Clojure Forward Declarations

Whilst working on Eliza I found I had a couple of functions that were mutually recursive; this meant I couldn't compile them because one didn't have the declaration of the other.

The solution is declare e.g.

  (declare my-odd? my-even?)
  (defn my-even? [x]
    (if (zero? x)
      true
      (my-odd? (dec x))))

  (defn my-odd? [x]
    (if (zero? x)
      false
      (my-even? (dec x))))

(Update after comment from Matt)

As pointed out, this doesn't actually work because the stack can blow:

user> (my-even? 1000000000) ==> stackoverflow!

Using trampoline can fix the issue if you rewrite it so that the tail calls are actually closures

  (declare my-odd?)
  (defn my-even? [x] (if (zero? x) true #(my-odd? (dec x))))
  (defn my-odd? [x] (if (zero? x) false #(my-even? (dec x))))

  user> (trampoline #(my-even? 100000000)) ==> true

The syntax #(my-even? 1000000000) creates an anonymous function of no arguments (a thunk to be evaluated later) e.g.

  user> (apply #(+ 1 1) ) ==> 2