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Version: ZIO 2.x

ZState

ZState[S] models a value of type S that can be read from and written to during the execution of an effect. This is a higher-level construct built on top of FiberRef and the environment type to support using ZIO where we might have traditionally used state monad transformers.

Let's try a simple example of using ZState:

import zio._

import java.io.IOException

object ZStateExample extends zio.ZIOAppDefault {
val myApp: ZIO[ZState[Int], IOException, Unit] = for {
s <- ZIO.service[ZState[Int]]
_ <- s.update(_ + 1)
_ <- s.update(_ + 2)
state <- s.get
_ <- Console.printLine(s"current state: $state")
} yield ()

def run = ZIO.stateful(0)(myApp)
}

The idiomatic way to work with ZState is as part of the environment using operators defined on ZIO to access the ZState from the environment, and finally, allocate the initial state using the ZIO.stateful operator.

Because we typically use ZState as part of the environment, it is recommended to define our own state type S such as MyState rather than using a type such as Int to avoid the risk of ambiguity:

import zio._

import java.io.IOException

final case class MyState(counter: Int)

object ZStateExample extends zio.ZIOAppDefault {

val myApp: ZIO[ZState[MyState], IOException, Unit] =
for {
counter <- ZIO.service[ZState[MyState]]
_ <- counter.update(state => state.copy(counter = state.counter + 1))
_ <- counter.update(state => state.copy(counter = state.counter + 2))
state <- counter.get
_ <- Console.printLine(s"Current state: $state")
} yield ()

def run = ZIO.stateful(MyState(0))(myApp)
}

The ZIO data type also has some helper methods to work with ZState as the environment of ZIO effect such as ZIO.updateState, ZIO.getState, and ZIO.getStateWith:

import zio._

import java.io.IOException

final case class MyState(counter: Int)

val myApp: ZIO[ZState[MyState], IOException, Int] =
for {
_ <- ZIO.updateState[MyState](state => state.copy(counter = state.counter + 1))
_ <- ZIO.updateState[MyState](state => state.copy(counter = state.counter + 2))
state <- ZIO.getStateWith[MyState](_.counter)
_ <- Console.printLine(s"Current state: $state")
} yield state

An important note about ZState is that it is on top of the FiberRef data type. So it will inherit its behavior from the FiberRef.

For example, when a fiber is going to join to its parent fiber, its state will be merged with its parent state:

import zio._

case class MyState(counter: Int)

object ZStateExample extends ZIOAppDefault {
val myApp = for {
_ <- ZIO.updateState[MyState](state => state.copy(counter = state.counter + 1))
fiber <-
(for {
_ <- ZIO.updateState[MyState](state => state.copy(counter = state.counter + 1))
state <- ZIO.getState[MyState]
_ <- Console.printLine(s"Current state inside the forked fiber: $state")
} yield ()).fork
_ <- ZIO.updateState[MyState](state => state.copy(counter = state.counter + 5))
state1 <- ZIO.getState[MyState]
_ <- Console.printLine(s"Current state before merging the fiber: $state1")
_ <- fiber.join
state2 <- ZIO.getState[MyState]
_ <- Console.printLine(s"The final state: $state2")
} yield ()

def run =
ZIO.stateful(MyState(0))(myApp)
}

The output of running this snippet code would be as below:

Current state before merging the fiber: MyState(6)
Current state inside the forked fiber: MyState(2)
The final state: MyState(2)