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* module/srfi/srfi-45.scm (promise?): Export. * doc/ref/srfi-modules.texi (SRFI-45): Update docs. * test-suite/tests/srfi-45.test: Add test. Add FSF copyright for 2010 and 2013. Add missing year to André van Tonder's copyright notice.
78 lines
3 KiB
Scheme
78 lines
3 KiB
Scheme
;;; srfi-45.scm -- Primitives for Expressing Iterative Lazy Algorithms
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;; Copyright (C) 2010, 2011, 2013 Free Software Foundation, Inc.
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;; Copyright (C) 2003 André van Tonder. All Rights Reserved.
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;; Permission is hereby granted, free of charge, to any person
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;; obtaining a copy of this software and associated documentation
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;; files (the "Software"), to deal in the Software without
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;; restriction, including without limitation the rights to use, copy,
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;; modify, merge, publish, distribute, sublicense, and/or sell copies
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;; of the Software, and to permit persons to whom the Software is
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;; furnished to do so, subject to the following conditions:
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;; The above copyright notice and this permission notice shall be
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;; included in all copies or substantial portions of the Software.
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;; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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;; EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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;; MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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;; NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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;; BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
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;; ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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;; CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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;; SOFTWARE.
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;;; Commentary:
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;; This is the code of the reference implementation of SRFI-45, modified
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;; to use SRFI-9 and to add 'promise?' to the list of exports.
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;; This module is documented in the Guile Reference Manual.
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;;; Code:
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(define-module (srfi srfi-45)
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#:export (delay
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lazy
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force
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eager
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promise?)
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#:replace (delay force promise?)
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#:use-module (srfi srfi-9))
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(cond-expand-provide (current-module) '(srfi-45))
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(define-record-type promise (make-promise val) promise?
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(val promise-val promise-val-set!))
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(define-record-type value (make-value tag proc) value?
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(tag value-tag value-tag-set!)
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(proc value-proc value-proc-set!))
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(define-syntax-rule (lazy exp)
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(make-promise (make-value 'lazy (lambda () exp))))
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(define (eager x)
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(make-promise (make-value 'eager x)))
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(define-syntax-rule (delay exp)
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(lazy (eager exp)))
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(define (force promise)
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(let ((content (promise-val promise)))
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(case (value-tag content)
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((eager) (value-proc content))
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((lazy) (let* ((promise* ((value-proc content)))
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(content (promise-val promise))) ; *
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(if (not (eqv? (value-tag content) 'eager)) ; *
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(begin (value-tag-set! content
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(value-tag (promise-val promise*)))
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(value-proc-set! content
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(value-proc (promise-val promise*)))
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(promise-val-set! promise* content)))
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(force promise))))))
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;; (*) These two lines re-fetch and check the original promise in case
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;; the first line of the let* caused it to be forced. For an example
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;; where this happens, see reentrancy test 3 below.
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