Macros missing for Scala 3

The following code compiles and works as expected on Scala 2.13.x, but fails on 3.0.0-RC1:

import eu.timepit.refined.numeric._
import eu.timepit.refined.api.Refined
import eu.timepit.refined.auto._

val x: Int Refined Positive = 5

[error] – [E007] Type Mismatch Error [error] |Found: (5 : Int) [error] |Required: eu.timepit.refined.api.Refined[Int, eu.timepit.refined.numeric.Positive]

It looks like the 2.x macros haven’t been ported to Scala 3 yet.

[note]

I took a look at what it would take to migrate macros to Scala 3.

The biggest issue is stumbled upon is lack of equivalent of Evals.eval in Scala 3. eval is used here in refined. On this page that is considered a major blocker for migrating macros in a few projects. You can more for example here and here.

Anyway, I managed to create a very incomplete draft (https://github.com/fthomas/refined/pull/959), it supports only one use case and almost everything is hardcoded there. It shows the basic idea though - since we cannot evaluate the tree we have to manually pattern match against each possible predicate which is very cumbersome. Also, I don’t see a way to support user defined custom predicates in this solution. Implementing non-leaf predicates (as e.g. And) might be tricky.

It’s definitely a problematic solution, I would love to see something simpler.

The above attempt assumed we are constrained by current encoding. #921 suggests that @fthomas was experimenting with new encodings. I played around with Scala 3 features that seems relevant and you can see the outcome here. Predicates for Ints can be written exclusively in terms of scala.compiletime - link. With Strings writing macros are required but they are actually very straightforward - link. All in all, it looks promising. The biggest limitation of that encoding is that it uses closed world assumption - I don’t see an easy way to support custom user-defined predicates.

A friend of mine hinted me about multi-stage programming. I’ve had no idea about it but it sounds like something worth looking into as a way of overcoming limitations of Scala 3 macros.

[armanbilge]

Polite bump on this 😃 if folks are happy with this Predicate strategy I can work on a PR.