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?
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).
It's not Scala-specific, but I'd reach for AtomicReference first in Java land. How much of atoms functionality do you want/need?
