Scala: store call-by-name variable as class field

Question

In my progress in Scala learning I try to implement a simple DSL with callbacks

object Button {...} // apply 
class Button(val name: String) {
    private val: => Unit; // doesn't work

    def click(f: => Unit) = {
        _click_cb = f
        this
    }

    def onClick() = this._click_cb()
}

Button("Click me!") click {println("Clicked!")}

I create a new object, pass it a callback to store. My demo framework fires onClick method, that should call the stored one

It works with () => Unit but my DSL looks ugly:

Button("Click me!") click (() => println("Clicked!"))

Sure, I could do onClick abstract and implement an anonymous class later

new Button("Click me!") {def onClick = println("Clicked!")}

But I want to play with some DSL and such

The questions are:

• How do I store f in _click_cb ?
• How do I provide initial "empty" function for _click_cb ?
• And maybe there's a more scala-way to achieve this? (without anonymous classes)

Answer 1

An uglier version just to show that lazy val can hold the by name parameter value without evaluating it:

case class Button(val name: String) {
  def clickCallback(): Unit = ()

  def click(f: => Unit) = {
    lazy val notEvaluated = f
    new Button(name) { override def clickCallback() = notEvaluated }
  }

  def onClick(): Unit = clickCallback()
}

A cleaner and more functional implementation:

class Button(val name: String) {
  def click(f: => Unit) = new Button(name) { override def onClick() = f }

  def onClick(): Unit = ()
}