> For the complete documentation index, see [llms.txt](https://php-fhir-tools.ardenexal.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://php-fhir-tools.ardenexal.net/fhirpath/status.md).

# Implementation Status & Known Issues

The FHIRPath component implements the core of the FHIRPath 2.0 language — a complete lexer, parser, and evaluator — plus a large library of built-in functions and the FHIR R4 FHIRPath extensions. It is a partial implementation: most everyday navigation, filtering, and conversion expressions work, while a small number of spec edge cases remain open.

## What's implemented

* **Operators** — arithmetic, comparison, equivalence (`~` / `!~`), logical, string concatenation, union, membership, and type (`is` / `as`). See [Expressions & Operators](/fhirpath/expressions.md).
* **Functions** — a broad library spanning existence, filtering, subsetting, string, math, date/time, type, tree navigation, utility, combining, type conversion, FHIR-specific, and precision/boundary categories. The full, code-verified list is in the [Function Reference](/fhirpath/functions.md).
* **Language features** — path navigation, indexing, function calls, literals (including date/time and quantity), collection literals `{}`, external constants (`%context` …), reserved identifiers (`$this`, `$index`, `$total`), and expression compilation/caching.

{% hint style="info" %}
Treat the [Function Reference](/fhirpath/functions.md) as the authoritative inventory of registered functions rather than a fixed count.
{% endhint %}

## Known issues

{% hint style="warning" %}

* **Semantic validation** — the parser does not reject every ambiguous expression that the spec requires parentheses for (e.g. constructs like `1 > 2 is Boolean`). A handful of conformance cases that exercise this fail.
* **FHIR deserialization** — many specification test cases cannot run because the test fixtures fail to deserialize from XML/JSON. This is a limitation in the Serialization component, not in FHIRPath itself, so those cases are skipped rather than failed.
  {% endhint %}

{% hint style="info" %}
The `FunctionRegistry` is a singleton, which historically caused intermittent static-state leakage between test suites. The registry now exposes a `reset()` method (used in test teardown) to clear that state, mitigating the previously reported flakiness.
{% endhint %}

## Specification conformance

The component ships with the official FHIR FHIRPath specification test suite (loaded from the `fhir/fhir-test-cases` package) and tracks progress against it. Conformance is **in progress**: core expression and function tests pass, a small number fail on the semantic-validation edge cases noted above, and a large block is skipped because of the deserialization limitation rather than evaluator defects.

{% hint style="info" %}
Exact pass/fail/skip counts shift as work continues, so they are intentionally not quoted here. Run the spec suite locally to get current numbers.
{% endhint %}


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