letfn.clj

; https://stackoverflow.com/questions/4899113/fixed-point-combinator-for-mutually-recursive-functions
(defn Y* [& fs]
  (map (fn [f] (f))
       ((fn [x] (x x))
        (fn [p]
          (map
            (fn [f]
              (fn []
                (apply f
                  (map
                    (fn [ff]
                      (fn [& y] (apply (ff) y)))
                    (p p)))))
            fs)))))

(defmacro my-letfn* [bindings & body]
  (let [bindings (partition 2 bindings)
        names (map first bindings)
        fns (map second bindings)]
   `(let [[~@names] (Y* ~@(for [f fns] `(fn [~@names] ~f)))]
    ~@body)))

(my-letfn* [my-even? (fn [n] (or (= 0 n) (my-odd? (dec n))))
            my-odd? (fn [n] (and (not= 0 n) (my-even? (dec n))))]
  (even? 122)) ; => true