Implement conformsTo

README.md

@@ -157,6 +157,15 @@ specification across different FHIR versions. In particular, DateTime and Time i
 include partial time (e.g. missing minutes and seconds), which is not allowed in FHIR. Therefore,
 new implementations are needed.
 
+### Profile validation
+
+The `conformsTo()` function supports the base FHIR profiles
+(`http://hl7.org/fhir/StructureDefinition/<Type>`): the input element's type is compared to the
+type named by the structure. Custom profiles (e.g. US Core) would require profile validation,
+which is not implemented. Passing a custom profile URL results in an error, consistent with the
+specification's requirement to error when a structure cannot be resolved
+(https://hl7.org/fhir/fhirpath.html#functions).
+
 ### Timezone offset in date time values
 
 This FHIRPath implementation adopts a strict, safety-first approach to date time comparisons,
@@ -296,7 +305,6 @@ documented in the table below.
 | `testType22`                       | Implementation     |     |                                                        | `is` with an unknown `System` type should evaluate to false, but the type resolver throws.                                                                                                             |
 | `testType23`                       | Implementation     |     |                                                        | As `testType20`.                                                                                                                                                                                       |
 | `testTypeA*`                       | Implementation     |     |                                                        | Evaluating `Parameters.parameter[x].value` crashes with `NoSuchElementException`.                                                                                                                      |
-| `testConformsTo*`                  | Implementation     |     |                                                        | Function `conformsTo` is not implemented.                                                                                                                                                              |
 | `LowBoundaryDateTimeMillisecond1`  | Specification/Test |     |                                                        | Diverges from FHIRPath specification. See [Discussion](https://chat.fhir.org/#narrow/channel/179266-fhirpath/topic/lowBoundary.20and.20highBoundary.20with.20incomplete.20date.20time/with/611113639). |
 | `HighBoundaryDateTimeMillisecond1` | Specification/Test |     | As above.                                              | As above.                                                                                                                                                                                              |
 | `HighBoundaryDateTimeMillisecond3` | Specification/Test |     | As above.                                              | As above.                                                                                                                                                                                              |

fhir-path-core/src/commonMain/kotlin/dev/ohs/fhir/fhirpath/functions/FhirFunctions.kt

@@ -17,6 +17,9 @@
 package dev.ohs.fhir.fhirpath.functions
 
 import dev.ohs.fhir.fhirpath.model.FhirModelNavigator
+import dev.ohs.fhir.fhirpath.toFhirPathType
+import dev.ohs.fhir.fhirpath.types.FhirPathTypeResolver
+import dev.ohs.fhir.fhirpath.types.FhirType
 
 /**
  * Returns the extensions with the given url on each item in the input collection.
@@ -40,3 +43,43 @@ internal fun Collection<Any>.extension(
     }
     .filter { fhirModelNavigator.accessProperty(it, "url") == url }
 }
+
+private const val BASE_STRUCTURE_DEFINITION_PREFIX = "http://hl7.org/fhir/StructureDefinition/"
+
+/**
+ * Returns whether the single input element conforms to the profile specified by the structure
+ * argument.
+ *
+ * Only the base FHIR profiles (`http://hl7.org/fhir/StructureDefinition/<Type>`) are supported: the
+ * input's type is compared to `<Type>`. An error is thrown if the structure cannot be resolved, as
+ * the specification requires. Custom profiles would need profile validation, which is not
+ * implemented.
+ *
+ * See [specification](https://hl7.org/fhir/fhirpath.html#functions).
+ */
+internal fun Collection<Any>.conformsTo(
+  params: List<Any>,
+  fhirPathTypeResolver: FhirPathTypeResolver,
+): Collection<Boolean> {
+  check(size <= 1) { "conformsTo() cannot be called on a collection with more than 1 item" }
+  val item = singleOrNull() ?: return emptyList()
+  // The structure argument can be a FHIR string (e.g. from a resource element), so it is
+  // converted before use like any other string parameter.
+  val structure =
+    params.singleOrNull()?.toFhirPathType(fhirPathTypeResolver) as? String
+      ?: error("conformsTo() requires a structure argument")
+
+  if (!structure.startsWith(BASE_STRUCTURE_DEFINITION_PREFIX)) {
+    error("Cannot resolve structure definition: $structure")
+  }
+  // resolveFromString throws for an unknown type name, satisfying the specification's
+  // requirement to error when the structure cannot be resolved. It falls back to System types
+  // for names that are not FHIR types (e.g. `String` rather than `string`), which are not valid
+  // structure definitions either, so those must error as well.
+  val targetType =
+    fhirPathTypeResolver.resolveFromString(structure.removePrefix(BASE_STRUCTURE_DEFINITION_PREFIX))
+  if (targetType !is FhirType) {
+    error("Cannot resolve structure definition: $structure")
+  }
+  return listOf(fhirPathTypeResolver.resolveFromObject(item) == targetType)
+}

