FHIRPath

Pathling leverages the FHIRPath language in order to abstract away some of the complexity of navigating and interacting with FHIR data structures.

Pathling implements the FHIRPath subset within the Sharable View Definition profile of the SQL on FHIR view definition, plus additional terminology and utility functions.

Supported language features

Path navigation

Pathling supports standard FHIRPath path navigation using dot notation:

Patient.name.given
Observation.code.coding.system

Array indexing is supported using bracket notation:

Patient.name[0].given

The $this special variable is supported for referring to the current context within expressions.

Literals

Pathling supports the following literal types:

Type	Syntax	Examples
Boolean	true, false	active = true
String	Single quotes with escapes	'hello', 'it\'s'
Integer	Whole numbers	123, -45
Decimal	Numbers with decimal point	3.14, -0.5
Date	@ prefix, ISO 8601	@2023-01-15
DateTime	@ prefix, ISO 8601	@2023-01-15T14:30:00
Time	@T prefix	@T14:30:00
Quantity	Number with unit	10 'mg', 4 days

Operators

See Operators in the FHIRPath specification for detailed semantics.

Comparison operators

Operator	Description
=	Equality
!=	Inequality
<	Less than
<=	Less than or equal to
>	Greater than
>=	Greater than or equal to

Boolean operators

Operator	Description
and	Logical AND
or	Logical OR
xor	Exclusive OR
implies	Logical implication

Arithmetic operators

Operator	Description
+	Addition
-	Subtraction
*	Multiplication
/	Division
mod	Modulus

Unary + and - are also supported for numeric values.

String operators

Operator	Description
&	String concatenation

Collection operators

Operator	Description
|	Union of two collections
in	Test if element is in collection
contains	Test if collection contains element

Type operators

Operator	Description
is	Type checking
as	Type casting

Standard functions

The following standard FHIRPath functions are implemented. See Functions in the FHIRPath specification for detailed semantics.

Existence functions

Function	Description
exists(criteria?)	Returns true if the collection has any elements, optionally filtered by criteria
empty()	Returns true if the collection is empty

Filtering and projection functions

Function	Description
where(criteria)	Filter collection by criteria expression
ofType(type)	Filter collection by type

Subsetting functions

Function	Description
first()	Returns the first element of the collection

Boolean functions

Function	Description
not()	Boolean negation

String functions

Function	Description
join(separator?)	Join strings with optional separator

Type functions

Function	Description
is(type)	Type checking (equivalent to is operator)
as(type)	Type casting (equivalent to as operator)

Conversion functions

Function	Description
toBoolean()	Convert to Boolean
toInteger()	Convert to Integer
toDecimal()	Convert to Decimal
toString()	Convert to String
toDate()	Convert to Date
toDateTime()	Convert to DateTime
toTime()	Convert to Time
toQuantity(unit?)	Convert to Quantity with optional unit conversion using UCUM

Function	Description
convertsToBoolean()	Check if convertible to Boolean
convertsToInteger()	Check if convertible to Integer
convertsToDecimal()	Check if convertible to Decimal
convertsToString()	Check if convertible to String
convertsToDate()	Check if convertible to Date
convertsToDateTime()	Check if convertible to DateTime
convertsToTime()	Check if convertible to Time
convertsToQuantity(unit?)	Check if convertible to Quantity

Limitations

The following FHIRPath features are not currently supported:

• Equivalence operators: ~ and !~
• Lambda expressions
• Aggregate functions: count(), sum(), avg(), min(), max()
• Special variables: $index, $total
• Quantity arithmetic: Math operations on Quantity types
• DateTime arithmetic: DateTime math operations
• Full resolve(): Traversal of resolved references

Additional functions

Pathling also supports additional functions beyond the standard FHIRPath specification:

• FHIR-specific functions - Functions defined in the FHIR specification for use with FHIR data, including extension, resolve, memberOf, subsumes, and subsumedBy.
• Extension functions - Functions unique to Pathling, including terminology functions like designation, display, property, and translate, plus the Coding literal data type.