从clojure映射使init成为java构建器类的更好方法?
每个人 我正试着写一个函数来包装clojure 函数将采用如下映射:{:header true:locale“US”},函数将根据映射配置生成器从clojure映射使init成为java构建器类的更好方法?,clojure,clojure-java-interop,Clojure,Clojure Java Interop,每个人 我正试着写一个函数来包装clojure 函数将采用如下映射:{:header true:locale“US”},函数将根据映射配置生成器 (defn reader-options [ opts ] (let [ b (CsvReadOptions$Builder.)] (cond-> b (contains? opts :locale ) (.locale (:locale opts)) (contains? opts :header ) (.h
(defn reader-options [ opts ]
(let [ b (CsvReadOptions$Builder.)]
(cond-> b
(contains? opts :locale ) (.locale (:locale opts))
(contains? opts :header ) (.header (:header opts))
true (.build ))
)
)
抱歉,如果要求太多,clojure中是否有更好的方法来实现这一点?因为键在单行中重复工作
(contains? opts :locale ) (.locale (:locale opts))
再次感谢您的建议。您可以在let
中使用:
(let [{:keys [a b c]} {:a 1 :b false}]
[a b c])
;; => [1 false nil]
或在函数参数中:
(defn args-demo [{:keys [a b c]}]
[a b c])
(args-demo {:a 1 :b false})
;; => [1 false nil]
问题是,如果地图中不存在特定的键,它将绑定到nil
。如果您的值可以有nil
值,那么它将不起作用
对于非当前值,可以使用一些“标记”值:
(let [{:keys [a b c] :or {a ::absent b ::absent c ::absent}} {:a 1 :b nil}]
(cond-> {}
(not= a ::absent) (assoc :a2 a)
(not= b ::absent) (assoc :b2 b)
(not= c ::absent) (assoc :c2 c)))
;; => {:a2 1, :b2 nil}
您还可以创建宏:
(defmacro when-key-present
[k m & body]
`(let [{:keys [~k] :or {~k ::not-found}} ~m]
(when (not= ~k ::not-found)
~@body)))
(when-key-present a {:a false :b nil}
(println "a was" a))
;; prints "a was false"
(when-key-present b {:a false :b nil}
(println "b was" b))
;; prints "b was nil"
(when-key-present c {:a false :b nil}
(println "c was" c))
;; doesn't print anything
你的职能是:
(defn reader-options [opts]
(let [builder (CsvReadOptions$Builder.)]
(when-key-present locale opts
(.locale builder locale))
(when-key-present header opts
(.header builder header))
(.build builder)))
您甚至可以创建一个宏,该宏假定opts map中的键与应该调用的生成器方法相同,这样您就可以像这样使用它:
(let [builder (CsvReadOptions$Builder.)]
(doseq [k (keys opts)]
(set-builder-property builder k opts)))
但这变得越来越神奇,越来越难理解,所以在使用宏之前,我会三思而后行。在这个问题中,有几个因素存在:
locale
和header
的原因。我想不出一个简单的方法来减少这种重复。如果这是应用程序中的常见模式,则可以编写宏(编译器扩展)以使其更容易。除非代码中经常出现这种情况,否则成本(在复杂性、文档、误解等方面)将大大超过收益
有点像买一辆新车而不是清洗旧车。除了在极端情况下,成本可能不值得收益
)() 理论上,您可以编写一个宏,将其扩展为所需的代码:
(defmacro defbuilder [fn-name builder-class fields]
(let [opts (gensym)]
`(defn ~fn-name [~opts]
(cond-> (new ~builder-class)
~@(mapcat
(fn [field-sym]
(let [field-kw (keyword (name field-sym))]
`((contains? ~opts ~field-kw)
(. ~field-sym (get ~opts ~field-kw)))))
fields)
true (.build)))))
现在,
将产生:
(clojure.core/defn options-from-map [G__12809]
(clojure.core/cond-> (new CsvReadOptions$Builder)
(clojure.core/contains? G__12809 :header)
(. header (clojure.core/get G__12809 :header))
(clojure.core/contains? G__12809 :locale)
(. locale (clojure.core/get G__12809 :locale))
...
true (.build)))
然而,在实践中,该规范是:
- 可读性和可维护性较差(有些库大量使用宏,读起来很痛苦),以及
- 您可能希望为某些方法添加额外的特定处理(例如,将区域设置字符串转换为
对象)locale
(clojure.core/defn options-from-map [G__12809]
(clojure.core/cond-> (new CsvReadOptions$Builder)
(clojure.core/contains? G__12809 :header)
(. header (clojure.core/get G__12809 :header))
(clojure.core/contains? G__12809 :locale)
(. locale (clojure.core/get G__12809 :locale))
...
true (.build)))