Implicit encoder for a trait type parameter

Question

I would like to encode to json a field of type List[E] using argonaut lib.

sealed trait Msg[E] {
  val contents: List[E]    

  def send(): Unit = {
    val json = contents.asJson
    println("Sending json: " + json.toString())
  }
}

Then I have a StringMsg case class:

case class StringMsg(contents: List[String]) extends Msg[String]

The argonaut lib defines the JsonIdentity[J] trait:

trait JsonIdentity[J] {
  val j: J

  /**
   * Encode to a JSON value using the given implicit encoder.
   */
  def jencode(implicit e: EncodeJson[J]): Json =
    e(j)
}

When I create a new instance of StringMsg and call the send() method, I have the following error:

StringMsg(List("a","b")).send()

could not find implicit value for parameter e: argonaut.EncodeJson[List[E]]

I've never tried this lib but looking at the doc it seems you have to import argonaut.EncodeJson._. No? — Hiura
No the import does not solve the problem. The type parameter E should be resolved to String in order to use an encoder like EncodeJson[List[String]], i think i have to deal with the type erasure or something else. — mfudi

dk14 dk14 · Accepted Answer · 2015-04-07T14:10:07

Your API should require implicit argonaut.EncodeJson[List[E]] from client code:

sealed trait Msg[E] {
  val contents: List[E]
  implicit def encodeJson: argonaut.EncodeJson[List[E]] //to be implemented in subclass   

  def send(): Unit = {
    val json = contents.asJson
    println("Sending json: " + json.toString())
  }
}

//or

abstract class Msg[E](implicit encodeJson: argonaut.EncodeJson[List[E]]) {
  val contents: List[E]

  def send(): Unit = {
    val json = contents.asJson
    println("Sending json: " + json.toString())
  }
}

//or

sealed trait class Msg[E] {
  val contents: List[E]

  def send()(implicit encodeJson: argonaut.EncodeJson[List[E]]): Unit = {
    val json = contents.asJson
    println("Sending json: " + json.toString())
  }
}

Somewhere in the client-code:

case class StringMsg(contents: List[String]) extends Msg[String] { 
   implicit val encodeJson = argonaut.StringEncodeJson 
}

//or

import argonaut.StringEncodeJson //or even import argonaut._
case class StringMsg(contents: List[String]) extends Msg[String]() //implicit will be passed here

//or

import argonaut.StringEncodeJson //or even import argonaut._
case class StringMsg(contents: List[String]) extends Msg[String]
val a = StringMsg(Nil)
a.send() //implicit will be passed here

Implicit encoder for a trait type parameter

1 Answers