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Version: 2.0.x


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 UIO is equal to a ZIO that doesn't need any requirement (because it accepts Any environment) and that cannot fail (because in Scala the Nothing type is uninhabitable, i.e. there can be no actual value of type Nothing). It succeeds with A.

ZIO values of type UIO[A] are considered infallible. Values of this type may produce an A, but will never fail.

Let's write a Fibonacci function. In the following example, the fib function is an unexceptional effect, since it has no requirements, we don't expect any failure, and it succeeds with a value of type Int:

import zio.{UIO, ZIO}

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
Principle of 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 the other hand, the type aliases are a way of specializing the ZIO type for less powerful workflows.

Often, 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 appropriate specialized type alias.

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