FHIR Chat · operand errors · fhirpath

Stream: fhirpath

Topic: operand errors

Michael Lawley (May 31 2018 at 01:23):

I am getting an error message like:
Problem processing expression (derivation = 'constraint') or (kind = 'logical') or (url = 'http://hl7.org/fhir/StructureDefinition/'+id) in profile http://hl7.org/fhir/StructureDefinition/StructureDefinition path StructureDefinition: Error performing +: right operand has no value
from the validation engine because there's no value for id. But this is coming as an exception rather than a validation error. Is it valid to treat any PathEngineException as a validation error, or should the constraint check that id has a value before using it?

Bryn Rhodes (May 31 2018 at 01:35):

In general, an operator in FHIRPath will return empty ({ }) if any of its arguments are empty, so I would think the concatenate there ought to return an empty, rather than throw an exception like that.

Michael Lawley (May 31 2018 at 01:43):

That's certainly not the behaviour of FHIRPathEngine.java

private List<Base> opPlus(List<Base> left, List<Base> right) throws PathEngineException {
  if (left.size() == 0)
    throw new PathEngineException("Error performing +: left operand has no value");
  if (left.size() > 1)
    throw new PathEngineException("Error performing +: left operand has more than one value");
  // ...

Grahame Grieve (May 31 2018 at 08:10):

I believe that this is the correct implementation of + as defined in the fhirpath specificatino

Grahame Grieve (May 31 2018 at 08:10):

I believe we argued about this

Michael Lawley (May 31 2018 at 08:20):

In that case the constraint needs re-writing as:
(derivation = 'constraint') or (kind = 'logical') or (exists(id) and url = 'http://hl7.org/fhir/StructureDefinition/'+id)
?

Michael Lawley (May 31 2018 at 08:23):

But 4.4.1 of fhirpath says

For functions or operators that take a single values as input, this means in general if the input is empty, then the result will be empty as well.

Grahame Grieve (May 31 2018 at 08:24):

• has a different definition, I think

Michael Lawley (May 31 2018 at 08:25):

6.6.7. & (string concatenation)
For strings, will concatenate the strings, where an empty operand is taken to be the empty string. This differs from + on two strings, which will result in an empty collection when one of the operands is empty.
http://hl7.org/fhirpath/#math

Grahame Grieve (May 31 2018 at 08:29):

guess I should change that to & not + then

Michael Lawley (May 31 2018 at 08:32):

which will create a different bug, no? It would result in url being compared with "http://hl7.org/fhir/StructureDefinition/"when there's no id

Michael Lawley (May 31 2018 at 22:31):

My take on the code above is that the second test is correct - more than one value is an error, but that the first case should result in the empty list.

Last updated: Apr 12 2022 at 19:14 UTC