fibo-memo

( defn fib [n]
  
  (def memo (ref  {})) ;create a ref to memo
  
  (for [k  (range  (inc n))] 
  
      (do 
        (if (> k 1) 
          (dosync (alter memo  conj [k (+ (@memo (- k 1)) (@memo (- k 2)))])  ;update memo with new pair
            )
          (dosync (alter memo conj  [k k])    ; update memo with first two pairs
            ) 
          )
        (when (= k n) (println "Fibonacci " n " " (@memo n)))  ; I am trying to look up the value associated with n 
                                                               ; but it appears that memo is being dereferenced on each
                                                               ; iteration? Where are the nulls coming from?
        
        )

      )
              
        
    )

GFredericks immutable version
(defn fib [n](get
%28reduce %28fn [memo n] %28assoc memo n %28+ %28memo %28dec n%29%29 %28memo %28- n 2%29%29%29%29%29 {0 1N 1 1N} %28range 2 %28inc n%29%29%29 n))