ring

• handler fn: req -> resp
• middleware fn: handler -> handler
  • request middleware

    (defn wrap-user [handler user-info]
    (fn [request]
      (handler
        (assoc request :user user-info))))
    

  • response middleware

    (defn wrap-content-type [handler content-type]
    (fn [request]
      (let [response (handler request)]
        (assoc-in response [:headers "Content-Type"] content-type))))
    

  • req+resp middleware

    (defn wrap-cookies [handler options]
    (fn [request]
      (-> request
          (cookies-request options)
          handler
          (cookies-response options))))
    

• middleware order: bottom up

  ; let said we have 2 middleware
  (defn wrap-1 [handler]
    (fn [req]
      (handler
        (assoc req :a 1))))
  
  (defn wrap-2 [handler]
    (fn [req]
      (handler
        (assoc req :a 2))))
  
  ; when use it, order will be apply for wrap-2 first, then wrap-1
  ; it seem confuse at first, as it's reverse with -> order
  (-> handler
      wrap-1
      wrap-2)
  
  ; let see why
  (wrap-2 (wrap-1 handler))
  
  (wrap-2 #(handler (assoc % :a 1)))
  
  (fn [req]
    (#(handler (assoc % :a 1))
      (assoc req :a 2)))
  
  (fn [req]
    (handler (assoc (assoc req :a 2) :a 1)))
  
  ; so after all, fn from wrap-2 is apply to req first, then wrap-1