;; referring to the application function via #' means that ring sees
;; the variable user/app rather than the function (fn[x]{:status 200})
;; which that variable evaluates to. And that means that if we
;; reevaluate the definition, the behaviour the browser sees will
;; change.

http://www.learningclojure.com/2013/01/getting-started-with-ring.html

Thanks!