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Version: 1.0.18


UIO[A] is a type alias for ZIO[Any, Nothing, A], which represents an Unexceptional effect that doesn't require any specific environment, and cannot fail, but can 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 UIO type alias is defined:

type UIO[+A] = ZIO[Any, Nothing, A]

So the UIO just equal to ZIO which doesn't need any requirement and cannot fail because in the Scala the Nothing type has no inhabitant, we can't create an instance of type Nothing.

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

Let's write a fibonacci function. As we don't expect any failure, it is an unexceptional effect:

In the following example, the fib, doesn't have any requirement, as it is an unexceptional effect, we don't except any failure, and it succeeds with value of type Int:

import zio.UIO
def fib(n: Int): UIO[Int] =
if (n <= 1) {
} else {
for {
fiber1 <- fib(n - 2).fork
fiber2 <- fib(n - 1).fork
v2 <- fiber2.join
v1 <- fiber1.join
} yield v1 + v2

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.