type mismatch; found : cats.Show[shapeless.CNil] required: cats.Show[A] after adding scalac option -Ypartial-unification in sbt

Question

I have below code.

import enumeratum.{Enum, EnumEntry}

sealed abstract class AppEnvironment extends EnumEntry

object AppEnvironment extends Enum[AppEnvironment] {
  case object Local extends AppEnvironment
  case object Testing extends AppEnvironment
  case object Production extends AppEnvironment

  override val values: Vector[AppEnvironment] =
    findValues.toVector
}

import java.net.InetAddress
import ciris.Secret
import eu.timepit.refined.types.net.UserPortNumber
import scala.concurrent.duration.Duration

final case class ApiConfig(
  host: InetAddress,
  port: UserPortNumber,
  apiKey: Secret[ApiKey],
  timeout: Duration
)


import java.net.InetAddress
import cats.Show
import cats.derived._
import cats.implicits._
import ciris.Secret
import ciris.cats._
import enumeratum.EnumEntry
import eu.timepit.refined.auto._
import eu.timepit.refined.cats._
import eu.timepit.refined.types.net.UserPortNumber
import eu.timepit.refined.types.string.NonEmptyString
import is.cir.example.domain.config.AppEnvironment.{Local, Production, Testing}
import scala.concurrent.duration._

final case class Config(
  appName: NonEmptyString,
  environment: AppEnvironment,
  api: ApiConfig
)


object Config {

  import cats.implicits._

  implicit val showConfig: Show[Config] = {

    implicit val showInetAddress: Show[InetAddress] =
      Show.fromToString

    implicit def showEnumEntry[E <: EnumEntry]: Show[E] =
      Show.show(_.entryName)

    semi.show
  }
}

I have to add the scalac option -Ypartial-unification in build.sbt to resolve another issue after adding I get another exception which was not happening earlier(without the flag).

The error is -

[error] /Users/rajkumar.natarajan/Documents/Coding/OS/ciris-example/src/main/scala/is/cir/example/domain/config/Config.scala:38:10: type mismatch;
[error]  found   : cats.Show[shapeless.CNil]
[error]  required: cats.Show[is.cir.example.domain.config.Config]
[error]     semi.show
[error]          ^
[error] one error found

How can I resolve this error?

Answer 1

You should call it specifying type parameter

semi.show[Config]

At least error message is different then

Error:(82, 14) diverging implicit expansion for type cats.derived.MkShow[is.cir.example.domain.config.Config]
starting with value tagRefType in object RefType
    semi.show[Config]

You can try to play with your Show instances. In order to have a Show for "bigger" case class you should have Show for "smaller" ones.

implicitly[Show[Config]]
implicitly[Show[AppEnvironment]]
implicitly[Show[AppEnvironment.Local.type]]
//...
implicitly[Show[ApiConfig]]
implicitly[Show[InetAddress]]
//...