letfn/letfn*

gistfile1.txt

user=> (source letfn)
(defmacro letfn
  "fnspec ==> (fname [params*] exprs) or (fname ([params*] exprs)+)

  Takes a vector of function specs and a body, and generates a set of
  bindings of functions to their names. All of the names are available
  in all of the definitions of the functions, as well as the body."
  {:added "1.0", :forms '[(letfn [fnspecs*] exprs*)],
   :special-form true, :url nil}
  [fnspecs & body]
  `(letfn* ~(vec (interleave (map first fnspecs)
                             (map #(cons `fn %) fnspecs)))
           ~@body))

;; letfn*: https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/Compiler.java#L5816

user=> (letfn [(a [n] (when (> n 0) (b (dec n)))) (b [n] (when (> n 0) (a (dec n))))] (a 1000))
nil
user=> (letfn [(a [n] (when (> n 0) (b (dec n)))) (b [n] (when (> n 0) (a (dec n))))] (a 10000))
StackOverflowError   clojure.lang.Numbers.gt (Numbers.java:3747)

user=> (letfn [(a [n] (when (> n 0) #(b (dec n)))) (b [n] (when (> n 0) #(a (dec n))))] (trampoline a 10000))
nil