Newtype incompatibility with ZIO 2.0.0-M6

[LukasDBaker]

The ZIO 2.0 update changes the allowed types for all ZIO effects and ZLayers. These changes are incompatible with zio-prelude's Newtype.

As of the 2.0 update, in types of the form ZIO[R, E, A] and ZLayer[R, E, A] neither R nor A is allowed to be an intersection type in almost all of ZIO's functionality (for example conversion of function to layer, ZIO.apply methods, etc.).

Since prelude's Newtype[A].Type is an intersection type (type Type = Base with Tag), ZIO 2.0 cannot easily be used to interface with prelude's Newtype.

Is there a plan to fix this in the future?

Environment:

• Scala version: 2.12
• ZIO version: 2.0.0-M6
• zio-prelude version: 1.0.0-RC8

[kitlangton]

Gooood point. Will explore this. Thanks for the report!

[juliano]

Hello there, I've migrated a project recently and started dealing with a related problem, when using zio newtypes in my tests (initially from here):

object UserToken extends Subtype[String] { ... }

// then, for tests I have the following layer
object CreateUser {
  val layer: ZLayer[Random with Sized, Throwable, UserToken] =
    (for {
      req <- RequestsGen.anyCreateUserRequest.runHead.some
        .orDieWith(_ => new RuntimeException("Failure creating request"))
      resp <- ApiClient.send(req)
    } yield resp.userToken).provideSome[Random with Sized](env).toLayer
}

wherever ZIO.service[UserToken] is used now I get:

A Tag may not contain an intersection type, yet have provided: example.model.UserToken.Base with example.model.UserToken.Tag

Maybe it can be useful :)

[alphaho]

A workaround I come up with is like below. But not sure whether it breaks the ZEnvironment's mechanism or not in this way.
If it's a viable option, maybe we can put it into the NewType's definition, so that all zio-prelude users can benefit from this.

Maybe @kitlangton can help shed some light on this.
I can raise a PR if this is a good enough solution.

import zio.prelude.Subtype
import zio.test.{Spec, TestEnvironment, ZIOSpecDefault, assertTrue}
import zio.{Scope, ULayer, ZIO, ZLayer}

abstract class SubtypeWithTag[T] extends Subtype[T] { self =>
  // steal from zio.Tag's companion object
  implicit def tag(implicit tag0: zio.EnvironmentTag[self.Type]): zio.Tag[self.Type] = zio.Tag[self.Type](tag0)
}

object ZIOTagCompatibilityTest extends ZIOSpecDefault {
  object UserToken extends SubtypeWithTag[String]
  type UserToken = UserToken.Type

  object ClientName extends SubtypeWithTag[String]
  type ClientName = ClientName.Type

  def userTokenLayer(value: String): ULayer[UserToken]   = ZLayer.succeed(UserToken(value))
  def clientNameLayer(value: String): ULayer[ClientName] = ZLayer.succeed(ClientName(value))

  override def spec: Spec[TestEnvironment with Scope, Any] =
    // my best try to verify that it doesn't break ZEnvironment's mechanism by 
    // getting two services out of the environment
    test("should get the subtype as a service") {
      for {
        userToken  <- ZIO.service[UserToken]
        clientName <- ZIO.service[ClientName]
      } yield assertTrue(userToken == UserToken("Foo")) &&
        assertTrue(clientName == ClientName("Bar"))

    }.provide(userTokenLayer("Foo"), clientNameLayer("Bar"))
}

[sideeffffect]

@kitlangton I've put you as the assignee because you seem to understand this the most of us.

[kitlangton]

I wonder if this will be fixed by #949. I'll check this out this week.

[ashwinbhaskar]

Facing the same issue as well. Is there a temporary work around for this?

[soujiro32167]

Still not working; sadness