Support reflection

[johngrimes]

Spec link: https://build.fhir.org/ig/HL7/FHIRPath/#reflection

Implement the type() reflection function based on the CI build FHIRPath specification, returning SimpleTypeInfo/ClassInfo structs with namespace, name, and baseType fields.

The trace() function has been split to #2580.

[johngrimes]

There seems to be a difference between https://hl7.org/fhirpath/#reflection and https://build.fhir.org/ig/HL7/FHIRPath/#reflection.

Perhaps we should start by implementing the version in the CI build and then review.

[piotrszul]

Excerpt from CI Build FHIRPath Specification — Reflection

Note: The contents of this section are Standard for Trial Use (STU)

FHIRPath supports limited reflection to provide the ability for expressions to access basic type information of values. The ability to access child elements or walk type trees to parent definitions using this mechanism is outside the scope of this specification. The ofType() function can be used with type-names or their subtypes to filter content.

Structures

TypeInfo { baseType: string }
SimpleTypeInfo extends TypeInfo { namespace: string, name: string }
ClassInfo extends TypeInfo { namespace: string, name: string }

Note: These structures are a subset of the abstract metamodel used by the Clinical Quality Language Tooling. Although the SimpleTypeInfo and ClassInfo appear the same here, implementations may have additional information (as is the case in the referenced CQL structures).

type() : collection

The type function returns the type information for each item of the input collection, using concrete subtypes of TypeInfo.

Note: using X.ofType(Y) or X as Y to filter content is better supported than using X.type().name = 'Y'.

If the input collection is empty ({ }), or access to type information is unavailable, the result is empty.

For primitive types such as String and Integer, the result is a SimpleTypeInfo:

('John' | 'Mary').type()

Results in:

[
  SimpleTypeInfo { namespace: 'System', name: 'String', baseType: 'System.Any' },
  SimpleTypeInfo { namespace: 'System', name: 'String', baseType: 'System.Any' }
]

Note: The base type for primitives is defined as System.Any.

For complex types such as a FHIR CodeableConcept, the result is a ClassInfo:

Patient.maritalStatus.type()

Results in:

{
  ClassInfo { namespace: 'FHIR', name: 'CodeableConcept', baseType: 'FHIR.Element' }
}

[piotrszul]

Implementation Notes

The type() reflection function has been implemented in PR #2578 based on the CI build FHIRPath specification.

Design decisions

Single TypeInfo class for both SimpleTypeInfo and ClassInfo: The CI spec defines identical fields for both subtypes (namespace, name, baseType) and notes: "Although the SimpleTypeInfo and ClassInfo appear the same here, implementations may have additional information." Since no distinguishing fields are needed at this time, a single TypeInfo class is used. If a future spec revision adds distinguishing fields, the implementation can be split.

Simplified baseType system: Based on the R4 FHIR type hierarchy, only three base types are used:

• System.Any — for System primitives (String, Integer, Boolean, Decimal, Date, DateTime, Time, Quantity, Coding)
• FHIR.Element — for FHIR data types, complex types, and backbone elements
• FHIR.Resource — for FHIR resource types

This is sufficient for the CI spec's scoped-down reflection model, which explicitly states: "The ability to access child elements or walk type trees to parent definitions using this mechanism is outside the scope of this specification."

Not implemented

• trace() function — not mentioned in the CI build spec excerpt, can be added separately if needed.