That almost looks like C code.

register_module(Name, Pred, L, !IO) :-

    ( semipure ready(L) ; impure init(L) ),

    semipure (ref(Ref) = function(Pred)
        ; unexpected($module, $pred,
        "function/1 did not return a ref.").
    ),
    impure lua_getregistry(L, LUA_RIDX_MR_MODULE), /* table -3 */
    impure lua_pushstring(L, N), /* key -2 */
    impure luaMR_pushref(L, R), /* value -1 */
    impure lua_settable(L, -3), /* table -1 */
    impure lua_pop(L, 1). /* empty stack */

:- pragma promise_pure(register_module/5).