Conformance

Scope and the canonical source​

The engine implements the FHIRPath Specification v3.0.0 (STU3) and, for the FHIR-specific additions, the FHIRPath section of FHIR R5. Conformance is verified against the official FHIR test-case suite on JVM and Android.

The repository maintains the authoritative per-feature conformance table, listing every function and operator with its status and linking implemented ones to their source, in docs/conformance.md. This page summarizes it. Check the repository document for a specific feature before relying on it.

What is solidly covered​

The core language. That means path navigation, filtering and projection (where, select, repeat), existence (exists, all, empty, count), subsetting (first, last, tail, skip, take, indexing), combining, string manipulation, math, type operators (is, as, ofType), boolean logic, comparison and equivalence, date and time arithmetic, aggregate, environment variables, trace, and reflection over types. UCUM-based quantity comparison and conversion is implemented.

Notable gaps​

Features that require external services are absent by design, because the engine is purely in-memory.

• resolve(), since following references needs a resource store
• memberOf, subsumes, and subsumedBy, since terminology operations need a terminology server
• The terminology and general server API bindings

Features from newer spec drafts and less-traveled corners that are unimplemented in the current release include the following.

• defineVariable, coalesce, repeatAll
• The aggregate shorthands sum(), min(), max(), and avg(). Use aggregate() instead.
• Date and time component extractors (yearOf, monthOf, and siblings)
• toLong and convertsToLong, lastIndexOf, instance and object creation syntax, %rootResource, elementDefinition, slice, checkModifiers, conformsTo, htmlChecks

Known divergences​

Places where behavior differs from a literal spec reading, tracked in the conformance document with upstream issue links.

• The indexer [n] raises on out-of-bounds access instead of returning {}.
• combine does not deduplicate.
• Some numeric conversions omit edge-case handling. Long inputs are not handled, toDecimal can lose precision on large mantissas, and toQuantity with a unit argument ignores the requested unit on bare numbers, hardcoding the default unit instead.
• Timezone comparisons are deliberately stricter than the spec permits, as described under evaluation semantics.
• Resource equality compares full object structure, including id.

A further set of divergences exists against the official test suite itself, where the engine's authors judged the test or the spec to be wrong. Each is documented, with links to the upstream discussions where they exist.

What to do at a gap​

If an expression you need touches an unimplemented feature, first confirm its status in the repository's conformance table. Then the options in practice are to rewrite the expression within the supported surface, to evaluate the offending fragment in your own code against the model classes, or, for resolve()-style needs, to restructure so the referenced resource is fetched by your application first.