ThunkVar.scala

object Bar {
  private var graphFun: Function0[Any] = () => ()
  def graph: () => Any = graphFun
  def graph_=(fun: => Any) {
    graphFun = () => fun
  }
  def boing() = graphFun()
}

Bar.graph = { println("Hallo") }
Bar.boing()

//////////////////

trait ZeroOut

object Out extends ZeroOut

object GE {
  implicit def const(f: Float): GE = Const(f)
  final case class Const(f: Float) extends GE
}
trait GE

object GraphFunction {
  object Result {
    implicit def in[T](implicit view: T => GE) = In(view)
    implicit case object Out extends Result[ZeroOut]
    final case class In[T](view: T => GE) extends Result[T]
  }
  sealed trait Result[-T]
}

object Foo {
  private var res: GraphFunction.Result[_] = GraphFunction.Result.Out
  private var fun: () => Any = () => ()
  def graph: () => Any = fun
  def graph_=[T](body: => T)(implicit result: GraphFunction.Result[T]) {
    res = result
    fun = () => body
  }
  
  def apply() = fun()
}

Foo.graph = { println("Schoko"); 33f }
Foo()