Why compacting keeps `format` term prefix CURIE in this case?

[jmcanterafonseca-iota]

Example available in the JSON-LD Playground at:

https://json-ld.org/playground/#startTab=tab-compacted&json-ld=%7B%22%40context%22%3A%7B%22dcat%22%3A%22http%3A%2F%2Fwww.w3.org%2Fns%2Fdcat%23%22%2C%22odrl%22%3A%22http%3A%2F%2Fwww.w3.org%2Fns%2Fodrl%2F2%2F%22%2C%22dcterms%22%3A%22http%3A%2F%2Fpurl.org%2Fdc%2Fterms%2F%22%7D%2C%22%40id%22%3A%22dataset%3Adataset1%22%2C%22%40type%22%3A%22dcat%3ADataset%22%2C%22dcat%3Adistribution%22%3A%5B%7B%22%40type%22%3A%22dcat%3ADistribution%22%2C%22dcterms%3Aformat%22%3A%22example%3Aformat%22%2C%22dcat%3AaccessService%22%3A%22service%3Aaccess1%22%7D%5D%2C%22odrl%3AhasPolicy%22%3A%7B%22%40context%22%3A%22http%3A%2F%2Fwww.w3.org%2Fns%2Fodrl.jsonld%22%2C%22%40type%22%3A%22Offer%22%2C%22uid%22%3A%22policy%3Apolicy1%22%2C%22permission%22%3A%5B%7B%22action%22%3A%22use%22%7D%5D%7D%2C%22dcterms%3Atype%22%3A%22https%3A%2F%2Fvocabulary.uncefact.org%2FConsignment%22%2C%22dcterms%3Apublisher%22%3A%22did%3Aiota%3A0x123456789abcdef%22%7D&context=%7B%22%40context%22%3A%5B%22https%3A%2F%2Fw3id.org%2Fdspace%2F2025%2F1%2Fcontext.jsonld%22%5D%7D

The unexplainable is that format is kept as dct:format but it should be just compacted to format.

--- Explanation -----

let it be

{
  "@context": {
    "dcat": "http://www.w3.org/ns/dcat#",
    "odrl": "http://www.w3.org/ns/odrl/2/",
    "dcterms": "http://purl.org/dc/terms/"
  },
  "@id": "dataset:dataset1",
  "@type": "dcat:Dataset",
  "dcat:distribution": [
    {
      "@type": "dcat:Distribution",
      "dcterms:format": "example:format",
      "dcat:accessService": "service:access1"
    }
  ],
  "odrl:hasPolicy": {
    "@context": "http://www.w3.org/ns/odrl.jsonld",
    "@type": "Offer",
    "uid": "policy:policy1",
    "permission": [
      {
        "action": "use"
      }
    ]
  },
  "dcterms:type": "https://vocabulary.uncefact.org/Consignment",
  "dcterms:publisher": "did:iota:0x123456789abcdef"
}

that it is compacted through

{
  "@context": ["https://w3id.org/dspace/2025/1/context.jsonld"]
}

result is below. Why dcterms:format is not compacted to be just format and why is it prefixed? because the compaction LD Context has a term definition for it at https://github.com/eclipse-dataspace-protocol-base/DataspaceProtocol/blob/main/artifacts/src/main/resources/context/dspace.jsonld#L415

{
  "@context": "https://w3id.org/dspace/2025/1/context.jsonld",
  "@id": "dataset:dataset1",
  "@type": "Dataset",
  "dct:publisher": "did:iota:0x123456789abcdef",
  "dct:type": "https://vocabulary.uncefact.org/Consignment",
  "distribution": [
    {
      "@type": "Distribution",
      "dct:format": "example:format",
      "accessService": "service:access1"
    }
  ],
  "hasPolicy": [
    {
      "@id": "policy:policy1",
      "@type": "Offer",
      "permission": [
        {
          "action": "use"
        }
      ]
    }
  ]
}

[davidlehn]

For easier experimentation, here's a simplified example.

The @vocab type on format triggers this behavior. I have trouble reasoning about how that is working. Perhaps someone else has more insight.

This probably isn't jsonld.js specific. Might need to ask in the spec groups. (We probably need a better documented place to send general JSON-LD questions.)

[jmcanterafonseca-iota]

thanks @davidlehn ! which group should I address? could you give me a contact point?

@msporny any insight?

[dlongley]

This could be directed at either https://github.com/w3c/json-ld-syntax or https://github.com/w3c/json-ld-api (at their respective issues pages).

However, I think the problem here is that the @type defined for the format term is @vocab but the value in the input does not match any term in the new context.

When compacting, a term definition will only be selected if the value type matches the term. When using "@type": "@vocab" in a term definition, this means that the value must map to a term in the context, and the value "example:format" does not.

If you were to change the value to say:

{"@id": "https://w3id.org/dspace/2025/1/ACCEPTED"}

That would match a URL in the new context that is mapped to the term ACCEPTED, so I'd expect that to cause the format term to then be selected during compaction.

Note: Using "@type": "@vocab" is similar to declaring that the value for the term is of an "enumerated type", with any term in the context being in the enumeration.

[BigBlueHat]

So, there's a lot of side quests in this thread...which I'll skip for now. The primary/original question was:

The unexplainable is that format is kept as dct:format but it should be just compacted to format.

AFAICT, this is being caused by the format definition changing from the original definition to the new/different mapping.

The original definition (with everything else stripped away) is just the CURIE mapping (so no "@type": "@vocab" stuff):

{
  "@context": {
    "dcterms": "http://purl.org/dc/terms/"
  }
}

but the definition changes in the new context (https://w3id.org/dspace/2025/1/context.jsonld) to...

