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;;; srfi-45.scm -- Primitives for Expressing Iterative Lazy Algorithms
;; Copyright (C) 2010, 2011, 2013 Free Software Foundation, Inc.
;; Copyright (C) 2003 André van Tonder. 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.
;;; Commentary:
;; This is the code of the reference implementation of SRFI-45, modified
;; to use SRFI-9 and to add 'promise?' to the list of exports.
;; This module is documented in the Guile Reference Manual.
;;; Code:
(define-module (srfi srfi-45)
#:export (delay
lazy
force
eager
promise?)
#:replace (delay force promise?)
#:use-module (srfi srfi-9)
#:use-module (srfi srfi-9 gnu))
(cond-expand-provide (current-module) '(srfi-45))
(define-record-type promise (make-promise val) promise?
(val promise-val promise-val-set!))
(define-record-type value (make-value tag proc) value?
(tag value-tag value-tag-set!)
(proc value-proc value-proc-set!))
(define-syntax-rule (lazy exp)
(make-promise (make-value 'lazy (lambda () exp))))
(define (eager x)
(make-promise (make-value 'eager x)))
(define-syntax-rule (delay exp)
(lazy (eager exp)))
(define (force promise)
(let ((content (promise-val promise)))
(case (value-tag content)
((eager) (value-proc content))
((lazy) (let* ((promise* ((value-proc content)))
(content (promise-val promise))) ; *
(if (not (eqv? (value-tag content) 'eager)) ; *
(begin (value-tag-set! content
(value-tag (promise-val promise*)))
(value-proc-set! content
(value-proc (promise-val promise*)))
(promise-val-set! 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.
(define* (promise-visit promise #:key on-eager on-lazy)
(define content (promise-val promise))
(case (value-tag content)
((eager) (on-eager (value-proc content)))
((lazy) (on-lazy (value-proc content)))))
(set-record-type-printer! promise
(lambda (promise port)
(promise-visit promise
#:on-eager (lambda (value)
(format port "#<promise = ~s>" value))
#:on-lazy (lambda (proc)
(format port "#<promise => ~s>" proc)))))
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