Add update method for updating box fields in Actor

core/src/main/scala/lacasa/Actor.scala

@@ -66,6 +66,15 @@ abstract class Actor[T] {
     throw new NoReturnControl
   }
 
+  /* Variant of `open` for more convenient access to box fields.
+   *
+   * `fun` must satisfy the same safety constraints as closures passed to `open`
+   */
+  def update[S](select: => Box[S])(fun: Spore[S, S])(implicit noCapture: Safe[fun.Captured]): Unit = {
+    val theBox = select
+    theBox.updateInstance(fun)
+  }
+
   // swap for accessing fields of type Box[S]
   def swap[S](select: => Box[S])(assign: Box[S] => Unit, newBox: Box[S])(
     fun: Spore[Packed[S], Unit] { type Excluded = newBox.C })(

core/src/main/scala/lacasa/Box.scala

@@ -8,7 +8,7 @@ import scala.reflect.{ClassTag, classTag}
 import scala.spores._
 
 import scala.util.control.ControlThrowable
-
+import scala.annotation.unchecked.uncheckedVariance
 
 private class NoReturnControl extends ControlThrowable {
   // do not fill in stack trace for efficiency
@@ -63,7 +63,7 @@ object Safe {
   implicit def tuple2IsSafe[T, S](implicit one: Safe[T], two: Safe[S]): Safe[(T, S)] = new Safe[(T, S)] {}
 }
 
-sealed class Box[+T] private (private val instance: T) {
+sealed class Box[+T] private (private var instance: T @uncheckedVariance) {
   self =>
 
   type C
@@ -77,6 +77,11 @@ sealed class Box[+T] private (private val instance: T) {
     }
   }
 
+  // trusted operation
+  private[lacasa] def updateInstance(fun: T => T @uncheckedVariance): Unit = {
+    instance = fun(instance)
+  }
+
   def open(fun: Spore[T, Unit])(implicit access: CanAccess { type C = self.C }, noCapture: Safe[fun.Captured]): Box[T] = {
     fun(instance)
     self