gotchas.md

FHIRPath Gotcha’s

Empty collections are propagated

• In FHIRPath, whenever an empty collection is encountered, rather than raising an error it gets propagated throughout the rest of the expression. This may make some issues difficult to catch.
• Eg. given Patient.name -> {}, Patient.name.family + ' MD' -> {}

Equality sometimes returns an empty collection { }, rather than false

• If either collection is empty
• If the **precision_ _**of Date, Time, or DateTime objects are mismatched
• If the dimension of a Quantity unit is mismatched

FHIR type specifiers are case-sensitive

• Primitive types are denoted with lower case specifiers.
• Primitive types that are written as upper case will be resolved as System types, not FHIR types.
• Eg. Patient.birthDate is date = **true** but Patient.birthDate is Date = **false**
• Case should match what’s listed here
• System types always begin with an uppercase letter

As Expression is not a filter, expects singleton input

• The as expression (Observation.value as integer) expects a singleton as input. For example, if you pass in a resource with multiple value fields, it will raise an error.
• It doesn’t filter out things that don’t match the type. For this purpose, the where() function should be used -> where(value is integer)