Customizing union generation

[abarreau]

Hello,

I'm trying to customize the way unions are generated. More specifically, my unions are annotated with a specific trait that would need to change the whole structure of the generated typescript code.

Here is my contract:

@discriminated("_type")
union MyUnion {
  package: PackageDTO
  simulation: SimulationDTO
}

Currently the UnionGenerator generates this:

export namespace MyUnion {
    export interface PackageMember {
        package: PackageDTO;
        simulation: never;
    } 

    export interface SimulationMember {
        package: never;
        simulation: SimulationDTO
    }

    // UnknownMember and Visitor interfaces + function ommited
}

What I would like to generate:

export type MyUnion = (PackageDTO & { "_type": "package" }) | (SimulationDTO & { "_type": "simulation" });

I tried to look into interceptors to see if any CodeSection would help me intercept this part of the generation, but unfortunaly the UnionGenerator has no call to TypescriptWriter.injectSection.

What would be the best approach here? Is this doable without reimplementing the whole DirectedCodegen? (The TypescriptDirectedCodegen class is protected and final which makes it painful to customize behavior and I didn't want to rewrite the whole codegen, just override the generateUnions method).

I'm willing to open a PR if that's needed to add new CodeSections in UnionGenerator.

Thanks for your help.

[kuhe]

can you show a TypeScript example of what the current output is and your desired output would be?

[abarreau]

@kuhe I updated the original post with what the current generated output is and what I would like to have.

[kuhe]

Related: smithy-lang/smithy#1082

[kuhe]

Because this is not universally applicable to all union shapes due to backwards compatibility, I agree that a trait is needed to distinguish between existing and discriminator-field based unions.

We don't currently have plans to support this in either TypeScript or across multiple programming languages that have Smithy generators.

I am open to reviewing a PR enabling this. I must warn that we are not guaranteeing acceptance of the PR, and that the code is tricky to understand. I request that if you contribute a PR, the feature should be trait based, and the trait should be called @typescriptDiscriminatedUnion. This leaves more room for the feature to be superseded by a general Smithy feature.