scheme thunks (delayed evaluation)

scheme_thunks.scm

;; delay and force are defined in r5rs,
;; but if we were defining them for ourselves


(define-syntax my-delay
  (syntax-rules ()
    ((_ expr) 
      (lambda () expr))))

(define (my-force expr)
  (expr))

(define (not-a-delay expr)
  ;; expr will be evaluated before the closure
  (lambda () expr))

;; leys assume we have an infinite proc
(define (infinite-proc x) (infinite-proc x))

(define my-thunk (my-delay (infinite-proc 10)))

;; DON'T DO THIS
(define this-wont-work (not-a-delay (infinite-proc 10)))

;; streams --- 

(define-syntax my-lazy-cons
  (syntax-rules ()
    ((_ x xs) 
      (cons x (my-delay xs)))))