Introduction to ZIO's Built-in Services

ZIO already provides four built-in services:

1. Console — Operations for reading/writing strings from/to the standard input, output, and error console.
2. Clock — Contains some functionality related to time and scheduling.
3. Random — Provides utilities to generate random numbers.
4. System — Contains several useful functions related to system environments and properties.

All four follow the same design: a service trait, a companion object with a Tag and accessor methods, a Live implementation, and a synchronous UnsafeAPI. See Anatomy of a Built-in Service for how that shared design works.

When we use these services we don't need to provide their corresponding environment explicitly. ZIO provides built-in live version of ZIO services to our effects, so we do not need to provide them manually. This works because ZIO seeds a FiberRef of default services, DefaultServices.live, with Live implementations of all four services before a program ever runs, so every accessor reads from that FiberRef unless something upstream overrides it — see Anatomy of a Built-in Service for the details.

import zio._

import java.io.IOException

object MainApp extends ZIOAppDefault {
  val myApp: ZIO[Any, IOException, Unit] = 
    for {
      date <- Clock.currentDateTime
      _    <- ZIO.logInfo(s"Application started at $date")
      _    <- Console.print("Enter your name: ")
      name <- Console.readLine
      _    <- Console.printLine(s"Hello, $name!")
    } yield ()

  def run = myApp
}

myApp above needs both Clock and Console, so its environment type composes them with an intersection type — Clock with Console in Scala 2, Clock & Console in Scala 3. Anatomy of a Built-in Service covers this environment composition in detail.

Testing These Services

Each of these four services has a deterministic test double — TestClock, TestConsole, TestRandom, and TestSystem — that ZIO Test swaps in for Live behind the same trait, so code written against Clock, Console, Random, and System is testable without any changes. See Testing These Services for how each test double works.

See Also

• Anatomy of a Built-in Service — The shared trait/companion/Live/UnsafeAPI design behind all four services.
• Writing ZIO Services — Guide to ZIO Service Pattern: define maintainable services using interfaces and ZLayer for automatic dependency injection.