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/*===========================================================================
* Filename : promise.c
* About : R5RS delayed evaluation
*
* Copyright (C) 2006 YAMAMOTO Kengo <yamaken AT bp.iij4u.or.jp>
* Copyright (c) 2007-2008 SigScheme Project <uim-en AT googlegroups.com>
*
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of authors nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS
* IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
* OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
* WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
* OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
* ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
===========================================================================*/
#include <config.h>
#include "sigscheme.h"
#include "sigschemeinternal.h"
/*=======================================
File Local Macro Definitions
=======================================*/
#define PROMISE_FORCEDP(p) (!EQ(CAR(p), l_tag_unforced))
/*=======================================
File Local Type Definitions
=======================================*/
/*=======================================
Variable Definitions
=======================================*/
#include "functable-r5rs-promise.c"
SCM_GLOBAL_VARS_BEGIN(static_promise);
#define static
static ScmObj l_tag_unforced;
#undef static
SCM_GLOBAL_VARS_END(static_promise);
#define l_tag_unforced SCM_GLOBAL_VAR(static_promise, l_tag_unforced)
SCM_DEFINE_STATIC_VARS(static_promise);
/*=======================================
File Local Function Declarations
=======================================*/
/*=======================================
Function Definitions
=======================================*/
SCM_EXPORT void
scm_init_promise(void)
{
SCM_GLOBAL_VARS_INIT(static_promise);
scm_register_funcs(scm_functable_r5rs_promise);
/* Use a pair as the unique tag. The symbol %%unforced-promise is only for
* human-readability. */
scm_gc_protect_with_init(&l_tag_unforced,
LIST_1(scm_intern("%%unforced-promise")));
}
/*===========================================================================
R5RS : 4.2 Derived expression types : 4.2.5 Delayed evaluation
===========================================================================*/
SCM_EXPORT ScmObj
scm_s_delay(ScmObj exp, ScmObj env)
{
ScmObj proc;
DECLARE_FUNCTION("delay", syntax_fixed_1);
proc = scm_s_lambda(SCM_NULL, LIST_1(exp), env);
/* (result . proc) */
return CONS(l_tag_unforced, proc);
}
/*===========================================================================
R5RS : 6.4 Control features
===========================================================================*/
SCM_EXPORT ScmObj
scm_p_force(ScmObj promise)
{
ScmObj proc, result;
DECLARE_FUNCTION("force", procedure_fixed_1);
ENSURE_CONS(promise);
proc = CDR(promise);
ENSURE_PROCEDURE(proc);
if (PROMISE_FORCEDP(promise))
return CAR(promise);
/* R5RS:
* Rationale: A promise may refer to its own value, as in the last
* example above. Forcing such a promise may cause the promise to be
* forced a second time before the value of the first force has been
* computed. This complicates the definition of `make-promise'. */
result = scm_call(proc, SCM_NULL);
if (PROMISE_FORCEDP(promise))
return CAR(promise);
SET_CAR(promise, result);
return result;
}
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