Memoize Thunk

scheme.rkt

(define (force-it obj)
  (if (thunk? obj)
      (define-in-environment (name thunk-exp) list('evaluated-thunk ((actual-value (thunk-exp obj) (thunk-env obj))))
      obj))
...

(define (evaluated-thunk? obj)
  (eq? (car obj) 'evaluated-thunk))
  
(define (thunk-value evaluated-thunk) (cadr evaluated-thunk))