Basic Scala STM Syntax

stmbasic.scala

import scala.concurrent.stm._
 
val x = Ref(0) // allocate a Ref[Int]
val y = Ref.make[String]() // type-specific default
val z = x.single // Ref.View[Int]
 
atomic { implicit txn =>
  val i = x() // read
  y() = "x was " + i // write
  val eq = atomic { implicit txn => // nested atomic
    x() == z() // both Ref and Ref.View can be used inside atomic
  }
  assert(eq)
  y.set(y.get + ", long-form access")
}
 
// only Ref.View can be used outside atomic
println("y was '" + y.single() + "'")
println("z was " + z())
 
atomic { implicit txn =>
  y() = y() + ", first alternative"
  if (x getWith { _ > 0 }) // read via a function
    retry // try alternatives or block 
} orAtomic { implicit txn =>
  y() = y() + ", second alternative"
}
 
val prev = z.swap(10) // atomic swap
val success = z.compareAndSet(10, 11) // atomic compare-and-set
z.transform { _ max 20 } // atomic transformation
val pre = y.single.getAndTransform { _.toUpperCase }
val post = y.single.transformAndGet { _.filterNot { _ == ' ' } }