arrow-fx / arrow.fx.typeclasses / MonadDefer / Ref

Ref

open fun <A> Ref(a: A): Kind<F, Ref<F, A>>

Create a Ref a pure atomic reference to safely manage mutable state. It’s a pure version of java.util.concurrent.atomic.AtomicReference that works in context F.

It’s always initialized to a value, and allows for concurrent updates.

import arrow.Kind
import arrow.core.Tuple2
import arrow.fx.*
import arrow.fx.extensions.io.monadDefer.monadDefer
import arrow.fx.typeclasses.MonadDefer

fun main(args: Array<String>) {
  fun <F> MonadDefer<F>.refExample(): Kind<F, Tuple2<Int, Int>> =
    //sampleStart
    fx.monad {
      val ref = !Ref(1)
      val initial = !ref.getAndUpdate(Int::inc)
      val updated = !ref.get()
      Tuple2(initial, updated)
    }

  //sampleEnd
  IO.monadDefer().refExample()
    .fix().unsafeRunSync().let(::println)
}