Version: 2.x

Defects

By providing a Throwable value to the ZIO.die constructor, we can describe a dying effect:

object ZIO {
  def die(t: => Throwable): ZIO[Any, Nothing, Nothing]
}

Here is an example of such effect, which will die because of encountering divide by zero defect:

import zio._

val dyingEffect: ZIO[Any, Nothing, Nothing] =
  ZIO.die(new ArithmeticException("divide by zero"))

The result is the creation of a ZIO effect whose error channel and success channel are both Nothing. In other words, this effect cannot fail and does not produce anything. Instead, it is an effect describing a defect or an unexpected error.

Let's see what happens if we run this effect:

import zio._

object MainApp extends ZIOAppDefault {
  def run = ZIO.die(new ArithmeticException("divide by zero"))
}

If we run this effect, ZIO runtime will print the stack trace that belongs to this defect. So, here is the output:

timestamp=2022-02-16T13:02:44.057191215Z level=ERROR thread=#zio-fiber-0 message="Exception in thread "zio-fiber-2" java.lang.ArithmeticException: divide by zero
	at MainApp$.$anonfun$run$1(MainApp.scala:4)
	at zio.ZIO$.$anonfun$die$1(ZIO.scala:3384)
	at zio.internal.FiberContext.runUntil(FiberContext.scala:255)
	at zio.internal.FiberContext.run(FiberContext.scala:115)
	at zio.internal.ZScheduler$$anon$1.run(ZScheduler.scala:151)
	at <empty>.MainApp.run(MainApp.scala:4)"

The ZIO.die constructor is used to manually describe a dying effect because of a defect inside the code.

For example, assume we want to write a divide function that takes two numbers and divides the first number by the second. We know that the divide function is not defined for zero dominators. Therefore, we should signal an error if division by zero occurs.

We have two choices to implement this function using the ZIO effect:

We can divide the first number by the second, and if the second number was zero, we can fail the effect using ZIO.fail with the ArithmeticException failure value:

import zio._

def divide(a: Int, b: Int): ZIO[Any, ArithmeticException, Int] =
  if (b == 0)
    ZIO.fail(new ArithmeticException("divide by zero"))
  else
    ZIO.succeed(a / b)

We can divide the first number by the second. In the case of zero for the second number, we use ZIO.die to kill the effect by sending a signal of ArithmeticException as a defect:

import zio._

def divide(a: Int, b: Int): ZIO[Any, Nothing, Int] =
  if (b == 0)
    ZIO.die(new ArithmeticException("divide by zero")) // Unexpected error
  else
    ZIO.succeed(a / b)

So what is the difference between these two approaches? Let's compare the function signature:

def divide(a: Int, b: Int): ZIO[Any, ArithmeticException, Int]   // using ZIO.fail
def divide(a: Int, b: Int): ZIO[Any, Nothing,             Int]   // using ZIO.die

The first approach, models the divide by zero error by failing the effect. We call these failures expected errors or typed error.
While the second approach models the divide by zero error by dying the effect. We call these kinds of errors unexpected errors, defects or untyped errors.

We use the first method when we are handling errors as we expect them, and thus we know how to handle them. In contrast, the second method is used when we aren't expecting those errors in our domain, and we don't know how to handle them. Therefore, we use the let it crash philosophy.

In the second approach, we can see that the divide function indicates that it cannot fail because it's error channel is Nothing. However, it doesn't mean that this function hasn't any defects. ZIO defects are not typed, so they cannot be seen in type parameters.

Note that to create an effect that will die, we shouldn't throw an exception inside the ZIO.die constructor, although it works. Instead, the idiomatic way of creating a dying effect is to provide a Throwable value into the ZIO.die constructor:

import zio._

val defect1 = ZIO.die(new ArithmeticException("divide by zero"))       // recommended
val defect2 = ZIO.die(throw new ArithmeticException("divide by zero")) // not recommended

Also, if we import a code that may throw an exception, all the exceptions will be translated to the ZIO defect:

import zio._

val defect3 = ZIO.succeed(throw new Exception("boom!"))

Therefore, in the second approach of the divide function, we do not require to manually die the effect in case of the dividing by zero, because the JVM itself throws an ArithmeticException when the denominator is zero.

When we import any code into the ZIO effect, any exception is thrown inside that code will be translated to ZIO defects by default. So the following program is the same as the previous example:

import zio._

def divide(a: Int, b: Int): ZIO[Any, Nothing, Int] =
  ZIO.succeed(a / b)

Another important note is that if we map/flatMap a ZIO effect and then accidentally throw an exception inside the map operation, that exception will be translated to a ZIO defect:

import zio._

val defect4 = ZIO.succeed(???).map(_ => throw new Exception("Boom!"))
val defect5 = ZIO.attempt(???).map(_ => throw new Exception("Boom!"))

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