gistfile1.clj

(def validations
     [[:username #(< (count %) 3) "Username must be more then two characters."]
      [:username #(> (count %) 50) "Username must be less then fifty characters."]
      [:password #(< (count %) 3) "Password must be more then two characters."]
      [:password #(> (count %) 50) "Password must be lass then fifty characters."]])

(defn- valid
  [validation]
  (let [[value pred message] validation]
    (if (pred value)
      message
      nil)))

(defn validate
  [fields]
  (filter #(not (nil? %))
	  (map #(valid %)
	       (map #(replace fields %)
		    (filter #(not (nil? (some (set (keys fields)) %))) validations)))))


;; Example usage:

(validate {:username "me"}) ; -> (Username must be more then two characters.)

(validate {:username "user" :password "p"}) ; -> (Password must be more then two characters)

(validate {:username "me" :password "pa"}) ; -> (Username must be more then two characters, Password must be more then two characters)