Shouldn't aliases expand inside other alias definitions

[keithboone]

Alias: $Base = http://hl7.org/fhir/us/saner
ValueSet: Populations
Title: "Measure Populations"
Description: "Measure Populations defined by The SANER Project"

• codes from system PopulationSystem
  Alias: Populations = $Base/ValueSet/Populations

Doesn't seem to work the way I expect. I have to expand to:
ValueSet: Populations
Title: "Measure Populations"
Description: "Measure Populations defined by The SANER Project"

• codes from system PopulationSystem
  Alias: Populations = http://hl7.org/fhir/us/saner/ValueSet/Populations

[ngfreiter]

As another example of how Alias expansion might work, see this thread: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Bug.20when.20using.20SNOMED.20Version.20URLs. The example below does not work, because the individual Aliases do not resolve within the rule. The whole sequence is resolves as one alias SCT|SCTIntlMar2020, which of course does not exist. If the functionality of Alias expansion is expanded, we should consider cases like the one shown below.

Alias: SCT = http://snomed.info/sct
Alias: SCTIntlMar2020 = http://snomed.info/sct/900000000000207008/20200309

ValueSet:  SCTBodySitesVS
Id: sctBodySitesVS
Title: "Seitenlokalisation nach SNOMED CT"
Description:  "Seitenlokalisation z.B. einer Prozedure"

* SCT|SCTIntlMar2020#85421007 "Structure of right half of body (body structure)"
* SCT|SCTIntlMar2020#31156008 "Structure of left half of body (body structure)"

[cmoesel]

This is an interesting question. I was initially a little concerned about unintentional expansion of aliases (e.g., if the value of an alias just happens to have another alias as a substring) but I think we can avoid that by smart implementation and rules. Using $ prefix would help, but I think the following rules would likely be sufficient:

• For an alias to be expanded inside an alias:
  • it must be at the start of the alias value
  • it must be followed by a /, ?, or whitepace

So... imagining Keith's example (but without the $, just to make it trickier)... Given:

Alias: Base = http://hl7.org/fhir/us/saner

Then the alias Base would be expanded in these situations:

Alias A: Base/ValueSet/Populations // http://hl7.org/fhir/us/saner/ValueSet/Populations
Alias B: Base?vs=Populations // http://hl7.org/fhir/us/saner?vs=Populations
Alias C: Base // http://hl7.org/fhir/us/saner

but it would not be expanded in these situations:

Alias A: http://example.org/ValueSet/BasePopulations 
Alias B: http://example.org/ValueSet/Base/Populations 
Alias C: BasePopulations/ValueSet/BP
Alias D: BasePopulations
// etc.

@markkramerus -- what do you think?

(BTW: This doesn't address the issue @ngfreiter pointed out for using aliases as a CS version).

[cmoesel]

This is an interesting question. I was initially a little concerned about unintentional expansion of aliases (e.g., if the value of an alias just happens to have another alias as a substring) but I think we can avoid that by smart implementation and rules. Using $ prefix would help, but I think the following rules would likely be sufficient:

• For an alias to be expanded inside an alias:
  • it must be at the start of the alias value
  • it must be followed by a /, ?, or whitepace

So... imagining Keith's example (but without the $, just to make it trickier)... Given:

Alias: Base = http://hl7.org/fhir/us/saner

Then the alias Base would be expanded in these situations:

Alias A: Base/ValueSet/Populations // http://hl7.org/fhir/us/saner/ValueSet/Populations
Alias B: Base?vs=Populations // http://hl7.org/fhir/us/saner?vs=Populations
Alias C: Base // http://hl7.org/fhir/us/saner

but it would not be expanded in these situations:

Alias A: http://example.org/ValueSet/BasePopulations 
Alias B: http://example.org/ValueSet/Base/Populations 
Alias C: BasePopulations/ValueSet/BP
Alias D: BasePopulations
// etc.

@markkramerus -- what do you think?

(BTW: This doesn't address the issue @ngfreiter pointed out for using aliases as a CS version).

[keithboone]

Added to ballot comments.