ZIO Direct Style

ZIO Direct Style is a library that allows using directly-style i.e. imperative programming with ZIO effects which is based on the Monadless paradigm.

Installation

To use zio-direct, add the following to your build.sbt file.

libraryDependencies += "dev.zio" %% "zio-direct" % "1.0.0-RC7"

You can also use zio-direct with ZStream and ZPure by importing the following modules.

Currently this is only supported in Scala 3.

// ZStream
libraryDependencies += "dev.zio" %% "zio-direct-streams" % "1.0.0-RC7"
// ZPure
libraryDependencies += "dev.zio" %% "zio-direct-pure" % "1.0.0-RC7"

See the Other Supported Monads section for more details.

IDE Support

The preferred IDE to use with ZIO-Direct is Visual Studio Code + Metals. This is because Metals correctly reads the returns from defer calls directly from the Scala compiler which is not the case of IntelliJ. To remedy this issue, a Library Extension is provided for ZIO-Direct. See the IntelliJ Support section for more details.

Introduction

Talk at Functional Scala 2022:

ZIO-Direct allows direct style programming with ZIO. This library provides a syntactic sugar that is more powerful than for-comprehensions as well as more natural to use. Simply add the .run suffix to any ZIO effect in order to retrieve its value.

ZIO-Direct works by using macros to rewrite sequential code into flatMap-chains based on the Monadless paradigm. The values resulting in .run calls from the ZIO effects are not actually awaited. Instead, they are rolled-up into a chain of flatMaps.

For example, in imperative programming operations typically are done in a simple set of steps.

object FileOps:
  def read(file: File): String
  def write(file: File, content: String): Unit

val textA = read(fileA)
val textB = read(fileB)
write(fileC, textA + textB)

Using functional programming, the equivalent of this functionality is a set of nested flatMap-chains.

object FileOps
  def read(file: File): ZIO[Any, Throwable, String]
  def write(file: File, content: String): ZIO[Any, Throwable, Unit]

read(fileA).flatMap { textA =>
  read(fileB).flatMap { textB =>
    write(fileC, textA + textB)
  }
}

In order to avoid this complexity scala provides a for-comprehension syntactic sugar.

for {
  textA <- read(fileA)
  textB <- read(fileB)
  _ <- write(fileC, textA + textB)
} yield ()

Unfortunately this syntactic sugar is limited in many cases, for example, inserting a conditional value inside is impossible.

for {
  textA <- read(fileA)
  // Not a possible syntax
  if (fileA.contains("some string")) {
    textB <- read(fileB)
    _ <- write(fileC, textA + textB)
  }
} yield ()

ZIO-Direct offers an equivalent syntactic sugar that is more ergonomic and allows many constructs that for-comprehensions do not.

defer {
  val textA = read(fileA).run
  if (fileA.contains("some string")) {
    val textB = read(fileB).run
    write(fileC, textA + textB).run
  }
}

ZIO-Tailored

ZIO-Direct is specifically tailored to ZIO capabilities as it supports Environment and Error composition in ZIO effects similar to the for-comprehension.

val out: ZIO[CustomerConfig & DistributorConfig, CustomerGetException | DistrubutorGetException, (Customer, Distributor)] =
  defer {
    // Get a customer-configuration object from the environment and extract its .url field
    val custUrl: String = ZIO.service[CustomerConfig].run.url
    // Get a distributor-configuration from the environment and extract its .url field
    val distUrl: String = ZIO.service[DistributorConfig].run.url
    (
      // Use the two configurations to make an HTTP-call
      parseCustomer(httpGetCustomer(custUrl).run),
      parseDistrubutor(httpGetDistributor(distUrl).run)
    )
  }

Branching and Looping Support

Unlike the for-comprehension, ZIO-Direct supports branching and looping in the use of flatMaps composition. Let's have a look at a another non-trivial example.

class Database:
  def nextRow(): ZIO[Any, Throwable, Row]
  def hasNextRow(): ZIO[Any, Throwable, Boolean]
  def lockNextRow(): ZIO[Any, Throwable, Boolean]
object Database:
  def open: ZIO[Any, Throwable, Database]

defer {
  // Open a database connection
  val db = Database.open().run
  // See if there is is a next-row
  while (db.hasNextRow().run) {
    // try to lock, if aquired continue
    if (db.lockNextRow().run)
      val nextRow = db.nextRow().run
      doSomethingWith(nextRow)
    else
      waitT()
  }
}

NOTE: The above database-api is imaginary.

The above code needs to be translated into something like this:

Database.open.flatMap { db =>
  def whileFun(): ZIO[Any, Throwable, Unit] =
    db.hasNextRow().flatMap { hasNextRow =>
      if (hasNextRow)(
        db.lockNextRow().flatMap { lockNextRow =>
          if (!lockNextRow)
            db.nextRow().map(nextRow => doSomethingWith(nextRow))
          else
            ZIO.succeed(waitT())
        }
      ).flatMap(_ => whileFun())
      else
        ZIO.unit
    }
  whileFun()
}

Note that normally this is the exact code that would have to be written to capture such functionality For-comprehensions do not provide a way to do looping and branching so in such cases a combination of flatMaps and recursion is necessary to avoid calling effects unnecessarily.

Great for Refs and FiberRefs!

ZIO-direct makes it much easier to use ZIO mutable Ref and FiberRef variables. Since retrieveing and updating Ref and FiberRef variables requries a flatMap/for-comprehension call, it is typically very difficult to use them with branching/looping constructs. ZIO-direct makes these cases much easier.

class DatabaseApi {
  val connRef = FiberRef.make[Option[Connection]](None)
  def openConnection(): Connection = lowLevelDatabaseApi.openConnection()
  def transaction(action: Action) =
    defer {
      val conn = connRef.get.run
      if (conn == None) {
        connRef.set(Some(openConnection())).run
      }
      val conn1 = connRef.get.run
      conn.execute(action).run
    }
}

Instead of having to write the following code:

class DatabaseApi {
  val connRef = FiberRef.make[Option[Connection]](None)
  def openConnection(): Connection = lowLevelDatabaseApi.openConnection()
  def transaction(action: Action) =
    connRef.get.flatMap { conn =>
      (if (conn == None) {
        connRef.set(Some(openConnection()))
      } else {
        ZIO.unit
      }).flatMap(_ => connRef.get.flatMap { conn1 =>
        conn1.execute(action)
      })
    }

  // Note that for-comprehensions do not help very much in this use-case
  def transaction0(action: Action) =
    for {
      conn <- connRef.get
      _ <- if (conn == None) {
        connRef.set(Some(openConnection()))
      } else {
        ZIO.unit
      }
      conn1 <- connRef.get
      _ <- conn1.execute(action)
    }
}

Documentation

Learn more on the ZIO Direct Style homepage!

Contributing

For the general guidelines, see ZIO contributor's guide.

Code of Conduct

See the Code of Conduct

Support

Come chat with us on .

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