Activity Streams/Primer/Inverse Activities and Undo

Copied from Inverse Activities and "Undo"

Several of the core Activity types are defined as natural inversions of one another. These include:

• Accept and Reject,
• Arrive and Leave,
• Join and Leave,
• Create and Delete,
• Like and Dislike

It is important to note that these types of activities are semantically distinct from one another and have no direct relationship on the other. That is, for example, if an actor "likes" a note at one point in time then later "dislikes" it, the "dislike" activity does not "undo" or negate out the prior "like".

The appropriate interpretation for the following is that Sally first liked, then later disliked John's note:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "summary": "History of John's note",
  "type": "Collection",
  "items": [
    {
      "summary": "Sally liked John's note",
      "type": "Like",
      "actor": "http://sally.example.org",
      "id": "http://activities.example.com/1",
      "published": "2015-11-12T12:34:56Z",
      "object": {
        "summary": "John's note",
        "type": "Note",
        "id": "http://notes.example.com/1",
        "attributedTo": "http://john.example.org",
        "content": "My note"
      }
    },
    {
      "summary": "Sally disliked John's note",
      "type": "Dislike",
      "actor": "http://sally.example.org",
      "id": "http://activities.example.com/2",
      "published": "2015-12-11T21:43:56Z",
      "object": {
        "summary": "John's note",
        "type": "Note",
        "id": "http://notes.example.com/1",
        "attributedTo": "http://john.example.org",
        "content": "My note"
      }
    }
  ]
}

The Undo activity type is defined to provide the specific ability to undo or cancel out a prior activity. The appropriate interpretation for the following, then, is that Sally liked John's note at one point but has explicitly redacted that like later on.

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "summary": "History of John's note",
  "type": "Collection",
  "items": [
    {
      "summary": "Sally liked John's note",
      "type": "Like",
      "id": "http://activities.example.com/1",
      "actor": "http://sally.example.org",
      "published": "2015-11-12T12:34:56Z",
      "object": {
        "summary": "John's note",
        "type": "Note",
        "id": "http://notes.example.com/1",
        "attributedTo": "http://john.example.org",
        "content": "My note"
      }
    },
    {
      "summary": "Sally no longer likes John's note",
      "type": "Undo",
      "id": "http://activities.example.com/2",
      "actor": "http://sally.example.org",
      "published": "2015-12-11T21:43:56Z",
      "object": "http://activities.example.com/1"
    }
  ]
}

The end result of the former example is that Sally has indicated that she changed her opinion about John's note and now dislikes it, while in the latter example she currently neither likes or dislikes it.