it looks like it returns a Process type https://github.com/leonoel/missionary/blob/master/src/missionary/impl/Observe.cljs#L6-L10

interesting, javascript functions are clojure functions but the opposite is not true

I'm not sure this can be solved without modifying the clojurescript compiler

Maybe a helper that returns #(kill this) could be a workaround?

(get-cancel-fn ((observe)))

Not great admittedly

you could also have a useEffect helper that checks to see if the return value is an IFn and wraps it in a function

(defn use-effect
  [f deps]
  (useEffect
   (fn []
     (let [ret (f)]
       (cond
         (ifn? ret) #((ret))
         (nil? ret) js/undefined
         :else ret)))
   deps))

I think the downside to that is it might be too much of a catch all, e.g. maps implement IFn