memoized fibonacci

fib-memo.rkt

#lang racket
;; memoized fibonacci
(define Fib (make-hash))
(define (fib n)
  (let ([v (hash-ref Fib n #f)])
    (cond [v v]
          [else (let ([v (if (< n 2) n (+ (fib (- n 2)) (fib (- n 1))))])
                  (hash-set! Fib n v)
                  v)])))