The Using CQL with FHIR IG specifies a set of conversions between CQL and FHIR data types. Some of the FHIR types are not directly serializable themselves, so must be represented as sub-parameters (or parameter parts) of FHIR Parameters resources. The Type Mapping Example shows the expected results for all the defined types.

There's an implementation of CQL to FHIR type conversion here but it predates the above spec. It doesn't use the entire context of the CQL evaluation to produce a single, unified Parameters resource. Instead, it assumes each definition is converted one at a time.

This is currently being used in downstream code, so can't yet be migrated wholesale. We need to provide a correct implementation and then update the downstream code.