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doto和有条件地设置属性

clojure

Kurt Schelfthout · 技术社区 · 14 年前

(defn download-web-page
    "Downloads the webpage at the given url and returns its contents."
    [^String url ^String user ^String password]
    (with-open [client (doto (WebClient.)
                       (when user (.set_Credentials (NetworkCredential. user password ""))))]
    (.DownloadString client url)))

因此,我只想将凭据设置为函数的参数。然而,它似乎不是这样工作的-当我用if替换when时也不是这样。

如果我完全删除when,函数就可以正常工作。

4 回复 | 直到 14 年前

dreish 14 年前

(defn download-web-page
  "Downloads the webpage at the given url and returns its contents."
  [^String url ^String user ^String password]
  (with-open [client (WebClient.)]
    (when user
      (.set_Credentials client (NetworkCredential. user password "")))
    (.DownloadString client url)))

with-open 没有任何特别的要求 client close 方法,因此不需要在绑定向量中“完成”它。

MichaÅ Marczyk 14 年前

(注意:这应该都是有希望的工作,但我不能在这个时候测试它。请自己检查一下。)

(defn download-web-page
  "Downloads the webpage at the given url and returns its contents."
  ([^String url] (download-web-page url nil nil))
  ([^String url ^String user ^String password]
     (with-open [client (doto (WebClient.)
                          (-> (.set_Credentials
                               (NetworkCredential. user password ""))
                              (->> (when user))))]
       (.DownloadString client url))))

不过,这对我来说似乎很复杂。另一种方法:

(defn download-web-page
  "Downloads the webpage at the given url and returns its contents."
  ([^String url] (download-web-page url nil nil))
  ([^String url ^String user ^String password]
    (with-open [client (let [c (WebClient.)]
                         (when user
                           (.set_Credentials
                            (NetworkCredential. user password "")))
                         c)]
      (.DownloadString client url))))

-> / ->> 第一个版本的模式可以用宏抽象出来:

(defmacro doto-guard [guard action]
  `(-> ~action ~guard))

然后你就可以写了

(doto (WebClient.)
  (doto-guard (when user) (.setCredentials ...)))

它有一个很好的特性,你可以在一个单独的时间内多次使用它 doto 在有规律地混合时形成 let -基础版本应该做的很好。

真正地 较少的 灵活,但更漂亮,比如说 ~guard 具有 (when ~guard) (doto-guard user (.setCredentials ...)) . 然而,选择某个特定版本的任何深层原因都必须来自更广泛的背景。)

nil nil 当没有实际提供凭据时。

wilkes 14 年前

(defn download-web-page
  "Downloads the webpage at the given url and returns its contents."
  [^String url ^String user ^String password]
  (let [client (WebClient.)]
    (when user 
      (.set_Credentials client (NetworkCredential. user password "")))
    (with-open [client client]
      (.DownloadString client url)))

(with-open [client client]... doto 它经常出现保证一个宏,但我会先跳转到一个生成器函数:

(defn build-web-client
  [^String user ^String password]
  (let [client (WebClient.)]
    (when user 
      (.set_Credentials client (NetworkCredential. user password "")))
    client))

(defn download-web-page
  "Downloads the webpage at the given url and returns its contents."
  [^String url ^String user ^String password]
    (with-open [client (build-web-client user password)]
      (.DownloadString client url)))

Didier A. 5 年前

你可以筑巢 cond-> doto 像这样:

(doto (java.util.Vector.)
  (cond-> (= 1 1) (.add "bla")))
;; => ["bla"]

(doto (java.util.Vector.)
  (cond-> (= 1 2) (.add "bla")))
;; => []