Cross-publish for Scala 3

[jrudolph]

We thought we could rely on Scala 2.13 compatibility for a while. Even though this works well for applications, it turned out that it isn't a viable solution for libraries that depend on Akka HTTP (see e.g. scala-js/scala-js-dom#451).

Unfortunately, we are pretty stuck on parboiled2 Scala 3 support.

Our options:

• Port parboiled2 to Scala 3 (or at least help in porting), see Publish for Dotty sirthias/parboiled2#210
• Move away from parboiled2 to another parser engine like cats-parse
  • Needs testing that performance will be good enough
  • Requires rewriting all parsers (or somehow generating the parsers from what we have right now)
• Implement a mini-parboiled2 without macros for Scala 3, losing performance but getting something ready soon.

In the meantime, we can:

• Replace parsing with a stub for Scala 3
• Try to get the rest of the code compiling with Scala 3 (most tests are likely to fail, parsing is too important).

Remaining steps:

• dependencies have Scala 3 artifacts
  • Akka dependencies
  • parboiled2 Ongoing Scala 3 support sirthias/parboiled2#280
  • spray-json
• port our own macros Ongoing Scala 3 migration effort #4079
• update our version of parboiled2
• get parsing code to compile
• fix other issues

[armanbilge]

Is there still interest in sharing a parser with http4s? http4s/http4s#4733

[jrudolph]

I'm not opposed, but I think it might be more work than useful. The main problem seems to be that most of the parser usage is not the parsing itself but instantiating the model from the data. I don't see a good way for abstracting that away without either

• creating either an intermediate representation (which we want to avoid for performance reasons)
• or, adding some underlying trait-based base infrastructure (which would be a complex / binary incompatible change that we'd like to avoid for now for compatibility reasons)

We will probably have a serious attempt at getting parboiled2 up to speed first before reevaluating the other options, if necessary.

[armanbilge]

or, adding some underlying trait-based base infrastructure

@jrudolph Are you referring to something like Jawn's Facade type class? Which lets it parse JSON to any AST. Not sure why this would be binary incompatible, couldn't this be hidden away from public APIs?

[armanbilge]

What are your thoughts on this proposal? http4s/http4s#4745 (comment)

[SethTisue]

Thrilled to see that parboiled2 now supports Scala 3.1: sirthias/parboiled2#280 (comment)

👏 @jrudolph

[liosedhel]

Hello :) now as parboiled2 is published for Scala 3 what are the next steps? This steps in ticket description seems to be outdated - e.g. spray and akka deps are also migrated now. How can one help here @jrudolph ? :)

[jrudolph]

There's still a crazy amount of work left, I've got a branch open of Scala 3 work at #4079 but it's currently quite unorganized. When I find time I'll try to clean it up so that it can be easier broken down into distinct steps to finish,