ikarus/scheme/ikarus.promises.ss

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;;; Ikarus Scheme -- A compiler for R6RS Scheme.
;;; Copyright (C) 2006,2007,2008 Abdulaziz Ghuloum
;;;
;;; This program is free software: you can redistribute it and/or modify
;;; it under the terms of the GNU General Public License version 3 as
;;; published by the Free Software Foundation.
;;;
;;; This program is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;;; General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with this program. If not, see <http://www.gnu.org/licenses/>.
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(library (ikarus promises)
(export force make-promise)
(import
(except (ikarus) force make-promise))
(define (force x)
(unless (procedure? x)
(die 'force "not a procedure" x))
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(x))
(define (make-promise proc)
(unless (procedure? proc)
(die 'make-promise "not a procedure" proc))
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(let ([results #f])
(lambda ()
(if results
(apply values results)
(call-with-values proc
(lambda x*
(if results
(apply values results)
(begin
(set! results x*)
(apply values x*))))))))))