Scala 3 mode with _ as a placeholder and ? as wildcard?

[neko-kai]

Since dotty/Scala 3 is going to swap the meaning of these two (already does with a flag -source:3.2), would it be possible to support it in kind-projector, to be able to cross-compile Scala 2 / dotty projects with new syntax directly, without going through intermediate * period.

That would be convenient for those projects that are more comfortable making 1 syntax jump right now, instead of 2 syntax jumps in the future.

[SethTisue]

maybe we can nerd-snipe @martijnhoekstra into this (since he volunteered for #172, and no good deed should go unpunished)

[martijnhoekstra]

dammit Seth

[SethTisue]

sounds like it's working :-)

[martijnhoekstra]

I'm punting on this one.

[SethTisue]

maybe someone else watching the repo would like to volunteer...

[neko-kai]

Is this even technically possible to implement in a compiler plugin? There must've been a reason in the first place to choose the longer ? -> * -> _ hop over a shorter ? -> _ hop

[neko-kai]

@SethTisue
I've checked and it appears possible to swap the meaning of _ and ?, although it's not as trivial as I hoped due to the wrapping ExistentialTypeTree(...) node that appears once _ is used and must be taken care of.

However, it's not possible to implement +_ and -_ without changing scalac because they're not getting parsed as valid Scala!

[error] kind-projector/src/test/scala/functor.scala:7:56: ']' expected but '_' found.
[error] class EitherRightFunctor[L] extends Functor[Either[L, +_]] {

They're very frequent in my code, so even if a ?->_ swap is merged, without support for variance I wouldn't be able to migrate. Dotty supports some of kind-projector's syntax with -Ykind-projector, maybe it would be a solution to change scalac and add Scala 3.2 type lambda syntax and meaning swap for ?->_ under a flag -source:3.2 or -Ynew-placeholders

[SethTisue]

@neko-kai I wish I had time to properly think through all your aspects of your suggestion but I don't, so the following is only a partial response.

I can't think of a reason why a scala/scala PR wouldn't be accepted that allowed +_ and -_ through the parser, as long as it also ensures that a later phase gives a comprehensible error (when kind-projector isn't enabled)

In Scala 2, we generally discourage -Y language flags that essentially fork the language, because of the maintenance trouble they cause as well as the trouble they make for tooling authors. But if you can make a case that we should make an exception, we'll listen.

[smarter]

Scala 2.13.6 and 2.12.14 will interpret ? as a wildcard when using the -Xsource:3 flag (cf scala/scala#9560), I've also opened a PR to remove the deprecated support for ? in kind-projector: #186

[neko-kai]

@smarter Thanks! This solves one half of the puzzle.
@SethTisue I've opened a PR to allow +_ and -_ through (scala/scala#9605), for now just for the 2.13 branch, I will submit a backport for 2.12 branch if that one gets merged.

[neko-kai]

There is now a PR that implements this #188