法拉第扫描功能的来源 (https://github.com/ptaoussanis/faraday/blob/master/src/taoensso/faraday.clj#L1197) 有一个我很难理解的解构形式...
(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}}}]]
...)
:or {span-reqs {:max 5}} 是做什么的？
【问题讨论】：
标签： clojure
这是默认值。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
【讨论】：
:or {span-reqs {:max 5}} 指定如果 opts 没有键 :span-reqs，span-reqs 将绑定到映射 {:max 5}。
请注意，span-reqs 不直接引用密钥 :span-reqs - 这也是可能的：
(defn scan
[client-opts table
& [{:keys [attr-conds last-prim-kvs return limit total-segments
filter-expr
segment return-cc?] :as opts
foo :span-reqs
:or {foo {:max 5}}}]])
请注意，在:or 映射中，您通常可以为您在解构形式中绑定的符号提供默认表达式。
N。 B.：将运行时表达式放入:or 时要小心。从 Clojure 1.9-alpha14 开始，无论是否需要它们，它们仍然会被评估（参见 http://dev.clojure.org/jira/browse/CLJ-1676）。
【讨论】：
为了把它放在上下文中，我将把它包装在一个示例函数中：
user> (defn foo [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}}}]]
(println "client-opts")
(clojure.pprint/pprint client-opts)
(println "table")
(clojure.pprint/pprint table)
(println "opts")
(clojure.pprint/pprint opts)
(println "the symbol attr-conds is bound to:" attr-conds)
(println "the symbol limit is bound to:" limit)
(println "the symbol span-reqs is bound to:" span-reqs))
#'user/foo
user> (foo :I'm-a-client-opt
:I'm-a-table
{:attr-conds 1
:span-reqs [1 2 3]
:limit 47}
{:attr-conds :i-will-be-ignored}
{:limit :i-will-also-be-ignored})
client-opts
:I'm-a-client-opt
table
:I'm-a-table
opts
{:attr-conds 1, :span-reqs [1 2 3], :limit 47}
the symbol attr-conds is bound to: 1
the symbol limit is bound to: 47
the symbol span-reqs is bound to: [1 2 3]
nil
现在我们看到它将一些名称绑定到列表中第一个映射的部分，所以让我们把这个破坏性表达式分开：
& ;; this symbol collects all the rest of the arguments into a list
[ ;; this one then does list destructuring on the list indicated by the &
{:keys [attr-conds ;; This block destructures the first (and only the first) entry in the vector.
last-prim-kvs ;; Because this is a map it will do map destructuring and
span-reqs ;; bind (assigns) values to symbols in this list
return
limit
total-segments
filter-expr
segment return-cc?]
:as opts ;; the entire map in the first position in the list created by the & will be bound to the name opts
:or {span-reqs {:max 5}}}] ;; if span-reqs is not in the map at all,
;; then the map {:max 5} will be bound to the name
;; span-reqs instead
因此，如果函数的第三个参数是一个不为关键字 :span-reqs 提供值的映射，则该嵌套解构表达式中的关键字 :or 会为符号分配默认值。如果关键字 :span-reqs 存在并且包含其他键但没有为 :max 提供值，则它什么也不做。它确实不在这种情况下将默认值合并到地图中，如果它完全丢失，它只会提供一个值：
这是根本没有指定的值：
user> (foo :I'm-a-client-opt
:I'm-a-table
{:attr-conds 1
;; :span-reqs [1 2 3] ;; removed this one
:limit 47}
{:attr-conds :i-will-be-ignored}
{:limit :i-will-also-be-ignored})
client-opts
:I'm-a-client-opt
table
:I'm-a-table
opts
{:attr-conds 1, :limit 47}
the symbol attr-conds is bound to: 1
the symbol limit is bound to: 47
the symbol span-reqs is bound to: {:max 5}
nil
再次指定一个值，其中该值不包括 :max
user> (foo :I'm-a-client-opt
:I'm-a-table
{:attr-conds 1
:span-reqs {:min -7} ;; present but contains something else
:limit 47}
{:attr-conds :i-will-be-ignored}
{:limit :i-will-also-be-ignored})
client-opts
:I'm-a-client-opt
table
:I'm-a-table
opts
{:attr-conds 1, :span-reqs {:min -7}, :limit 47}
the symbol attr-conds is bound to: 1
the symbol limit is bound to: 47
the symbol span-reqs is bound to: {:min -7}
nil
【讨论】：