Dependency Propagation

When we write an application, our application has a lot of dependencies. We need a way to provide implementations and to feed and propagate all dependencies throughout the whole application. We can solve the propagation problem by using ZIO environment.

During the development of an application, we don't care about implementations. Incrementally, when we use various effects with different requirements on their environment, all parts of our application compose together, and at the end of the day we have a ZIO effect which requires some services as an environment. Before running this effect by unsafeRun we should provide an implementation of these services into the ZIO Environment of that effect.

ZIO has some facilities for doing this. ZIO#provide is the core function that allows us to feed an R to an effect that requires an R.

Notice that the act of provideing an effect with its environment, eliminates the environment dependency in the resulting effect type, represented by type Any of the resulting environment.

Using ZIO#provideEnvironment Method

The ZIO#provideEnvironment takes an instance of ZEnvironment[R] and provides it to the ZIO effect which eliminates its dependency on R:

trait ZIO[-R, +E, +A] {
  def provideEnvironment(r: => ZEnvironment[R]): IO[E, A]
}

This is similar to dependency injection, and the provide* function can be thought of as inject.

Assume we have the following services:

trait EmailService {
  def send(email: String, content: String): UIO[Unit]
}

object EmailService {
  def send(email: String, content: String) = ZIO.serviceWithZIO[EmailService](_.send(email, content))
}

Let's write a simple program using EmailService service:

val app: ZIO[EmailService, Nothing, Unit] = EmailService.send("john@doe.com", "Hello John!")

We can provide implementation of EmailService service into the app effect:

val loggingImpl = new EmailService {
  override def send(email: String, content: String): UIO[Unit] = ???
}

val effect = app.provideEnvironment(ZEnvironment(loggingImpl))

Most of the time, we don't use ZIO#provideEnvironment directly to provide our services; instead, we use ZLayer to construct the dependency graph of our application, then we use methods like ZIO#provide, ZIO#provideSome and ZIO#provideCustom to propagate dependencies into the environment of our ZIO effect.

Using ZIO#provide Method

Unlike the ZIO#provideEnvironment which takes a ZEnvironment[R], the ZIO#provide takes a ZLayer to the ZIO effect and translates it to another level.

Assume we have written this piece of program that requires Foo and Bar services:

trait Foo {
  def foo(): UIO[String]
}

object Foo {
  def foo(): ZIO[Foo, Nothing, String] = ZIO.serviceWithZIO[Foo](_.foo())
}

case class FooLive() extends Foo {
  override def foo(): UIO[String] = ZIO.succeed("foo")
}

object FooLive {
  val layer: ULayer[Foo] = ZLayer.succeed(FooLive())
}

trait Bar {
  def bar(): UIO[Int]
}

object Bar {
  def bar(): ZIO[Bar, Nothing, Int] = ZIO.serviceWithZIO[Bar](_.bar())
}

case class BarLive() extends Bar {
  override def bar(): UIO[Int] = ZIO.succeed(1)
}

object BarLive {
  val layer: ULayer[Bar] = ZLayer.succeed(BarLive())
}

val myApp: ZIO[Foo & Bar, Nothing, Unit] = for {
  foo  <- Foo.foo()
  bar  <- Bar.bar()
  _    <- ZIO.debug(s"foo: $foo, bar: $bar")
} yield ()

We provide implementations of Foo, Bar services to the myApp effect by using ZIO#provide method:

val mainEffect: ZIO[Any, Nothing, Unit] = 
  myApp.provide(FooLive.layer, BarLive.layer)

As we see, the type of our effect converted from ZIO[Foo & Bar, Nothing, Unit] which requires two services to ZIO[Any, Nothing, Unit] effect which doesn't require any services.

Using ZIO#provideSome Method

Sometimes we have written a program, and we don't want to provide all its requirements. In these cases, we can use ZIO#provideSome to partially apply some layers to the ZIO effect.

In the previous example, if we just want to provide the Foo, we should use ZIO#provideSome:

val mainEffectSome: ZIO[Bar, Nothing, Unit] = 
  myApp.provideSome(FooLive.layer)

caution

When using ZIO#provideSome[R0], we should provide the remaining type as R0 type parameter. This workaround helps the compiler to infer the proper types.