Request map passed to my anonymous function inside a Compojure handler

Question

I'm two days into learning Clojure by writing a simple REST server using ring-clojure and Compojure with ring-json's wrap-json-body middleware.

So far, I have:

A vector users containing the users (with a couple of default users):

(def users [{:id 0 :username "aname"}
            {:id 1 :username "anothername"}])

A function (form?) save-user that accepts a map (user) and looks for existing users with the same username. If the username is available, I overwrite the users vector to include the new user before returning HTTP 201. If the username is taken, I simply return HTTP 400:

(defn save-user [user]
  (prn users)
  (if
    (not-any? #(= (:username %) (:username user)) users)
    (fn [request]
      (def users (conj users user))
      (status
        (response (str "Saved user with username: " (:username user)))
        201))
    (status
      (response (str "User with username '" (:username user) "' already exists"))
      400)))

A route for POST /users which calls save-user with the received map:

(defroutes app-routes
       (POST "/users" request (save-user (:body request))))

I don't think it matters, but the middleware is applied like this:

(def app
  (-> app-routes
      (wrap-cors :access-control-allow-origin "*" :access-control-allow-methods "*")
      (wrap-json-response)
      (wrap-keyword-params)
      (wrap-json-body {:keywords? true :bigdecimals? true})
      (wrap-defaults (assoc site-defaults :security false))))

My problem:

For whatever reason, the entire request map is passed to the function i pass as then inside the if. Printing it:

  (if
    (not-any? #(= (:username %) (:username user)) users)
    (fn [request]
      (prn request))
    ...

... gives me this:

{:ssl-client-cert nil, :cookies {}, :remote-addr "0:0:0:0:0:0:0:1", :params {}, :flash nil, :route-params {}, :headers {"host" "localhost:3000", "accept" "*/*", "content-length" "42", "content-type" "application/json", "user-agent" "curl/7.43.0"}, :server-port 3000, :content-length 42, :form-params {}, :compojure/route [:post "/users"], :session/key nil, :query-params {}, :content-type "application/json", :character-encoding nil, :uri "/users", :server-name "localhost", :query-string nil, :body {:username "testusername", :password "testpassword"}, :multipart-params {}, :scheme :http, :request-method :post, :session {}}

The same happens if I pass an anonymous function as the if's else. However, nothing wrong happens when I only pass (status ...), like in the code above.

From what I understand, the request map shouldn't be available inside save-user at all, since it's not passed as an argument. Why is it passed to my anonymous function, and is there any way to simply ignore it?

Not sure what the question is. Why are you returning a function from the save-user function at all? You could just return the response directly (replace (fn [request] with (do). — glts
By the way, you should wrap the users vector in an atom and then operate on the atom. Redefining global vars from inside a function is a big no-no in Clojure. — glts
@glts I'm not returning a function from save-user, but I'm passing one to the if because I need to both save the user and return the HTTP response object (returned from the status function). As for wrapping the list in an atom: thanks! That was because I'm a newbie at this. — Martin Lehmann
Hi Martin, I’ve converted my comment into an answer. You are returning a function from the if: in Clojure each conditional branch is an expression that returns a value. — glts

glts glts · Accepted Answer · 2016-04-16T18:43:06

In save-user you are returning a function that takes a request and returns a response (a handler).

I suppose you should instead just return the response directly. Just replace (fn [request] with (do to wrap multiple expressions in a single form.

(def users (atom [{:id 0 :username "aname"}
                  {:id 1 :username "anothername"}]))

(defn save-user [user]
  (if (not-any? #(= (:username %) (:username user)) @users)
    (do
      (swap! users conj user)
      (status
        (response (str "Saved user with username: " (:username user)))
        201))
    (status
      (response (str "User with username '" (:username user) "' already exists"))
      400)))

I’ve also changed the global users var to be an atom. Redefining global vars from inside a function is a big no-no in Clojure.

Request map passed to my anonymous function inside a Compojure handler

So far, I have:

My problem:

1 Answers