Support ValueSet as a first-class type

[cmoesel]

(Updated description in Oct 2022 during backlog cleanup)

The CQL ValueSet type represents a reference to a value set, allowing that value set to be passed into CQL functions without the need to do a value set expansion (as in CQL 1.4).

To demonstrate how this might work, consider the following example:

library MyLibrary version '0.0.1'
valueset MyVS: 'http://example.org/vs/myvs'
codesystem MyCS: 'http://example.org/cs/mycs'
code Foo: 'Foo' from "MyCS" display 'Foo'

define function FooInVS(vs ValueSet):
  Foo in vs

define IsFooInMyVS:
  FooInVS(MyVS)

This results in the following JSON:

[
  {
    "name": "FooInVS",
    "context": "Patient",
    "accessLevel": "Public",
    "type": "FunctionDef",
    "expression": {
      "type": "InValueSet",
      "code": {
        "name": "Foo",
        "type": "CodeRef"
      },
      "valueset": {
        "name": "vs",
        "type": "OperandRef"
      }
    },
    "operand": [
      {
        "name": "vs",
        "operandTypeSpecifier": {
          "name": "{urn:hl7-org:elm-types:r1}ValueSet",
          "type": "NamedTypeSpecifier"
        }
      }
    ]
  },
  {
    "name": "IsFooInMyVS",
    "context": "Patient",
    "accessLevel": "Public",
    "expression": {
      "name": "FooInVS",
      "type": "FunctionRef",
      "operand": [
        {
          "name": "MyVS",
          "type": "ValueSetRef"
        }
      ]
    }
  }
]

Note that the function indicates that it takes a ValueSet, but when you invoke the function, it's actually a ValueSetRef that is passed in.

Initial Zulip conversation: https://chat.fhir.org/#narrow/stream/179220-cql/topic/ValueSet.20Typing

[JSRankins]

Hi @cmoesel. We were discussing ValueSetRef and CodeSystemRef as first-class types at the Connecathon today. An update is being made to fix some issues with ValueSetRef and CodeSystemRef as first-class types in CQL with version 1.5.2. 1.5.2 correlates to CQL-to-ELM Translator 1.5.6. Bryn had mentioned this functionality wasn't available in cql-execution yet. He said a ticket needed to be created, so I volunteered. See Issue 259. I didn't see this ticket until afterward. Not sure if you want to merge tickets or, since this ticket seems to be specific to ValueSet references, leave Issue 259 open for the CodeSystem reference piece. Thoughts?

[cmoesel]

I think it's likely we might implement these separately (one at a time), so it probably makes sense to have one issue for VS references and one for CS references. @JSRankins - can you update #259 to be specific to CodeSystems? And also include a reference to this ticket so that at least they are linked in some way?

[JSRankins]

@cmoesel Done.

[IrickNcqa]

Adding a note of support to this request from NCQA. A use case from our measures prompted this language update, and as such we are looking for engine support for the feature.