Make "Reference" type generic?

[preston]

Would it be possible to make the TypeScript version of Reference generic? It is very difficult to enforce constraints without this, and TypeScript supports complex "A | B | C | D"-style generics.

We are trying to strictly enforce all manner of constraints in our information models for both IG publishing and software development purposes, and such refinements would be very welcome!

[GinoCanessa]

I like the idea, but I am struggling with what the output should look like.

In Bundle, we use the generic to apply type information to Bundle.entry.resource, which actually contains a FHIR object.

However, the datatype Reference doesn't contain an object - it just has an optional type element (which points to the URI of the reference type).

Is the goal that Reference<Patient> forces the literal Patient into Reference.type?

Thoughts: @bkaney @jdjkelly @ir4y @jmandel ?

[preston]

Yup, exactly. Reference itself would be a generic type. Something along the lines of..

export interface Reference<T extends FhirResource> extends Element {
...
_type?: T;
...
}

..with the general pattern repeated for other resource types to match more closely with the FHIR specification and be more directly implementable without casting madness.

[GinoCanessa]

Updating here for visibility. @jmandel suggested the following:

interface Reference<T extends Resource> {
 ...
  type?: `${T["resourceType"]}`,
 ...
}

Which, I believe is in line with what we are looking for (e.g., the type field is actually a string value). I haven't tested it yet, but it looks like a step in the right direction. I believe it will need a little more logic around undefined though.

[jdjkelly]

From the string PR, this is a bit stricter (edit: tho I'm not sure is valid TS lol)

type?: `${T["resourceType"]}` as FhirResourceType

Although I suppose if you have T that is accessing ["resourceType"] you're probably okay already. Or did you mean something else by the undefined case @GinoCanessa ?

[ir4y]

It would be nice to check Reference.reference.

interface Reference<T extends Resource> {
 ...
  reference?: `${T["resourceType"]}/\s+`,
 ...
}

Now it is not possible because microsoft/TypeScript#41160 has to be approved first.
Also, we need to handle local references somehow.

Regarding defining the Reference.type personally, I like the @jmandel idea.

interface Reference<T extends Resource> {
 ...
  type?: `${T["resourceType"]}`,
 ...
}

However, based on type definition https://www.hl7.org/fhir/references-definitions.html#Reference.type the valueset binding for Reference.type is extensible.
Accordingly this the most close definition to FHIR is

interface Reference<ResourceType extends (FhirResourceType | string) = FhirResourceType> {
...
type: ResourceType;
...
};

let r1:Reference<"Patient">;
let r2:Reference<"AnotherValue">;

I don't this. | string usage makes Reference.type weak and it will no catch typos.
However, it is how FHIR defines Reference.type and we have to support all FHIR valid cases.

I would like to hear your thoughts.

@GinoCanessa @jdjkelly @bkaney @preston

[bkaney]

Yes, this would be great @ir4y! I am looking forward to microsoft/TypeScript#41160, but wonder how we could do this to be somewhat backward compatiable?