snippet.mdown

Clojure now supports mutual recursive local functions using letfn:

(defn ring [n] 
  (letfn [(a [n] (if (zero? n) n (b (dec n)))) 
          (b [n] (if (zero? n) n (c (dec n)))) 
          (c [n] (if (zero? n) n (a (dec n))))] 
     (c n))) 
(ring 1000)

Using trampoline, you can even make this stackless:

(defn ring [n]
  (letfn [(a [n] #(if (zero? n) n (b (dec n)))) 
          (b [n] #(if (zero? n) n (c (dec n)))) 
          (c [n] #(if (zero? n) n (a (dec n))))] 
     (trampoline c n)))
(ring 100000)