the middleware in ring provides another way to understand function that return function.
Here I am gonno explain a little bit.
The code below comes from stackoverflow.

(let [post-wrap (fn [handler]
                  (fn [request]
                    (str (handler request) ", post-wrapped")))
;this function will call handler first, then to form string
      pre-wrap  (fn [handler]
                  (fn [request]
                    (handler (str request ", pre-wrapped"))))
;this function will form string first, then to call handler
      around    (fn [handler]
                  (fn [request]
                    (str (handler (str request ", pre-around")) ", post-around")))
;this function will first form a string, then call handler,
;and append another string lastly
      handler   (-> 
                  (pre-wrap identity)
                  around) ]
;this can be rewritten as (around (post-wrap (pre-wrap identity)))
     (println (handler "(Rugal Bernstein)")))

(Rugal Bernstein), pre-around, pre-wrapped, post-wrapped, post-around

The most important thing is that each function in code will return a function and wait for one more parameter to activate the inner one.

So after wrap, the pre-wrap function actually looks like this:

(fn [request] (str (identity request) ", post-wrapped"))

And post-wrap becomes:

(fn [request] (str (pre-wrap request) ", post-wrapped"))

So as around:

(fn [request] (str (post-wrap (str request ", pre-around")) ", post-around"))

After this, when we call (handler "(Rugal Bernstein)"), of course it will be a invert order execution.
This is a good example for non-funtional programmer to think from the perspective of functional programming.
Try to understand it by yourself, you will learn more than just copying.


06 May 2015