Example of ring middleware

ring.clj

(defn one
  [handler]
  (println "handler one")
  (fn [request]
    (println "request one")
    (handler request)))

(defn two
  [handler]
  (println "handler two")
  (fn [request]
    (println "request two")
    (handler request)))

(def app1 (one (two (fn [& args] args))))
(app1 {:test "test1"})

; let's break that out a little, and replace 'two' with an inline
; version of itself. We unfortunatly loose the println to let us know that
; handler two has been called...

(def app2
  (one
    (fn [request]
      (println "request two")
      ((fn [& args] args) request))))


(app2 {:test "test2"})

; now let's replace 'one'

(def app3
  (fn [outer-request] ; one

    (println "request one")

    ((fn [inner-request] ; two

      (println "request two")

      ((fn [& args] args)
        inner-request))

      outer-request)))

(app3 {:test "test3"})

; clear so far!? Probably not, but let's hope.


(def app4
  (fn [outer-request] ; one

    (println "request one - before: " outer-request)

    (let [outer-request (assoc outer-request :outer "we added this in (outer)")]

      (println "request one - after" outer-request)

      ((fn [inner-request] ; two

        (println "request two - before: " inner-request)

        (let [inner-request (assoc inner-request :inner "we added this (inner)")]

          (println "request two - after" inner-request)

          ((fn [& args] args) inner-request)))

      outer-request))))

(app4 {:test "test4"})