letrec.clj

(defn tramp-fn [f]
  (if (fn? f)
    (fn [& args] #(apply f args))
    f))

(defn tramp-fn2 [f]
  (if (fn? f) (partial trampoline f)  f))

(defmacro letrec [bindings & body]
  (let [bcnt (quot (count bindings) 2)
        arrs (gensym "bindings_array")
        arrv `(make-array Object ~bcnt)
        bprs (partition 2 bindings)
        bssl (map first bprs)
        bsss (set bssl)
        bexs (map second bprs)
        arrm (zipmap bssl (range bcnt))
        btes (map #(prewalk (fn [f]
                                   (if (bsss f)
                                     `(tramp-fn (aget ~arrs ~(arrm f)))
                                     f))
                                 %)
                  bexs)]
    `(let [~arrs ~arrv]
       ~@(map (fn [s e]
                `(aset ~arrs ~(arrm s) ~e))
              bssl
              btes)
       (let [~@(mapcat (fn [s]
                         [s `(tramp-fn2 (aget ~arrs ~(arrm s)))])
                       bssl)]
         ~@body))))