build.sbt

libraryDependencies += "org.typelevel" %% "cats-effect" % "2.0.0"

scalaVersion := "2.13.1"

CatsRefUnsafeExample.scala

package example

import cats.effect.concurrent.Ref
import cats.effect._
import cats.syntax.traverse._
import cats.instances.list._

// https://github.com/typelevel/cats-effect/blob/v2.0.0/core/shared/src/main/scala/cats/effect/concurrent/Ref.scala#L208
object CatsRefUnsafeExample extends IOApp {
  override def run(args: List[String]) = {
    val unsafe = {
      val x = Ref.unsafe[IO, Int](0)
      for {
        _ <- x.update(_ + 1)
        a <- x.get
      } yield {
        println("unsafe = " + a)
        a
      }
    }

    val safe = {
      for {
        x <- Ref[IO].of(0)
        _ <- x.update(_ + 1)
        a <- x.get
      } yield {
        println("safe = " + a)
        a
      }
    }

    List(unsafe, unsafe, unsafe, safe, safe, safe).sequence.map{ _ =>
      ExitCode.Success
    }
  }
}

log.txt

[info] running example.CatsRefUnsafeExample 
unsafe = 1
unsafe = 2
unsafe = 3
safe = 1
safe = 1
safe = 1