David R. MacIver

class ErrRef[S](s : S){
  private[this] var contents : Either[Throwable, S] = Right(s);
 
  def update(value : =>S){
    contents = try { Right(value) } 
              catch { case (e : Throwable) => Left(e) }
  }
 
  def apply() = contents match {
    case Right(s) => s;
    case Left(e) => throw e.fillInStackTrace();
  }
}
 
object ER{
  def apply[S](s : S) = new ErrRef(s);
}

scala> ER(1)
res0: ErrRef[Int] = ErrRef@a96606
 
scala> res0()
res1: Int = 1
 
scala> res0() = 3
 
scala> res0()    
res3: Int = 3
 
scala> res0() = { println("Hello world"); 3} 
Hello world
 
scala> res0()                               
res5: Int = 3
 
scala> res0() = error("Lets see what happens here...")
 
scala> res0()
java.lang.RuntimeException: Lets see what happens here...
	at ErrRef.apply(RefExcept.scala:11)
	at .<init>(<console>:6)
	at .<clinit>(<console>)
	at RequestResult$.<init>(<console>:3)
	at RequestResult$.<clinit>(<console>)
	at RequestResult$result(<console>)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAcc...
scala> res0() = throw new IllegalArgumentException("Go away")
 
scala> res0()
java.lang.IllegalArgumentException: Go away
	at ErrRef.apply(RefExcept.scala:11)
	at .<init>(<console>:6)
	at .<clinit>(<console>)
	at RequestResult$.<init>(<console>:3)
	at RequestResult$.<clinit>(<console>)
	at RequestResult$result(<console>)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorI...