Use of $index always returns last element in list

[jpnarkinsky]

The following fragment is expected to return '[0]' (the index of the element foo.bar.com.) Instead, it returns '[1]'. When used with longer arrays, it apparently always returns the index of the last element in the array.

> const fhirpath = require('fhirpath')
undefined
>     const resource = {
...       resourceType: 'Practitioner',
...       telecom: [{
.....         'use': 'work',
.....         'rank': 2,
.....         'value': 'foo.bar.com',
.....         'system': 'url',
.....       },
...       {
.....         'use': 'work',
.....         'rank': 1,
.....         'value': 'bar.foo.com',
.....         'system': 'url',
.....       }],
...     };
undefined
> fhirpath.evaluate(resource, `Practitioner.telecom.where(value='foo.bar.com')`)  # this returns the element at index 0
[ { use: 'work', rank: 2, value: 'foo.bar.com', system: 'url' } ]
> fhirpath.evaluate(resource, `Practitioner.telecom.where(value='foo.bar.com').$index`)  # this returns index 1
[ 1 ]
>  

Am I completely misundertanding the purpose of $index, or is this a bug? Any ideas would be appreciated since this is a show-stopper for us.

[plynchnlm]

The behavior of $index outside of a expression passed into an function is not defined. The specification says, "If the function takes an expression as a parameter, the function will evaluate the expression passed for the parameter with respect to each of the items in the input collection. These expressions may refer to the special $this and $index elements, which represent the item from the input collection currently under evaluation, and its index in the collection, respectively."

In other words, you can do, ".where($index =1)", but you can't do ".where(...).$index."

[jpnarkinsky]

@plynchnlm In that case, shouldn't something like this work?

Practitioner.telecom.where(value='foo.bar.com').select($index)

If so, it doesn't appear to work -- it always returns 0.

> fhirpath.evaluate(r, 'Practitioner.telecom')
[
  { use: 'work', rank: 2, value: 'foo.bar.com', system: 'url' },
  { use: 'work', rank: 1, value: 'bar.foo.com', system: 'url' }
]
> fhirpath.evaluate(r, "Practitioner.telecom.where(value = 'foo.bar.com')")
[ { use: 'work', rank: 2, value: 'foo.bar.com', system: 'url' } ]
> fhirpath.evaluate(r, "Practitioner.telecom.where(value = 'foo.bar.com').select($index)")
[ 0 ]
> fhirpath.evaluate(r, "Practitioner.telecom.where(value = 'bar.foo.com').select($index)")
[ 0 ]

Our objective is to get the index of the matching element (for our open source fhirpatch library). Any suggestions on a path that would accomplish that?

[plynchnlm]

After the "where" function has run, you are left with an array of one element, so when the "select" call operates on that, the value of $index is zero.

[plynchnlm]

The question of how to get the $index of a element matching some expression is an interesting one. I suggest you post that question on the fhirpath stream of chat.fhir.org.