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?

Answer 1

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

1. Take :filename key from passed argument and bind its value to filename
2. 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