Clojure:映射作为函数参数

Question

我正在尝试运行“使用 Clojure 进行 Web 开发”一书中的一些代码。有一个我无法理解的功能:

(defn handle-upload [{:keys [filename] :as file}]
  (upload-page 
    (if (empty? filename)
      "please select a file to upload"
      (try 
        (upload-file (gallery-path) file)
        (save-thumbnail file)
        (db/add-image (session/get :user) filename)
        (image {:height "150px"}
          (str "/img/"
               (session/get :user)
               "/"
               thumb-prefix
               (url-encode filename)))
        (catch Exception ex 
          (str "error uploading file " (.getMessage ex)))))))

在哪里

(defn upload-page [info]
    (layout/common
      [:h2 "Upload an image"]
      [:p info]
      (form-to {:enctype "multipart/form-data"}
               [:post "/upload"]
               (file-upload :file)
               (submit-button "upload"))))

函数handle-upload的参数是什么意思？

改变之后

(defn handle-upload [{:keys [filename] :as file}]
...

到

(defn handle-upload [{:keys filename :as file}]
...

我收到一条错误消息:

java.lang.IllegalArgumentException:不知道如何从 clojure.lang.Symbol 创建 ISeq，编译:(picture_gallery/routes/upload.clj:32:1)

为什么？

Answer 1

{:keys [filename] :as file} 表示:

1. 从传递的参数中获取 :filename 键并将其值绑定(bind)到 filename
2. 将整个参数保留为 文件

所以如果你通过了:

{:filename "foo"
 :somethingelse "bar"}

作为参数，那么函数作用域中的filename 将等于foo 并且file 将等于整个 HashMap .

引用资料:

• Special Forms