Clojure: map as function parameter

Question

I'm trying to running some codes from the book "Web development with Clojure". There is a function which I can not understand:

(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)))))))

where

(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"))))

What is the meaning of the parameter of the function handle-upload?

And after the changing from

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

to

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

I got an error message:

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

Why?

zerkms · Accepted Answer · 2014-12-16 10:41:03Z

9

{:keys [filename] :as file} means:

Take :filename key from passed argument and bind its value to filename
Leave the whole argument available as file

So if you pass:

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

As an argument, then filename in the function scope will be equal to foo and the file will be equal to the whole hash map.

References:

Special Forms

answered Dec 16, 2014 at 10:41

zerkms

256k73 gold badges447 silver badges551 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

RedDeckWins Over a year ago

Also, you can do similar de-structuring with other collection types.

Collectives™ on Stack Overflow

Clojure: map as function parameter

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related