contract.clj

(defmacro defcontract [name & forms]
  (let [body (collect-bodies forms)]
    (list* 'defn name body)))

(declare build-contract)

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

(defn build-contract [[args & cs]]
  (list
   (into '[f] args)
   (apply merge
	  (for [[tag & conds] cs] 
	    (case tag
	      require {:pre (vec conds)}
	      ensure {:post (vec conds)}
	      (throw (Exception. (str "Unknown tag " tag))))))
   (list* 'f args)))

(defcontract doubler-contract
  [x]
  (require
   (pos? x))
  (ensure
   (= (* 2  x) %))
  [x y]
  (require
   (pos? x) (pos? y))
  (ensure
   (= (* 2 (+ x y)) %)))