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