Kotlin FHIRPath

Kotlin FHIRPath is an implementation of HL7® FHIR®'s FHIRPath on Kotlin Multiplatform.

Warning: The library is in alpha and subject to change. Use at your own risk.

Key features

• Built with an ANTLR-generated parser for strict adherence to the formal FHIRPath grammar
• Conforms strictly to the specification, with predictable and well-documented behavior
• Support for validation, conversion, and comparison between compatible UCUM units
• Designed for portability, providing a single engine across JVM, Android, iOS, Web (JS), and Native platforms
• Tested against the official FHIR test cases to guarantee correctness

FHIRPath version support

The implementation is based on the FHIRPath Normative Release. However, we also incorporate some of the latest features and clarifications from the Continuous Build wherever feasible. Please note the experimental nature of the sections marked as STU (Standard for Trial Use) in the Continuous Build.

FHIR version support

The library currently supports R4 only. Support for other versions will be added as the library matures.

Implementation

This project uses ANTLR Kotlin to generate the lexer, parser and visitor directly from the formal FHIRPath grammar. This automated approach ensures correctness, improves maintainability, and significantly reduces development time.

The FHIRPath Evaluator implements the visitor class generated by ANTLR, evaluating FHIRPath expressions by traversing the in-memory data model from the Kotlin FHIR library.

A key requirement for FHIRPath evaluation is the capability to access data elements by name. To achieve this with cross-platform compatibility (avoiding reflection), a codegen embedded in buildSrc generates helper functions to the Kotlin FHIR data model.

graph LR
    A[formal FHIRPath grammar] -- ANTLR Kotlin --> B(lexer, parser, visitor)
    C(Kotlin FHIR data model<br>com.google.fhir:fhir-model)
    subgraph buildSrc
        direction LRTB
        D[FHIR spec<br>in JSON] -- kotlinx.serialization --> E(instances of<br>StructureDefinition<br>Kotlin data class<br>)
        E -- KotlinPoet --> F(generated data model helper functions)
        G[ucum-essence.xml] --> H(generated UCUM helper functions)

    end
    B --> I(FHIRPath Evaluator)
    C --> I
    F --> I
    H --> I

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Figure 1: Architecture diagram

The following table lists the chosen internal types for the FHIRPath primitive types.

FHIRPath type	Internal type
Boolean	kotlin.Boolean
String	kotlin.String
Integer	kotlin.Int
Long	kotlin.Long
Decimal	kotlin.Double
Date	FhirDate(*)
DateTime	FhirPathDateTime(**)
Time	FhirPathTime(**)
Quantity	Quantity(*)

(*): Classes defined in Kotlin FHIR (**): Classes defined in this project

Classes from the Kotlin FHIR are used for more complex types that do not have direct representations in Kotlin. For DateTime and Time, the requirements in FHIRPath are more lenient than in the FHIR specification. As a result, custom classes need to be authored to handle cases where the minutes, seconds, or milliseconds are not present (allowed in FHIRPath but not allowed in FHIR).

Timezone offset in date time values

This FHIRPath implementation adopts a strict, safety-first approach to date time comparisons, especially around the handling of timezones and date time values with different precisions.

Date time values without timezone offset

The FHIRPath specification allows implementations to provide a default timezone offset for date time values that do not have one. See the relevant sections on equality, equivalence, and comparison.

To prioritise safety and correctness, when comparing date time values without a timezone offset with date time values with a timezone offset, this implementation does not assume a default timezone offset (such as UTC or the system's timezone offset). This is because the data could have originated from a different system or context unknown to this implementation, making any "guess" potentially incorrect and unsafe.

This leads to the following behavior:

• Equality (=, !=) and comparison (<=, <, >, >=) operators will return an empty result {} to indicate uncertainty
• Equivalence (~) operator will return false since equivalence cannot be proven. Likewise, !~ will return true.

@2025-01-01T00:00:00.0+00:00 = @2025-01-01T00:00:00.0  // returns {} 
@2025-01-01T00:00:00.0+00:00 ~ @2025-01-01T00:00:00.0  // returns false
@2025-01-01T00:00:00.0+00:00 > @2025-01-01T00:00:00.0  // returns {}

Note: While comparing two date time values without timezone offset, the implementation will treat them as if they had the same timezone offset. This compromise is made so that local date time values can be compared:

@2025-01-01T00:00:00.0 = @2025-01-01T00:00:00.0`  // returns true

Date time values with timezone offsets but different precisions

According to the specification, two date time values should be compared at each precision, starting from years all the way to seconds. However, this becomes problematic when the date time values at hourly precision have half-hour or quarter-hour timezone offsets. Consider @2025-01-01T00+05:30 and @2025-01-01T00+05:45. In no timezone can both values still be represented as partial date time values at the same precision in order to carry out the comparison algorithm.

Whilst it is possible to implement precision based timing in CQL using intervals, it is not part of the FHIRPath specification. For simplicity, this implementation returns an empty result for comparing partial date time values with timezone offsets.

// Indian Standard Time (IST) and Nepal Time (NPT)
@2025-01-01T00+05:30 = @2025-01-01T00+05:45   // returns {}

Conformance

Due to the library's WIP status, not all test cases from the published official test suites are passing. The failures are documented in the table below.

