Expression discovery

Finds every expression in a Questionnaire and records where it is and what kind it is. Each one becomes an expression site, and the list of sites is what the analysis and the rules work on.

The walk goes depth-first through the Questionnaire's JSON, so the sites come out in document order. There are three kinds:

• Catalogued: an Expression held by an SDC extension the linter knows, such as a variable, a calculatedExpression or an enableWhenExpression, where SDC allows it. The extension catalog lists these locations. Every check can run here. An Expression with a name at a catalogued location declares that name for other expressions.
• Uncatalogued: any other object with a language and an expression or reference, including one in a contained resource. Only its syntax and function names can be checked. sdc/uncatalogued-expression reports it as info.
• Fragment: each {{…}} in an x-fhir-query is a FHIRPath site of its own, listed right after its query.

For example, this item gives two sites:

{
  "linkId": "weight",
  "extension": [{
    "url": "http://hl7.org/fhir/StructureDefinition/variable",
    "valueExpression": {
      "name": "obs",
      "language": "application/x-fhir-query",
      "expression": "Observation?subject={{%patient.id}}"
    }
  }]
}

1. The variable: catalogued, an x-fhir-query that declares obs, on item weight.
2. Its fragment %patient.id: FHIRPath, at its offsets within the query.

Supporting a new location means adding an entry to the catalog, not changing the walk. The site model is public API: plugin rules receive the same frozen sites as ctx.sites.

Schema

Code

In packages/core/src/analysis/:

• expression-sites.ts: findExpressionSites walks a Questionnaire and returns its sites.
• extension-catalog.ts: extensionCatalog is data, not code. Each entry gives an extension's URL, the elements it may sit on, its sub-extension when it is complex, its use cases (rendering, population or extraction), and the languages SDC allows there. definedLanguages and languagesAllowedAt, which plugins can import, are here too.
• expression-site.ts: the site model, public API: QuestionnaireDocument, ExpressionSite, CataloguedLocation, QueryFragment and SiteItem.

Expression discovery in the architecture map has the details.