om ClojureScript ref-cursor error when trying to update vector using om/transact

Question

I am seeing the following error when trying to update a cursor:

Uncaught Error: No protocol method ITransact.-transact! defined for type function: function comments(){return om.core.ref_cursor.call(null,new cljs.core.Keyword(null,"comments-data","comments-data",1871210833).cljs$core$IFn$_invoke$arity$1(om.core.root_cursor.call(null,cljs_playground.core.app_state)));

I am using a ref-cursor to point to the comments-data vector in my application state:

(def app-state
  (atom
    {:comments-data [{ :author "Commenter 1" :text "comment 1" }
                     { :author "Commenter 2" :text "comment 2" }]}))

(defn comments []
  (om/ref-cursor (:comments-data (om/root-cursor app-state))))

Now, I would like to update this cursor when a user submits some data via an input in my input component. I use (let [foo (->> comments)] to refer to the cursor inside render-state. I pass it into a function call that handles the user submit and I would like to simply add one more item to this vector.

(defn handle-submit [e owner {:keys [text]} foo]
  (om/transact! foo #(concat % {:author "foo" :text "bar"}))
  (om/set-state! owner :text ""))

(defn Input
  [data owner]
  (reify
    om/IInitState
    (init-state [_]
      {:text nil})
    om/IRenderState
    (render-state [this state]
     (let [foo (->> comments)]
       (dom/div nil
         (dom/input #js
           { :type "text"
             :ref "text-field"
             :value (:text state)
             :onChange (fn [event] (handle-change event owner state))})
         (dom/button #js 
           { :onClick (fn [event] (handle-submit event owner state foo))} "submit"))))))

However, when I make the om/transact! call I am seeing the above error.

Answer 1

You want to call your comments function and use its return value (the cursor), not use the function itself. Change this line:

 (let [foo (->> comments)]

to:

 (let [foo (->> (comments))]

and that should work for you.

Answer 2

Change

(let [foo (->> comments)]

to

(let [foo (comments)]

and keep it simpler. There is no need for the ->> operator.