pre.clj

user> (clojure.pprint/pprint (macroexpand-all '(defn constrained-fn [f x]
  {:pre  [(pos? x)]
   :post [(= % (* 2 x))]}
  (f x))))
(def
 constrained-fn
 (fn*
  ([f x]
   (if
    (pos? x)
    nil
    (do
     (throw
      (new
       java.lang.AssertionError
       (clojure.core/str
        "Assert failed: "
        (clojure.core/pr-str '(pos? x)))))))
   (let*
    [% (f x)]
    (if
     (= % (* 2 x))
     nil
     (do
      (throw
       (new
        java.lang.AssertionError
        (clojure.core/str
         "Assert failed: "
         (clojure.core/pr-str '(= % (* 2 x))))))))
    %))))
nil
user>