clojure - :or mean in Clojure destructuring? 是什么意思

Question

法拉第扫描函数的来源( https://github.com/ptaoussanis/faraday/blob/master/src/taoensso/faraday.clj#L1197 )有一个我正在努力理解的解构形式......

(source far/scan)
(defn scan
  "..."
  [client-opts table
   & [{:keys [attr-conds last-prim-kvs span-reqs return limit total-segments
              filter-expr
              segment return-cc?] :as opts
       :or   {span-reqs {:max 5}}}]]
...)

那是什么 :or {span-reqs {:max 5}}做？

Answer 1

它是默认值。更多详情请见http://clojure.org/guides/destructuring

(def my-map {:a "A" :b "B" :c 3 :d 4})
(let [{a :a, x :x, :or {x "Not found!"}, :as all} my-map]
  (println "I got" a "from" all)
  (println "Where is x?" x))
;= I got A from {:a "A" :b "B" :c 3 :d 4}
;= Where is x? Not found!

使用 :keys我们得到

(let [{:keys [a x] :or {x "Not found!"}, :as all} my-map]
  (println "I got" a "from" all)
  (println "Where is x?" x))

结果相同