FHIR Chat · Adding a 'let' statement

Stream: mapping-framework

Topic: Adding a 'let' statement

Ewout Kramer (Aug 20 2019 at 16:24):

Yesterday during the FHIR-I teleconf, we discussed GF#21642, asking for the addition of a "const" or "let" to the mapping language, so you can define, say, a named constant for an often-used extension url in your mapping.

like so:

let TariffFormExtensionUrl = 'http://uk-koeln.de/fhir/StructureDefinition/Extension/nNGM/pkvTarifform/0.1'

We have several things we want to discuss before we decide on adding this:

Where do we allow a let? Just at the top of the mapping file with a global scope? Inside each group or nested set of dependent rules?
The let would just define a read-only label, you cannot re-assign another value to it later on.
It would need to be defined at the beginning of a group, it makes no sense to insert a let between rules.
Would this only work with constant values? E.g. should we let the right hand side of the let contain function calls? Or refer to variables/aliases defined in the parent scope of the let?
If a constant, shall we call it 'const'?

Currently, my engine supports constant values only, and they need to be included just before the first group in a map. When serialized to a StructureMap, I emit an extension:

<StructureMap xmlns="http://hl7.org/fhir">
    <extension url="http://hl7.org/fhir/5.0/StructureDefinition/extension-StructureMap.let">
        <extension url="name">
            <valueString value="TariffFormExtensionUrl" />
        </extension>
        <extension url="value">
            <valueString value="http://uk-koeln.de/fhir/StructureDefinition/Extension/nNGM/pkvTarifform/0.1" />
        </extension>
    </extension>
    <url value="http://fire.ly/tests/AllFeatures" />

Grahame Grieve (Aug 21 2019 at 19:23):

what type can it have? primitive only?

Grahame Grieve (Aug 21 2019 at 19:24):

can it be defined at group scope?

Simone Heckmann (Sep 16 2019 at 14:50):

right hand side should allow at least the function uuid().
When your target is a Bundle, you may want to assign fullUrls to your entries that you can later use within different grous to link your resources together.

David Hay (Dec 05 2019 at 05:39):

global scope would mean that they could all go at the top of the mapping file, making them easier to see. Would also support use across different groups...

Last updated: Apr 12 2022 at 19:14 UTC

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