Circe: Moving an Implicit Encoder into A Generic Class

Question

I am working with the Circe library and want to learn the ropes. Consider the following code:

import io.circe.generic.auto._
import io.circe.syntax._
import io.circe.{Decoder, Encoder, Json}

sealed trait Something
case class Name(val name: String) extends Something

class myClass[T](name: Option[Name] = None, data: Option[Seq[T]] = None) {
  // convert an arbitrary sequence to json
  def seqToJson[T](lst: Seq[T])(implicit encoder: Encoder[T]): Json = {
    lst.asJson
  }

  val mydata: Json = seqToJson(data.get)
}

object Trace extends App {
  val name = new myClass(Some(Name("Noob")))
}

Following on from the excellent answer here: Link, I am now trying an example where I can build an encoder into a class.

However when I run the above code, it says:

could not find implicit value for parameter encoder: io.circe.Encoder[T] [error] val mydata: Json = seqToJson(data.get) [error] ^

Now my question is, why is this happening. When I move the definition of the implicit encoder inside the class, why can the compiler not pick up how to use it?

I tried something else too, which was to move where I define the implicit encoder:

import io.circe.generic.auto._
import io.circe.syntax._
import io.circe.{Decoder, Encoder, Json}

sealed trait Something
case class Name(val name: String) extends Something

class myClass[T](name: Option[Name] = None, data: Option[Seq[T]] = None)(implicit encoder: Encoder[T]) {
  // convert an arbitrary sequence to json
  def seqToJson[T](lst: Seq[T]): Json = {
    lst.asJson
  }

  val mydata: Json = seqToJson(data.get)
}

object Trace extends App {
  val name = new myClass(Some(Name("Noob")))
}

This gives the following error:

could not find implicit value for parameter encoder: io.circe.Encoder[Seq[T]] ambiguous implicit values:

[error]  both lazy value encodeDuration in object Encoder of type io.circe.Encoder[java.time.Duration]
[error]  and lazy value encodeInstant in object Encoder of type io.circe.Encoder[java.time.Instant]
[error]  match expected type io.circe.Encoder[T]
[error] Error occurred in an application involving default arguments.
[error]   val name = new myClass(Some(Name("Noob")))

So my question is, how would I get the encoder that is defined implicitly into scope for the Class. Any answers with an explanation of the theory would be much appreciated!

EDIT: Used Ivan's structure and it works!

import io.circe.syntax._
import io.circe.{Decoder, Encoder, Json}

class myClass[T](data: Seq[T])(implicit encoder: Encoder[T]) {
  def seqToJson(lst: Seq[T]): Json = {
    lst.asJson
  }
}

object Trace extends App {
  println(new myClass[Int](data = Seq(1,2,3)))
}

Ivan Stanislavciuc Ivan Stanislavciuc · Accepted Answer · 2020-06-10T17:37:41

Now my question is, why is this happening. When I move the definition of the implicit encoder inside the class, why can the compiler not pick up how to use it?

The answer is that there is no implicit Encoder[T] available in the scope of class myClass.

You can fix it by moving (implicit encoder: Encoder[T]) to constructer, which you did.

Additionally, you're defining generic type T in two places

class myClass[T]

and

def seqToJson[T]

You should keep just one at myClass

could not find implicit value for parameter encoder: io.circe.Encoder[Seq[T]] ambiguous implicit values:

This is caused by the problem of missing generic type. You didn't specify the type T when creating a new instance of myClass. It will compile if you do it.

For example,

val name = new myClass[Int](Some(Name("Noob")))

This will fail during runtime on line seqToJson(data.get) because you're not passing in any data.

how would I get the encoder that is defined implicitly into scope for the Class

Constructor is a good option

Circe: Moving an Implicit Encoder into A Generic Class

2 Answers