recursion - How do I generate memoized recursive functions in Clojure?

Question

I'm trying to write a function that returns a memoized recursive function in Clojure, but I'm having trouble making the recursive function see its own memoized bindings. Is this because there is no var created? Also, why can't I use memoize on the local binding created with let?

This slightly unusual Fibonacci sequence maker that starts at a particular number is an example of what I wish I could do:

(defn make-fibo [y]
  (memoize (fn fib [x] (if (< x 2)
             y
             (+ (fib (- x 1))
                (fib (- x 2)))))))

(let [f (make-fibo 1)]
  (f 35)) ;; SLOW, not actually memoized

Using with-local-vars seems like the right approach, but it doesn't work for me either. I guess I can't close over vars?

(defn make-fibo [y]
  (with-local-vars [fib (fn [x] (if (< x 2)
                                  y
                                  (+ (@fib (- x 1))
                                     (@fib (- x 2)))))]
    (memoize fib)))

(let [f (make-fibo 1)]
  (f 35)) ;; Var null/null is unbound!?! 

I could of course manually write a macro that creates a closed-over atom and manage the memoization myself, but I was hoping to do this without such hackery.

question from:https://stackoverflow.com/questions/3906831/how-do-i-generate-memoized-recursive-functions-in-clojure

Answer 1

This seems to work:

(defn make-fibo [y]
  (with-local-vars
      [fib (memoize
            (fn [x]
              (if (< x 2)
                y
                (+ (fib (- x 2)) (fib (dec x))))))]
    (.bindRoot fib @fib)
    @fib))

with-local-vars only provides thread-local bindings for the newly created Vars, which are popped once execution leaves the with-local-vars form; hence the need for .bindRoot.