容器类型的类型类

Question

I want to build an interface that doesn't have any dependencies, only scala library

Let's imagine this is what i want:

iface.jar

trait jsonIface[JsValue] {

  def turnJsonIntoClass[T](t: JsValue)

}

As you see it doesn't contains any imports.

Let's go to implementation:

iface_implementation1.jar

import play.api.libs.json._

trait myPlayJsonImpl extends jsonIface[JsValue] {

      def turnJsonIntoClass[T](t: JsValue) { t.as[T] }

}

But this wouldn't compile because as[T] needs implicit Reads[T]

So i rewrote my iface like that:

trait jsonIface[JsValue] {

  type metaInfo[T]

  def turnJsonIntoClass[T](t: JsValue)(implicit meta: metaInfo[T])

}

and play json impl looks like that:

import play.api.libs.json._

trait myPlayJsonImpl extends jsonIface[JsValue] {

  type conv[M] = Reads[M]

  def turnJsonIntoClass[T](t: JsValue)(implicit reads: Reads[T]) { t.as[T] }    

}

and json4s looks like that:

import org.json4s.JsonAST._

trait json4sImpl extends jsonIface[JValue] {

  type conv[M] = Manifest[M]

  def turnJsonIntoClass[T](t: JsValue)(implicit reads: Manifest[T]) { t.extract[T] }    

}

This compiles but it looks cumbersome

Answer 1

Normally when you start to work with type-class traits you continue to do so rather than work with OOP traits:

import org.json4s.Formats
import org.json4s.JsonAST.JValue
import play.api.libs.json.{JsValue, Reads}

trait jsonIface[JsValue, T] {
  def turnJsonIntoClass(t: JsValue): T
}

object jsonIface {
  implicit def json4sImpl[T](implicit formats: Formats, manifest: Manifest[T]): jsonIface[JValue, T] = new jsonIface[JValue, T] {
    def turnJsonIntoClass(t: JValue): T = t.extract[T]
  }

  implicit def myPlayJsonImpl[T](implicit reads: Reads[T]): jsonIface[JsValue, T] = new jsonIface[JsValue, T] {
    def turnJsonIntoClass(t: JsValue): T = t.as[T]
  }
}

or

object jsonIface {
  implicit def json4sImpl[T](implicit formats: Formats, manifest: Manifest[T]): jsonIface[JValue, T] = (t: JValue) => t.extract[T]

  implicit def myPlayJsonImpl[T](implicit reads: Reads[T]): jsonIface[JsValue, T] = (t: JsValue) => t.as[T]
}