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Version: 2.x

Automatic Layer Construction

ZIO also has an automatic layer construction facility, which takes care of building dependency graphs from the individual layers and building blocks. So instead of manually composing layers together to build the final layer, we can only provide individual layers to the ZIO application, and it will do the rest.

The automatic layer construction takes place at the compile-time, so if there is a problem in providing a layer, we will receive an error or warning message. So it helps us to diagnose the problem. Additionally, it has a way to print the dependency graph using built-in debug layers.

Providing Individual Layers to a ZIO Application​

When we provide individual layers using ZIO#provide, ZIO#provideCustom, or ZIO#provideSome to a ZIO application, the compiler will create the dependency graph automatically from the provided layers:

info

We have a separate section that describes different methods for providing layers to the ZIO application.

Assume we have written the following services (Cake, Chocolate, Flour, and Spoon):

import zio._

trait Cake

object Cake {
  val live: ZLayer[Chocolate & Flour, Nothing, Cake] =
    for {
      _ <- ZLayer.environment[Chocolate & Flour]
      cake <- ZLayer.succeed(new Cake {})
    } yield cake
}

trait Spoon

object Spoon {
  val live: ULayer[Spoon] =
    ZLayer.succeed(new Spoon {})
}

trait Chocolate

object Chocolate {
  val live: ZLayer[Spoon, Nothing, Chocolate] =
    ZLayer.service[Spoon].project(_ => new Chocolate {})
}

trait Flour

object Flour {
  val live: ZLayer[Spoon, Nothing, Flour] =
    ZLayer.service[Spoon].project(_ => new Flour {})
}

The Cake service has the following dependency graph:

          Cake
          /   \
   Chocolate   Flour
       |         |
     Spoon     Spoon

Now we can write an application that uses the Cake service as below:

import zio._

import java.io.IOException

val myApp: ZIO[Cake, IOException, Unit] = for {
  cake <- ZIO.service[Cake]
  _    <- Console.printLine(s"Yay! I baked a cake with flour and chocolate: $cake")
} yield ()

The type of myApp indicates we should provide Cake to this ZIO application to run it. Let's give it that and see what happens:

object MainApp extends ZIOAppDefault {
  def run =
    myApp.provide(Cake.live)
}

// error:
// 
// ──── ZLAYER ERROR ────────────────────────────────────────────────────
// 
//  Please provide layers for the following 2 types:
// 
//    Required by Cake.live
//    1. Chocolate
//    2. Flour
//    
// ──────────────────────────────────────────────────────────────────────

Here are the errors that will be printed:

──── ZLAYER ERROR ────────────────────────────────────────────────────

 Please provide layers for the following 2 types:

   Required by Cake.live
   1. Chocolate
   2. Flour
   
──────────────────────────────────────────────────────────────────────

It says that we missed providing Chocolate and Flour layers. Now let's add these two missing layers:

import zio._

object MainApp extends ZIOAppDefault {
  def run =
    myApp.provide(
      Cake.live,
      Chocolate.live,
      Flour.live
    )
}

// error:
// 
// ──── ZLAYER ERROR ────────────────────────────────────────────────────
// 
// Please provide a layer for the following type:
// 
// Required by Flour.live
// 1. Spoon
// 
// Required by Chocolate.live
// 1. Spoon
// 
// ──────────────────────────────────────────────────────────────────────

Again, the compiler asks us to provide another dependency called Spoon:

──── ZLAYER ERROR ────────────────────────────────────────────────────

Please provide a layer for the following type:

Required by Flour.live
1. Spoon

Required by Chocolate.live
1. Spoon

──────────────────────────────────────────────────────────────────────

Finally, our application compiles without any errors:

import zio._

object MainApp extends ZIOAppDefault {
  def run =
    myApp.provide(
      Cake.live,
      Chocolate.live,
      Flour.live,
      Spoon.live  
    )
}

Note that the order of dependencies doesn't matter. We can provide them in any order.

Now, let's compare the automatic layer construction with the manual one:

import zio._

object MainApp extends ZIOAppDefault {

  val layers: ULayer[Cake] =
      (((Spoon.live >>> Chocolate.live) ++ (Spoon.live >>> Flour.live)) >>> Cake.live)

  def run = myApp.provideLayer(layers)

}

Automatically Assembling Layers​

  1. ZLayer.make[R] — Using ZLayer.make[R], we can provide a type R and then provide individual layers as arguments, it will automatically assemble these layers to create a layer of type R.

For example, we can create a Cake layer as below:

import zio._

val cakeLayer: ZLayer[Any, Nothing, Cake] =
  ZLayer.make[Cake](
    Cake.live,
    Chocolate.live,
    Flour.live,
    Spoon.live
  )

We can also create a layer for intersections of services:

import zio._

val chocolateAndFlourLayer: ZLayer[Any, Nothing, Chocolate & Flour] =
  ZLayer.make[Chocolate & Flour](
    Chocolate.live,
    Flour.live,
    Spoon.live
  )
  1. ZLayer.makeSome[R0, R] — Automatically constructs a layer for the provided type R, leaving a remainder R0:
import zio._

val cakeLayer: ZLayer[Spoon, Nothing, Cake] =
  ZLayer.makeSome[Spoon, Cake](
    Cake.live,
    Chocolate.live,
    Flour.live
  )

ZLayer Debugging​

To debug ZLayer construction, we have two built-in layers, i.e., ZLayer.Debug.tree and ZLayer.Debug.mermaid.

Let's include the ZLayer.Debug.tree layer into the layer construction:

import zio._

object MainApp extends ZIOAppDefault {
  def run =
    myApp.provide(
      Cake.live,
      Chocolate.live,
      Flour.live,
      Spoon.live,
      ZLayer.Debug.tree
    )
}

The following debug messages will be generated by the compiler:

[info]   ZLayer Wiring Graph
[info]
[info] â—‰ Cake.live
[info] ├─◑ Chocolate.live
[info] │ ╰─◑ Spoon.live
[info] ╰─◑ Flour.live
[info]   ╰─◑ Spoon.live
[info]

If we use the ZLayer.Debug.mermaid layer, it will generate the following debug messages:

[info]   ZLayer Wiring Graph  
[info] 
[info] â—‰ Cake.live
[info] ├─◑ Chocolate.live
[info] │ ╰─◑ Spoon.live
[info] ╰─◑ Flour.live
[info]   ╰─◑ Spoon.live
[info] 
[info] Mermaid Live Editor Link
[info] https://mermaid-js.github.io/mermaid-live-editor/edit/#eyJjb2RlIjoiZ3JhcGhcbiAgICBDb25zb2xlLmxpdmVcbiAgICBDYWtlLmxpdmUgLS0+IENob2NvbGF0ZS5saXZlXG4gICAgQ2FrZS5saXZlIC0tPiBGbG91ci5saXZlXG4gICAgRmxvdXIubGl2ZSAtLT4gU3Bvb24ubGl2ZVxuICAgIFNwb29uLmxpdmVcbiAgICBDaG9jb2xhdGUubGl2ZSAtLT4gU3Bvb24ubGl2ZVxuICAgICIsIm1lcm1haWQiOiAie1xuICBcInRoZW1lXCI6IFwiZGVmYXVsdFwiXG59IiwgInVwZGF0ZUVkaXRvciI6IHRydWUsICJhdXRvU3luYyI6IHRydWUsICJ1cGRhdGVEaWFncmFtIjogdHJ1ZX0=