luaL_opt

luaL_opt[-0, +0, e]

T luaL_opt (L, func, arg, dflt);

This macro is defined as follows:

(lua_isnoneornil(L,(arg)) ? (dflt) : func(L,(arg)))

In words, if the argument arg is nil or absent, the macro results in the default dflt. Otherwise, it results in the result of calling func with the state L and the argument index arg as parameters. Note that it evaluates the expression dflt only if needed.