Unsafe and safe concurrent threads for experimenting in the Scala REPL

ThreadSafety.scala

var shared = 0
val lock = new java.lang.Object

def unsafe =
  val local = shared
  Thread.sleep(0)
  shared = local + 1

def safe =
  lock.synchronized:
    val local = shared
    Thread.sleep(0)
    shared = local + 1

def runConcurrently(block: => Unit)(howMany: Int) =
  shared = 0
  val ps = Seq.fill(howMany):
    new java.lang.Thread:
      override def run() = block
  ps.foreach(_.start())
  ps.foreach(_.join())
  shared

// first paste up to here, then the experiments below

val vu = Iterator.continually(runConcurrently(unsafe)(4)) // infinite stream of results of running runUnsafe
vu take 1000 count ( _ != 4 )          // see how many of the first 1000 are not 4

val vs = Iterator.continually(runConcurrently(safe)(4)) // infinite stream of results of running runSafe
vs take 1000 count ( _ != 4 )          // see how many of the first 1000 are not 4