🪄 zio-magic

ANNOUNCEMENT: This library is officially included as a first-class feature of ZIO 2.0 😊

Construct ZLayers automagically, with friendly compile-time hints!

// build.sbt
libraryDependencies += "io.github.kitlangton" %% "zio-magic" % "0.3.12"

What's all this then?

// Given a dependency graph (Cake needs Chocolate & Flour, which in turn need Spoon)*
//
//          Cake
//          /   \
//   Chocolate   Flour
//       |         |
//     Spoon     Spoon
//
// *Not an actual recipe.
import zio._
import zio.magic._

def run(args: List[String]): URIO[ZEnv, ExitCode] = {
  
  // An effect requiring Cake and Console. Yum!
  val program: URIO[Console with Cake, Unit] =
    Cake.isDelicious.flatMap { bool => console.putStrLn(s"Cake is delicious: $bool") }

  // The old way
  val manually: ULayer[Cake with Console] =
    ((Spoon.live >>> Flour.live) ++ (Spoon.live >>> Chocolate.live)) >>> Cake.live ++ Console.live

  // The magical way (The order doesn't matter)
  val magically: UIO[Unit] =
    program.inject(
      Cake.live,
      Flour.live,
      Chocolate.live,
      Spoon.live,
      Console.live
    )

  magically.exitCode
}

---

And if you leave something off, a compile time clue!

val magically: UIO[Unit] =
  program.inject(
    Cake.live,
    //Flour.live, <-- Oops
    Chocolate.live,
    Spoon.live,
    Console.live
  )

   ZLayer Wiring Error

>  provide zio.magic.Example.Flour.Service
>      for Cake.live

---

Versus leaving out a dependency when manually constructing your layer...

 val manually: ULayer[Cake with Console] =
   (Flour.live ++ (Spoon.live >>> Chocolate.live)) >>> Cake.live ++ Console.live
 // ^ A Spoon is missing here! 

type mismatch;
 found   : zio.ZLayer[zio.magic.Example.Spoon.Spoon with Any,Nothing,zio.magic.Example.Cake.Cake with zio.console.Console]
    (which expands to)  zio.ZLayer[zio.Has[zio.magic.Example.Spoon.Service] with Any,Nothing,zio.Has[zio.magic.Example.Cake.Service] with zio.Has[zio.console.Console.Service]]
 required: zio.ULayer[zio.magic.Example.Cake.Cake with zio.console.Console]
    (which expands to)  zio.ZLayer[Any,Nothing,zio.Has[zio.magic.Example.Cake.Service] with zio.Has[zio.console.Console.Service]]
      ((Flour.live) ++ (Spoon.live >>> Chocolate.live)) >>> Cake.live ++ Console.live

Also

You can also directly construct a ZLayer (However you must annotate the call to ZLayer.wire[LikeThis], because macros).

val layer = ZLayer.wire[Flour with Console](Console.live, Flour.live, Spoon.live)

To construct URLayer[In, Out] use ZLayer.wireSome[In, Out] this way:

val layer = ZLayer.wireSome[CommonEnv, Flour with Console](Console.live, Flour.live, Spoon.live)

Alternatively you can provide environment partially with injectSome[Rest](l1, l2, l3) - similarly to .provideSomeLayer.

There's also .injectCustom for which behaves similarly to .provideCustomLayer, only it also provides ZEnv.any to all transitive dependencies.

val program: URIO[Console with Car, Unit] = ???

val carLayer: URLayer[Blocking with Wheels, Car] = ???
val wheelLayer: ULayer[Wheels] = ???

// The ZEnv you use later will provide both Blocking to carLayer and Console to the program
val provided: URIO[ZEnv, Unit] = 
  program.injectCustom(carLayer, wheelLayer)

Specs

inject, injectCustom, injectSome, injectShared, injectCustomShared and injectSomeShared all work for zio-test's Spec.

Debug!

Try ZLayer.wireDebug[Cake] or ZLayer.wireSomeDebug[Blocking with Console, Cake] to print out a pretty tree! Ooh la la!

ZLayer Wiring Graph
===================

 ◉ Cake.live
 ├─◑ Flour.live
 │ ├─◑ Spoon.live
 │ │ ╰─◑ Blocking.live
 │ ╰─◑ Console.live
 ╰─◑ Berries.live
   ╰─◑ Spoon.live
     ╰─◑ Blocking.live

Let me know if you can think of any helpful variants, and I'll give 'em a whirl!