rules via lua's pcall

1. pcheck.clj

(defn pcall [f & args]
  (try [true (apply f args)]
  (catch Exception e
    [false e])))

(defn matched? [[status value]]
  (if status
   (when (and value 
              (not= value :nginx-null))
     value)
   (println "ouch:" value)))

2. cases.clj

(defn nginx-null [& args]
  :nginx-null)

(defn just-false [& args]
  false)

(defn just-null [& args]
  nil)

3. running-with-it.clj

;; running it:

(matched? (pcall / 1 3))
;; => 1/3

(matched? (pcall nginx-null 1 0))
;; => nil

(matched? (pcall just-null 1 0))
;; => nil

(matched? (pcall just-false 1 0))
;; => nil

(matched? (pcall / 1 0))
;; => ouch: #error { :cause Divide by zero ... }
;; => nil