sample code of Cats Effect LiftIO.liftIO

LiftIODemo.scala

package cats_effect_typeclass_exericise.liftio

import java.time.format.DateTimeFormatter

import cats.data.{Chain, Kleisli, WriterT}
import cats.effect._
import scala.language.higherKinds
import scala.util.Try
import LiftIODemoUtil._
import cats.instances.either._

object LiftIODemoUtil {
  val formatter = DateTimeFormatter.ofPattern("mm:ss")

  def putIn(milli: Int, s: Any): String = {
    Thread.sleep(milli)
    val threadId      = Thread.currentThread.getId
    val currentMinSec = java.time.LocalTime.now.format(formatter)
    println(s"in thread($threadId) at $currentMinSec): $s")
    s.toString
  }
  def putLine(s: Any): String = putIn(1000, s)
  def putIn1(s: Any):  String = putIn(1000, s)
  def putNow(s: Any):  String = putIn(0,    s)
  def putRnd(s: Any):  String = putIn((math.random * 1000 + 500).toInt, s)

  // Either[Throwable, ?] の LiftIO インスタンスがスコープにあれば...
  type ErrorOr[T] = Either[Throwable, T]
  implicit val errorOrLiftIO: LiftIO[ErrorOr] = new LiftIO[ErrorOr] {
    def liftIO[A](ioa: IO[A]): ErrorOr[A] = ioa.attempt.unsafeRunSync()
  }
}

object LiftIOKleisliDemo extends IOApp {
  type MyKleisli[T] = Kleisli[ErrorOr, String, T]

  def tryParse(s: String): ErrorOr[Int] = Try(s.toInt).toEither

  // Either[Throwable, ?] の LiftIO インスタンスがスコープにあれば
  // Kleisli との合成インスタンスが暗黙に提供される
  def testLiftIO(ios: IO[String])(implicit L: LiftIO[MyKleisli]): MyKleisli[Int] =
    L.liftIO(ios).flatMapF(tryParse)

  def run(args: List[String]): IO[ExitCode] = {
    putLine(testLiftIO(IO("100")).run("dummy")) // Right(100)
    putLine(testLiftIO(IO("NaN")).run("dummy")) // Left(NumberFormatException: For input string: "NaN")
    putLine(testLiftIO(IO.raiseError(new Exception("boom!"))).run("dummy")) // Left(Exception: boom!)
    IO(ExitCode.Success)
  }
}
object LiftIOWriterTDemo {
  type MyWriterT[T] = WriterT[ErrorOr, Chain[String], T]

  def tryParse(s: String): ErrorOr[Int] =
    Try { putNow(s"parsing $s"); s.toInt }.toEither

  // Either[Throwable, ?] の LiftIO インスタンスさえスコープにあれば
  // WriterT との合成インスタンスが暗黙に提供される
  def testWriterT(ios: IO[String])(implicit L: LiftIO[MyWriterT]): MyWriterT[Int] =
    L.liftIO(ios)
      .tell(Chain("log: hello"))
      .flatMap(x => WriterT.liftF(tryParse(x)))

  def main(args: Array[String]): Unit = {
    val ios = IO { putNow("now, ios's block is being executed"); "1234" }
    putLine("ios is initialized, but execution of its block is deferred")
    putLine(testWriterT(ios).run)
  }
  //  in thread(1) at 49:18): ios is initialized, but execution of its block is deferred
  //  in thread(1) at 49:19): now, ios's block is being executed
  //  in thread(1) at 49:19): parsing 1234
  //  in thread(1) at 49:20): Right((Chain(log: hello),1234))
}