Union operator on valuesets does not work

[sorliog]

Good afternoon.

I am developing rule that use external valuesets. In my expression, I need to union valuesets together, ideally into a list of codes. However, I discoved that the union operator on valuesets throws a Null error. I am using cql-execution version 3.1.0 with the vsac.CodeService() to resolve valuesets from NLM. All valuesets in the CQL do resolve successfully and are not null.

This is the CQL rule that I am using:

library "Union issue" version '1.0.0'
using FHIR version '4.0.1'

codesystem "Snomed": 'http://snomed.info/sct'
codesystem "CVX": 'http://hl7.org/fhir/sid/cvx'

code "Single Pregnancy": '237244005' from "Snomed" display 'Single Pregnancy'
code "Pregnant woman": '255409004' from "Snomed" display 'Pregnant woman'
valueset "Pregnant State VS": 'https://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113883.3.3157.4055'
valueset "Pregnancy VS": 'https://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113762.1.4.1078.264'

// Does not work
define "Union on valuesets":
    "Pregnant State VS" union "Pregnancy VS" // Here is the issue

// Does work
define "Union on codes":
    "Single Pregnancy" union "Pregnant woman"

// Does work
define "Union on numbers":
     1 union 3

// Does work
define "Union on strings":
     'hello' union 'world'

This is the error message:

Error Message:	Cannot read properties of null (reading 'execute')
	CQL Library:	Union issue|1.0.0
	Expression:	Union


    at Union.execute (projectPath\node_modules\cql-execution\src\elm\expression.ts:49:13)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)
    at ExpressionDef.exec (projectPath\node_modules\cql-execution\src\elm\reusable.ts:18:45)
    at ExpressionDef.execute (projectPath\node_modules\cql-execution\src\elm\expression.ts:41:27)
    at Executor.exec_patient_context (projectPath\node_modules\cql-execution\src\runtime\executor.ts:99:28)
    at Executor.exec (projectPath\node_modules\cql-execution\src\runtime\executor.ts:60:15)
    at executeCql (projectPath\src\cdss-module.js:253:20)
    at Object.<anonymous> (projectPath\src\test\debug.test.js:34:22)

Does the union operator have an implementation to deal with valuesets? I seem to recall that it did work in the near past.

[cmoesel]

If you look at the ELM that CQL-to-ELM produces for the example above, you'll notice that performing a union on two value sets results in calls to ExpandValueSet for each value set. Unfortunately, cql-execution does not currently support the ExpandValueSet operator (which is marked as trial-use in CQL 1.5). I apologize that the error it produces doesn't convey what's really happening.

In practice, I've avoided this issue by doing unions of retrieve statements instead. In other words, instead of:

define "Union of Pregnancy VS":
    "Pregnant State VS" union "Pregnancy VS"

define PregnancyOperations: [Observation: "Union of Pregnancy VS"]

you can do:

define PregnancyOperations: [Observation: "Pregnant State VS"] union [Observation: "Pregnancy VS"]

We'd also be glad to accept a PR for this if you'd like to consider taking a crack at implementing it yourself.

[sorliog]

Thank you @cmoesel. The workarounds did not work in my use case. Instead I implemented the ExpandValueset function in this PR