25 lines
717 B
Scheme
25 lines
717 B
Scheme
;; All implementations here are "borrowed" from
|
|
;; husk-scheme (github.com/justinethier/husk-scheme).
|
|
(define force
|
|
(lambda (object)
|
|
(object)))
|
|
|
|
(define-syntax delay
|
|
(syntax-rules ()
|
|
((delay expression)
|
|
(make-promise (lambda () expression)))))
|
|
|
|
(define make-promise
|
|
(lambda (proc)
|
|
(let ((result-ready? #f)
|
|
(result #f))
|
|
(lambda ()
|
|
(if result-ready?
|
|
result
|
|
(let ((x (proc)))
|
|
(if result-ready?
|
|
result
|
|
(begin (set! result x)
|
|
(set! result-ready? #t)
|
|
result))))))))
|