hmemcpy / zio-magic

Construct ZLayers automagically (w/ helpful compile-time errors)

🪄 zio-magic

Construct ZLayers automagically, with friendly compile-time hints!

// build.sbt
libraryDependencies += "io.github.kitlangton" %% "zio-magic" % "0.1.8"

What's all this then?

// Given a dependency graph (Pie needs Berries & Flour, which in turn need Spoon)*
//
//           Pie
//          /   \
//     Berries   Flour
//       |         |
//     Spoon     Spoon
//
// *Not an actual recipe.

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

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

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

  magically.exitCode
}

---

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

val magically: UIO[Unit] =
  program.provideMagicLayer(
    Pie.live,
    //Flour.live, <-- Oops
    Berries.live,
    Spoon.live,
    Console.live
  )

🪄  ZLayer Magic Missing Components
🪄
🪄  provide zio.magic.Example.Flour.Service
🪄      for Pie.live

---

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

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

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

Also

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

val layer = Zlayer.fromMagic[Flour with Console](Console.live, Flour.live, Spoon.live)

To construct URLayer[In, Out] use Zlayer.fromSomeMagic[In, Out] this way:

val layer = Zlayer.fromSomeMagic[CommonEnv, Flour with Console](Console.live, Flour.live, Spoon.live)

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

There's also .provideCustomMagicLayer 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.provideCustomMagicLayer(carLayer, wheelLayer)

Specs

provideMagicLayer, provideCustomMagicLayer, provideSomeMagicLayer, provideMagicLayerShared, provideCustomMagicLayerShared and provideSomeMagicLayerShared all work for zio-test's Spec.

Debug!

Try ZLayer.fromMagicDebug[Pie] or ZLayer.fromSomeMagicDebug[Blocking with Console, Pie] to print out a pretty graph! Ooh la la!

      Your Delicately Rendered Graph

                   Pie.live                   
               ┌───────┴──────────────┐       
          Flour.live            Berries.live  
        ┌──────┴───────┐              │       
   Spoon.live    Console.live    Spoon.live   
       │                             │       
 Blocking.live                Blocking.live 

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