Cannot generate an implementation for parameterized traits

Lagom Version 1.3.4 Scala API, with Scala 2.12.2 OpenJDK 64-Bit Server VM (Zulu 8.20.0.5-macosx) (build 25.121-b15, mixed mode)

Expected Behavior

With these traits

trait GenericService[T] extends Service {
  def get(id: String, t: T): ServiceCall[NotUsed, T]
  def baseDescriptor(name: String)(implicit ft: Format[T], pt: PathParamSerializer[T]) =
    named(name).withCalls(
      pathCall( s"/api/$name/get/:id/:t", this.get _ ))
}

trait StringService extends GenericService[String] {
  override def descriptor = baseDescriptor("String")
}

I would expect this to compile

  val stringService = serviceClient.implement[StringService]

Actual Behavior

However, it produces this error

object creation impossible, since method get in trait GenericService of type (id: String, t: String)com.lightbend.lagom.scaladsl.api.ServiceCall[akka.NotUsed,String] is not defined

It would be nice if type parameters were supported, as it would allow to factor out similarities between a few services into a common base trait.

[octonato]

Thanks @crfeliz I will have a look into that.

[patrick-premont]

My use case does not exactly relate to endpoints generated from another tool. Those would likely have distinct methods of the same form, but I don’t think it would be possible to write a generic descriptor for those.

It relates to services that have similar interfaces and for which we can rely on inheritance to share parts of the interface and implementation. More concretely, I have a polymorphic repository class that I want to put behind a service layer, and that would lead the services to be mostly identical (especially the read-only methods that do not need to do validation).

Since Lagom does not support it currently, I have opted to duplicate the methods, so I do not have an urgent need for it anymore. Still, if it’s not too challenging, I think it would be nice to allow for this.

Perhaps eventually I will want to move many of my similar methods into a single service, and then even a solution to the present issue would not allow me to reuse code. I would then need reuse through aggregation (nested service traits) instead of inheritance, which is a whole other story.