fhir-path-core/src/commonMain/kotlin/dev/ohs/fhir/fhirpath/functions/FirstOrderFunctions.kt

@@ -133,6 +133,7 @@ internal fun Collection<Any>.invoke(
     // FHIR-specific functions
     // https://hl7.org/fhir/fhirpath.html#functions
     "extension" -> this.extension(params, fhirModelNavigator)
+    "conformsTo" -> this.conformsTo(params, fhirPathTypeResolver)
 
     // Utility functions
     // https://hl7.org/fhirpath/N1/#utility-functions

fhir-path/src/commonTest/kotlin/dev/ohs/fhir/fhirpath/ConformsToTest.kt

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2026 Open Health Stack Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package dev.ohs.fhir.fhirpath
+
+import dev.ohs.fhir.model.r4.Resource
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlinx.serialization.json.Json
+
+private val fhirPathEngine = FhirPathEngine.forR4()
+
+private val patient: Resource =
+  Json { ignoreUnknownKeys = true }
+    .decodeFromString("""{"resourceType": "Patient", "name": [{"family": "Chalmers"}]}""")
+
+class ConformsToTest {
+
+  @Test
+  fun `element conforms to its base data type profile`() {
+    assertEquals(
+      listOf(true),
+      fhirPathEngine
+        .evaluateExpression(
+          "name.first().conformsTo('http://hl7.org/fhir/StructureDefinition/HumanName')",
+          patient,
+        )
+        .toList(),
+    )
+    assertEquals(
+      listOf(false),
+      fhirPathEngine
+        .evaluateExpression(
+          "name.first().conformsTo('http://hl7.org/fhir/StructureDefinition/Address')",
+          patient,
+        )
+        .toList(),
+    )
+  }
+
+  @Test
+  fun `unresolvable structure throws`() {
+    assertFailsWith<Exception> {
+      fhirPathEngine.evaluateExpression(
+        "conformsTo('http://hl7.org/fhir/StructureDefinition/NotARealType')",
+        patient,
+      )
+    }
+  }
+
+  @Test
+  fun `structure resolving only to a System type throws`() {
+    // `String` is not a FHIR structure definition (FHIR's is lowercase `string`), so it must
+    // error rather than fall back to the System type and return false. The input element is a
+    // FHIR string, so with the lowercase URL this would return true; the error is about the
+    // structure being unresolvable, not about the input.
+    assertFailsWith<Exception> {
+      fhirPathEngine.evaluateExpression(
+        "name.first().family.conformsTo('http://hl7.org/fhir/StructureDefinition/String')",
+        patient,
+      )
+    }
+  }
+
+  @Test
+  fun `empty input returns empty`() {
+    assertEquals(
+      emptyList(),
+      fhirPathEngine
+        .evaluateExpression(
+          "{}.conformsTo('http://hl7.org/fhir/StructureDefinition/Patient')",
+          patient,
+        )
+        .toList(),
+    )
+  }
+}

fhir-path/src/commonTest/kotlin/dev/ohs/fhir/fhirpath/FhirPathEngineTest.kt

@@ -46,7 +46,6 @@ val skippedTestGroupToReasonMap =
   mapOf(
     "testEscapeUnescape" to "Unimplemented",
     "testVariables" to "Unimplemented",
-    "testConformsTo" to "Unimplemented",
     "Comparable" to "Unimplemented",
     "Precision" to "Unimplemented",
   )