Thunk

scheme.rkt

; "Thunk:" An unevaluated expression along with an environment (where it would evaluate)
(define (delay-it sexp env)
  (list 'thunk sexp env))

(define (thunk? obj)
  (and (pair? obj) (eq? (car obj) 'thunk)))

(define (thunk-exp obj)
  (cadr obj))

(define (thunk-env obj)
  (caddr obj))

; Evaluation of thunks
(define (force-it obj)
  (if (thunk? obj)
      (actual-value (thunk-exp obj) (thunk-env obj))
      obj))