No access to default arguments in Mirrors

[dwijnand]

Minimized code

case class WithDefault(id: Int, a: String = "a", b: Option[String] = Some("b"))

Output

scala> import scala.compiletime._, scala.deriving._

scala> val m = summon[Mirror.Of[WithDefault]]
val m:
  (
    deriving.Mirror{
      MirroredType = WithDefault;
      MirroredMonoType = WithDefault;
      MirroredElemTypes = (Int, String, Option[String]);
      MirroredLabel = "WithDefault";
      MirroredElemLabels = ("id", "a", "b")
    }
  ) = WithDefault

Expectation

I would expect a way to access to default arguments, such that it may be used in a derived typeclass instance.

[odersky]

I agree, but I believe this is also feature requests. We should probably look at it only once we have revamped default parameters in Tasty.

[SethTisue]

We should probably look at it only once we have revamped default parameters in Tasty

(Has that happened, by any chance?)

[bishabosha]

Not yet, it's still the same encoding with name mangled methods

[godenji]

Any progress here?

This is definitely a pain point when attempting to serialize case classes with default arguments to/from json (which is a fairly common scenario in practice).

[bishabosha]

no one is working on this as far as I know. Do you have a specific API in mind? I could imagine something untyped at least like def defaultArgument(arg: String): Option[Any]. Perhaps with another type like type DefaultMappings = (("a", String), ("b", Option[String])) to generate type safe casting?

[godenji]

Do you have a specific API in mind?

yes, Play JSON, but I assume any Scala 3 json library that attempts to extract case class defaut arguments at compile time is affected.

[odersky]

Changing the default parameter scheme is a very large undertaking. I would guess on the order of 3-6pm. I won't have the time to do it personally. I am not sure how we will be able to fund/resource this. Suggestions are welcome.

[Lasering]

While this is not implemented see this solution using macros: https://stackoverflow.com/questions/68421043/type-class-derivation-accessing-default-values

[adampauls]

I'm not sure if this makes anything easier, but a slightly more limited option would be to expose a fromNamedProduct method on mirrors that takes a map from names to values. You wouldn't be able to access the default values directly, but you could at least construct an instance where the default are filled in for you. Might that avoid the need for any major refactors?

[odersky]

The problem is that the "default values" are hidden in default getter methods. And, if the default depends on previous parameters it will be a function over those parameter values instead of being a simple value.

For the moment, I think the only solution would be the one @Lasering describes.