ZIO 2: Passing a ZLayer value to an application, getting a return value, and handling possible errors

By Alvin Alexander. Last updated: August 18, 2024

As a little note today, here’s an example ZIO 2 application where I do the following:

Pass a value to the ZIO application (named app) from run using a ZLayer
Use that ZLayer value in the application
Pass a result value from the application back to the run value
Use that value in run
Handle any possible errors in run with foldZIO
Show other ways to write the run value/function

ZIO/ZLayer example

Given that introduction, here’s the ZIO source code:

import zio.*

case class EmailConfig(address: String)

def emailLive(address: String): ULayer[EmailConfig] =
    ZLayer.succeed(EmailConfig(address))

/**
 * A test where I (a) provide a value to `app` using ZLayer and also
 * (b) get a value back from `app` and use that value inside `run`,
 * while also (c) accounting for possible errors.
 */
object ZioPassValueBackToRun extends ZIOAppDefault:

    val app: ZIO[EmailConfig, Throwable, Int] = for
        emailConfig <- ZIO.service[EmailConfig]
        _           <- Console.printLine(s"EMAIL: ${emailConfig.address}")
    yield
        42  // pass this value back to `run`

    // [1]
    val run: ZIO[Any, Throwable, Unit] =
        app.provide(emailLive("coyote@acme.com"))
           .foldZIO(
                failure => Console.printLineError(s"FAILURE = $failure"),
                success => Console.printLine(     s"SUCCESS = $success")
            )

    // [2]
    // val run: ZIO[Any, Throwable, Unit] =
    //    app.provide(emailLive("coyote@acme.com")).tap { result =>
    //        Console.printLine(s"RESULT: $result")
    //    }.unit    // OR .exitCode here

    // [3]
    // val run: ZIO[Any, Throwable, Unit] =
    //    app.provide(emailLive("coyote@acme.com")).mapError { err =>
    //        Exception(s"ERROR: $err")
    //    }.flatMap { result =>
    //        Console.printLine(s"SUCCESS = $result")
    //    }

Note that I use option 1 to handle run in this code, and I also show two other ways the run value could be implemented, especially when (a) you want to get a value back from the application, and (b) also account for possible errors in the application.

Note that I also show all the ZIO type signatures in the application, so you can follow along more easily here.

As a little summary, if you needed a ZIO 2 example that does these things, I hope this is helpful.