/usr/share/scheme48-1.9/srfi/srfi-45.scm is in scheme48 1.9-5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 | ;;; SRFI 45, which this code is part of, bears the following
;;; copyright/license notice:
;;; Copyright (C) André van Tonder (2003). All Rights Reserved.
;;;
;;; Permission is hereby granted, free of charge, to any person
;;; obtaining a copy of this software and associated documentation
;;; files (the "Software"), to deal in the Software without
;;; restriction, including without limitation the rights to use, copy,
;;; modify, merge, publish, distribute, sublicense, and/or sell copies
;;; of the Software, and to permit persons to whom the Software is
;;; furnished to do so, subject to the following conditions:
;;;
;;; The above copyright notice and this permission notice shall be
;;; included in all copies or substantial portions of the Software.
;;;
;;; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
;;; EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
;;; MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
;;; NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
;;; HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
;;; WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
;;; OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
;;; DEALINGS IN THE SOFTWARE.
(define-record-type promise :promise
(make-promise pair)
promise?
;; This pair has as a car either 'EAGER (in which case the cdr is
;; the value) or 'LAZY (in which case the cdr is a thunk).
(pair promise-ref set-promise!))
(define-record-discloser :promise
(lambda (r)
(case (car (promise-ref r))
((eager)
(list 'promise 'eager (cdr (promise-ref r))))
((lazy)
(list 'promise 'lazy)))))
;;;=========================================================================
;;; Primitives for lazy evaluation:
(define-syntax lazy
(syntax-rules ()
((lazy exp)
(make-promise (cons 'lazy (lambda () exp))))))
(define (eager x)
(make-promise (cons 'eager x)))
(define-syntax delay
(syntax-rules ()
((delay exp) (lazy (eager exp)))))
(define (force promise)
(let ((content (promise-ref promise)))
(case (car content)
((eager) (cdr content))
((lazy) (let* ((promise* ((cdr content)))
(content (promise-ref promise))) ; *
(if (not (eqv? (car content) 'eager)) ; *
(begin (set-car! content (car (promise-ref promise*)))
(set-cdr! content (cdr (promise-ref promise*)))
(set-promise! promise* content)))
(force promise))))))
; (*) These two lines re-fetch and check the original promise in case
; the first line of the let* caused it to be forced. For an example
; where this happens, see reentrancy test 3 below.
|