Alex Rivera | Logout

Why can't I assign to var in Scala subclass?

Asked 2011-07-11T21:31:49.057
15

Suppose I have the following abstract class:

abstract class A (var is_happy : Boolean) {
  def toggle_happiness();
}

And now I want to define a concrete class which implements the toggle_happiness() method:

class B (is_happy : Boolean) extends A (is_happy) {
  def toggle_happiness() = {
    is_happy = !is_happy
  }
}

Scala's compiler gives me:

error: reassignment to val
   is_happy = !is_happy
            ^

What's going on here? I thought that is_happy referred to a var in my class that is set by my constructor. Do I have a conflict with the name is_happy?

Thanks, Dan

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1 Answer

1

There is another simple solution. One that only requires to modify B (no need to change the base class) and won't modify the interface (no parameter renaming). Just introduce a private method that returns this, and explicitly dereference this:

class B (var is_happy : Boolean) extends A {
  private def self = this
  def toggle_happiness() = {
    self.is_happy = !self.is_happy
  }
}

Note that this work around is localized to B. Everywhere else (included in derived classes) you can keep using just is_happy (no need for self.is_happy).

As a side note, we should really be able to directly dereference this, as in this.is_happy (instead of adding the self method and doing self.is_happy). But for some reason the compiler will blindly treat this.is_happy the same as is_happy, so we get back to square one and this.is_happy actually still point to B's parameter rather than A's variable. This very much looks like a compiler bug to me.

answered 2013-03-05T23:51:19.543

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