For anyone curious, here's what it would take for the inclusion of Public to actually make sense:

• Dropping the as prefix entirely
• Using @base = https://www.w3.org/ns/activitystreams/ (and rewriting all @id IRIs to avoid using compact IRIs)

Such an example is here:

https://json-ld.org/playground/#startTab=tab-compacted&copyContext=true&json-ld={"@context":[{"@base":"https://www.w3.org/ns/activitystreams/","Public":{"@id":"https://www.w3.org/ns/activitystreams/Public"},"to":{"@id":"https://www.w3.org/ns/activitystreams/to","@type":"@id"},"cc":{"@id":"https://www.w3.org/ns/activitystreams/cc","@type":"@id"},"audience":{"@id":"https://www.w3.org/ns/activitystreams/audience","@type":"@id"}}],"to":"Public","audience":"https://www.w3.org/ns/activitystreams/Public"}

Obviously, this is completely untenable and would break a ton of things, as well as making all @id values potentially relative to this @base. So there's no way to get Public to "work right". It has to be as:Public (or, if the prefix were removed, https://www.w3.org/ns/activitystreams#Public)

EDIT: Apparently it can be done with @type: @vocab!

https://json-ld.org/playground/#startTab=tab-expanded&copyContext=true&json-ld={"@context":[{"as":"https://www.w3.org/ns/activitystreams#","Public":"https://www.w3.org/ns/activitystreams#Public","to":{"@id":"https://www.w3.org/ns/activitystreams#to","@type":"@vocab"},"cc":{"@id":"https://www.w3.org/ns/activitystreams#cc","@type":"@vocab"},"audience":{"@id":"https://www.w3.org/ns/activitystreams#audience","@type":"@vocab"}}],"to":["Public","https://example.com/1"],"cc":["as:Public","https://example.com/2"],"audience":["https://www.w3.org/ns/activitystreams#Public","https://example.com/3"]}

JSON-LD 1.1 Section 4.2.3 "Type Coercion" describes this in further detail: https://www.w3.org/TR/json-ld11/#type-coercion

Examples 65 and 66 show this in further detail.

In order to make use of this, the context document needs to change to, cc, audience (and bto and bcc?) from @type: @id to @type: @vocab. As explained in the above section, @type: @vocab is like @type: @id but it does term expansion first (whereas @type: @id will expand against @base instead)

Example doc:

{
  "@context": [
    {
      "as": "https://www.w3.org/ns/activitystreams#",
      "Public": "https://www.w3.org/ns/activitystreams#Public",
      "to": {
        "@id": "https://www.w3.org/ns/activitystreams#to",
        "@type": "@vocab"
      },
      "cc": {
        "@id": "https://www.w3.org/ns/activitystreams#cc",
        "@type": "@vocab"
      },
      "audience": {
        "@id": "https://www.w3.org/ns/activitystreams#audience",
        "@type": "@vocab"
      }
    }
  ],
  "to": [
    "Public",
    "https://example.com/1"
  ],
  "cc": [
    "as:Public",
    "https://example.com/2"
  ],
  "audience": [
    "https://www.w3.org/ns/activitystreams#Public",
    "https://example.com/3"
  ]
}

Expands to:

[
  {
    "https://www.w3.org/ns/activitystreams#audience": [
      {
        "@id": "https://www.w3.org/ns/activitystreams#Public"
      },
      {
        "@id": "https://example.com/3"
      }
    ],
    "https://www.w3.org/ns/activitystreams#cc": [
      {
        "@id": "https://www.w3.org/ns/activitystreams#Public"
      },
      {
        "@id": "https://example.com/2"
      }
    ],
    "https://www.w3.org/ns/activitystreams#to": [
      {
        "@id": "https://www.w3.org/ns/activitystreams#Public"
      },
      {
        "@id": "https://example.com/1"
      }
    ]
  }
]

Compacts to:

{
  "@context": {
    "as": "https://www.w3.org/ns/activitystreams#",
    "Public": "https://www.w3.org/ns/activitystreams#Public",
    "to": {
      "@id": "https://www.w3.org/ns/activitystreams#to",
      "@type": "@vocab"
    },
    "cc": {
      "@id": "https://www.w3.org/ns/activitystreams#cc",
      "@type": "@vocab"
    },
    "audience": {
      "@id": "https://www.w3.org/ns/activitystreams#audience",
      "@type": "@vocab"
    }
  },
  "audience": [
    "Public",
    "https://example.com/3"
  ],
  "cc": [
    "Public",
    "https://example.com/2"
  ],
  "to": [
    "Public",
    "https://example.com/1"
  ]
}

This issue has been mentioned on SocialHub. There might be relevant details there:

https://socialhub.activitypub.rocks/t/fep-7502-limiting-visibility-to-authenticated-actors/3784/2

@nightpool — Please edit your #404 (comment) and codefence every instance of any @word (single backticks will do, a la `@context`), including those in quoteblocks, such that every comment made here does not ping those GitHub users who did not choose to participate in this discussion.

One thing I am not entirely clear on wrt @type: @vocab is whether it makes sense to close the door entirely on addressing IRIs relative to the @base. For example there is the possibility of doing something like

@context: [
  activitystreams, {
    @base: https://example.com/
  }]
to: [users/1, users/2]

or doing something like

GET https://example.com/actor

@context: activitystreams
inbox: inbox  # expands to https://example.com/actor/inbox
outbox: outbox  # expands to https://example.com/actor/outbox

My immediate take is that these relative IRI references should probably be avoided, even though they are valid. But in order to keep the possibility open, one would have to go with the "do nothing, use as:Public" option (and keep the coercion as @type: @id)

@nightpool — Please edit your new #404 (comment) and delete the quoted email content, which includes a number of @name, which cannot be codefenced because GitHub doesn't handle Markdown in emailed comments. Sorry to keep banging on this drum, but it's the only way to prevent pinging those uninvolved GitHub users.