Test case	Root cause	STU	Tracking issue / PR	Note
testPolymorphismAsB	Test		To be raised	No error should be thrown according to specification.
testLiteralDecimalGreaterThanNonZeroTrue	Implementation		#7
testLiteralDecimalGreaterThanZeroTrue	Implementation		#7
testLiteralDecimalGreaterThanIntegerTrue	Implementation		#7
testLiteralDecimalLessThanInteger	Implementation		#7
testDateTimeGreaterThanDate1	Specification/Test		To be raised	Unclear in the specification whether the result should still be empty if two values have different precisions but the comparison can still be "certain" (e.g. 2025 is greater than 2024-01).
testDecimalLiteralToInteger	Test		To be raised	The result should be true.
testStringIntegerLiteralToQuantity	Specification/Test			Discussion
testQuantityLiteralWkToString	Specification/Test			As above.
testQuantityLiteralWeekToString	Specification/Test			As above.
testQuantity4	Test		PR
testExists3	Implementation			Equality of enum and string is not implemented.
testSubSetOf3	Implementation
testQuantity9	Implementation			Quantity multiplication is not implemented.
testQuantity10	Implementation			Quantity division is not implemented.
testQuantity11	Implementation			As above.
testDistinct2	Implementation			Function descendants is not implemented.
testDistinct3	Implementation			As above.
testDistinct5	Implementation			As above.
testDistinct6	Implementation			As above.
testSelect3	Implementation
testRepeat*	Implementation			Function repeat is not implemented.
testAggregate*	Implementation			Function aggregate is not implemented.
testIif10	Implementation
testIif11	Implementation
testMatchesSingleLineMode1	Implementation
testReplace5	Implementation
testReplace6	Implementation
testEncode*	Implementation	STU		Function encode is not implemented.
testDecode*	Implementation	STU		Function decode is not implemented.
testEscape*	Implementation	STU		Function escape is not implemented.
testUnescape*	Implementation	STU		Function unescape is not implemented.
testTrace2	Implementation
testNow1	Specification/Test			As testDateTimeGreaterThanDate1.
testSort*	Specification/Test			Function sort is not defined in the specification.
testEquality28	Implementation
testNEquality24	Implementation
testEquivalent11	Implementation
testEquivalent22	Implementation
testLessThan22	Implementation
testLessThan24	Implementation
testLessOrEqual22	Implementation
testLessOrEqual24	Implementation
testGreaterOrEqual22	Implementation
testGreaterOrEqual24	Implementation
testGreaterThan22	Implementation
testGreaterThan24	Implementation
testGreaterThan6	Implementation
testCombine*	Implementation			Function combine is not implemented.
testPlusDate*	Implementation			Date time arithmetic not implemented.
testMinus3	Implementation
testMinus5	Implementation
testMod4	Implementation
testAbs3	Implementation
testPrecedence3	Implementation
testPrecedence4	Implementation
testPrecedence6	Implementation
testVariables*	Implementation			Variables are not implemented.
testExtension*	Implementation			Function extension is not implemented.
testType*	Implementation			Function type is not implemented.
testConformsTo*	Implementation			Function conformsTo is not implemented.
testLowBoundary*	Implementation	STU		Function testLowBoundary is not implemented.
testHighBoundary*	Implementation	STU		Function testHighBoundary is not implemented.
testComparable*	Implementation			Function testComparable is not implemented.
testPrecision*	Implementation			Function precision is not implemented.
testIndex	Implementation
testPeriod*	Implementation
testFHIRPathIsFunction*	Implementation
testFHIRPathAsFunction*	Implementation
testContainedId	Implementation

The root cause column documents if the test failure is caused by issues with the implementation (this repository), the tests, the specification itself, or is under investigation. We exclude test cases that would fail due to issues in the tests and the specification itself. But we track the ongoing discussions and resolutions in this table.

User Guide

Developer Guide

ANTLR

To generate the lexer, parser, and visitor locally using ANTLR Kotlin:

./gradlew generateKotlinGrammarSource

The generated code will be placed in fhir-path/build/generated under package com.google.fhir.fhirpath.parsers.

Model extensions

To run the model extension codegen in buildSrc locally:

./gradlew generateR4Helpers

The generated code will be located in fhir-path/build/generated under packages com.google.fhir.fhirpath and com.google.fhir.fhirpath.ext.

UCUM helpers

To run the UCUM helper codegen in buildSrc locally:

./gradlew generateUcumHelpers

The generated code will be located in fhir-path/build/generated under package com.google.fhir.fhirpath.ucum.

Dependencies

Dependencies must be kept in sync between the buildSrc/build.gradle.kts file and the gradle/libs.versions.toml file. The former cannot use the latter since the buildSrc directory is precompiled separately in Gradle.

Tests

XmlUtil is used to load the XML test cases from the third_party directory. To run the tests:

./gradlew :fhir-path:jvmTest

The number of passing test cases is displayed on a badge at the top of this page.

Publishing

To create a maven repository from the generated FHIR model, run:

./gradlew :fhir-path:publish

This will create a maven repository in the fhir-path/build/repo directory with artifacts for all supported platforms.

To zip the repository, run:

./gradlew :fhir-path:zipRepo

This will generate a .zip file in the fhir-path/build/repoZip directory.

Third Party

The third_party directory includes resources from the FHIRPath specification and related repositories for code generation and testing purposes:

• fhir-test-cases: content from the fhir-test-cases repo
  • tests-fhir-r4.xml: R4 test cases (commit)
  • resources JSON versions of the relevant test resources generated using Anton V.'s FHIR Converter alongside the XML versions (commit)
• fhirpath-2.0.0: the formal antlr grammar from the FHIRPath Normative Release N1 (v2.0.0) including
• hl7.fhir.r4.core: content from FHIR R4 for code generation
• ucum: content from the UCUM repo

Disclaimer

This is not an officially supported Google product. This project is not eligible for the Google Open Source Software Vulnerability Rewards Program.