To be clear, by "alternate wire format and client library for protobuf" you mean "reuse the protobuf IDL but change everything else", right?
There are a few major reasons I didn't go that route:
* The .proto language has a lot of weird quirks that I don't like. Some of the quirks are specific to the protobuf encoding (e.g. int32 vs. sint32 vs. fixed32 being different types), while other quirks have no particular rationale behind them. I didn't want that baggage.
* The .proto language does not treat interfaces (aka services) as a first-class type. That is, you cannot define a field whose type is an RPC interface type -- a reference to a remote object. The ability to do this is a critical part of Cap'n Proto's interface design.
* It's highly unlikely that the protobuf team would be interested in accepting changes to the language which were not actually supported by protobuf. This means that if I shared the language, I would have my hands tied when it comes to new features -- or I'd also have to implement Protobuf equivalents to make them happy.
For cases that don't use features specific to one format or the other, it should be easy to write a tool that automatically converts between schema formats, FWIW.
There are a few major reasons I didn't go that route:
* The .proto language has a lot of weird quirks that I don't like. Some of the quirks are specific to the protobuf encoding (e.g. int32 vs. sint32 vs. fixed32 being different types), while other quirks have no particular rationale behind them. I didn't want that baggage.
* The .proto language does not treat interfaces (aka services) as a first-class type. That is, you cannot define a field whose type is an RPC interface type -- a reference to a remote object. The ability to do this is a critical part of Cap'n Proto's interface design.
* It's highly unlikely that the protobuf team would be interested in accepting changes to the language which were not actually supported by protobuf. This means that if I shared the language, I would have my hands tied when it comes to new features -- or I'd also have to implement Protobuf equivalents to make them happy.