Re: Notification of metatable change?

[Marc Lepage <mlepage@...>]

You mean, a metatable reaches back into the object to look for fields, in certain cases? Yeah, I prototyped that and it works. That's what I meant in my original post, about adding extension hooks to the metatable. It means that client code has to be careful to edit the hooks and not the metatable directly. I might end up having to do it that way, but I'm investigating alternatives.

Or, is this technique slightly different than I am thinking?

On Mon, Oct 15, 2012 at 5:17 PM, Coda Highland <chighland@gmail.com> wrote:

Wouldn't a meta-metatable be less invasive for this kind of thing?
Have a singular metatable that you assign to everything you care about
that delegates its behaviors to an element of the table it's assigned
to.

I suppose performance might be somewhat lower, if you get a LOT of
metatable accesses, but it wouldn't be all that hard.

/s/ Adam

On Mon, Oct 15, 2012 at 2:12 PM, Marc Lepage <mlepage@antimeta.com> wrote:
> Yeah, it's just that being notified of setmetatable would facilitate using
> metatables to build state machines (getting notification of the state
> change), and wrapping metatables allows states to be composed, but the
> notification is also necessary for maintaining the wrapping.
>
>
> On Mon, Oct 15, 2012 at 5:05 PM, Rena <hyperhacker@gmail.com> wrote:
>>
>> On 2012-10-15 4:50 PM, "Marc Lepage" <mlepage@antimeta.com> wrote:
>> >
>> > I'm wondering if there's a good way to know if a metatable for an object
>> > is changed.
>> >
>> > Suppose the case is that the metatable is not protected. An object wants
>> > to wrap its metatable with another, so it can be extended. But if the
>> > metatable is changed, it needs to re-wrap it.
>> >
>> > Is there a good way the object can know when that occurs, so it can
>> > perform the re-wrapping?
>> >
>> > I thought of re-writing setmetatable to perform a notification. I guess
>> > that would work, but it seems heavy-handed.
>> >
>> > I thought of requiring clients to call a different function that does
>> > the notification (or even the wrapping), but that requires editing clients.
>> >
>> > It would be cool if objects could implement __setmetatable, which would
>> > be called if available, when the metatable is set. It would effectively
>> > replace __metatable, because you could always just ignore it (do nothing) if
>> > you don't want the metatable to be changeable.
>> >
>> > Does anyone have advice on this topic? Thanks.
>>
>> Wrapping setmetatable to call some type of notification metamethod on the
>> old metatable before applying the new one seems like the best idea. Then you
>> can do things like filter or modify the new metatable (have the notification
>> method take the new table as parameter and return a value that then gets set
>> as the new metatable), while client code doesn't need to know anything about
>> it.
>>
>> Of course when you modify a global/standard method you have to consider
>> modules loaded earlier that might keep a local reference, lua_setmetatable,
>> and potential client headache when they aren't aware of the new behaviour...
>
>