Functions
Once you have a value, or a collection of values, you often would like to do something more specific with that value. So, besides getting data (out) of a resource, you might also want to change that data. For that FhirPath has the concept of functions. Sometimes a function, just limits the data you get, but often it helps you chnage it or tells you something about that data.
Syntax
A function is called by using a dot after a FhirPath field name follwed by two round brackets ( and ). Between those braces you might have to provide some additional information, depending on the function. Here is how it looks.
The exists() function
The function exists() tells you whether the value you are looking for is actually there. if it is, the function gives back the value True if found it, and False if it did not. So the following expression checks if a patient record has a birthDate:
Patient.birthDate.exists()
The count() function
And the follwoing function count() is even more interesting, it tells you how many elements where found. Imagine a patient with two names, and for each name, it has two given names. Take it for example a patient has the following four given names:
Patient.name[0].given[0] = 'William'
Patient.name[0].given[1] = 'John'
Patient.name[1].given[0] = 'Bill'
Patient.name[1].given[1] = 'Jack'
This expression
Patient.name.given.count()
will return the value 4.
Optional parameters
If you supply a value, or another expression inside of the brackets of a function, that is called a parameter. Some functions have mandatory parameters, and some have optional. Take for example the exists() function, you may provide an additional condition. This function
checks if the Patient has a phone listed in his telecomunication channels.
Patient.telecom.exists(system = 'phone')
Evaluation
Since the primary design goal for FhirPath was validation - expressing how a resource should look - there are a lot of functions that tell you whether something is true.