62 lines
1.7 KiB
C
62 lines
1.7 KiB
C
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/*
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*
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* p r o m i s e . c -- Promises management
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*
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*
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* Copyright <EFBFBD> 1993-1996 Erick Gallesio - I3S-CNRS/ESSI <eg@unice.fr>
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*
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*
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* Permission to use, copy, and/or distribute this software and its
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* documentation for any purpose and without fee is hereby granted, provided
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* that both the above copyright notice and this permission notice appear in
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* all copies and derived works. Fees for distribution or use of this
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* software or derived works may only be charged with express written
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* permission of the copyright holder.
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* This software is provided ``as is'' without express or implied warranty.
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*
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* This software is a derivative work of other copyrighted softwares; the
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* copyright notices of these softwares are placed in the file COPYRIGHTS
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*
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*
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* Author: Erick Gallesio [eg@kaolin.unice.fr]
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* Creation date: 2-Jun-1993 12:27
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* Last file update: 10-Dec-1995 22:31
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*
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*/
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#include "stk.h"
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/*
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* Delay is implemented by syntax_delay in syntax.c
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*/
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PRIMITIVE STk_force(SCM promise)
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{
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SCM z;
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if (NPROMISEP(promise)) return promise;
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if (promise->storage_as.promise.resultknown)
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/* promise was alraedy evaluated. Simply return expr field */
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return promise->storage_as.promise.expr;
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z = Apply(promise->storage_as.promise.expr, NIL);
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if (promise->storage_as.promise.resultknown)
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/* R4RS: "A promise may refer to its own value.... Forcing such
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* a promise may cause the promise to be forced a second time before
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* the first value has been computed.
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*/
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return promise->storage_as.promise.expr;
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else {
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promise->storage_as.promise.expr = z;
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promise->storage_as.promise.resultknown = 1;
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return z;
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}
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}
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PRIMITIVE STk_promisep(SCM promise)
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{
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return PROMISEP(promise)? Truth: Ntruth;
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}
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