libguile/boolean.h

   1 /* classes: h_files */
   2
   3 #ifndef SCM_BOOLEAN_H
   4 #define SCM_BOOLEAN_H
   5
   6 /* Copyright (C) 1995,1996,2000, 2006, 2008, 2009, 2010 Free Software Foundation, Inc.
   7  *
   8  * This library is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public License
  10  * as published by the Free Software Foundation; either version 3 of
  11  * the License, or (at your option) any later version.
  12  *
  13  * This library is distributed in the hope that it will be useful, but
  14  * WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with this library; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  21  * 02110-1301 USA
  22  */
  23
  24 \f
  25
  26 #include "libguile/__scm.h"
  27
  28 \f
  29
  30 /* Boolean Values. Obviously there are #t and #f, but there is also nil to deal
  31  * with. We choose to treat nil as a false boolean. All options might silently
  32  * break existing code, but this one seems most responsible.
  33  *
  34  */
  35
  36 /*
  37  * Use these macros if it's important (for correctness)
  38  * that #nil MUST be considered true
  39  */
  40 #define scm_is_false_and_not_nil(x)     (scm_is_eq ((x), SCM_BOOL_F))
  41 #define scm_is_true_or_nil(x)          (!scm_is_eq ((x), SCM_BOOL_F))
  42
  43 /*
  44  * Use these macros if #nil will never be tested,
  45  * for increased efficiency.
  46  */
  47 #define scm_is_false_assume_not_nil(x)  (scm_is_eq ((x), SCM_BOOL_F))
  48 #define scm_is_true_assume_not_nil(x)  (!scm_is_eq ((x), SCM_BOOL_F))
  49
  50 /*
  51  * See the comments preceeding the definitions of SCM_BOOL_F and
  52  * SCM_MATCHES_BITS_IN_COMMON in tags.h for more information on
  53  * how the following macro works.
  54  */
  55 #define scm_is_false_or_nil(x)    \
  56   (SCM_MATCHES_BITS_IN_COMMON ((x), SCM_ELISP_NIL, SCM_BOOL_F))
  57 #define scm_is_true_and_not_nil(x) (!scm_is_false_or_nil (x))
  58
  59 /*
  60 #nil is false. */
  61 #define scm_is_false(x)  (scm_is_false_or_nil (x))
  62 #define scm_is_true(x)   (!scm_is_false (x))
  63
  64 /*
  65  * Since we know SCM_BOOL_F and SCM_BOOL_T differ by exactly one bit,
  66  * and that SCM_BOOL_F and SCM_ELISP_NIL differ by exactly one bit,
  67  * and that they of course can't be the same bit (or else SCM_BOOL_T
  68  * and SCM_ELISP_NIL be would equal), it follows that SCM_BOOL_T and
  69  * SCM_ELISP_NIL differ by exactly two bits, and these are the bits
  70  * which will be ignored by SCM_MATCHES_BITS_IN_COMMON below.
  71  *
  72  * See the comments preceeding the definitions of SCM_BOOL_F and
  73  * SCM_MATCHES_BITS_IN_COMMON in tags.h for more information.
  74  *
  75  * If SCM_ENABLE_ELISP is true, then scm_is_bool_or_nil(x)
  76  * returns 1 if and only if x is one of the following: SCM_BOOL_F,
  77  * SCM_BOOL_T, SCM_ELISP_NIL, or SCM_XXX_ANOTHER_BOOLEAN_DONT_USE_0.
  78  * Otherwise, it returns 0.
  79  */
  80 #define scm_is_bool_or_nil(x)  \
  81   (SCM_MATCHES_BITS_IN_COMMON ((x), SCM_BOOL_T, SCM_ELISP_NIL))
  82 #define scm_is_bool_and_not_nil(x)  \
  83   (SCM_MATCHES_BITS_IN_COMMON ((x), SCM_BOOL_F, SCM_BOOL_T))
  84
  85 SCM_API int scm_is_bool (SCM);
  86
  87 #define scm_is_bool(x)   (scm_is_bool_or_nil (x))
  88
  89 #define scm_from_bool(x) ((x) ? SCM_BOOL_T : SCM_BOOL_F)
  90 SCM_API int scm_to_bool (SCM x);
  91
  92 \f
  93
  94 /*
  95  * The following macros efficiently implement boolean truth testing as
  96  * expected by most lisps, which treat '() aka SCM_EOL as false.
  97  *
  98  * Since we know SCM_ELISP_NIL and SCM_BOOL_F differ by exactly one
  99  * bit, and that SCM_ELISP_NIL and SCM_EOL differ by exactly one bit,
 100  * and that they of course can't be the same bit (or else SCM_BOOL_F
 101  * and SCM_EOL be would equal), it follows that SCM_BOOL_F and SCM_EOL
 102  * differ by exactly two bits, and these are the bits which will be
 103  * ignored by SCM_MATCHES_BITS_IN_COMMON below.
 104  *
 105  * See the comments preceeding the definitions of SCM_BOOL_F and
 106  * SCM_MATCHES_BITS_IN_COMMON in tags.h for more information.
 107  *
 108  * scm_is_lisp_false(x) returns 1 if and only if x is one of the
 109  * following: SCM_BOOL_F, SCM_ELISP_NIL, SCM_EOL or
 110  * SCM_XXX_ANOTHER_LISP_FALSE_DONT_USE.  Otherwise, it returns 0.
 111  */
 112 #define scm_is_lisp_false(x)  \
 113   (SCM_MATCHES_BITS_IN_COMMON ((x), SCM_BOOL_F, SCM_EOL))
 114
 115 \f
 116
 117 SCM_API SCM scm_not (SCM x);
 118 SCM_API SCM scm_boolean_p (SCM obj);
 119
 120 SCM_INTERNAL void scm_init_boolean (void);
 121
 122 #endif  /* SCM_BOOLEAN_H */
 123
 124 /*
 125   Local Variables:
 126   c-file-style: "gnu"
 127   End:
 128 */