2.2 of as2-core says

Relative IRI (and URL) references SHOULD NOT be used within an Activity Streams 2.0 document due to the fact that many JSON parser implementations are not capable of reliably preserving the base context necessary to properly resolve relative references.

so that's a SHOULD NOT, not a MUST NOT, but good catch anyway! still worth keeping in mind that type:vocab makes things relative to the vocab mapping and not the base mapping.

"expected" language is good for non-normative discussion (primer or the like), instead of "should" or "must" that are often read as normative even in non-normative documents (or non-normative sections of normative documents) and even when lowercase

I think the best steps for us right now are as follows:

1. A Primer page that lays out the guidance in option 1 -- that publishers ought to use as:Public for maximum interop, and that consumers ought to expect all three options.

2. We consider the normative change of adding @type: @vocab to the AS2 context document once a new working group is called. We can mark this issue as "Next version" so we remember to consider it at that time (even if it's closed).

3. Review the AS2 and AP specs for examples that use Public or the https://www.w3.org/ns/activitystreams#Public and consider making ERRATA for those examples, since they are technically not compliant since they aren't equivalent to the compacted form.

2. We consider the normative change of adding @type: @vocab to the AS2 context document once a new working group is called. We can mark this issue as "Next version" so we remember to consider it at that time (even if it's closed).

Speaking of considerations and next versions, a more ideal path forward might be to consider not putting the Public pseudo-collection in the addressing properties at all, but instead defining a new property visibilityClass/readClass which can be either Public or null/missing (or extended with other classes like acl:AuthenticatedAgent, etc). This is kind of like a halfway-point between what we have now vs. adopting an additional access control framework like Web Access Control or something inspired by it (since WAC might not be ideal in its current form wholesale... it's certainly more verbose than it needs to be for a simple "visibility check"!)

If we go "all the way" and adopt WAC in some form, it could look something like this at ~minimum:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "acl": "https://www.w3.org/ns/auth/acl#",
      "accessControl": {
        "@id": "acl:accessControl",
        "@type": "@id"
      },
      "aclMode": {
        "@id": "acl:mode",
        "@type": "@vocab"
      },
      "agentClass": {
        "@id": "acl:agentClass",
        "@type": "@vocab"
      },
      "agent": {
        "@id": "acl:agent",
        "@type": "@id"
      },
      "Agent": "http://xmlns.com/foaf/0.1/Agent", // this is technically the same thing as Public, so it could be considered redundant, but presumably the WAC side of things will include this anyway
      "AuthenticatedAgent": "acl:AuthenticatedAgent",
      "accessTo": {
        "@id": "acl:accessTo",
        "@type": "@id"
      }
    }
  ],
  "id": "https://domain.example/some-object",
  "type": "Note",
  "attributedTo": "https://domain.example/the-author",
  "content": "This is Public, as signaled by WAC and the ACL ontology",
  "accessControl": [
    {
      "type": "acl:Authorization",
      "agentClass": ["Public", "Agent"], // these mean the same thing
      "aclMode": "acl:Read",
      "accessTo": "https://domain.example/some-object"
    },
    {
      "type": "acl:Authorization",
      "agent": "https://domain.example/the-author",
      "aclMode": ["acl:Read", "acl:Write"],
      "accessTo": "https://domain.example/some-object"
    }
  ]
}

If we go with something more bespoke:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "visibilityClass": {
        "@id": "as:visibilityClass",
        "@type": "@vocab"
      }
    }
  ],
  "id": "https://domain.example/some-object",
  "type": "Note",
  "attributedTo": "https://domain.example/the-author",
  "content": "This is Public, as signaled by a new property",
  "visibilityClass": "Public"  // no more as:Public, this actually uses the vocab mapping
}

Over time, consumers should be encouraged to support this new property. After some time has passed, producers should be discouraged from addressing as:Public and encouraged to use the new property. Producers can also adopt the property early if they are okay with falling back to having their activities and objects be interpreted as non-public.

There's no seamless way to do this, but that's more because addressing as:Public was a mistake and should not have been used to signal visibility. There is a slight consideration for an exemption, which is to have clients explicitly signal that they are opting into the "deliver to all shared inboxes" behavior that is lightly touched upon in the spec as a MAY.

We did not find any examples of Public in AS2 core or vocabulary.

Uses of full URI in AP:

• example 6
• example 10
• explanatory note in 5.6 "Public Addressing"
• example 15
• example 16

I've added errata for these examples and note to https://www.w3.org/wiki/ActivityPub_errata/Proposed

• In Examples 6, 10, 15, 16 the public collection address should be compacted to "as:Public".
• The Note in section 5.6 "Public Addressing" should read Compacting an ActivityStreams object using the ActivityStreams JSON-LD context will result in "https://www.w3.org/ns/activitystreams#Public" being represented as "as:Public". Publishers might erroneously produce representations with the full URI or with the unprefixed value "Public". To maximize interoperability, consumers can accept all three representations.

I might suggest (without the parenthetical) —

To maximize interoperability (and comply with Postel's Law), consumers SHOULD accept all three representations.