60 lines
1.6 KiB
C
60 lines
1.6 KiB
C
/*
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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 © 1993-1999 Erick Gallesio - I3S-CNRS/ESSI <eg@unice.fr>
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*
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*
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* Permission to use, copy, modify, distribute,and license this
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* software and its documentation for any purpose is hereby granted,
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* provided that existing copyright notices are retained in all
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* copies and that this notice is included verbatim in any
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* distributions. No written agreement, license, or royalty fee is
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* required for any of the authorized uses.
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* This software is provided ``AS IS'' without express or implied
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* warranty.
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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: 3-Sep-1999 20:22 (eg)
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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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