An example ZIO 2’s collectAll method (and types and type aliases)

By Alvin Alexander. Last updated: January 28, 2024

As a brief note, here’s an example of the ZIO (ZIO 2) collectAll method.

To make things more interesting, I also show several examples of how to use ZIO types and type aliases, both with the printLine function and the theEnd value at the end:

//> using scala "3"
//> using lib "dev.zio::zio::2.0.19"

// Set up your IDE:
//     scala-cli setup-ide .
// Run me like this:
//     scala-cli ThisFilename.scala

// `Task[A]` is a type alias for `ZIO[Any, Throwable, A]`
// `UIO[A]`  is a type alias for `ZIO[Any, Nothing, A]`
import zio.{Task, UIO, ZIO, ZIOAppDefault}

/**
  * This started as a ZIO `collectAll` example, and it also shows
  * type alternatives on the `printLine` function and the `theEnd` result.
  */
object ZioCollectAll extends ZIOAppDefault:

    // `ZIO.attempt` yields either of these types:
    // -------------------------------------------
    // def printLine(line: String): Task[Unit]                = ZIO.attempt(println(line))
    // def printLine(line: String): ZIO[Any, Throwable, Unit] = ZIO.attempt(println(line))

    // `ZIO.succeed` yields either of these types:
    // -------------------------------------------
    // def printLine(line: String): ZIO[Any, Nothing, Unit] = ZIO.succeed(println(line))
    def printLine(line: String): UIO[Unit]                  = ZIO.succeed(println(line))

    val run =

        val rez: ZIO[Any, Nothing, List[Int]] = ZIO.collectAll(
            List(
                ZIO.succeed(1),
                ZIO.succeed(2),
                ZIO.succeed(3)
            )
        )

        // when `printLine` uses `ZIO.attempt`, use either of these:
        // ---------------------------------------------------------
        // val theEnd                            = rez.flatMap(sum => printLine(s"$sum"))
        // val theEnd: ZIO[Any, Throwable, Unit] = rez.flatMap(sum => printLine(s"$sum"))
        // val theEnd: Task[Unit]                = rez.flatMap(sum => printLine(s"$sum"))

        // when `printLine` uses `ZIO.succeed`, use either of these:
        // ---------------------------------------------------------
        // val theEnd                          = rez.flatMap(sum => printLine(s"$sum"))
        // val theEnd: ZIO[Any, Nothing, Unit] = rez.flatMap(sum => printLine(s"$sum"))
        val theEnd: UIO[Unit]                  = rez.flatMap(sum => printLine(s"$sum"))

        theEnd