fep

FEP-a070: Ordered properties for plain JSON consumers

Summary

In a [Github-issue] filed against the normative AS2 context, it was pointed out that attachment and tag are unordered by default, although some implementations of “fediverse” software blindly assume them to always be ordered. This can be made unambiguous by using @list in JSON-LD, but for plain JSON consumers, a separate shorthand term must be defined. This FEP attempts to disambiguate between unordered and ordered arrays for those plain JSON consumers.

Prior art in items vs orderedItems

(This section is non-normative.)

In the [AS2-vocab], there is items, which is used to express the items included in a Collection or OrderedCollection. However, in [ActivityPub], the OrderedCollection type is mandated to be reverse chronological order. Tangentially, it is valid to have a Collection with items that are either ordered or unordered.

The normative [AS2-context] deals with issue by defining two different terms: items and orderedItems. Both of these terms have the same @id of https://www.w3.org/ns/activitystreams#items and the same @type of @id to indicate that they contain nodes on the graph, but orderedItems is additionally defined with a @container of an ordered @list, overriding the default @container of an unordered @set.

The use of orderedItems is present in the examples for items within [AS2-vocab], but it is not defined separately because it is not a separate term. Following this pattern, we can define similarly “ordered” counterparts to “unordered” existing properties.

An exploration of properties that producers may wish to explicitly order

(This section is non-normative)

attachment
There is nothing that requires attachments to an object to be ordered; however, it is a popular expectation in several cases that the attachments should be ordered. For example, someone authoring an object may wish to attach three images in order, representing a triptych. If the array order of attachment were to be changed, this would change the author’s intended representation.
tag
Tags are generally freeform and not meant to be parsed in any order, but some use-cases may wish to order tags as well. For example, Tumblr allows users to set the order their tags are presented in, and users sometimes use these tags to communicate a series of comments or thoughts without having them be present in the conversational context.
name
A thing may have multiple names, some of which are preferred more than others. Having an array for name is likely to confuse existing implementations at the time of writing this FEP, but it is something that could conceivably be useful for several use-cases.
oneOf/anyOf
When a Question represents a poll with predefined options for possible answers, the order of the options may be relevant to understanding the poll. Consider a self-referential poll that asks you to predict which option will receive the most votes: “option 2”, “option 3”, or “option 1”.

Terms defined by this FEP

In the associated context, we define the following terms:

orderedAttachment

URI
https://www.w3.org/ns/activitystreams#attachment
Notes
https://www.w3.org/TR/activitystreams-vocabulary/#dfn-attachment

JSON-LD definition:

{
	"@context": {
		"orderedAttachment": {
			"@id": "https://www.w3.org/ns/activitystreams#attachment",
			"@type": "@id",
			"@container": "@list"
		}
	}
}

orderedTag

Term
orderedTag
URI
https://www.w3.org/ns/activitystreams#tag
Notes
https://www.w3.org/TR/activitystreams-vocabulary/#dfn-tag

JSON-LD definition:

{
	"@context": {
		"orderedTag": {
			"@id": "https://www.w3.org/ns/activitystreams#tag",
			"@type": "@id",
			"@container": "@list"
		}
	}
}

References

CC0 1.0 Universal (CC0 1.0) Public Domain Dedication

To the extent possible under law, the authors of this Fediverse Enhancement Proposal have waived all copyright and related or neighboring rights to this work.

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