Re: Differences in IRI compaction of "@id" and "@type"

> On Aug 26, 2021, at 7:20 AM, Jindřich Mynarz <mynarzjindrich@gmail.com> wrote:
> 
> Hi,
> 
> I'm wondering about the differences in JSON-LD API's IRI compaction (https://www.w3.org/TR/json-ld-api/#iri-compaction <https://www.w3.org/TR/json-ld-api/#iri-compaction>) of "@id" and "@type". When I compact the following data:
> 
> {
>   "@type": "http://example.com/A <http://example.com/A>",
>   "http://example.com/property <http://example.com/property>": {"@id": "http://example.com/B <http://example.com/B>"}
> }
> 
> With the following JSON-LD context:
> 
> {
>   "@vocab": "http://example.com/ <http://example.com/>",
>   "ex": "http://example.com/ <http://example.com/>",
>   "AliasedA": "A",
>   "AliasedB": "B",
>   "property": {"@type": "@id"}
> }
> 
> The JSON-LD Playground (https://json-ld.org/playground <https://json-ld.org/playground>, running jsonld.js compliant with JSON-LD 1.1) returns the following compacted data:
> 
> {
>   "@context": {
>     "@vocab": "http://example.com/ <http://example.com/>",
>     "ex": "http://example.com/ <http://example.com/>",
>     "AliasedA": "A",
>     "AliasedB": "B",
>     "property": {
>       "@type": "@id"
>     }
>   },
>   "@type": "AliasedA",
>   "property": "ex:B"
> }
> 
> While the value of "@type" is compacted to its alias term, the value of "@id" is only compacted using a prefix definition, not its alias term.

The rules for interpreting strings as IRIs are described in 4.2.3 Type Coercion [1]. Terms used as properties can be defined using with either “@type”: “@id” or “@type”: “@vocab” indicate that they should be interpreted as IRI references, and are either relative to the document base or vocabulary base. By default, values of “@type” are interpreted as IRI references relative to the vocabulary base, and “@id” relative to the document base.

> It is important to note that terms are only used in expansion for vocabulary-relative positions, such as for keys and values of map entries. Values of @id are considered to be document-relative, and do not use term definitions for expansion.

Because @id is document relative, without specifying a document base, it can’t be turned into a relative IRI, but it can potentially be turned into a compact IRI, given a suitable term definition to use as the prefix. The value of @type, and other properties (which use “@type”: “@vocab”) can potentially be compacted as IRI references/relative IRIs.

When compacting, the most suitable term definition matching the value of a given node key is used for creating the compacted result, through a fairly involved process.

> Link into JSON-LD Playground: https://tinyurl.com/yfrzckbr <https://tinyurl.com/yfrzckbr>
> 
> I'm guessing values of "@type" are treated differently by IRI compaction, but I cannot find support for that in the JSON-LD API specification. Can you help?

Gregg Kellogg
gregg@greggkellogg.net

[1] https://www.w3.org/TR/json-ld11/#type-coercion <https://www.w3.org/TR/json-ld11/#type-coercion>

> Best regards,
> 
> Jindrich
> 
> -- 
> Jindrich Mynarz
> https://mynarz.net/#jindrich <https://mynarz.net/#jindrich>