add Scala3 equivalent of ScalaObjectMapper

[pjfanning]

ScalaObjectMapper is a mixin for Jackson ObjectMapper that provides some helper functions that use Scala Manifests to implicitly access type information.

Scala Manifests are not supported for Scala3.

The options would appear to be.

• write Scala3 macros
• use https://github.com/zio/izumi-reflect

[gaeljw]

I didn't look closely to the code yet but what do you use from Manifests ?

If you need to access type parameters of a generic type, this is exactly the use case I'm currently dealing with in a library (Cucumber Scala) and after looking at izumi I built a minimal library to cover the need: https://github.com/gaeljw/typetrees.

Maybe you can grab some ideas there 🤷 (Not saying you should use my library, I wouldn't be confident using it in such a big project as Jackson for now 😅 )

[pjfanning]

@gaeljw the manifests are basically used here - https://github.com/FasterXML/jackson-module-scala/blob/2.12/src/main/scala/com/fasterxml/jackson/module/scala/ScalaObjectMapper.scala#L67

I see this as a convenience because in most cases, it's possible to use the Jackson ObjectMapper java methods directly and pass the type reference explicitly.

[gaeljw]

I've open a draft proposal without using macros at #518 but I may be missing some stuff that make this proposal inadequate.

[pjfanning]

This code is merged but causes build problems with Scala 2.13 - see #522

[gaeljw]

That's weird. I'll give it a look event though I'm not sure I have useful knowledge to understand such behaviour.

[pjfanning]

@gaeljw in #522, I have added a comment with a link to a commit that disables the unit test that seems to cause the issue - a test with mapper.readValue[Map[String, Any]]

[pjfanning]

jackson-module-scala 2.13.0-rc1 is released

[tnielens]

scala.reflect.Manifest[T] seems still supported by scala3, see https://dotty.epfl.ch/api/scala/reflect/Manifest.html . TypeTag not though. Wasn't Manifest[T] part of scala3 at the time of this issue creation? Could this library consider adding ScalaObjectMapper again?

[pjfanning]

@tnielens try doing a PR of your own - ScalaObjectMapper has never compiled under Scala 3 for me - we have ClassTagExtensions as a like for like replacement for ScalaObjectMapper and that works with Scala 3 (see the jackson-scala-module 2.13.0-rc2 release)

[tnielens]

My bad, indeed two level of nesting seems to be unsupported List[GenericTestClass[Int]].

[gaeljw]

@tnielens the trait still exists but at compile time no value for the implicits instances will be provided by the compiler (or is it at runtime that there's an error? I don't remember exactly). But pretty sure that it doesn't work anymore and even if it works it's been deprecated since Scala 2.11, thus we should not rely on it.