Enum values and proto3 Any types

[Josh Humphries]

I have a question about Any types in proto3. I'm guessing they aren't quite fully-fleshed out since I haven't found a lot in the way of doc or real support in the Java and Go implementations.

My question: can you use enums or scalar types as values of an Any field? If so, how do they look?

For scalar types, I can understand if the answer is to use the wrapper well-known types instead. But there doesn't appear to be a wrapper for enum values. There is a well-known type named google.protobuf.EnumValue, however it is part of the "schema description" well-known types and doesn't carry enough data for wrapping an enum value (mainly, it's missing a reference to its enum type, e.g. a type URL).

It would be nice if the Any message's type_url field could be a URL that returned an enum description, not just a message description. But the docs state that the contents at that URL are an encoded google.protobuf.Type, which describes a message and does not support enum types. Should it instead have a single one-of field that can be an enum or message description?

Similarly, the type_url field for google.protobuf.Field is in a similar conundrum. The doc states it is "the field type URL, without the scheme, for message or enumeration types", but nothing really talks about enumeration types. The naive assumption is that the URL contents are an enum description (e.g. an encoded google.protobuf.Enum). But if that's the case, I could try to put an enum type into an Any field and indicate that same URL. I then get into trouble because I'd be looking to parse a google.protobuf.Type, in order to understand the Any message contents, but actually get back the bytes for a google.protobuf.Enum. 

That doesn't seem sound.

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Josh Humphries
jh...@bluegosling.com

[Adam Cozzette]

I think the right solution for putting an enum or primitive type in an Any field would be to use a wrapper type like you mentioned. Since there's no real way to create a generic wrapper for all enums, you would probably need to create a specific wrapper for each enum you want to store in an Any field. Does that work for your situation?

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[Josh Humphries]

That totally works. I don't actually need anything at the moment. But I was looking at the spec and implementing some stuff around it -- like for downloading and resolving type schemas from type URLs -- and noticed it as an omission. The main omission is that nothing describes the contents of google.protobuf.Field#type_url when the field's kind is enum. If it is an encoded google.protobuf.Enum (which is the only thing that really makes sense), it would be nice to state so in the spec. It might also be nice to clearly call out in the doc for google.protobuf.Any that scalar types must be boxed via wrappers (and enum types must be boxed via custom message types).

FWIW, a generic wrapper type for an enum is certainly possible and would look something like this:

message EnumValue {

  // The URL, optionally omitted scheme, for the enumeration type.

  type_url string

  value    int32

}

Unfortunately, EnumValue is a name conflict with the existing type that describes an element in the enumeration. Maybe WrappedEnumValue?

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Josh Humphries
jh...@bluegosling.com