#FHIR: Comments in JSON

We use XML comments extensively in FHIR for the examples in the specification:

  <Patient xmlns="http://hl7.org/fhir">
    <!-- this example patient shows how to use ... -->
  </Patient>

Generally, comments are only useful in examples for the specification and implementation guides.

Unlike XML, JSON doesn’t have a comments syntax. Standing advice is to put the comment in some property, so that’s what we did:

  {
    "resourceType" : "Patient",
    "fhir_comments" : "this example patient shows how to use ..."
  }

But there’s problems with that approach – you’re parser has to parse them, and many people have trouble with the idea that you can truly ignore them. Then, if you’re using JSON schema, you have to allow for them. Also, if you’re round-tripping with XML, you lose the exact position of the comment. Finally, must human readers miss the comment anyway, since it’s in a property – and the human readers are the only point of having comments. (So many people use custom variations to include comments, but that’s not an option we have. The JSON community is discussing that, but it seems unlikely it will change, or if it changes, be widely adopted).

The cumulative effect of all this is that, as a consequence of a ballot comment (disclosure: from me) the HL7 ITS committee proposes to remove support for comments from the FHIR JSON format. Specifically, we propose to remove this paragraph:

There is no inherent support in JSON for a comment syntax. As a convention, content that would be comments in an XML representation is represented in a property with the name fhir_comments, which is an array of strings, which can appear on any JSON object. This is heavily used in example instances, e.g. in this specification, but not usually used in production systems (and production systems may choose to reject resources with comments in them)

We don’t know of anyone using this in practice, and for practical reasons, I removed comments from the actual JSON examples in the current version earlier this year, and we’ve had no comments about that. Never-the-less, the JSON format is labelled FMM 5 (see the specification definition), so we need to consult with the implementation community about this.

So we are calling for comment from implementers about this change. If you wish to comment on this change, please send an email to the FHIR email list before Octover 11th 2016. If you comment against this change, make sure you identify the production implementation that would be affected by the change, and how it would be affected.

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