Failures

When writing ZIO application, we can model a failure, using the ZIO.fail constructor:

trait ZIO {
  def fail[E](error: => E): ZIO[Any, E, Nothing]
}

Let's try to model some failures using this constructor:

import zio._

val f1: ZIO[Any, String, Nothing] = ZIO.fail("Oh uh!")
val f2: ZIO[Any, String, Int]     = ZIO.succeed(5) *> ZIO.fail("Oh uh!")

Now, let's try to run a failing effect and see what happens:

import zio._

object MainApp extends ZIOAppDefault {
  def run = ZIO.succeed(5) *> ZIO.fail("Oh uh!")
}

This will crash the application and print the following stack trace:

timestamp=2022-03-08T17:55:50.002161369Z level=ERROR thread=#zio-fiber-0 message="Exception in thread "zio-fiber-2" java.lang.String: Oh uh!
    at <empty>.MainApp.run(MainApp.scala:4)"

We can also model a failure using Exception:

val f3: ZIO[Any, Exception, Nothing] =
  ZIO.fail(new Exception("Oh uh!"))

Or using user-defined failure types (domain errors):

import zio._

case class NegativeNumberException(msg: String) extends Exception(msg)

def validateNonNegative(input: Int): ZIO[Any, NegativeNumberException, Int] =
  if (input < 0)
    ZIO.fail(NegativeNumberException(s"entered negative number: $input"))
  else
    ZIO.succeed(input)

In the above examples, we can see that the type of the validateNonNegative function is ZIO[Any, NegativeNumberException, Int]. It means this is an exceptional effect, which may fail with the type of NegativeNumberException.

The ZIO.fail constructor is somehow the moral equivalent of throw for pure codes. We will discuss this further.