What is the Scala equivalent of Clojure's Atom?

clojure, concurrency, multithreading, scala

Solution

As @Shepmaster and @om-nom-nom said, it's a wrapper around `java.util.concurrent.atomic.Atomic...`.

An equivalent wrapper could look like this:

import java.util.concurrent.atomic._
import scala.annotation.tailrec

object Atom {
  def apply[A](init: A): Atom[A] = new Impl(new AtomicReference(init))

  private class Impl[A](state: AtomicReference[A]) extends Atom[A] {
    def apply(): A = state.get()
    def update(value: A): Unit = state.set(value)
    def transformAndGet(f: A => A): A = transformImpl(f)

    @tailrec private final def transformImpl(fun: A => A): A = {
      val v    = state.get()
      val newv = fun(v)
      if (state.compareAndSet(v, newv)) newv
      else transformImpl(fun)
    }
  }
}
trait Atom[A] {
  def apply(): A
  def update(value: A): Unit
  def transformAndGet(f: A => A): A
}

Ex:

val myAtom = Atom(0)
myAtom()  // --> 0
myAtom.transformAndGet(_ + 1) // --> 1
myAtom()  // --> 1
myAtom.transformAndGet(_ * 4) // --> 4

If you use Scala-STM, that functionality is built into STM references, by using the `.single` view:

scala> import scala.concurrent.stm._
import scala.concurrent.stm._

scala> val myAtom = Ref(0).single
myAtom: scala.concurrent.stm.Ref.View[Int] = 
        scala.concurrent.stm.ccstm.CCSTMRefs$IntRef@52f463b0

scala> myAtom()
res0: Int = 0

scala> myAtom.transformAndGet(_ + 1)
res1: Int = 1

scala> myAtom()
res2: Int = 1

scala> myAtom.transformAndGet(_ * 4)
res3: Int = 4

The advantage is that `Ref.apply` will already give you specialised cells for the primitive types, e.g. `Int` instead of `AnyRef` (boxed).

Problem

Clojure has an Atom for changing state between threads in a synchronous and independent manner, that is not part of the STM. You use it like this: ``` user=> (def my-atom (atom 0)) #'user/my-atom user=> @my-atom 0 user=> (swap! my-atom inc) 1 user=> @my-atom 1 user=> (swap! my-atom (fn [n] (* (+ n n) 2))) 4 ``` My question is: What is the Scala equivalent of Clojure's Atom?

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