Re: __update metamethod — intercept writes to exis ting keys (patch for 5.5.0)

I also like this idea, if I am allowed to comment :).

(I am not sure about the solution presented... of course it should be
added only if it is not adding too much overhead).

(I also recently considered to add a bit handling metaclass in form of
"array style" - but finally I decided against array style, as in this
case I preferred to create new bit fields with a special "init / open
access function", and not allow creating them just by definition of a
new array element (in case of such new meta functions - I want to
avoid the possibility that a new class is defined by accidential name
typing error)).

(so if there would be the possibility to use a function like
"__update", but NOT support "__newindex", this would solve my
objection here).

Am Mi., 8. Apr. 2026 um 12:23 Uhr schrieb chen chen
<[email protected]>:
>
> Hello,
>
> I've been thinking about the gap between `__newindex` (new keys only) and the need to intercept updates to existing keys. The proxy-table workaround works but has notable overhead. I put together a small patch (~95 lines) for Lua 5.5.0 to explore what a native solution might look like — I'd love to hear if this direction has been considered or if there are concerns I'm missing.
>
> The patch adds an `__update` metamethod that fires when assigning to an existing key (including deletion via nil), complementing `__newindex`. Same-value writes are skipped as a fast-path optimization.
>
> ```lua
> local t = setmetatable({}, {
>     __update = function(t, k, v)
>         rawset(t, k, v)  -- or do validation, logging, etc.
>     end
> })
> t.x = 1   -- fires (new key, no __newindex)
> t.x = 2   -- fires (existing key, different value)
> t.x = 2   -- no fire (same value, fast-path skip)
> t.x = nil -- fires (existing key, deletion)
> ```
>
> This eliminates the proxy-table pattern (`__newindex` + `__index` + `__pairs` on an empty table). Tables without `__update` have zero overhead — a single bit flag gates the fast path.
>
> Full proposal, implementation details, and benchmarks:
> https://github.com/Ne9roni/lua-update-metamethod
>
> Feedback welcome!
>
> --
> You received this message because you are subscribed to the Google Groups "lua-l" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to [email protected].
> To view this discussion visit https://groups.google.com/d/msgid/lua-l/e5ca8f49-c6d6-42c7-8524-11308ef59914n%40googlegroups.com.

-- 
You received this message because you are subscribed to the Google Groups "lua-l" group.
To unsubscribe from this group and stop receiving emails from it, send an email to [email protected].
To view this discussion visit https://groups.google.com/d/msgid/lua-l/CAOnDXmG3A0P1bXKDpm%3D8rNS8SZ2Jg6sPEB6e2hJNLk6iHYHuAw%40mail.gmail.com.