Missing union type support for Kotlin codegen

[madisp]

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Execution failed for task ':pkl-playground:genKotlin'.
> Pkl union types are not supported by the Kotlin code generator.

---

Bit of a tall ask, I know, but it'd be wonderful if we could map union types into some form of Kotlin structure.

[bioball]

It'd be great to support this, but it's not clear how they should turn into Kotlin. Feel free to provide suggestions, though.

[jamesward]

For now this will have to be implemented with sealed class https://kotlinlang.org/docs/sealed-classes.html

[madisp]

yup, most likely. sealed class needs a name though, maybe take a similar approach to string union to enum mapping that all union types should be defined via a type alias?

E.g. this wouldn't still map to Kotlin:

class Foo {
  items: Listing<Bar | Baz>
}

but this would?

typealias FooItem = Bar | Baz

class Foo {
  items: Listing<FooItem>
}

[giordano-newday]

Hi all, I'm evaluating pkl as a configuration language for an API GW. My server is in Kotlin and not having union mapped to Kotlin (or Java) is a show stopper. There any thoughts about adding them? or what would be a workaround for something with the same semantic but supported by Kotlin? My scenario is something like typealias HttpMethod = "GET" | "POST" | "PUT" | "DELETE" Thanks

[bioball]

@giordano-newday: We don't have a great way to model most union types as Kotlin, because it's missing anonymous sum types. However, typealiases of string literal unions are a special case, and are no problem to represent.

This pkl:

typealias HttpMethod = "GET" | "POST" | "PUT" | "DELETE"

Turns into the following Kotlin:

  enum class HttpMethod(
    val value: String
  ) {
    GET("GET"),

    POST("POST"),

    PUT("PUT"),

    DELETE("DELETE");

    override fun toString() = value
  }

[giordano-newday]

I think it would for my scenarios. Thanks @bioball.