Exhaustive checks in oneof fields

[scmorse]

Is there a way to configure Wire to generate either an Enum / Sealed Class for oneof fields, to provide Kotlin developers with an exhaustive API for selecting the oneof field? It would be very helpful for detecting new cases at compile-time, rather then as errors at run-time. 🙏

I see that the sealed classes approach used to exist, but was removed, and several developers have asked for similar "exhaustive check" support in this issue: #998. I have never contributed to this project before, but I would be willing to invest some time into this feature if it will be well-received.

[scmorse]

@oldergod Do you think this could work with the boxOneOfsMinSize feature from #1866?

[scmorse]

CC @Egorand Since I saw you had a PR to remove Sealed Classes in #1002

[oldergod]

We never had sealed classes for that, and currently have no plan to.

boxed oneofs will provide a set of all entries but that's it.

[scmorse]

@oldergod thanks for your reply!

It wouldn't have to be a sealed class necessarily, but I think exhaustive checks that are verified at compile-time are a very strong feature, and worth some consideration. Because if someone adds a new option to a oneof, any clients of that proto would like to be asked how they want to handle the new case, rather than defaulting to some else -> throw case, crashing at runtime. (The else -> case need not even be written when the compiler can guarantee the check is exhaustive, as I'm sure you know)

[scmorse]

@oldergod If sealed classes aren't palatable, what would you think declaring an enum where the value corresponds to which one of the oneof cases is provided? For example, if a field is declared like this:

oneof my_field {
  MessageOne message_one = 1;
  MessageTwo message_two = 2;
}

Then in addition to the message_one and message_two properties on the generated classes, there would also be a new my_field_case property, having an enum generated value so that we could check the case like:

when (obj.my_field_case) {
  MyFieldCase.MESSAGE_ONE -> handle(obj.message_one!!)
  MyFieldCase.MESSAGE_TWO -> handle(obj.message_two!!)
}

Which is an exhaustive check, so that we can handle new oneof cases at compile time rather than failing at runtime!

[oldergod]

We have no plan to add that at the moment.
This is however something you could build yourself for your project by injecting a custom handler which would generate extension members for your messages.

[friscoMad]

I am also struggling with this, not only for exhaustiveness in message handling but also for making it clear for devs which ones are oneOf fields and enforcing a single oneOf value is set during message construction.
I see that the custom handler can be used to create some extension functions for the map proposed by @scmorse but if I am not wrong there is no way to change constructor and getters.

For reference, all other alternative protoc compilers for Kotlin that I know do use sealed interfaces for oneOfs (ProtoKt, KrotoDC and PBAndK)

[lily-es]

Generating enums for oneof fields would allow for exhaustive type checking while maintaining backwards compatibility
Would be nice to have support for this