Binding Enums To Lua

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Here is a very simple (and incomplete) example. The goal was to create a cron-like facility, with a very simple interface:

-- add a task to be run on a given day of the week
cron.add("Monday", task)

-- return an iterator of tasks for a given day of the week
cron.tasks("Wednesday")

The following code does not actually implement these functions; it simply sketches a way of introducing the enums for the days of the week. Most of it is boilerplate code.

/*
   The auxiliary function to convert an argument to an enum
   This function assumes that the weekday table is the first
   upvalue, which in this case it is.
 */
static int get_weekday (lua_State *L, int argno)
{
  int weekday;
  lua_pushvalue(L, argno);
  lua_gettable(L, lua_upvalueindex(1));
  weekday = lua_tonumber(L, -1);
  lua_pop(L, 1);
  if (weekday == 0) {
    /* This works because there is no 0 in the weekday
        table and lua_tonumber returns 0 for any non-number.
        We could have used a lua_isnil() test instead, had 0
        been a possible return.
     */
    luaL_typerror(L, argno, "weekday");
  }
  return weekday;
}

/*
   These functions actually need to be implemented. All
   that is illustrated here is how to get the enum value.
 */ 
static int cron_add (lua_State *L) 
{
  int weekday = get_weekday(L, 1);
  // ...
}

static int cron_tasks (lua_State *L)
{
  int weekday = get_weekday(L, 1);
  // ...
}


/* As usual, a luaL_reg of function names and functions */
static const luaL_reg cron_funcs[] = {
  {"add", cron_add},
  {"tasks", cron_tasks}
  {NULL, NULL}
};
 

/*
   The function which creates the library; it must start by
   creating the enum table. This function needs to be added
   to the list of open functions called when the lua_State is
   created (unless the library is going to be loaded dynamically,
   of course, but that requires the same function.)
 */
int luaopen_cron (lua_State *L) {
  /* make the weekday table */
  lua_newtable(L);
  lua_pushliteral(L, "Monday"); lua_pushnumber(L, 1); lua_settable(L, -3);
  lua_pushliteral(L, "Tuesday"); lua_pushnumber(L, 2); lua_settable(L, -3);
  lua_pushliteral(L, "Wednesday"); lua_pushnumber(L, 3); lua_settable(L, -3);
  lua_pushliteral(L, "Thursday"); lua_pushnumber(L, 4); lua_settable(L, -3);
  lua_pushliteral(L, "Friday"); lua_pushnumber(L, 5); lua_settable(L, -3);
  lua_pushliteral(L, "Saturday"); lua_pushnumber(L, 6); lua_settable(L, -3);
  lua_pushliteral(L, "Sunday"); lua_pushnumber(L, 7); lua_settable(L, -3);
  /* Register the library */
  luaL_openlib(L, "cron", cron_funcs, 1);
  return 1;
}
        

The sequence of pushliteral, pushnumber, settable is a big ugly and hard to maintain. Fortunately, we can solve that with a macro.

#define LUA_ENUM(L, name, val) \
  lua_pushlstring(L, #name, sizeof(#name)-1); \
  lua_pushnumber(L, val); \
  lua_settable(L, -3);

Now we can even do this:

  lua_newtable(L);
  {
    int i = 1;
    LUA_ENUM(L, Monday, i++);
    LUA_ENUM(L, Tuesday, i++);
    LUA_ENUM(L, Wednesday, i++);
    LUA_ENUM(L, Thursday, i++);
    LUA_ENUM(L, Friday, i++);
    LUA_ENUM(L, Saturday, i++);
    LUA_ENUM(L, Sunday, i++);
  }

But we can do even better than that, with a little bit of CPP magic. We could use the same data to create both a C enum and the Lua conversion table.

#define C_ENUM_HELPER(cname, luaname)  cname,
#define LUA_ENUM_HELPER(cname, luaname) LUA_ENUM(L, luaname, cname)

#define WEEKDAY \
  E(WEEKDAY_MONDAY, Monday) \
  E(WEEKDAY_TUESDAY, Tuesday) \
  E(WEEKDAY_WEDNESDAY, Wednesday) \
  E(WEEKDAY_THURSDAY, Thursday) \
  E(WEEKDAY_FRIDAY, Friday) \
  E(WEEKDAY_SATURDAY, Saturday) \
  E(WEEKDAY_SUNDAY, Sunday)

/* In the c header file: */
#define E C_ENUM_HELPER

enum Weekday {
WEEKDAY
WEEKDAY_LAST  /* Avoid the trailing comma problem */ 
};

#undef E

/* In the Lua file: */
#define E LUA_ENUM_HELPER

  lua_newtable(L);
  WEEKDAY

#undef E