Protobuf import doesn't deal with Empty case

[ghostdogpr]

Checklist

• [x] I read the documentation at https://chimney.readthedocs.io/ and checked that the functionality exists
• [x] I verified that the behavior for my use case doesn't match the documentation
• [x] I checked the https://github.com/scalalandio/chimney/issues and haven't found the issue reported
• [x] I confirmed that the bug is not related to functionality that was deprecated: lifted transformers (TransformerFs) or unsafeOption flags

Describe the bug Apologies if I misunderstood something in the docs.

In the oneof part of the docs (https://chimney.readthedocs.io/en/stable/cookbook/#oneof-fields), it says we can handle Empty manually like this:

.withCoproductInstancePartial[pb.addressbook.AddressBookType.Value.Empty.type](
  _ => partial.Result.fromEmpty
)

Or using a simple import:

import io.scalaland.chimney.protobufs._ // includes support for empty scalapb.GeneratedMessage

While the first one works well, the second doesn't.

It looks like the import provides partialTransformerFromEmptyOneOfInstance for a GeneratedOneof with ValueType = Nothing, but the protobuf generated code doesn't look like that. It's just a simple case object Empty.

Reproduction An easy way to reproduce is to modify the existing unit test ProtobufOneOfSpec.

Before:

pbStatus
  .intoPartial[order.CustomerStatus]
  .withCoproductInstancePartial[pb.order.CustomerStatus.Empty.type](_ => partial.Result.fromEmpty)
  .withCoproductInstance[pb.order.CustomerStatus.NonEmpty](_.transformInto[order.CustomerStatus])
  .transform
  .asOption ==> Some(domainStatus)

After:

import io.scalaland.chimney.protobufs.*
pbStatus
 .intoPartial[order.CustomerStatus]
//  .withCoproductInstancePartial[pb.order.CustomerStatus.Empty.type](_ => partial.Result.fromEmpty)
  .withCoproductInstance[pb.order.CustomerStatus.NonEmpty](_.transformInto[order.CustomerStatus])
  .transform
  .asOption ==> Some(domainStatus)

Expected behavior According to the docs, it should compile.

Actual behavior

[error]    |      pbStatus
[error]    |      ^
[error]    |Chimney can't derive transformation from io.scalaland.chimney.examples.pb.order.CustomerStatus to io.scalaland.chimney.fixtures.order.CustomerStatus
[error]    |
[error]    |io.scalaland.chimney.fixtures.order.CustomerStatus
[error]    |  derivation from empty: io.scalaland.chimney.examples.pb.order.CustomerStatus.Empty to io.scalaland.chimney.fixtures.order.CustomerStatus is not supported in Chimney!
[error]    |
[error]    |
[error]    |io.scalaland.chimney.fixtures.order.CustomerStatus
[error]    |  can't transform coproduct instance io.scalaland.chimney.examples.pb.order.CustomerStatus.Empty to io.scalaland.chimney.fixtures.order.CustomerStatus

Which Chimney version do you use 0.8.0

Which platform do you use

• [x] JVM
• [ ] Scala.js
• [ ] Scala Native

If you checked JVM 17

Additional context Looking at the generated code from scalapb, Empty is defined like this:

case object Empty extends io.scalaland.chimney.examples.pb.order.CustomerStatus

There is also another Empty which is used internally:

  sealed trait SealedValue extends _root_.scalapb.GeneratedOneof {
    ...
  }
  object SealedValue {
    @SerialVersionUID(0L)
    case object Empty extends io.scalaland.chimney.examples.pb.order.CustomerStatusMessage.SealedValue {
      type ValueType = _root_.scala.Nothing

But it's not the one used when we convert types with Chimney so the import doesn't help.

[MateuszKubuszok]

It is rather clear to me that your case is described here: https://chimney.readthedocs.io/en/stable/cookbook/#sealed_value-oneof-fields.

ScalaPB can generate oneOf in 3 different ways:

• the default one uses non-flat ADT with scalapb.GeneratedOneof { type ValueType = Nothing } for empty value, and we can use this common subtype to handle this Empty case - it's the case you are quoting and which is handled by the implicit
• sealed_value creates an extra Empty subtype - with nothing that we could use to distinguish it from any other Empty case object, which perhaps shouldn't be automatically mapped to a failed transformation - this is the case you have, and you have to handle it manually, sorry
• sealed_value_optional - since ScalaPB does not use nulls, all fields are non-nullable, and empty value has to be represented somehow, the former 2 approaches do this by adding extra empty subtype to ADT, and this one uses Option in every place that this oneOf would be used - this case is handled by PartialTransformers and their automatic unwrapping of Options (no import required).

So no, it is not a bug. Your protobuf definition explicitly opted-in with the only representation of 3 possible, that Chimney cannot automatically handle and it is described in the documentation. (I say explicitly because you have to name oneOf sealed_value, and I don't think someone would come up with such name accidentally). You can map things manually or, if you have the ability, change sealed_value name in your protobufs to sealed_value_optional. If you look at the ProtobufOneOfSpec you mentioned, you'll see that there are 3 different tests for 3 different representations. We are generating these protobufs from sources, and update dependencies regularly so any (unlikely) change in behavior would be discovered.

[ghostdogpr]

Thanks for the detailed explanation! I wrongly assumed the import was relevant to the second case. This is all clear now 🙏

[MateuszKubuszok]

NP, I can add some warning to the docs to make it clearer.