Skip to main content
Version: ZIO 2.x


IO[E, A] is a type alias for ZIO[Any, E, A], which represents an effect that has no requirements, and may fail with an E, or succeed with an A.


In Scala, the type alias is a way to give a name to another type, to avoid having to repeat the original type again and again. It doesn't affect the type-checking process. It just helps us to have an expressive API design.

Let's see how the IO type alias is defined:

type IO[+E, +A] = ZIO[Any, E, A]

So the IO just equal to ZIO which doesn't need any requirement.

ZIO values of type IO[E, Nothing] (where the value type is Nothing) are considered unproductive, because the Nothing type is uninhabitable, i.e. there can be no actual values of type Nothing. Values of this type may fail with an E, but will never produce a value.

Note: Principle of The Least Power

The ZIO data type is the most powerful effect in the ZIO library. It helps us to model various types of workflows. On other hand, the type aliases are a way of subtyping and specializing the ZIO type, specific for a less powerful workflow.

Lot of the time, we don't need such a piece of powerful machinery. So as a rule of thumb, whenever we require a less powerful effect, it's better to use the proper specialized type alias.

So there is no need to convert type aliases to the ZIO data type, whenever the ZIO data type is required, we can use the most precise type alias to fit our workflow requirement.