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Version: ZIO 2.x

Running Effects

ZIO effects are precise plans that describe a computation or interaction. Ultimately, every effect must be executed by the ZIO runtime.

In this section, you will learn about the several ways that ZIO provides for you to execute effects in your application.


If you construct a single effect for your whole program, the most natural way to run the effect is to extend ZIOAppDefault.

This class provides Scala with a JVM-compatible main function, so it can be called from IDEs and launched from the command-line. All you have to do is implement the run method by returning the effect to run.

import zio._
import zio.Console._

object MyApp extends ZIOAppDefault {

def run =
for {
_ <- printLine("Hello! What is your name?")
name <- readLine
_ <- printLine(s"Hello, ${name}, welcome to ZIO!")
} yield ()

If you are using a custom environment for your application, you will have to supply your environment to the effect (using ZIO#provideEnvironment or, if you are using layers, ZIO#provide) before you return it from run.

ZIOAppDefault does not know how to supply custom environments.

Default Runtime

Most applications are not greenfield, meaning they must integrate with legacy code and procedural libraries and frameworks.

In these cases, a better solution for running effects is to create a Runtime, which can be passed around and used to run effects wherever required.

ZIO contains a default runtime called Runtime.default.

To access it, merely use

val runtime = Runtime.default

Once you have a runtime, you can use it to execute effects:

Unsafe.unsafe { implicit unsafe =>"Hello World!"))).getOrThrowFiberFailure()

In addition to run, which is for synchronous execution, there are other methods available on Runtime that support asynchronous execution.

Custom Runtime

If you are using a custom environment for your application, you may find it useful to create a Runtime specifically tailored for that environment.

A custom Runtime[R] can be created with a ZEnvironment[R] (which holds the context required in order to execute your effects), as well as fiber refs and runtime flags (which can generally be set to default values).

For example, the following creates a Runtime that can provide an Int to effects :

val myRuntime: Runtime[Int] = 
Runtime(ZEnvironment[Int](42), FiberRefs.empty, RuntimeFlags.default)

Error Reporting

The ZIO runtime system automatically logs all errors encountered when executing your effects, so long as those errors are not handled by your ZIO code.

You can specify a custom logger easily using ZIO Logging, which can intercept these logged errors and handle them as configured by your logging backend.

Next Steps

If you are comfortable with running effects, congratulations!

You are now ready to dive into other sections on the ZIO website covering data types, use cases, and interop with other systems.

Refer to the Scaladoc for detailed documentation on all the core ZIO types and methods.