Typed Errors Guarantees

Typed errors don't guarantee the absence of defects and interruptions. Having an effect of type ZIO[R, E, A], means it can fail because of some failure of type E, but it doesn't mean it can't die or be interrupted. So the error channel is only for failure errors.

In the following example, the type of the validateNonNegativeNumber function is ZIO[Any, String, Int] which denotes it is a typed exceptional effect. It can fail of type String but it still can die with the type of NumberFormatException defect:

import zio._

def validateNonNegativeNumber(input: String): ZIO[Any, String, Int] =
  input.toIntOption match {
    case Some(value) if value >= 0 =>
      ZIO.succeed(value)
    case Some(other) =>
      ZIO.fail(s"the entered number is negative: $other")
    case None =>
      ZIO.die(
        new NumberFormatException(
          s"the entered input is not in the correct number format: $input"
        )
      )
  }

Also, its underlying fiber can be interrupted without affecting the type of the error channel:

import zio._

val myApp: ZIO[Any, String, Int] =
  for {
    f <- validateNonNegativeNumber("5").fork
    _ <- f.interrupt
    r <- f.join
  } yield r

Therefore, if we run the myApp effect, it will be interrupted before it gets the chance to finish.