Obligatory mentions to trwnh@mastodon.social and helge@mymath.rocks because I called out multi-typing, which is a whole other conversation in and of itself.

@julian activitypub has no concept of replies either. neither `inReplyTo`/`replies` nor `context` were specced for activitypub. these are more generally terms from the activitystreams vocabulary, and they have no side effects defined or associated with them

@julian in theory there shouldn't be anything wrong with saying a Collection is also something else, but activitypub's delivery algorithm will have unintended consequences if you address a Collection that has its own inbox

also Remove is defined with respect to object+target, not object+origin. yes, this is somewhat confusing because in english we remove "from" rather than remove "to", but that's far from the only slip-up. (currently AS2 also defines you Invite an Event to a Person...)

@julian for what it's worth, this is what i was trying to head off in the past several months of discussion, with regards to as:context *being* an as:Collection, as opposed to *having* an associated as:Collection. it's why i wrote https://w3id.org/fep/2931 to extend from https://w3id.org/fep/7888

changing the activitypub delivery algorithm would probably end up needing new properties: one that delivers directly (deliverToActor) and one that delivers to expanded items (deliverToCollection)

@julian prior art: Web Access Control differentiates subjects who are being granted authorization to access a certain resource. they have separate properties

- agent = every value is a foaf:Agent, and access is granted directly to each entity
- agentGroup = every value is a foaf:Group, and access is granted indirectly to each foaf:member
- agentClass = every value is a rdfs:Class, and access is granted according to vocab

the classes are Agent (as:Public) and AuthenticatedAgent (logged in only)

@julian so we have kind of a problem of data modeling, where we have actors (inbox), collections (items[*].inbox), possibly groups which could either deliver directly (inbox) or indirectly per foaf (member[*].inbox) or double-indirectly via swicg/groups task force's current proposal (members.items[*].inbox)

we could *maybe* avoid this by directly specifying which inboxes to deliver to, a la multibox (https://w3id.org/fep/0499 is one take on this), but it's unclear which if any/all are supported.

@julian hopefully this demonstrates why the distinction between "is a" and "has a" matters a lot. in the group case you would need to distinguish between delivering to a group vs delivering to a group's members. currently there's a lot of ambiguity around this in 1b12 applications.

trwnh@mastodon.social You're speaking theoretically... that the spec says that if a collection is addressed, then all members in that collection are by definition addressed.

Do you have an example of this in the wild? This has the potential to be a speed bump that is entirely theoretical, and can be ignored in advance of its removal from the AS2 (or AP?) spec(s).

trwnh@mastodon.social oh doy... of course it exists in the wild, we address to follower collections all the time.

... but crucially, when you address a follower collection, you don't resolve the follower collection itself, you rely on the follower to forward your activity onwards... so, again, perhaps resolving a collection for addressing isn't actually used in-practice?

@julian yes, this is how followers-only posts in mastodon work. they address your followers collection, which gets expanded to all items in the followers collection.

trwnh@mastodon.social I am going to argue technical details here — when Mastodon sends a followers-only post, they are addressing the followers collection but they aren't parsing it to retrieve targets. They already know them internally and build the targets separately.

I still know of no known implementation that sends an activity addressing a collection, and resolves that collection to find the actor ids.

@julian you could, and arguably you should/must, but "no one currently does this" is not the same as "no one will ever do this". consider the case of a public collection.example, and you author an activity to:[collection.example] which you HTTP POST to outbox.example... the specified behavior is that outbox.example fetches collection.example with your credentials and discovers inbox on any items[*]. there is undefined behavior when collection.example has an inbox.

@julian mastodon doesn't resolve anything directly over HTTP but they do the equivalent internally with an SQL query. the case where a Collection has an inbox is unhandled because mastodon ignores anything that isn't Person/Group/Organization/Application/Service. mastodon i think also disallows addressing collections that aren't your own followers collection (potentially overzealous spam protection?)

@julian in "technical terms" the reason no one has to worry about this issue is because no one implements activitypub, and when they deliver, they generally disallow delivering to any collections except your own followers collection.

imagine addressing someone else's followers collection, or your own following collection, or a group's members collection, or a public collection. those are all possible in activitypub and would behave according to the specified delivery algorithm for any AP outbox

@julian i mean, if i sent nodebb an activity addressed to collection.example right now, what would you do with it? mastodon would use that as a signal to upgrade any "direct" or "followers" post to a "limited" post. this was implemented as part of their early support for "circles", which i think are so far only a thing on fedibird and bonfire maybe? i'm not sure how audience would be inherited in further interactions but mastodon would probably default to followers-only as overridden by API

trwnh@mastodon.social but that's exactly it, nobody does it.

That's perhaps not a compelling reason to remove it from the spec, but if it's already on the chopping block then I don't feel as strongly about sticking to that part of it.

trwnh@mastodon.social I can't speak for how Mastodon would handle it, but if an arbitrary collection were addressed, I don't see why it would change visibility of the object.

Public and unlisted are containing as:Public in to and cc respectively. Followers-only is sender's follower collection addressed, and otherwise it's mentioned-only post.

Would Mastodon try to resolve the collection for actors? Good question. If it did it would likely resolve it, see that it isn't an Actor, and give up. However I'm venturing into conjecture now.

trwnh@mastodon.social said in Context deletion vs. Removal brainstorming:
> also Remove is defined with respect to object+target, not object+origin.

That's fine, I'll make the corresponding change.

I was basing it off this line in the AS spec:

> If specified, the origin indicates the context from which the object is being removed. [[source](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-remove)]

@julian if "no one POSTs to outbox" is an argument for axing the outbox, then i don't know what we'd be discussing, because what would be left? i mean, maybe we can say "addressing collections no longer expands delivery to items", but then we presumably need an alternative that doesn't involve addressing actors one-by-one.

@julian that's pretty much exactly what happens iirc, except instead of "it isn't an actor", the check mastodon does is "it isn't a Person/Group/Organization/Application/Service".

multityping [OrderedCollection, Service] as you propose would cause mastodon to try to process it as an actor, but likely fail when it doesn't pass the webfinger assertion and therefore can't be converted to an Account entity.