ZIO Quickstart: Hello World

This is the simplest quickstart for a ZIO application. You can download and run it very quickly. This will give you an idea of how to write similar apps.

Running The Example

First, open the console and clone the ZIO Quickstarts project using git (or you can simply download the project) and then change the directory:

$ git clone https://github.com/zio/zio-quickstarts.git
$ cd zio-quickstarts/zio-quickstart-hello-world

Once you are inside the project directory, run the application:

sbt run

Alternatively, to enable hot-reloading and prevent port binding issues, you can use:

sbt reStart

note

If you encounter a "port already in use" error, you can use sbt-revolver to manage server restarts more effectively. The reStart command will start your server and reStop will properly stop it, releasing the port.

To enable this feature, we have included sbt-revolver in the project. For more details on this, refer to the ZIO HTTP documentation on hot-reloading.

Testing The Quickstart

When we prompt the sbt run the sbt search for the executable class and will find the zio.dev.quickstart.MainApp which contains the following code:

import zio._

object MainApp extends ZIOAppDefault {
  def run = Console.printLine("Hello, World!")
}

The MainApp is the main object of our application which extends the ZIOAppDefault trait. This trait provides the main function which can be invoked from IDEs or the command-line, e.g. the sbt run.

This example is the simplest, example we can run. Let's change it so that we ask the name of the user and print a hello along with the user's name:

import zio._

object MainApp extends ZIOAppDefault {
  def run =
    for {
      _    <- Console.print("Please enter your name: ")
      name <- Console.readLine
      _    <- Console.printLine(s"Hello, $name!")
    } yield ()
}

In this example, we used for-comprehension to compose print, readLine and printLine workflows. We can think of it as the following flatMap operations:

import zio._

object MainApp extends ZIOAppDefault {
  def run =
    Console.print("Please enter your name: ")
      .flatMap { _ =>
        Console.readLine
          .flatMap { name =>
            Console.printLine(s"Hello, $name!")
          }
      }
}