DI with zio ZLayer

zio-layers.scala

// val ZIOVersion = "1.0.0-RC17+319-8917936d-SNAPSHOT"
// resolvers += Resolver.sonatypeRepo("snapshots")
// 
// libraryDependencies ++= Seq(
//   // zio
//   "dev.zio" %% "zio"         % ZIOVersion,
//   "dev.zio" %% "zio-streams" % ZIOVersion,
// )


import zio._
import zio.console._
import zio.clock._
import java.io.IOException
import zio.duration.Duration
import zio.duration.Duration._


package object moduleA {
  type ModuleA = Has[ModuleA.Service]

  object ModuleA {
    trait Service {
      def letsGoA(v: Int): UIO[String]
    }

    val any: ZLayer[ModuleA, Nothing, ModuleA] =
      ZLayer.requires[ModuleA]

     val live: ZLayer.NoDeps[Nothing, ModuleA] = ZLayer.succeed {
       new Service {
         def letsGoA(v: Int): UIO[String] = UIO(s"done: v = $v ")
       }
     }
  }

   def letsGoA(v: Int): ZIO[ModuleA, Nothing, String] =
     ZIO.accessM(_.get.letsGoA(v))
}


import moduleA._

package object moduleB {
  type ModuleB = Has[ModuleB.Service]

  object ModuleB {
    trait Service {
      def letsGoB(v: Int): UIO[String]
    }

    val any: ZLayer[ModuleB, Nothing, ModuleB] =
      ZLayer.requires[ModuleB]

     val live: ZLayer[ModuleA, Nothing, ModuleB] = ZLayer.fromService { (moduleA: ModuleA.Service) =>
       Has(new Service {
         def letsGoB(v: Int): UIO[String] =
           moduleA.letsGoA(v)
       })
     }
  }

   def letsGoB(v: Int): ZIO[ModuleB, Nothing, String] =
     ZIO.accessM(_.get.letsGoB(v))
}


object ZLayersApp extends zio.App {

  import moduleB._

  val env = Console.live ++ Clock.live ++ (ModuleA.live >>> ModuleB.live)
  val program: ZIO[Console with Clock with Service, IOException, Unit]  =
    for {
      _ <- putStrLn(s"Welcome to ZIO!")
      _ <- sleep(Finite(1000))
      r <- letsGoB(10)
      _ <- putStrLn(r)
    } yield ()

  def run(args: List[String]) =
    program.provideLayer(env).fold(_ => 1, _ => 0)

}

// output: 
// [info] running ZLayersApp 
// Welcome to ZIO!
// done: v = 10 

Hi,
Thank you very much for this short contained example ! It really helped lowering the entry barrier into ZLayers.
What would be the purpose of the val any ? My slightly tweaked example runs fine without it.
Cheers,

Thanks.

Originally I asked the same question so here is an answer of Adam Fraser from ZIO discord:

The any environments can be useful when you want to express that you layer contains a certain value but just passes another value unchanged from a higher layer.
For example, here is an example of providing the Size module to a generator while leaving the Random module outstanding:

  def provideSize[A](zio: ZIO[Random with Sized, Nothing, A])(n: Int): ZIO[Random, Nothing, A] =
    zio.provideLayer[Nothing, Random, Random with Sized](Random.any ++ Sized.live(n))