Exceptional and Unexceptional Effects

Besides the IO type alias, ZIO has four different type aliases which can be categorized into two different categories:

• Exceptional Effect— Task and RIO are two effects whose error parameter is fixed to Throwable, so we call them exceptional effects.
• Unexceptional Effect— UIO and URIO have error parameters that are fixed to Nothing, indicating that they are unexceptional effects. So they can't fail, and the compiler knows about it.

So when we compose different effects together, at any point of the codebase we can determine this piece of code can fail or cannot. As a result, typed errors offer a compile-time transition point between this can fail and this can't fail.

For example, the ZIO.acquireReleaseWith API asks us to provide three different inputs: acquire, release, and use. The release parameter requires a function from A to URIO[R, Any]. So, if we put an exceptional effect, it will not compile:

object ZIO {
  def acquireReleaseWith[R, E, A, B](
    acquire: => ZIO[R, E, A],
    release: A => URIO[R, Any],
    use: A => ZIO[R, E, B]
  ): ZIO[R, E, B]
}