Type Checking Functions

Type checking functions allow you to test an element’s type and to cast elements to specific types. These are essential when working with polymorphic FHIR® elements (like value[x]) where the actual type may vary.

is

Returns true if the input element is of the specified type.

Signature:

is(type : TypeSpecifier) : Boolean

Parameters:

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(observation, "Observation.value.is(Quantity)")
// true if value is a Quantity

result, _ := fhirpath.Evaluate(observation, "Observation.value.is(CodeableConcept)")
// true if value is a CodeableConcept

result, _ := fhirpath.Evaluate(patient, "Patient.name.first().is(HumanName)")
// true

Edge Cases / Notes:

• Requires a singleton input (exactly one element). If the input contains more than one element, an error is raised.
• Returns empty collection if the input is empty.
• The type name is resolved by the evaluator directly from the expression AST, so type names like Patient or Quantity are used without quotes.
• Type matching uses the eval.TypeMatches function, which supports both simple type names and fully qualified FHIR® type names.
• The function form value.is(Quantity) is equivalent to the operator form value is Quantity.

as

Casts the input to the specified type. Returns the input if it matches the type, otherwise returns an empty collection.

Signature:

as(type : TypeSpecifier) : Collection

Parameters:

Return Type: Collection

Examples:

result, _ := fhirpath.Evaluate(observation, "Observation.value.as(Quantity)")
// Returns the value as a Quantity, or empty if not a Quantity

result, _ := fhirpath.Evaluate(observation, "Observation.value.as(Quantity).value")
// Accesses the numeric value if value[x] is a Quantity

result, _ := fhirpath.Evaluate(resource, "Bundle.entry.resource.as(Patient)")
// Returns only entries that are Patient resources

Edge Cases / Notes:

• Returns empty collection if the input is empty.
• Returns empty collection if none of the elements match the specified type.
• Unlike is(), the as() function works on collections with multiple elements – it filters and returns only matching elements.
• The function form value.as(Quantity) is equivalent to the operator form value as Quantity.
• The type name is typically handled specially by the evaluator, extracting it directly from the AST.

ofType

Filters the input collection, returning only elements that are of the specified type. This function is identical in behavior to as() but is the preferred form when filtering collections.

Signature:

ofType(type : TypeSpecifier) : Collection

Parameters:

Return Type: Collection

Examples:

result, _ := fhirpath.Evaluate(observation, "Observation.value.ofType(Quantity)")
// Returns value only if it is a Quantity

result, _ := fhirpath.Evaluate(resource, "Bundle.entry.resource.ofType(Patient)")
// Returns only Patient resources from a Bundle

result, _ := fhirpath.Evaluate(resource, "Bundle.entry.resource.ofType(Observation).status")
// Gets status from all Observation resources in a Bundle

Edge Cases / Notes:

• This function is also documented under Filtering Functions since it filters collections by type.
• Type matching compares the element’s runtime type name against the specified type name.
• Returns an empty collection if no elements match the type.
• Unlike is(), ofType() works on collections with multiple elements and never errors on non-singleton input.
• URI subtypes: FHIR® defines several URI subtypes (url, oid, uuid, id, code, markdown, etc.) that all map to String in FHIRPath. This library preserves the FHIR® type identity, so ofType(url) correctly returns only url values, not oid or other string subtypes.
• Complex types from JSON: When resolving polymorphic fields (e.g., value[x]), complex types like Quantity and CodeableConcept are recognized even when the JSON lacks optional fields like unit, system, or code.

Comparison: is vs. as vs. ofType

Example illustrating the differences:

// is() -- returns a boolean
result, _ := fhirpath.Evaluate(observation, "Observation.value.is(Quantity)")
// true or false

// as() -- returns the value if it matches, empty otherwise
result, _ := fhirpath.Evaluate(observation, "Observation.value.as(Quantity)")
// The Quantity object, or empty

// ofType() -- filters multiple elements by type
result, _ := fhirpath.Evaluate(resource, "Bundle.entry.resource.ofType(Patient)")
// All Patient resources from the Bundle

In practice, as() and ofType() behave identically in this implementation – both filter elements by type. The FHIRPath specification recommends using ofType() when filtering collections and as() when casting a single value.