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Version: ZIO 2.x


URIO[R, A] is a type alias for ZIO[R, Nothing, A], which represents an effect that requires an R, 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 URIO type alias is defined:

type URIO[-R, +A] = ZIO[R, Nothing, A]

So the URIO just equal to ZIO which requires R and cannot fail because in the Scala the Nothing type has no inhabitant, we can't create an instance of type Nothing. It succeeds with A.

In following example, the type of printLine is URIO[Console, Unit] which means, it requires Console service as an environment, and it succeeds with Unit value:

def printLine(line: => String): ZIO[Has[Console], IOException, Unit] =  ZIO.serviceWith(_.printLine(line))

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.