Is Actor a core type?

[silverpill]

Section 4. Model lists eight core types (Object, Link, Activity, etc). It is followed by sections describing these core types in detail: 4.1 Object, 4.2 Link and then 4.3 Actor, which is not included in the list of core types.

Actor is defined as a specialization of Object, but this is also true for Activity, and yet Activity is considered a core type.

[evanp]

So section 4 on Model does not have subsections for each of the core types listed.

It also has subsections about other topics, including natural language values and marking up languages.

So, I don't think that having a subsection on "Actor" necessarily means that Actor is a "core type". It is a little confusing that the "Actor" subsection is right in the middle of the other type subsections!

I also think that we have a weak definition of "core type". I think right now it is just an enumeration of those 8 types, but it might also mean types in the vocabulary that don't have a parent or supertype. It might also derive from the early OWL definition for AS2.

There might also be a definitional term that "core types" are "the types described and defined in AS2 Core". "Abstract types" is probably another fair term to use.

Finally, I think to be a "core type" Actor would have to be a type at all! We don't have one defined in the context document.

Actor is a "ghost type" in the hierarchy between the Object root type and the "actor types" Person, Group, etc. I think there was a lot of conversation in the Social Web WG about whether to define an Actor type, and we eventually decided not to do it. I don't remember at this point why!

@trwnh shared the link to the discussion on this:

• https://www.w3.org/wiki/Socialwg/2015-12-15-minutes
• Proposal: Drop "Actor" Type #277

I think there are a few outcomes for this ticket:

• Primer page or elsewhere defining what "core type" means besides the list of 8 types (why those 8?)
• Primer page on the "actor" types and why there is no Actor type
• Better explanation of why there is no Actor type in the vocabulary OR
• Define an Actor type in the next version of AS

[nightpool]

Does it matter what is a "core type"? What requirements even hinge on that definition?

[silverpill]

Core types mentioned many times throughout the ActivityPub spec. Quick analysis:

• "actor", 124 occurrences
• "activity": 84 occurrences
• "collection": 106 occurrences

Literally everything hinges on their definitions.

[TallTed]

Certainly, everything hangs on the definitions of the types/classes @silverpill listed (actor, activity, collection), whether or not they are core types/classes.

But how many times do the phrase "core type" or "core class" (case insensitive, perhaps with an intervening word or two, like "Core Actor Type") occur?

How much does rely on the definition(s) of "core type" or "core class", e.g., how much do the definitions of the listed types rely on the definition(s) of "core type" or "core class"?

Are the listed types really subtypes/subclasses of a "core type" or "core class"?

[nightpool]

I can't find any normative distinction in the spec between the 8 core types and the other "core vocabulary" types (the term the spec uses for the types defined in https://www.w3.org/TR/activitystreams-vocabulary/ e.g. ones that aren't part of an extension

From this, I conclude that the idea of the "core types" is simply a pedagogical concept for explaining the spec, and maybe for structuring the spec / extension types somewhat, and otherwise has no major value to implementors. So I don't see any reason to write a Primer section on this.

[silverpill]

Are the listed types really subtypes/subclasses of a "core type" or "core class"?

@TallTed Yes, they are. ActivityPub spec talks about the relationship to ActivityStreams in section 3. Objects:

Objects are the core concept around which both [ActivityStreams] and ActivityPub are built. Objects are often wrapped in Activities and are contained in streams of Collections, which are themselves subclasses of Objects. See the [Activity-Vocabulary] document, particularly the Core Classes; ActivityPub follows the mapping of this vocabulary very closely.

[strugee]

@silverpill I believe the point that @TallTed and @nightpool are making is that from an implementation perspective, it doesn't matter whether Actor is a core type because there is no requirement to alter behavior based on whether or not a type is a core type.

Nobody is disputing that the definition of e.g. Activity is important. The question is whether, since Activity is a core type, the definition of "core type" also matters (and the suggestion is that it does not).

[silverpill]

The definition of "core type" matters because according to AS2 there is a hierarchy of types:

Every JSON object in an Activity Streams 2.0 document is either an Object or a Link. All other types defined in the Activity Vocabulary, as well as all extension types, are derived from these two base types.

Then it describes Activity and Collection core types as specializations of Object with some properties inherited from Object, and other properties unique to a subtype. For example, items property is only allowed on Collection types, and target is only allowed on Activity types.

The existence of this type hierarchy implies that different core types do not overlap. They are located on the different branches of the tree, and there can't be an Activity which is also a Collection.

If actor is not a core type, and just an Object, then it can be also a Collection or an Activity.
If actor is a core type, then it can not be a Collection or an Activity.

The definition of actor supports the latter hypothesis:

Actor objects are specializations of the base Object type that represent entities capable of carrying out an Activity.

First, it is a specialization of Object, which is how Activity and Collection core types are described. Second, if actor can be an Activity, we get an awkward situation where activities can perform other activities. Therefore, I believe that removal of Actor from the list of core types was a mistake.

There is also an inconsistency between definition of the actor (type) and the actor property:

Describes one or more entities that either performed or are expected to perform the activity.

According to the definition of "actor", it is an entity that perform activities, but "Range" of actor property is Object | Link, meaning that any object can perform activity.

[nightpool]

The existence of this type hierarchy implies that different core types do not overlap. They are located on the different branches of the tree, and there can't be an Activity which is also a Collection.

If actor is not a core type, and just an Object, then it can be also a Collection or an Activity.
If actor is a core type, then it can not be a Collection or an Activity.

no idea where you're getting this from. I don't think the spec implies this and I also don't think whether Actor is a "core type" or not has any bearing on this.

Even if I did think the specification implied that core types are non-overlapping (which I don't), it's disproven by Activity and IntransitiveActivity, which are trivially overlapping and are both defined as "core" types.

[silverpill]

I don't think the spec implies this and I also don't think whether Actor is a "core type" or not has any bearing on this.

@nightpool If AS2 type hierarchy is modeled as a tree structure, then every element must have exactly one parent, and there can't be an object that is both Activity and Collection (two parents).

If Actor abstract type exists, and it is a subtype of Object, then it is adjacent to Activity and Collection in the hierarchy of types, and therefore there can't be an object that is both Activity and Actor (two parents).

If Actor abstract type does not exist, then any activity can also be an actor, because Activity is a subtype of Object, and actor is also an Object.

Even if I did think the specification implied that core types are non-overlapping (which I don't), it's disproven by Activity and IntransitiveActivity, which are trivially overlapping and are both defined as "core" types.

IntransitiveActivity is a subtype of Activity. Every intransitive activity is an activity, so they always overlap, but I was talking about different case (adjacent types).

It seems that in your view AS2 type hierarchy is not a tree structure, and any type can have more than one parent (more than one supertype). Am I correct?

I can't find anything in the specs that might support this view. All types in Activity Vocabulary have exactly one supertype (except Object and Link, of course, which are root types). And while the spec doesn't explicitly forbid the existence of types with two or more supertypes, it doesn't provide any guidance on how that should be interpreted or how objects with such types should be processed.