Existence Functions

Existence functions allow you to test whether collections contain elements, whether those elements meet certain criteria, and to retrieve distinct values. These are fundamental to FHIRPath expressions and are used extensively in FHIR® validation and data extraction.

empty

Returns true if the input collection is empty, false otherwise.

Signature:

empty() : Boolean

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(patient, "Patient.name.empty()")
// false (patient has at least one name)

result, _ := fhirpath.Evaluate(patient, "Patient.contact.empty()")
// true (if patient has no contacts)

result, _ := fhirpath.Evaluate(resource, "{}.empty()")
// true

Edge Cases / Notes:

• Always returns true or false, never an empty collection.
• This is the only existence function guaranteed to return a boolean even for empty input.

exists

Returns true if the input collection contains any elements. With an optional criteria expression, returns true if any element satisfies the criteria.

Signature:

exists([criteria : Expression]) : Boolean

Parameters:

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(patient, "Patient.name.exists()")
// true (patient has at least one name)

result, _ := fhirpath.Evaluate(patient, "Patient.name.exists(use = 'official')")
// true if any name has use = 'official'

result, _ := fhirpath.Evaluate(patient, "Patient.telecom.exists(system = 'phone')")
// true if patient has a phone telecom entry

Edge Cases / Notes:

• Without criteria, exists() is the inverse of empty().
• With criteria, it is equivalent to where(criteria).exists().
• Returns false for an empty input collection.
• The criteria expression is evaluated with $this set to each element.

all

Returns true if all elements in the collection satisfy the given criteria expression. Returns true for an empty collection (vacuous truth).

Signature:

all(criteria : Expression) : Boolean

Parameters:

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(patient, "Patient.name.all(use.exists())")
// true if every name entry has a 'use' field

result, _ := fhirpath.Evaluate(patient, "Patient.telecom.all(system = 'phone')")
// true only if every telecom entry is a phone

result, _ := fhirpath.Evaluate(patient, "Patient.contact.all(name.exists())")
// true if patient has no contacts (vacuous truth)

Edge Cases / Notes:

• An empty collection returns true (vacuous truth per FHIRPath specification).
• The criteria expression is evaluated with $this set to each element.
• This function is commonly used in FHIR® invariant definitions.

allTrue

Returns true if all items in the collection are boolean true.

Signature:

allTrue() : Boolean

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(resource, "Patient.active.allTrue()")
// true if active is true

result, _ := fhirpath.Evaluate(resource, "(true | true | true).allTrue()")
// true

result, _ := fhirpath.Evaluate(resource, "(true | false | true).allTrue()")
// false

Edge Cases / Notes:

• An empty collection returns true (vacuous truth).
• Non-boolean elements cause the function to return false.
• Commonly used after mapping a collection to boolean values.

anyTrue

Returns true if any item in the collection is boolean true.

Signature:

anyTrue() : Boolean

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(resource, "(true | false | false).anyTrue()")
// true

result, _ := fhirpath.Evaluate(resource, "(false | false).anyTrue()")
// false

result, _ := fhirpath.Evaluate(resource, "{}.anyTrue()")
// false

Edge Cases / Notes:

• An empty collection returns false.
• Returns true as soon as one boolean true is found.

allFalse

Returns true if all items in the collection are boolean false.

Signature:

allFalse() : Boolean

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(resource, "(false | false | false).allFalse()")
// true

result, _ := fhirpath.Evaluate(resource, "(false | true | false).allFalse()")
// false

result, _ := fhirpath.Evaluate(resource, "{}.allFalse()")
// true (vacuous truth)

Edge Cases / Notes:

• An empty collection returns true (vacuous truth).
• Non-boolean elements cause the function to return false.

anyFalse

Returns true if any item in the collection is boolean false.

Signature:

anyFalse() : Boolean

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(resource, "(true | false | true).anyFalse()")
// true

result, _ := fhirpath.Evaluate(resource, "(true | true).anyFalse()")
// false

result, _ := fhirpath.Evaluate(resource, "{}.anyFalse()")
// false

Edge Cases / Notes:

• An empty collection returns false.
• Returns true as soon as one boolean false is found.

count

Returns the number of items in the input collection.

Signature:

count() : Integer

Return Type: Integer

Examples:

result, _ := fhirpath.Evaluate(patient, "Patient.name.count()")
// Number of name entries (e.g., 2)

result, _ := fhirpath.Evaluate(patient, "Patient.telecom.count()")
// Number of telecom entries

result, _ := fhirpath.Evaluate(resource, "{}.count()")
// 0

Edge Cases / Notes:

• Always returns a non-negative integer, never an empty collection.
• An empty collection returns 0.

distinct

Returns a collection containing only the distinct (unique) elements from the input.

Signature:

distinct() : Collection

Return Type: Collection

Examples:

result, _ := fhirpath.Evaluate(resource, "(1 | 2 | 2 | 3 | 3 | 3).distinct()")
// { 1, 2, 3 }

result, _ := fhirpath.Evaluate(resource, "('a' | 'b' | 'a').distinct()")
// { 'a', 'b' }

result, _ := fhirpath.Evaluate(resource, "{}.distinct()")
// { } (empty)

Edge Cases / Notes:

• The order of elements in the result is implementation-dependent.
• Element equality is determined by FHIRPath’s equality rules.
• An empty collection returns an empty collection.

isDistinct

Returns true if all elements in the collection are distinct (no duplicates).

Signature:

isDistinct() : Boolean

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(resource, "(1 | 2 | 3).isDistinct()")
// true

result, _ := fhirpath.Evaluate(resource, "(1 | 2 | 2).isDistinct()")
// false

result, _ := fhirpath.Evaluate(resource, "{}.isDistinct()")
// true (empty collection is trivially distinct)

Edge Cases / Notes:

• Equivalent to count() = distinct().count().
• An empty collection returns true.

subsetOf

Returns true if all elements in the input collection are also present in the other collection.

Signature:

subsetOf(other : Collection) : Boolean

Parameters:

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(resource, "(1 | 2).subsetOf(1 | 2 | 3)")
// true

result, _ := fhirpath.Evaluate(resource, "(1 | 4).subsetOf(1 | 2 | 3)")
// false

result, _ := fhirpath.Evaluate(resource, "{}.subsetOf(1 | 2)")
// true (empty set is a subset of any set)

Edge Cases / Notes:

• An empty input collection is always a subset of any collection.
• Element comparison follows FHIRPath’s equality rules.

supersetOf

Returns true if all elements in the other collection are also present in the input collection.

Signature:

supersetOf(other : Collection) : Boolean

Parameters:

Return Type: Boolean

Examples:

result, _ := fhirpath.Evaluate(resource, "(1 | 2 | 3).supersetOf(1 | 2)")
// true

result, _ := fhirpath.Evaluate(resource, "(1 | 2).supersetOf(1 | 2 | 3)")
// false

result, _ := fhirpath.Evaluate(resource, "(1 | 2).supersetOf({})")
// true (any set is a superset of the empty set)

Edge Cases / Notes:

• a.supersetOf(b) is equivalent to b.subsetOf(a).
• Any collection is a superset of the empty collection.