tuple.c

#include <lua.h>
#include <lauxlib.h>

int tuple (lua_State *L) {
    /* first upvalue is number of tuple values */
    int argc = luaL_checkint(L, lua_upvalueindex(1));
    
    /* if arg is "#", return number of tuple values */
    const char *count = luaL_optstring(L, 1, "");
    if (count[0] == '#') {
        lua_pushnumber(L, argc);
        return 1;
    }
    
    int index = luaL_optint(L, 1, 0);
    /* if arg is nothing or 0, return all values */
    if (index == 0) {
        int i;
        for (i = 2; i <= argc + 1; i++) {
            lua_pushvalue(L, lua_upvalueindex(i));
        }
        return argc;
    }
    
    /* if arg is invalid index, return nothing */
    if (index < 0 || index > argc) {
        return 0;
    }
    
    /* else arg is valid index, return value at index */
    lua_pushvalue(L, lua_upvalueindex(index + 1));
    return 1;
}


int tuple_new (lua_State *L) {
    /* push number of args onto bottom of stack */
    int argc = lua_gettop(L);
    lua_pushnumber(L, argc);
    lua_insert(L, 1);
    /* return tuple function */
    lua_pushcclosure(L, tuple, argc + 1);
    return 1;
}

int luaopen_tuple (lua_State *L) {
    lua_pushcclosure(L, tuple_new, 0);
    return 1;
}

xample.lua

local tuple = require 'tuple'

local t = tuple(123, 'x', {})

print(t())      -- prints 123, x, table[...]
print(t(2))     -- prints x
print(t('#'))   -- prints 3