# URIO

> Type alias for ZIO[R, Nothing, A] representing an effect requiring environment R that cannot fail but succeeds with value A.

`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`.

:::note

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:

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

So `URIO` is equal to a `ZIO` that requires `R` and cannot fail (because in Scala the `Nothing` type has no inhabitant, so we can't create an instance of type `Nothing`). It succeeds with `A`.

:::note Principle of Least Power

[The `ZIO` data type](zio.md) 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.
:::

## See Also

- [Exceptional and Unexceptional Effects](../../error-management/exceptional-and-unexceptional-effects.md)