Stream: implementers
Topic: Optional fields that seems to require default value in base
Morten Ernebjerg (Sep 09 2020 at 11:17):
Hi :wave: Looking at Questionnaire, it seems that there are certain boolean elements that are optional, but for which processing systems and validators are de facto forced to assume a definitive element value if the element is absent. Examples are Questionnaire.experimental (even a modifier field) and Questionnaire.item.repeats. E.g. if the latter is missing, I must assume a value in order to judge whether a QuestionnaireResponse with multiple matching items is valid or not.
Accordingly, I expected there to be default values defined in the documentation (I thought I already knew the two examples above default to "false"). Failing to find them, I finally realized that there is a rule saying that base resource definition cannot have default values (sdf-22 on StructureDefinition). What is the reasoning behind this restrictions? And how is an implementations supposed to deal with (or just know) such apparently "tacit defaults"?
Lloyd McKenzie (Sep 09 2020 at 13:47):
FHIR doesn't support default values, but it does have a notion called "meaning if missing" (e.g. http://build.fhir.org/questionnaire-definitions.html#Questionnaire.item.repeats). There probably should be a "meaning if missing" on experimental - can you submit a change request?
Morten Ernebjerg (Sep 09 2020 at 14:12):
Ah right, I missed that (given that it isn't there for experimental). I see ElementDefinition.meaningWhenMissing is MarkDown. Is there any way of accessing this information in a machine-readable way or are tools generally expected to have this information hand-coded into them?
Will open a JIRA ticket for experimental.
Morten Ernebjerg (Sep 09 2020 at 14:22):
Ticket FHIR-28454
Lloyd McKenzie (Sep 09 2020 at 14:42):
The information isn't necessarily going to be straightforward to interpret - e.g. Resource.language. So no, no expectation for computable processing. We actually used to have explicit default values, but that created its own challenges.
Last updated: Apr 12 2022 at 19:14 UTC
