Function Utilities

(define (memo-proc proc)
  (let ((already-run? #f)
        (result '()))
    (lambda ()
      (if already-run?
          result
          (begin  (set! result (proc))
                  (set! already-run? #t)
                  result)))))     
  

memo-proc ensures that a given function proc will only be run once and it's value is cached for later reuse.

It is implemented as a special form where a closure is setup to keep track of when to execute the proc and when to return it's already computed value.