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

• Strict conformation to the FHIRPath specification, with predictable and well-documented behavior
• Built with an ANTLR-generated parser for adherence to the formal grammar
• Support for validation, conversion, and comparison between compatible UCUM units
• Multiplatform support across Android, iOS, Desktop (JVM), Server-side (JVM), and Web (Wasm/JS)
• Support for FHIR R4, R4B, R5, and future versions
• 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 supports FHIR R4, R4B and R5. Support will be added for future FHIR versions.

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	com.ionspin.kotlin.bignum.decimal.BigDecimal
Date	FhirPathDate
DateTime	FhirPathDateTime
Time	FhirPathTime
Quantity	FhirPathQuantity

This project defines Date, DateTime, Time, and Quantity classes in order to implement the FHIRPath specification across different FHIR versions. In particular, DateTime and Time in FHIRPath may include partial time (e.g. missing minutes and seconds), which is not allowed in FHIR. Therefore, new implementations are needed.

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 {}

Error handling

The FHIRPath specification does not specify the desired behavior when type checking errors occur, allowing the implementation to adopt a strict (e.g. throws an exception) or a lenient (e.g. returns an empty collection) approach. However, the official test suite include test cases that require lenient type checking. To accommodate such cases, this implementation returns an empty collection when the FHIRPath expression attempts to access a data element that does not exist.

Conformance

Test failures against the official FHIR test cases 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.
testDateTimeGreaterThanDate1	Implementation			Comparison of two date time values, one with a timezone offset one without; see Date time values without timezone offset
testStringIntegerLiteralToQuantity	Specification/Test			Discussion
testQuantityLiteralWkToString	Specification/Test			As above.
testQuantityLiteralWeekToString	Specification/Test			As above.
testQuantity4	Test		PR
testSubSetOf3	Specification/Test			The test resource is invalid and missing (FHIR/fhir-test-cases#247); the scope of "$this" is unclear (https://jira.hl7.org/browse/FHIR-44601)
testIif11	Implementation			https://jira.hl7.org/browse/FHIR-44774; https://jira.hl7.org/browse/FHIR-44601
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.
testTrace*	Implementation			Function trace is not implemented.
testNow1	Specification/Test			As testDateTimeGreaterThanDate1.
testSort*	Specification/Test			Function sort is not defined in the specification.
testPlusDate13	Specification/Test			https://chat.fhir.org/#narrow/channel/179266-fhirpath/topic/Definite.20durations.20above.20seconds.20in.20date.20time.20arithmetic/with/564095766
testPlusDate15	Specification/Test			As above.
testPlusDate18	Implementation			To be fixed together with testPlusDate13, testPlusDate15, testPlusDate21, testPlusDate22 for a consistent implementation.
testPlusDate19	Implementation			To be fixed together with testPlusDate13, testPlusDate15, testPlusDate21, testPlusDate22 for a consistent implementation.
testPlusDate20	Implementation			To be fixed together with testPlusDate13, testPlusDate15, testPlusDate21, testPlusDate22 for a consistent implementation.
testPlusDate21	Specification/Test			As testPlusDate13.
testPlusDate22	Specification/Test			As testPlusDate13.
testMinus5	Specification/Test			As testPlusDate13.
testPrecedence3	Specification/Test		FHIR/fhir-test-cases#249	https://chat.fhir.org/#narrow/channel/179266-fhirpath/topic/FHIRPath.20test.20suite.20for.20precedence.20correct.3F/with/564497251
testPrecedence4	Specification/Test		FHIR/fhir-test-cases#249	https://chat.fhir.org/#narrow/channel/179266-fhirpath/topic/FHIRPath.20test.20suite.20for.20precedence.20correct.3F/with/564497251
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.
LowBoundary*	Implementation	STU		Function lowBoundary is not implemented.
HighBoundary*	Implementation	STU		Function highBoundary is not implemented.
Comparable*	Implementation			Function comparable is not implemented.
Precision*	Implementation			Function precision is not implemented.
testIndex	Implementation			$index is not implemented.
testPeriodInvariantOld	Implementation			Function hasValue is not implemented.
testPeriodInvariantNew	Implementation			Function lowBoundary and function highBoundary are not implemented.
testFHIRPathIsFunction*	Implementation
testFHIRPathAsFunction*	Implementation
testContainedId	Implementation
testCombine2	Implementation			FHIR String and Kotlin String comparison issue in exclude() function.
testCombine3	Implementation			As above.

The root cause column documents if the test failure is caused by implementation issues in this repository, if the test cases themselves are problematic, or it is believed that the specification itself is ambiguous or inconsistent. For issues in the test cases and the specification, discussions and proposals should be linked in the table above.

User Guide

Adding the library dependency to your project

To use Kotlin FHIRPath, add it to the dependencies in your project. To do that, first make sure to include the Google Maven repository in the build.gradle.kts file in your project root.

// build.gradle.kts
repositories {
    // Other repositories such as mavenCentral() and gradlePluginPortal()
    google()
}

Next, follow the instructions for your specific project type.

Kotlin Multiplatform Projects

For Kotlin Multiplatform projects, add the dependency to the shared commonMain source set within the kotlin block of the module's build.gradle.kts file (e.g., composeApp/build.gradle.kts or shared/build.gradle.kts). This makes the library available across all platforms in your project.

// e.g., composeApp/build.gradle.kts or shared/build.gradle.kts
kotlin {
    sourceSets {
        commonMain.dependencies {
            implementation("com.google.fhir:fhir-path:1.0.0-alpha02")
        }
    }
}

Android projects

For Android projects, add the dependency to the dependency block in the module's build.gradle.kts file (e.g., app/build.gradle.kts).

// e.g., app/build.gradle.kts
dependencies {
    implementation("com.google.fhir:fhir-path:1.0.0-alpha02")
}

Evaluating FHIRPath expressions

To evaluate a FHIRPath expression, create a FhirPathEngine for the correct FHIR version and use evaluateExpression function:

import com.google.fhir.fhirpath.FhirPathEngine
import com.google.fhir.model.r4.FhirR4Json

val patientExampleJson = ... // Load "patient-example.json"
val patient = FhirR4Json().decodeFromString(patientExampleJson)
val fhirPathEngine = FhirPathEngine.forR4()
val results = fhirPathEngine.evaluateExpression("name.given", patient)  // ["Peter", "James", "Jim", "Peter", "James"]

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
./gradlew generateR4BHelpers
./gradlew generateR5Helpers

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

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). The XML and JSON resource files in the fhir-test-cases repository are inconsistent; we use XML files converted to JSON.
• 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
• hl7.fhir.r4b.core: content from FHIR R4B for code generation
• hl7.fhir.r5.core: content from FHIR R5 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.