Usefull macros for clojure

util.clj

(defn collect-bodies [forms]
  (for [form (partition 3 forms)]
    (build-contract form)))

(defn build-contract [c]
  (let [args (first c)]
    (list
      (into '[f] args)
      (apply merge (for [con (rest c)]
                     (cond
                       (= (first con) 'require) (assoc {} :pre (vec (rest con)))
                       (= (first con) 'ensure) (assoc {} :post (vec (rest con)))
                       :else (throw (Exception. (str "Unknown tag " (first con)))))))
      (list* 'f args))))

(defmacro contract [name & forms]
  (list* `fn name (collect-bodies forms)))

;; How to use
(def double-contract
  (contract doubler
    [x]
    (require (pos? x))
    (ensure (= (* 2 x) %)))

(def times2 (partial doubler-contract #(* 2 x)))

(times2 9) ;; => 18

From The Joy of Clojure second edition