john" :john "smith"} :age 40 :weight 155 :something {:a "a" :b "b" :c "c" :d "d"}}]
(select-in person [[:name [:first]] [:something [:a :b]]]))
;; => {:something {:b "b", :a "a"}, :name {:first "john"}}
这是 Jay Fields 的解决方案:
(defn select-nested-keys [m top-level-keys & {:as pairs}]
(reduce #(update-in %1 (first %2) select-keys (last %2)) (select-keys m top-level-keys) pairs))
它使用不同的语法:
(let [person {:name {:first "john" :john "smith"} :age 40 :weight 155 :something {:a "a" :b "b" :c "c" :d "d"}}]
(select-nested-keys person [:name :something] [:name] [:first] [:something] [:a :b]))
;; => {:something {:b "b", :a "a"}, :name {:first "john"}}
这是 Baishampayan Ghose 的解决方案:
(defprotocol ^:private IExpandable
(^:private expand [this]))
(extend-protocol IExpandable
clojure.lang.Keyword
(expand [k] {k ::all})
clojure.lang.IPersistentVector
(expand [v] (if (empty? v)
{}
(apply merge (map expand v))))
clojure.lang.IPersistentMap
(expand [m]
(assert (= (count (keys m)) 1) "Number of keys in a selector map can't be more than 1.")
(let [[k v] (-> m first ((juxt key val)))]
{k (expand v)}))
nil
(expand [_] {}))
(defn ^:private extract* [m selectors expand?]
(let [sels (if expand? (expand selectors) selectors)]
(reduce-kv (fn [res k v]
(if (= v ::all)
(assoc res k (m k))
(assoc res k (extract* (m k) v false))))
{} sels)))
(defn extract
"Like select-keys, but can select nested keys.
Examples -
(extract [{:b {:c [:d]}} :g] {:a 1 :b {:c {:d 1 :e 2}} :g 42 :xxx 11})
;=> {:g 42, :b {:c {:d 1}}}
(extract [:g] {:a 1 :b {:c {:d 1 :e 2}} :g 42 :xxx 11})
;=> {:g 42}
(extract [{:b [:c]} :xxx] {:a 1 :b {:c {:d 1 :e 2}} :g 42 :xxx 11})
;=> {:b {:c {:d 1, :e 2}}, :xxx 11}
Also see - exclude"
[selectors m]
(extract* m selectors true))
它使用另一种语法(并且参数颠倒了):
(let [person {:name {:first "john" :john "smith"} :age 40 :weight 155 :something {:a "a" :b "b" :c "c" :d "d"}}]
(extract [{:name [:first]} {:something [:a :b]}] person))
;; => {:name {:first "john"}, :something {:a "a", :b "b"}}