Question

我试图从书中运行一些代码＆＃34;使用Clojure进行Web开发＆＃34;。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)))))))

，其中

(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的参数含义是什么？

从

改变之后

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

到

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

我收到错误消息：

java.lang.IllegalArgumentException: Don't know how to create ISeq from: clojure.lang.Symbol, compiling:(picture_gallery/routes/upload.clj:32:1)

为什么？

Answer 1

{:keys [filename] :as file}表示：

从传递的参数中取出:filename键，并将其值绑定到filename
将整个参数保留为file

所以如果你通过：

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

作为参数，函数范围中的filename将等于foo，file将等于整个哈希映射。

参考文献：

Special Forms

Clojure：map作为函数参数

1 个答案: