Question

I’m currently learning Scala, and just discovered the way to create custom field getters/setters. I have a simple example working:

class Thing(private val a:Int){
  override def toString = "Thing[" + a + "]"
  private var _value = a
  def value = _value
  def value_= (newVal:Int) = _value = newVal
}

On the console I can do:

scala> var t = new Thing(2)
t: dylan.code.Thing = Thing[2]

scala> t.value
res1: Int = 2

scala> t.value = 3

scala> t.value
res2: Int = 3

Now I’m trying to bring this concept to a slightly more complicated example; I’ll try to whittle the code down to what’s relevant:

abstract class CellExpression[Type] extends Publisher[CellUpdateEvent[Type]] with Subscriber[CellUpdateEvent[Type], CellExpression[Type]]{
    protected var cachedValue: Type = recalculateValue() 
    protected def recalculateValue(): Type

    protected def changeValue(newValue: Type):Unit = {
        val oldValue = value()
        if(newValue != oldValue){
            cachedValue = newValue
            publish(new CellUpdateEvent(this, oldValue, newValue))
        }
    }

    def value() = cachedValue
    def notify(pub: CellExpression[Type], event: CellUpdateEvent[Type]) = changeValue(recalculateValue())
}
//....
class CellVariable[Type](private val initialValue:Type) extends CellExpression[Type]{
    cachedValue = initialValue
    protected def recalculateValue() = { cachedValue }
    override def toString = "CellVariable[" + value + "]"
    def value_= (newValue:Type) = {changeValue(newValue)}
}

As far as I can tell, I’ve done what I need to in order to be able to treate value as a field via its getter and setter. But when I try it out in the console, I get:

scala> var i = new CellVariable(2)
i: dylan.code.CellVariable[Int] = CellVariable[2]

scala> i.value = 3
<console>:11: error: reassignment to val
       i.value = 3
               ^

What have I done wrong, and how can I fix it?

Answer 1

I actually stumbled onto the solution.

The line where I declare my value function: def value() = cachedValue is the culprit.
If I remove the parentheses to make the line def value = cachedValue everything seems to work as I expected.