Compile error with PlayJson macro and cats.data.Newtype (via NonEmptySet)

[ronanM]

A minimal failing snippet:

build.sbt:

scalaVersion := "2.12.7"

scalacOptions += "-Ypartial-unification"

libraryDependencies += "org.typelevel" %% "cats-core" % "1.4.0"

libraryDependencies += "com.typesafe.play" %% "play-json" % "2.6.10"

NesPlayJson.scala:

import cats._
import cats.data._
import cats.implicits._
import play.api.libs.functional.syntax._
import play.api.libs.json.JsArray
import play.api.libs.json.JsError
import play.api.libs.json.JsResult
import play.api.libs.json.JsResult.applicativeJsResult
import play.api.libs.json.JsValue
import play.api.libs.json.Json.toJson
import play.api.libs.json.Reads
import play.api.libs.json.Writes
import play.api.libs.json._

object NesPlayJson {

  // --- NEL, details: https://www.cakesolutions.net/teamblogs/how-to-use-nonemptylist-in-a-model-class
  implicit object jsResultApplicative extends Applicative[JsResult] {
    override def pure[A](x: A): JsResult[A] = applicativeJsResult.pure(x)

    override def ap[A, B](ff: JsResult[A => B])(fa: JsResult[A]): JsResult[B] = applicativeJsResult(ff, fa)
  }

  implicit final def nelReads[T: Reads]: Reads[NonEmptyList[T]] =
    Reads {
      case JsArray(values) =>
        values.toList match {
          case head :: tail => NonEmptyList.of(head.validate[T], tail.map(_.validate[T]): _*).sequence
          case Nil          => JsError("expected a NonEmptyList but got empty list")
        }
      case other: JsValue => JsError(s"expected an array but got $other")
    }

  implicit final def nelWrites[T: Writes]: Writes[NonEmptyList[T]] =
    Writes(nel => toJson(nel.toList))

  // --- NES.  
  implicit final def nesFormat[T: Format: Order]: Format[NonEmptySet[T]] =
    implicitly[Format[NonEmptyList[T]]].inmap(nel => NonEmptySet.of(nel.head, nel.tail: _*), _.toNonEmptyList)

  // --- Test.

  final case class MyCaseClass(values: NonEmptySet[String], foo: Int)

  object MyCaseClass {
    implicit val MyCaseClassFmt: OFormat[MyCaseClass] = Json.format
  }

  def main(args: Array[String]): Unit =
    println(Json.toJson(MyCaseClass(NonEmptySet.of("a", "b", "c"), 42)))
}

Compile error:

Error:(46, 62) No instance of play.api.libs.json.Format is available for
cats.data.Newtype.Type[java.lang.String] in the implicit scope 

    implicit val MyCaseClassFmt: OFormat[MyCaseClass] = Json.format

Gitter:

• https://gitter.im/typelevel/cats?at=5bcdd30f6e5a401c2db0e99a
• https://gitter.im/playframework/playframework?at=5bce2836ae7be940169c3707

[kailuowang]

try write implicitly[Format[NonEmptyList[String]]] and nesFormat[String] directly see what error you get.

[ronanM]

I changed to:

  object MyCaseClass {
    implicitly[Format[NonEmptyList[String]]]
    nesFormat[String]

    implicit val MyCaseClassFmt: OFormat[MyCaseClass] = Json.format
  }

Same compile error.

[kailuowang]

looks like that play.json's Json.format macro failed to pick up the implicit instance in scope. As a last attempt, try create a type alias for NonEmptySet[String] and explicitly create an implicit format for that type alias, and use that type alias in the case class. if that still doesn't work, you might need to manually create a Format for your case class using combinators.

[ronanM]

I changed to:

  type NesString = NonEmptySet[String]
  
  implicit val nesStringFormat : Format[NesString] = nesFormat[String]
  
  final case class MyCaseClass(values: NesString, foo: Int)

Same compile error.

[ronanM]

Thank you @kailuowang.

The workaround using manual Format combinators:

  final case class MyCaseClass(values: NonEmptySet[String], foo: Int)

  object MyCaseClass {

    implicit val MyCaseClassFmt: OFormat[MyCaseClass] =
      ((__ \ "values").format[NonEmptySet[String]] and
        (__ \ "foo").format[Int])(MyCaseClass.apply, unlift(MyCaseClass.unapply))
  }

  def main(args: Array[String]): Unit = {
    val myCaseClass = MyCaseClass(NonEmptySet.of("a", "b", "c"), 42)
    val jsonStr     = Json.prettyPrint(Json.toJson(myCaseClass))
    println(jsonStr)
    println(Json.fromJson[MyCaseClass](Json.parse(jsonStr)))
  }

[armanbilge]

This seems stale with nothing actionable to do in cats. Feel free to re-open.