optional.h

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#pragma once
 
#include <assert.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
 
#define OPTIONAL(type)                                                         \
  struct {                                                                     \
    bool has_value;                             \
    type value_;                       \
  }
 
#define OPTIONAL_SET(me, value)                                                \
  do {                                                                         \
    assert((me) != NULL);                                                      \
    (me)->has_value = true;                                                    \
    (me)->value_ = (value);                                                    \
  } while (0)
 
#define OPTIONAL_VALUE(me)                                                     \
  (((me).has_value ? (void)0                                                   \
                   : (fprintf(stderr,                                          \
                              "%s:%d: internal error: attempted to read the "  \
                              "value of an empty optional type\n",             \
                              __FILE__, __LINE__),                             \
                      abort())),                                               \
   (me).value_)
 
#define OPTIONAL_VALUE_OR(me, fallback)                                        \
  ((me).has_value ? (me).value_ : (fallback))