{
  "@context": {
    "dct": "http://purl.org/dc/terms/",
    "format": {
      "@type": "@vocab",
      "@id": "dct:format"
    }
  }
}

Because there's not a direct match, the CURIE is kept, but if you remove the "@type": "@vocab" in the "new context" above, you'll get the output you want:

{
  "@context": {
    "dct": "http://purl.org/dc/terms/",
    "format": {
      "@id": "dct:format"
    }
  },
  "@id": "dataset:dataset1",
  "format": "ACCEPTED"
}

Try it with this example--and remove the "@type": "@vocab" to see what I mean:
https://json-ld.org/playground/#startTab=tab-compacted&json-ld=%7B%22%40context%22%3A%7B%22dcterms%22%3A%22http%3A%2F%2Fpurl.org%2Fdc%2Fterms%2F%22%7D%2C%22%40id%22%3A%22dataset%3Adataset1%22%2C%22dcterms%3Aformat%22%3A%22ACCEPTED%22%7D&context=%7B%22%40context%22%3A%7B%22dct%22%3A%22http%3A%2F%2Fpurl.org%2Fdc%2Fterms%2F%22%2C%22format%22%3A%7B%22%40type%22%3A%22%40vocab%22%2C%22%40id%22%3A%22dct%3Aformat%22%7D%7D%7D

So...I don't think it's a bug...

There's a similar shift in the output if you start with the "@type": "@vocab" in the original context and remove it in the "new context" (the value of format is forced into an object shape using @id...for some reason):
https://json-ld.org/playground/#startTab=tab-compacted&json-ld=%7B%22%40context%22%3A%7B%22dcterms%22%3A%22http%3A%2F%2Fpurl.org%2Fdc%2Fterms%2F%22%2C%22format%22%3A%7B%22%40type%22%3A%22%40vocab%22%2C%22%40id%22%3A%22http%3A%2F%2Fpurl.org%2Fdc%2Fterms%2Fformat%22%7D%7D%2C%22%40id%22%3A%22dataset%3Adataset1%22%2C%22format%22%3A%22ACCEPTED%22%7D&context=%7B%22%40context%22%3A%7B%22dct%22%3A%22http%3A%2F%2Fpurl.org%2Fdc%2Fterms%2F%22%2C%22format%22%3A%7B%22%40id%22%3A%22dct%3Aformat%22%7D%7D%7D

@davidlehn not sure if that sleuthing helps at all, but it does seem like things are working as expected--if the "new context" isn't a direct one-to-one match the nearest correct "match" is output--so, given the use of CURIEs those "win" since no exact match was found.

Or so goes my understanding of things. 😅

[jmcanterafonseca-iota]

I can confirm that if in the original LD Context dcterms:format is set to be @type: @vocab then the compaction works as expected.

https://json-ld.org/playground/#startTab=tab-compacted&json-ld=%7B%22%40context%22%3A%7B%22dcat%22%3A%22http%3A%2F%2Fwww.w3.org%2Fns%2Fdcat%23%22%2C%22odrl%22%3A%22http%3A%2F%2Fwww.w3.org%2Fns%2Fodrl%2F2%2F%22%2C%22dcterms%22%3A%22http%3A%2F%2Fpurl.org%2Fdc%2Fterms%2F%22%2C%22dcterms%3Aformat%22%3A%7B%22%40id%22%3A%22dcterms%3Aformat%22%2C%22%40type%22%3A%22%40vocab%22%7D%7D%2C%22%40id%22%3A%22dataset%3Adataset1%22%2C%22%40type%22%3A%22dcat%3ADataset%22%2C%22dcat%3Adistribution%22%3A%5B%7B%22%40type%22%3A%22dcat%3ADistribution%22%2C%22dcterms%3Aformat%22%3A%22https%3A%2F%2Fexample.org%2Fformat1%22%2C%22dcat%3AaccessService%22%3A%22service%3Aaccess1%22%7D%5D%2C%22odrl%3AhasPolicy%22%3A%7B%22%40context%22%3A%22http%3A%2F%2Fwww.w3.org%2Fns%2Fodrl.jsonld%22%2C%22%40type%22%3A%22Offer%22%2C%22uid%22%3A%22policy%3Apolicy1%22%2C%22permission%22%3A%5B%7B%22action%22%3A%22use%22%7D%5D%7D%2C%22dcterms%3Atype%22%3A%22https%3A%2F%2Fvocabulary.uncefact.org%2FConsignment%22%2C%22dcterms%3Apublisher%22%3A%22did%3Aiota%3A0x123456789abcdef%22%7D&context=%7B%22%40context%22%3A%5B%22https%3A%2F%2Fw3id.org%2Fdspace%2F2025%2F1%2Fcontext.jsonld%22%2C%7B%22fm1%22%3A%7B%22%40id%22%3A%22https%3A%2F%2Fexample.org%2Fformat1%22%7D%7D%5D%7D

So it seems that for the compaction algorithm the difference in the @typedefinition makes it consider the element as non-compactable as per the new LD Context

[BigBlueHat]

@jmcanterafonseca-iota is there anything else that needs addressing here? I think we solved the riddle. 😄 Feel free to reopen this or file new issues as needed. If you have general JSON-LD questions, I'd suggest posting to https://github.com/orgs/json-ld/discussions or to the JSON-LD CG's mailing list: https://lists.w3.org/Archives/Public/public-json-ld/

Thanks for bringing up this concern! It was a great exploration.